Samples from three (Mg,Fe)-carpholite [1,3,4] (high-P, low-T) and one Mn-carpholite outcrops [2] (low-P, low-T) have been investigated by TEM and EMP. Backscattered images revealed that chlorite and mica occurs pseudomorphically after (Mg,Fe)-carpholite surrounded by quartz. However, if the (Mg,Fe)-carpholite is investigated by HRTEM the structure is almost perfect and shows no reaction microstructures except for one individual grain that is in close contact to K-rich mica layers.
In contrast, Mn-carpholite reveals a variable amount of parageneses with sheet silicates as well as abnormal internal structural features:
1) Electron microscopy shows that chlorite and mica mainly precipitate topotactically with their layers parallel to the cleavage planes of Mn-carpholite (Mn-car). The interfaces between the sheet silicates (sh) and Mn-carpholite are tight and largely coherent. The sheet silicates tend to grow with their c*sh || to a*Mn-car or c*sh || to [110]*Mn-car and less often c*sh || to b*Mn-car.
2) Mn-carpholite exhibits locally clustered precipitates (size: 50-100Å) of serpentine-like layers that grew within the negative form of the host crystal. The serpentine-like layers almost precipitate along a*Mn-car or b*Mn-car. Other mineral structures may be present as precipitates but they could yet not be identified unequivocally.
3) HRTEM of Mn-carpholite revealed that silicate chain-width errors occur analog to those that have been found in pyriboles. These defects can be explained either by the insertion of talc-like slabs that grew successively in [010]-direction or by removing the octahedral walls within the carpholite structure.
We suggest that high-P favors the perfection of the (Mg,Fe)-carpholite crystals similar to other silicates. However, the lack of defects and topotactic growth of sheet silicates with (Mg,Fe)-carpholite may also be due to: (i) the small amount of (Mg,Fe)-carpholite samples investigated by us; (ii) the metastability of (Mg,Fe)-carpholite during retrograde metamorphism that leads to its "immediate" break down if hydration occurs; (iii) the differences in chemistry of the carpholites and the different P-T-t paths (see Ref.) of the carpholite outcrops that may favor the precipitation of serpentine-like layers in Mn-carpholite.
Theye et al., Eur. J. Min, 4, 487-507, (1992).
Theye et al., J. Petrol, 37, 767-783, (1996).
Theye et al., Eur. J. Min, 9, 859-873, (1997).
Weh et al., 3rd workshop on alpine geological studies, Biella, Italy, (1992).
Oscillatory mineral zonation is in most cases the result of crystal growth in an open system. Familiar examples are hydrothermal garnets grown in porous or fractured rocks which have been open to large scale fluid flow, and magmatic plagioclase grown during periods of magma mixing or degassing. In most cases such open systems will be out of thermodynamic equilibrium and often they may not even be in a time-invariant or steady state on the time-scales of crystal growth. To interpret an observed oscillatory zonation pattern in terms of the underlying geological processes operating at large or small scales, we need to understand both 1) The crystal growth dynamics and thus its sensitivity to variable boundary conditions (i.e. fluctuations in the environment) and 2) The coupling between the crystal growth processes and processes operating on a scale much larger than the size of the growing crystal. Although numerous crystal growth models have been forwarded that produce oscillatory zoning patterns, we still don't have an adequate understanding of the processes taking place near the crystal surface during the generation of such patterns (i.e. processes responsible for dissipation effects, lattice strain effects, autocatalysis etc.). In this contribution we have analysed the effects of variable boundary conditions on existing crystal growth models. Some of these models are very sensitive even to low amplitude noise, implying that an observed zonation pattern will be significantly affected by other processes than the local growth processes. In some cases fluctuations in the external environment may cause synchronisation so that different crystals develop similar zonation patterns even if their growth is controlled by highly non-linear local dynamics. This implies that intercrystalline similarity in zonation pattern is not always a sufficient argument for external (environmental) controls on the observed patterns. However, with the current lack of reliable crystal growth models, the origin of the patterns may only be deciphered from a detailed chemical characterisation of the zoned crystal, focusing particularly at concentration variations in externally derived components.
Orientation contrast imaging, using forescatter detectors in the Scanning Electron Microscope, facilitates imaging of the internal variations in crystallographic orientations in minerals. Electron Backscatter Diffraction enables the variations to be quantified. Analysis of garnet porphyroblasts from a variety of environments, using these techniques, show that garnets commonly contain internal substructures.
Deformation Microstructures: Deformed mantle nodules and continental eclogites contain garnets with 10 to 50 micron cellular subdomains of different crystallographic orientation. The mismatches between subdomains are small, typically less than 3 degrees. Where misorientation axes can be constrained, they correspond to rational crystallographic zone axes. The subdomains are more clearly developed and smaller where deformation is more intense. The subdomains are interpreted as subgrains. Individual dislocations and dislocation arrays are imaged lending support to this conclusion. Compositional variations correspond to the intensity of garnet substructure.
Nucleation Microstructures: Some amphibolite facies garnets contain substructures that can be related to the coalescence of individual garnets during growth. This is unsurprising. What is surprising is that many of the individual domains have very similar orientations. Misorientations are generally less than 5°. In contrast to the subgrains described above, misorientation axes do not correspond to rational crystallographic zone axes. Coalescence of the nuclei must be controlled by some mechanisms which favoured aggregation of similarly oriented nuclei and destroyed or reoriented strongly misoriented ones and could be driven by some surface energy mechanism. Garnet aggregates show concentric zonation patterns: individual garnets, even though sometimes concentrically zoned, have compositions correspondent to their position in the aggregate.
Growth Microstructures: Amphibolite facies garnets with sector zones defined by inclusions/intergrowths often contain radial crystallographic misorientation boundaries. The misorientation boundaries do not correspond in position to the sector boundaries. The misorientations across the boundaries vary between 1 and 15°. Misorientation axes correspond to rational crystallographic zone axes. These are interpreted as structures related to the propagation of growth defects. The garnet compositions are concentrically zoned with no compositional changes corresponding to misorientation boundaries.
Recognition of widespread garnet substructures has significant implications for the use of garnets in thermobarometry and in radiometric dating. Better understanding of garnet substructures will enable us to extract more information from the textures of metamorphic rocks.
Lewisian grey gneisses are a suite of tonalite, trondhjemite and granodiorite (TTG) rocks similar to those found in other Archaean terranes around the world. The mainland Lewisian is divided into three - the northern, central and southern regions. The northern and southern regions attained only amphibolite grade. It has been assumed that the entire central region was affected by a granulite-facies event, the Badcallian metamorphism (2700 Ma), and that some areas then underwent retrogression to amphibolite facies. P-T estimates for the Badcallian range from 850 to 1050°C and 0.8 to 1.5 GPa depending on the method employed. However, most estimates come from mafic bodies and not the predominant tonalites.
The central region actually displays a range of grades and evidence for varying degrees of partial melting. For example, in some areas the rocks contain a granulite-grade mineral assemblages and cm-scale felsic (presumed melt) segregations whereas, in other restricted areas, the rocks show a totally anhydrous mineralogy and no evidence for the (remaining) presence of melt. In view of the difficulty of retrogressing such rocks in the absence of melt, it is considered unlikely that rocks of this grade once occurred throughout the central region.
The presence of these highest-grade tonalites coincides with the proximity of mafic-ultramafic bodies that could, potentially, be the heat source for locally more intense metamorphism. They have, however, been described as predating the intrusion of the grey gneiss protoliths, although published ages (eg. Cohen et al. 1991) suggest some may post-date emplacement of the tonalites.
New experimental data suggest that T ~ 950 to 1000°C would have been required to 'dehydrate' the protoliths (assumed to be similar to northern-region amphibolite-facies biotite and hornblende tonalites). In view of the restricted occurrence of tonalites that would have exceeded 950°C, and their coincidence with mafic bodies, it is suggested that the Lewisian, already undergoing a regional high-grade event, was invaded by mafic to ultramafic magma, causing local ultra-high-grade metamorphism.
The experimental evidence also suggests that melt fractions would have been small, even at temperatures around 900°C. Since there is evidence for a small amount of melt retention in many areas, it seems probable that there was little melt segregation from all but the highest-grade areas of the central region. Thus, it seems likely that the 'depleted' nature of the terrain was not caused by the loss of a melt phase.
Cohen AS, O'Nions RK & O'Hara MJ, Contrib. Mineral. Petrol, 106, 142-153, (1991).
In the southern part of Harris of The Western Isles, Scotland, meta-igneous bodies intrude a series of meta-sediments. The bodies have elongate outcrop patterns and trend NW-SE. They comprise metagabbros, meta-anorthosite, metadiorite and amphibolite bodies. The meta-anorthosite comprises coarse polygonal plagioclase (An60) plus minor quartz, sphene and apatite with coarse garnet (ald)-cpx bands and schlieren with varying amounts of retrograde hornblende, biotite, prehnite, epidote and opaque. Banding becomes less frequent towards the centre of the body, from 0.5-1 m thick banding at the margins to cm thick bands, and the centre of the body is unbanded. The metagabbros comprise coarse gt (ald)-cpx (salite-augite)-opx-pl (An60)-qtz assemblages. The sediments are highly aluminous and are mostly metapelites with gt-ky-qtz-bi-ksp assemblages. Ca, Al rich and K deficient sediments with pl-hb-qtz assemblages and thin band of forsterite marble also occur.
Granulite facies metamorphism is indicated from thermobarometry and the assemblages. Mineral textures suggest an anticlockwise P-T path evolving from the sillimanite field to the kyanite stability field at peak conditions, with a period of initial isobaric cooling on retrogression.
Deformation prior to metamorphism is indicated by an LS fabric within a small (1.5 km long) granulite body which is overprinted by granulite cpx-gt banding. The nature of such deformation is hard to determine on the limited data available. The meta-anorthosite body has a polygonal texture and lacks fabric, yet fine banding within the meta-anorthosite is isoclinally folded on a cm-m scale. It is possible that such deformation occurred prior to metamorphism, and that any fabric has been overprinted. Crystallographic orientation data may reveal a pre-existing fabric.
Deformation during or after metamorphism was associated with the formation of a steeply dipping NW-SE trending LS fabric in the meta-sediments and the margins of the meta-igneous bodies. Fabric mapping reveals the presence of a km scale steeply NW plunging, tight, antiform in the meta-anorthosite. Banding trends NW-SE, dipping steeply NE and SW either side of the body. The fabric in the nose region of the anorthosite forms successive shear zones, whilst the body margins are stretched; banding is symmetrically boudinaged. This could support folding by shear. Witty (1975) also identifies an antiform, and suggests it is overturned based on grading in banding. Possible grainsize changes during metamorphism and deformation may render grading indeterminate. No evidence has been found to support overturning.
Continued retrogression after deformation is indicated by randomly oriented retrogression products and rounded coronas which crosscut the LS fabric. Arrays of NW-NNW trending cm-m scale shear zone in the meta-sediments and meta-anorthosite crosscut the LS fabric, so post-date large scale shear. They are amphibolite grade and may post-date or reset retrogression.
Witty GJ, Unpublished PhD thesis, (1975).
The Proterozoic Egersund Anorthosite Complex is composed of anorthosite and related rock types, which intrude intercalated charnockitic and garnetiferous migmatites. The highest metamorphic temperatures are spatially associated with the intrusion, and previous workers recognized high-temperature mineral isograds for pigeonite-in, osumilite (OSM)-in and orthopyroxene (OPX)-in. The P-T conditions at a distance of 16 km from the contact are about 700°C and 5 kbar and increase to more than 1000°C and 5 kbar at 2.5 km.
The high-temperature metamorphism in the country rocks found near the contacts the Rogaland intrusive complex (surface area of about 1000 square km) cannot be explained by assuming the heat source was a simple, single-phase of intrusion. In order to obtain the observed metamorphic temperatures and isograd distribution, thermal modelling indicates that the heat source must have had at least two main phases that were separated by a hiatus of about 3 - 4.5 m.y. In this model, emplacement and crystallization of the anorthosite (30 x 40 km) heats the region to 600-750°C. A second, smaller intrusion (9 x 12 km) that is present in the region provides sufficient heat to obtain the observed high-temperatures. Recent work indicates that the entire magmatic emplacement occurred over a time interval of about 10 m.y. (930-920 Ma) which is consistent with the thermal model of the metamorphism.
The P-T estimates from 5 and 10 km from the intrusive contact and data from thermal model were used to estimate cooling rates and time intervals. Maximum temperatures are 860°C and 800°C and temperatures greater than 750°C (apparent end of new garnet growth) were maintained for 4.5 to 8.5 m.y. About 5 km from the intrusion calculated post-peak cooling rates indicate a decrease from 30° to 10°/m.y. over about 7 m.y., while at about 10 km calculated post-peak cooling rates decrease from 12° to 7°/m.y. over about 7 m.y.
During this time interval, retrograde and fine-grained Gar-Qz rims formed around orthopyroxene and primary garnet (outermost 150-200 µ) present in assemblages of garnet-quartz-plagioclase-orthopyroxene ± spinel. Based on the widths of preserved zoning profiles in garnet and othopyroxene and diffusion data, estimates of cooling rates that can be made and these agree well with the calculated temperatures and cooling rates from the model.
Where garnet and orthopyroxene are in direct contact, the retrograde exchange may have continued as long as 15-25 m.y., which resulted in more highly developed Fe/Mg-zoning at the rims (outermost 40-240 µ) of both orthopyroxene and garnet.
Hercynitic spinel may be a minor phase in the Bt-Grt-Sil restitic xenoliths of El Joyazo, where it is commonly associated with biotite. The possible reaction relationship among biotite and hercynite was studied in well-preserved and straightforward reaction textures developed around biotite at the contact with patches of fibrolitic sillimanite and melt. In these textures, biotite crystals of about 1 mm show resorbed grain boundaries and are wrapped by a layer of glass <200 µ m in thickness, containing hercynite and ilmenite; the same glass also fills embayments in biotite. Hercynite forms euhedral crystals <100 µ m in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with hercynite. The melt-sillimanite aggregate ("mix" for short) appears like an homogeneous felt, richer in glass close to the reaction rim around biotite. Plagioclase and garnet are located >2 mm away from the reaction texture.Biotite is chemically zoned, the inner part (Bt1) has XFe = 0.65 and Ti = 0.58 a.f.u.; a thin rim (Bt2) has XFe = 0.55 and Ti = 0.64 a.f.u.. The hercynite-rich spinel (Her) has low ZnO (<0.50 wt.%), and XFe = 0.74±0.04. The variable chemical compositions of the mix aggregate represent a linear combination between stoichiometric sillimanite and a silica-rich melt. Such melt must be different (lower Si/Al) from that of the layer around biotite (melt), that has a composition at the limit trachydacite-rhyolite. Garnet (Grt) has low Ca and Mn, and XFe = 0.86. Plagioclase is characterized by large homogeneous cores (Pl1, An31±2) and a thin, more calcic rim (Pl2, An49±6).Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt1-Bt2-Grt-Her-Ilm-melt-mix-Pl1-Pl2) system provides the mass balance: 0.69Bt1 + 1.28Pl1 + 3.24 mix + 0.43 Grt = 0.23Ilm + 5.00 melt + 0.22Bt2 + 2.82Her + 0.64Pl2. This relationship is considered a good model for the biotite melting reaction, as it is in perfect agreement with the textures observed. The mass-balance indicates that Grt and Pl must be involved in the melting of biotite, so that the equilibration volume is larger that the reaction site. The incongruent melting of biotite, constrained at =850° and =5 kbar, is not a terminal reaction, as its variance in the multisystem is ~3.
High-silica phengite-3T, formed under ultrahigh-pressure conditions (P = 2.7 to 3.7 GPa and T = 1000 K, Schertl et al., 1991) occurs in metamorphic rocks from the Dora-Maira massif, Western Alps, Italy. A systematic HRTEM and AEM study of phengite-3T crystals (K0.93Na0.01) (Al1.44Mg0.56Ti0.02) (Si3.54Al0.46)O10(OH1.93F0.07) were carried out to explain the presence of quartz and talc suggested by recent neutron- and X-ray diffraction and electron-microprobe data obtained from apparently pure crystals (Pavese et al.,1997; Ferraris et al., 1998a, 1998b).
SAED diffraction pattern and HRTEM images show the presence of 100 to 700 Å thick <alpha>-quartz platelets elongated in the cleavage direction of the mica. The platelets have often well defined, rounded to slightly faceted terminations.
In close proximity to the quartz domains, and only there, the 29.7 Å period of the 3T-phengite matrix is faulted by a different type of layer silicate with 18.9 Å periodicity as determined from HRTEM images (Ferraris et al., 1998b). The faulted sections were less stable than phengite under the electron beam and AEM profiles (electron beam diameter= 3nm) taken across the layers showed a slight but statistically significant increase in Mg- and simultaneous decrease in K-concentration relative to the matrix phengite. The observed periodicity and the change in chemistry suggest the faulted sequences to be talc-2M.
Examination by optical microscopy of (001) sections of phengite in petrographic "thick" sections (50-60 µ m thick) reveals the presence of thin platelets, with characteristic "amoebic" contour, interlayered at various depths parallel to (001) of mica, and sometimes containing fluid inclusions showing a mobile vapour bubble (Ferraris et al., 1998b). The low birefringence, low refringence and enhanced Si EDS signal suggest that these platelets are quartz domains similat to those observed by TEM techniques. With the optical microscope, talc can be seen as a coarse-grained primary phase within the matrix and as late lamellae growing on the mica edges, but cannot be detected within the phengite grains.
The absence of deformation features in the phengite matrix around the quartz domains, the strict shape control exerted by the host mica on the quartz crystals and the coeval presence of talc suggest that quartz and talc are the products of a reaction that took place within mica, after the rock had left the coesite stability field, i.e. upon decompression. Taking into account the observed volume ratio between talc and quarz, the following mass-conserving reaction can be written:
3 alumino-celadonite = 1 muscovite + 1 talc + 5 quartz + 2KOH
The above net reaction corresponds to an increase in the muscovite component of the remaining phengite during decompression and is compatible with the trend observed in experiments (Massonne et al., 1989).
Petrographical examination of thick sections of white micas from other well-preserved high-pressure metamorphic rocks (Monte Mucrone and Sesia Zone in Western Alps and from Dabie Mountains in central China) revealed also the presence of similar quartz platelets. The features observed in the Dora-Maira phengites seem, therefore, to be typical decompression textures for white micas in UHP-rocks.
Phengite composition is an important pressure indicator for HP and UHP rocks but the possible presence of submicroscopic quartz and talc platelets has to be kept in mind, when analysing these micas and applying this geobarometer.
Ferraris C & Wessicken R, Suppl. no. 1 Terra Nova, 10, 419, (1998a).
Ferraris C & Chopin C, 17th IMA abstracts vol, A101, (1998b).
Massonne H-J & Schreyer W, Eur. J. Mineral, 1, 391, (1989).
Pavese A, Ferraris G, Prencipe M, Ibberson R, Eur. J. Mineral, 9, 1183, (1997).
Schertl H-P, Schreyer W & Chopin C, Contrib. Min. Petr, 108, 1, (1991).
Eclogite-facies metamorphism, traditionally defined in basic rocks, is difficult to study in non-basic lithologies due to easy retrogression of peak assemblages during exhumation. The Dabie Shan in China provides good opportunity to study such non-basic lithologies, since high pressure (HPM) and ultrahigh pressure metamorphism (UHPM) have been found in a number of different rocks, including meta-graywacke, meta-aplite and metamorphic veins.
In the Eclogite Complex of Dabie Shan, the most peculiar lithology preserving UHPM mineral assemblage is a meta-graywacke, which is part of a sedimentary association including marble and eclogite. The graywacke was converted to garnet-jadeite quartzite, with peak assemblage consisting of coesite, jadeite, garnet and accessory rutile, apatite and zircon. Rare zoned glaucophane to Mg-riebeckite porphyroblasts developed just after the peak assemblage. A minimum P of 2.8 GPa and T of 690±30°C have been estimated for the UHP metamorphic peak.
An unusual UHPM assemblage with the kyanite-talc association typical of "whiteschist" has been found in a leucocratric layer, crosscutting coesite-bearing eclogite, interpreted as deriving from a former aplitic dyke. The UHPM mineral assemblage consists of quartz and minor epidote, kyanite, talc and omphacite, with accessory rutile, apatite and zircon. The Fe-rich epidote is strongly zoned with up to 5wt% REE in the core and is crowded with inclusions of quartz after former coesite, minor kyanite, omphacite and rutile. REE- and Sr-rich epidote, a common mineral in felsic lithologies of the Dabie - Su Lu orogen (Nagasaki & Enami, 1998), may be a good marker of the UHPM in retrogressed rocks. A minimum P of 2.6 GPa and a T of 710±20°C have been estimated for the metamorphic peak of the meta-aplite.
Metamorphic veins, consisting of kyanite ± zoisite ± omphacite ± rutile ± apatite ± clinozoisite in a quartz matrix, locally occur in HPM quartz-bearing eclogite. The vein assemblage is considered to have formed during prograde metamorphism from breakdown of lawsonite, just before the eclogitic peak estimated at P of ca. 2.4 GPa and T of 700°C. The presence in the veins of minerals attaining pegmatite-like grain size suggests the local abundance of a wather-rich fluid during the HPM.
In all these lithologies the local occurrence of relic phases, together with a number of symplectites and reaction rims deriving from the breakdown of HPM and UHPM minerals, constrain a clockwise P-T path which is characterized by a post-eclogitic quasi-adiabatic decompression. This kind of retrograde path indicates a very fast exhumation from eclogite- to amphibolite-facies metamorphic conditions, the latter being responsible for the widespread retrogression of HPM and UHPM mineral assemblages.
Nagasaki A & Enami M, Amer. Mineral., 83, 240-247, (1998).
Micas are sheet silicates formed by stacking one octahedral (O) and two tetrahedral (T) sheets, so as to obtain TOT layers. Order/disorder phenomena and occurrence of one polytype rather than others have been suggested to be related to the environmental conditions (Sassi et al., 1994). The study of the p-V-T equations of state (EOS) of minerals is a very key to interpret petrologic data, to model the interior of the Earth and to provide insights on the thermodinamics and stability of all phases that experience non-ambient thermobaric conditions. Neutron (ISIS, ILL) and synchrotron (ESRF) powder diffraction measurements have been carried out to study Al/Si and Al/Mg ordering and the EOS of phengite-3T from the UHP outcrop of Dora-Maira massif (Italian western Alps; Compagnoni et al., 1995) and phengite-2 M1 from Val Passiria (Spiess & Bell, 1996). High pressure was achieved by Paris-Edinburgh-like cells; in particular the ESRF experiment was performed along seven isotherms (300 -1000 K) over 0-50 kbar p-range. Main results are: (1) The Dora-Maira phengite-3T shows both tetrahedral and octahedral cationic order up to 1000K. (2) The Val Passiria phengite-2 M1 shows tetrahedral order at high temperature only. (3) The bulk modulus shows that phengite-3T (about 58 Gpa) is stiffer than muscovite [about 50 GPa; Comodi & Zanazzi, 1995; Vaughan & Guggenheim, 1986]; (4) An isochor with slope about 0.02 kbar/K was calculated from the EOS of the phengite-3T. Apparent disagreement with the thermobaric conditions of 30 kbar and 1000 K experienced by the natural sample (Compagnoni et al., 1995) can be explained both by the obvious fact that the p-V retrograde path not necessarily is along an isochor and by recent results (Ferraris & Wessiken, 1998) showing that nanometric domains of <alpha>-quartz and talc are formed upon decompression from an originally Mg/Si-richer phengite which, therefore, would have a higher bulk modulus, as shown by point (3).
Petrologic thoughts: a) The room-conditions Al/Si disordered phengite-2 M1 (triggered to order by high T) can be interpreted as a metastable state preserving the situation of the formation conditions (6 kbar and 900 K). b) The higher bulk modulus observed for phengite in comparison to muscovite physically justifies the higher stability of the former at high p. c) Occurrence of cationic ordering in 3T (over a wide T range) but not in 2 M1 (at room conditions) support Sassi et al. (1994) hypothesis that on retrograde p/T conditions phengite may undergo a 3T -> 2 M1 order/disorder transition.
Comodi P & Zanazzi PF, Phys. Chem. Mineral., 22, 170-177, (1995).
Compagnoni R, Hirajima T & Chopin C, in Ultra High Pressure Metamorphism, Coleman RG & Wang X Eds, Cambridge Univ. Press, Cambridge, 206-243, (1995).
Ferraris C & Wessiken R, Terra Abs. 1 Suppl. to Terra Nova, 10, 15, (1998).
Sassi FP, Guidotti C, Rieder M & De Pieri R, Eur. J. Mineral, 6, 151-160, (1994).
Spiess R & Bell TH, 8, 165-186
Vaughan, MT & Guggenheim S, J. Geophys. Res, 91, 4657-4664, (1986).
The reaction sequence that a rock can experience during metamorphism is a strong function of the "effective bulk composition" of the rock. This is the bulk composition averaged over the length scale that is in thermodynamic equilibrium at a given time during the metamorphic evolution of the rock. This contribution deals with the changes of this "effective bulk compositon" due to temperature change.
During cooling of rocks, mineral grains may develop zoning profiles as successively larger parts of the grain "close" to the diffusive exchange with the rock. This is a very common observation made in rocks from many metamorphic terrains, but its implication for the retrograde reaction sequence in the whole rock rarely appreciated. One of the consequences of the development of retrograde zoning profiles is that successively larger parts of zoned minerals (depending on grain size) are effectively removed from the reacting part of the rock volume. Thus, the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size. The sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition. Thus, two rocks of identical overall bulk composition, but of different grain size, may experience a different sequence of reactions. Qualitatively identical peak paragenesis may react to form qualitatively different retrograde reaction textures.
The model is applied to examples in the pelitic system. There, garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition. It is shown that, during cooling of pelitic rocks from amphibolite facies conditions, typical aluminous peak parageneses of garnet-muscovite-kyanite-biotite may react to form either staurolite, chlorite or muscovite (or different combinations thereof), depending on grain size. During cooling from the granulite facies, aluminous peak parageneses of garnet-cordierite-sillimanite may form biotite, either on the expense of cordierite or garnet, also depending on grain size. The two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature.
(This contribution is supported by FWF Project P12846-GEO)
In the Swiss Helvetic Alps, a 9.5 km exploratory tunnel was drilled to gain detailed geologic information along a planned 32 km Lötschberg railway base tunnel. With the consent of ARGE, the private company in charge, sampling along the tunnel was executed and focussed on critical mineral-bearing greywackes (Taveyannaz sandstone), shales and vein quartz samples across two flysch zones and two Helvetic nappes. In addition to the investigations along the tunnel, several neighbouring drilling cores were sampled. In this study, preliminary results on mineral paragenesis, illite crystallinity, vitrinite reflectance, fluid inclusion thermometry, and fission track (FT) dating on apatites are presented and used to decipher the retrograde P-T-t path. Zircon FT ages turned out to be largely unreset by metamorphism.
At the northern end of the tunnel next to Frutigen, rocks underwent zeolite facies metamorphismm (lmt-pmp-ab-chl). Diagenetic illite crystallinities are in accordance with fluid inclusion data that indicate a transition from higher hydrocarbon-bearing to methane-dominated fluids. This transition occured at a temperature around 200°C. Along the tunnel towards south, methane-rich fluids indicate increasing peak temperature conditions, and illite crystallinities enter anchizonal grade in the Kandersteg area, where vitrinite reflectances indicate medium-grade coalification of semi-anthracitic to anthracitic rank.
Apatite FT ages decrease from 8 Ma in the north to 5 Ma in the south, revealing a considerable amount of post-metamorphic tilting of the Helvetic Alps in a N-S direction. The tilting is the result of an updoming of the western Aar-Gotthard massif in late Miocene.
Traditionally, the cause of regional metamorphism as been related to processes of crustal thickening based on modelling within convergent zones. In such models, burial by crustal thickening leads to rapid increases in pressure, followed by thermal relaxation with maximum temperatures attained during uplift and decompression. This results in a characteristic clockwise P-T-t path. More recently regional metamorphism occurring in response to extensional processes has been suggested in sediments deposited within extensional basins formed in the upper crust. The term diastathermal metamorphism was proposed in which lithospheric extension results in an enhanced thermal gradient and heat flow. In this model, strata deposited within extensional basins may be rapidly buried with sharp increases of temperature in relation to pressure, followed by slow cooling but increasing pressure during thermal relaxation and finally rapid cooling during uplift. This model shows an anticlockwise P-T-t path (Warr et al., 1991; Robinson, 1987; Robinson & Bevins, 1989).
Identification of a metamorphic facies series can be made by measurement of the b0 value, which is an indirect measure of the Mg+Fe2+ (celadonite) content in the octahedral position of the white micas. It has been shown that the celadonite content increases with an increase in pressure and Sassi & Scolari (1974) have empirically related b0 values to differing baric types of metamorphism. However there are various precautions that should be considered when using this method, and selection of samples from the Caledonian Brabant Massif in Belgium for b0 measurement has been made with regard to the guidelines given by Guidotti & Sassi (1976).
The mean b0 value of the anchizonal to epizonal samples from the Brabant Massif is 8.996 A, which is indicative of a low-P facies series, and is similar to the mean value for the metamorphism of the Bosost area in the Pyrenees (Sassi & Scolari, 1974) or the metamorphism of the Caledonian Welsh Basin (Robinson & Bevins, 1986). The b0 data show no significant variation with increasing metamorphic grade or stratigraphic age.
b0 data indicate a low-P facies series and thus high heat flow which imply high geothermal gradients (c. 50° C km-1). The general features of the metamorphism in the Brabant Massif as described in Van Grootel et al. (1997) are in accord with the diastathermal model of metamorphism suggested by Robinson (1987) and Robinson & Bevins (1989) in which high heat flow is linked to lithospheric extension leading to very low grade metamorphism with an anticlockwise P-T-t path.
Guidotti CV & Sassi FP, N. Jb. Mineral. Abh, 127, 97-142, (1976).
Robinson D, Geology, 15, 966-969, (1987).
Robinson D & Bevins RE, Earth and Planetary Science Letters, 92, 81-88, (1989).
Sassi FP & Scolari A, Contrib. Mineral. Petrol, 45, 143-152, (1974).
Van Grootel G, Verniers J, Geerkens B, Laduron D, Verhaeren M, Hertogen J & De Vos W, Geol. Mag, 134(5), 607-616, (1997).
Warr LN, Primmer TJ & Robinson D, J. metamorphic Geol, 9, 751-764, (1991).
Illite crystallinity (IC) is considered to be primarily a function of illite crystallite size which is in turn a direct function of metamorphic grade (temperature). Where other geological parameters such as tectonic stress affect IC however, conclusions regarding grade may be in error. In order to clarify the effect of tectonically-induced strain on IC, a combined XRD and TEM study has been carried out on low-grade metapelites belonging to three tectonic units in the Transantarctic Mountains which have undergone complex deformation. These include the Bowers Terrane (BT), the Robertson Bay Terrane (RBT), and the Millen Schist (MS). IC indices decrease westward from the Bowers Terrane (average value 0.24° 2<Gamma>) to the Robertson Bay Terrane (average value 0.21° 2<Gamma>) .The lowest IC values occur for the Millen Schist (average value 0.19° 2<Gamma>), which has been interpreted to be a shear zone juxtaposing BT and RBT. In fact, BT and RBT show a penetrative slaty cleavage, whereas MS shows two superimposed axial plane foliations. The mean illite-muscovite crystallite size measured on TEM images increases from BT (ca 500 Å) to MS, with MS and RBT rocks showing comparable results (ca 650 Å). All crystal size histograms are bell-shaped and skewed towards thickness values greater than 2000 Å, which is a sizes compatible with values typical of the epizone, contrary to conclusions based on IC data. TEM images showed that strain features in illite-muscovite crystals, e.g., kinks, microfolds, and sub-grain boundaries, are far more abundant in BT than in RBT and MS. High-angle sub-grain boundaries within former single-crystals (originally thicker than 1000 Å) result in decrease in effective crystal size as measured by XRD. This effect is greater in the BT sample and is consistent with its larger IC value. In the MS and RBT samples lattice strain is lower and the resultant mean crystal size is higher. These results indicate that strain-induced reduction in crystal size resulting from intracrystal slip and sub-grain formation was significant in BT samples. On the other hand, microtextures within the shear zone suggest recovery/recrystallization of subgrain boundaries, resulting in the smallest IC values.The TEM images reveal that metamorphic grade was approximately equal and corresponding to epizone grade in all three formations, but that anomalously high IC values in the BT samples are determined by strain-related crystal size reduction, and those in the MS samples by recovery of such strain. These data demonstrate that tectonic stress-induced strain may result in anomalous IC values relative to metamorphic grade. IC values should be treated with caution, therefore, especially where there is evidence of deformation.
The originality of the structure of the serpentine mineral antigorite (Kunze, 1961) is the regular corrugation of the TO-layers. The tetrahedral layer is always on the convex side of the corrugated octahedral sheet and consequently flips at each inversion point to the adjacent octahedral layer above or below. In order to get a continuous structure, the apical oxygens of the silicon tetrahedra, which flip at each reversal, have to conicide with one of the hydroxyl groups of the adjacent brucite-like layer. For a given wave curvature, a parameter depending mainly on temperature (Mellini et al., 1987), exact coincidence and, therefore, inversion of the wave, is only possible after a certain number n of tetrahedra or multiples thereof. It is possible that the flipping of a tetrahedron, for which coincidence would be achieved, is hindered because TO-layer growths faster or slower, than neighboring layers. The next possible (higher order) inversions are after <kappa> x n tetrahedra, with <kappa> = 2,3,4.... and the aperture angle of the higher order half-waves will be multiples of the basic aperture angle (<kappa> = 1) of 21°. The quantization of the wavelength should become less strict for long wavelengths for which flipping coincidences are possible for a range of n. Antigorite intergrowths found in ophicarbonates from St. Denis (Aosta Valley, NW Italy) and Val Ventina (Valtellina, NE Italy), are the first examples of retardation of tetrahedral flipping. Antigorite aggregates show a systematic topotactical relationship with parallel b-axes and an angle of <kappa> x 21° between the basal planes of adjacent grains. This angular relationship is compatible with a higher order wave connecting the neighboring grains. An epitaxial overgrowth of adjacent sectors of polygonal serpentine can be excluded, because the expected angle for such antigorite pseudomorphs is 12° or 24°. The basal plane of antigorites nucleating on, or replacing tremolite and chlorite in ophicarbonate from Val Ventina form the same angle of 21° with the (010) plane of tremolite and the (001) plane of chlorite. The first wave of the nucleating antigorite crystal suffered, thus, a retardation of the inversion leading to the observed orientation relationship. Antigorites with wavelengths greater then 10 nm present in the same sample show slight changes of orientation of the basal plane that coincide with the location of the inversion points of the wave. This is a characteristic sign for inhomogeneous flipping of tetrahedra, as predicted by Kunze (1961) for long wavelength antigorites.The serpentine texture found in Alpine ophicarbonate strengthen further the antigorite structure model proposed, but not fully refined, by Kunze (1961).
Kunze, G, Fortsch. der Min, 39, 206-324, (1961).
Mellini, M, Trommsdorff, V, and Compagnoni, R, Contrib. Mineral. Petrol, 97, 147-155, (1987).
The fact that metamorphic veins trace major sites of fluid-rock interaction and may represent fluid pathways has led to considerable work on fluid circulation in the crust, in view of constraining the way volatiles escape metamorphosing rocks. Besides, metamorphic veins appear essential in understanding the amplitude and extent of mass transfer during metamorphism. Yet, the complex interplay between metamorphic (devolatilization) reactions and deformation in triggering veining events was given comparably little attention.
In the following, we present new data pertaining to the mechanism of vein formation in the pervasively strained blueschist-facies (BS) metapelites from the french-italian Western Alps (Schistes lustrés unit, SL) marked by a large fluid budget, - since dense hydrous phase form during the prograde evolution (e.g. carpholite ; prograde metamorphic reactions are water-consuming, unlike in Barrovian metamorphism) and water retentivity is favoured by the almost isochoric prograde path (water-rich fluids) - which show marked differences between BS and greenschist (GS) segregations.
The numerous BS carpholite-quartz-bearing shear veins occur in blackschists as an evenly spaced network (one vein every ca. 50 cm), with up to 1 m large clusters of rotational, boudinage veins, and exhibit the same mineralogy as the wall rock. Incremental formation of the BS veins in response to deformation indicates that the crystallisation of carpholite (i.e. transferring elements to the vein) and subsequent crack sealing were relatively rapid when compared to the rate of progressive deformation. Vein products are shown to have formed from chlorite and white micas through a prograde reaction requiring the presence of a limiting water-rich phase (chlorite-whitemica-water ratio around 1:4:4). In contrast, quartz-calcite±chlorite GS veins are mostly extensional veins formed by hydraulic fracturing related to the breakdown of hydrous phases.
It is suggested that BS veining is triggered by continuous ductile deformation leading to porosity enhancement. Provided an intergranular fluid phase is present in the vicinity of the fracture once nucleation started, element transfer will be maintained by diffusive transport due to existing chemical potential gradients. In the case of the SL metapelites, mass transfer probably occurs at the mesoscopic scale at the most (migration of species < ca. 1 m) and the propagation of the fluid is likely to be limited since the fluid phase is rapidly used up by the crystallisation of hydrous minerals. It is proposed that BS veins reflect local (mesoscale) combined petrological and tectonic processes rather than effective fluid circulation. In the light of such an interpretation, BS vein formation could span the entire period of deformation, as long as rocks remain in the stability field of carpholite.
Amphibolites and various amphibole-bearing schists occur as large and small bodies and lenses within polymetamorphic amphibolite facies rock complex of Moslavacka gora (Croatia). Amphibolites are closely associated and intercalated with different types of gneisses, mica-schists, cordierite schists, hornfels and marbles beeing anclaved in pre-Alpine(?) andalusite/sillimanite bearing granitoids. The crystalline complex is strongly folded and tilted and shows a fairly significant migmatization.
Amphibole bearing schists containing zoned amphibole posses layered structure ("sandwich") with three different mineral associations: garnet association in "centre" of the rock (grossular-clinozoisite-clinopyroxene-plagioclase), surrounded by the clinopyroxene association (clinopyroxene-plagioclase-titanite) and embedded in the amphibole association (hornblende-plagioclase-clinopyroxene- ilmenite-quartz). A polymetamorphism is characterised by first stage metamorphic event with paragenesis metamorphosed with geothermal gradient 28-35°C/km. In the second metamorphic event intrusion of granite body caused HTLP metamorphism with geothermal gradients over 60°C/km. The pneumatolysis associated with granite pegmatites occurred in third phase. Changes in P-T conditions are recorded in the growth of amphibole grains (general prograde metamorphism) and changes in modal compositions of related parageneses.
Due to their geochemical features the amphibole-bearing rocks could be important for the correlation with other rocks of crystalline basement of Pannonian Basin in general, and especially for those from the Tisia unit. Distribution of main constituents together with distribution of minor and trace elements, petrographic data (relict textures) as well as field features observed in rock-columns, suggest that protholite of amphibole-bearing rocks was:1) basalt of oceanic tholeiite affinity for orthoamphibolites;2) marls or tuffs alternating with other calcareous metasediments for paraamphibolites.
This magmatic-sedimentary complex might have been partly generated in an oceanic realm and partly in arc trench and than metamorphosed in a presumed subduction zone. However to thoroughly validate this model more data are required to confirm especially the relationship between the P-T increments and the time scale.
Microfabric analysis of eclogites can be used to characterize the deformation parameters and events that have been operative in deep parts of subducted slabs. As garnet is known to act as a rigid body, attention has to focus on the deformation mechanisms of omphacite, the major stress-supporting mineral in eclogites. Four samples displaying different strain intensities were collected in a well-preserved shear zone (Monviso meta-ophiolitic complex, Western Alps) that formed during eclogite facies metamorphism (P>10 kbar, 400<T<550°C). All the samples contain the mineral assemblage omphacite, garnet, rutile and quartz. Incipient retrogressive metamorphism is indicated by minor amounts of amphibole, plagioclase, epidote and white mica. Evidences of intracrystalline deformation such as subgrains, undulous extinction and deformation lamellae are unevenly present in the four samples. In the low deformed sample, recrystallization started at the boundaries of large relict grains (500µ m - 2 mm), and narrow bands of recrystallized grains crosscut these porphyroclasts. In the more strained sample, both types of omphacite are recognized but recrystalllized grains form a matrix representing most of the rock. An average recrystallized grain diameter of 30-80µ m is characteristic of this sample. The higher strain samples consist almost completely of recrystallized grains exhibiting an elongate shape fabric and straight grain boundaries. Smaller grain sizes are observed in these samples (the grain diameter is about 10-30 µ m in the third sample and 20-60µ m in the most deformed one). The latter sample also presents frequent atoll-shape garnets, which include omphacite grains of a higher average diameter than the matrix.Electron backscatter diffraction (EBSD) was used to measure omphacite crystallographic preferred orientations (CPO). In both the porphyroclasts and the recrystallized grains of the low-strained samples, [001] is parallel to the lineation while (010) has a dominant orientation parallel to the foliation plane. Omphacite inclusions in garnets do not show any CPO. Higher strained samples display slightly weaker fabrics with [001] progressively dispersed within the foliation plane while (010) remains parallel to foliation. The omphacite isolated in the hollow garnet cavities present a similar yet weaker CPO as the matrix of the same sample. The observed trend towards an increasing homogeneous microstructure with smaller grain size will be discussed in terms of deformation mechanisms and associated growth and recrystallization processes.
A peculiar association of muscovite (Ms), paragonite (Pg) and chloritoid (Cld) occurs in a porphyroblast-bearing Ms and garnet (Grt) micaschists from the Greiner Unit outcropping north of the Lago di Neves (Eastern Alps, Italy). Two different metamorphic stages can be recognised in this rock: 1) a syn-kinematic crystallisation of Cld and Ms along an early foliation S1; 2) the development of a crenulation cleavage (S2) associated with the growth of chlorite (Chl) and porphyroblastic Grt and Ms along S2. Both Grt and Ms porphyroblasts show inclusion trails of Cld, quartz and ores, which are parallel to S1. Furthermore, Ms porphyroblasts include Pg polygranular domains, which are also parallel to S1 and display a rectangular stumpy shape in thin sections. When identified as inclusions in Ms by BSE and PLM images, these Pg domains elongate nearly perpendicular to the Ms (001) basal plane while adopting almost the same cleavage trace. The shape and size of these large Pg domains are similar to those of the Cld crystals occurring in the rock matrix. An HRTEM/SAED/AEM study of the Ms/Pg single crystals indicate that: 1) both are 2 M1 polytypes in structural continuity; 2) the Na-rich Ms and to a lesser extent the K-rich Pg are structurally and compositionally modulated along the layers as inferred from layer-thickness variations; 3) each type of coherent Ms or Pg domain minimises total elastic strain through aligning inclined to (001); 4) the angle between [00l]*Pg and [00l]*Ms varies from 0 to 5° to reabsorb the layer-thickness misfit. Such resulting "tweed texture" suggests a further, spinodal-like decomposition inside the Ms-Pg solvus, also related to the local destabilisation of Cld.
A preferred orientation of a relictual foliation (PORF) included by a regional porphyroblast population can, in principle, be determined by combining the (2D) preferred orientations of inclusion trail lines as measured in differently oriented thin sections. A problem arises, though, when porphyroblasts of different timing in a sample include different foliations, or the same foliation in specific orientations. In other words, multiple PORF planes can be associated with successive mineral growth phases in a sample, and these cannot be reliably discriminated if porphyroblasts have similar compositions and their internal foliations are discontinuous with external ones. To get around this problem, a statistical method was developed that resolves up to three different PORF planes in a sample through best-fit calculations for inclusion trail data from 7 differently oriented thin sections per sample. A FORTRAN program (FitPitch) compares inclusion trail data in all thin sections with the theoretical intersection lines of those sections with a very large number of possible plane combinations, and finds the particular combination that minimizes the difference between actual inclusion trails lines and theoretical intersections. Calculations for single, two and three plane combinations are performed separately, after which the success of each option is assessed by comparing standard deviations, and by checking 2D rose diagrams for inclusion trails on the presence of single, bimodal, or trimodal orientation maxima. Best-fit calculations for 20 samples covering 300 km distance in a particular tectonic unit of NW-Iberia revealed two interesting features: 1) PORF planes exhibit a distinctive preference for subvertical or subhorizontal positions. 2) PORF planes define two distinct spatial foliation fans with horizontal but differently oriented fan axes. The orientations of these fans were quantified using a second computer program (Aerden & Sanchez San Roman, 1998) that fitted the poles of all PORF planes to two planes. The computer results are fully consistent with results obtained with an existing technique to determine relative matrix-porphyroblast rotation axes (Hayward, 1990). The latter match the two foliation fan axes. Qualitative microstructural analysis demonstrates a different age of porphyroblasts defining both foliation fans and, consequently, a change in the direction of regional tectonic transport is inferred. PORFs and relative rotation axes are remarkably constant in a major oroclinal bend of 90° in the study area despite a younger age of this macrostructure. Their regional consistency and marked preference for subvertical and subhorizontal orientations suggests that PORF planes record ancient, pre-orocline orogen trends, and reflect an orogenic evolution characterized by alternating compression and extension. Similar conclusions are systematically indicated wherever this type of data has been collected to date in Australia, China, Europe and North America (Belll & Hickey, 1997; Aerden, 1998, and references in both works).
Aerden DGAM & Sanchez San Roman J, Computers & Geoscs, in rev
Hayward N, Tectonophysics, 179, 353-369, (1990).
Bell TH & Hickey KA, Tectonophysics, 274, 275-294, (1997).
Aerden DGAM, Tectonics, 17, 62-79, (1998).
Meter size boudins within variably deformed metagranitoids of the Malpica-Tui allochthon (Iberian Massif, NW Spain) record ductile deformation under eclogite-facies conditions. Eclogites have a heterogeneous fabric; the mylonitic foliation is defined by shape preferred orientation of matrix omphacite, amphibole, epidote, white mica and rutile. On the basis of microstructural investigations and major and trace element microanalysis, the mechanisms of amphibole development and its role in the trace element whole-rock budget have been unravelled.
The microstructural study reveals a complex recrystallization history involving various steps of amphibole growth from different types of omphacite-rich domains. Porphyroclastic omphacite (Omp1) mimetically replaced igneous Ti-rich Cpx. Finer grained, omphacite neoblasts (Omp2) dynamically grew after Omp1 and were also aligned in surrounding matrix. Single grain, patchy replacement of Omp1 by amphibole at cores (Amp1) may evolve into colourless to bluish poikiloblasts (Amp2) with inclusions of Omp1+Grt+Czo at the final stage. A similar growth process is inferred to have occurred between Omp2 and Amp2 poikiloblasts which are commonly oriented parallel to Omp2 lineation and enclose matrix garnet and rutile. The amphibole poikiloblasts are frequently thin-rimmed by green amphibole (Amp3).
Omp1 and Omp2 are nearly indistinguishable for major element chemistry, but exhibit distinct trace element features. Omp1 has relatively high concentrations of HREE, Y, Sc and Ti, which may be ascribed to a geochemical signature of the pristine igneous clinopyroxene. Omp2 shows a bell-shaped REE pattern, which attests the achievement of chemical equilibrium with matrix epidote and garnet, which respectively behave as a repository for LREE and HREE, in high diffusion domains.
The REE pattern of Amp1 (Mg-hornblende to barroisite) and Amp2 (barroisite) probably reflects that of parent clinopyroxene, i.e. amphibole is bell-shaped when developing after Omp2, and slightly HREE-enriched when replacing Omp1. Amp3 barroisitic rims, with higher pargasite and lower glaucophane substitutions, have higher Sc, Y, HREE, Ba and K than the cores (Amp2). These relatively high concentrations may be ascribed to the breakdown of garnet and white mica. Further, Amp3 has markedly lower Sr than Amp2 and no significant LREE variations, thus suggesting that its growth was accompanied by the development of feldspar in the presence of epidote.
Microstructural and chemical data reveal that amphibole formed at the expenses of omphacite under short range element diffusion, most likely in the garnet stability field. Late amphibole rims probably developed under lower pressure conditions, possibly in the epidote-amphibolite facies. On the basis of mass balance considerations, amphibole is inferred to have a minor role in the REE and HFSE budget of these eclogites.
The Mg analogue of CaMg-pumpellyite, MgMgAl-pumpellyite, ideally Mg5Al5Si6O21(OH)7, is considered to be a possible H2O-containing phase in cold subducting slabs (Schreyer et al., 1991). Recently, Fockenberg (1998) investigated the stability relations of this phase in the chemical system MgO-Al2O3-SiO2-H2O while preliminary thermodynamic data were estimated by Massonne (1995). In order to check these estimates and to be able to provide reliable thermodynamic calculations, a project was started to carry out measurements of the basic thermodynamic data of MgMgAl-pumpellyite.
Using about 200 mg of a stoichiometric gel as starting material a pumpellyite sample was synthesized at 5 GPa and 700°C in a piston cylinder apparatus. The chemical composition of the run product was determined on an electron microprobe; the water content of the sample was investigated by Karl-Fischer-titration. The resulting MgMgAl-pumpellyite with the formula Mg4.93Al5.15Si5.92O21(OH)7 was accompanied by traces of enstatite (less than 2 wt.%).
By using high temperature oxide melt solution calorimetry (Navrotsky, 1997) the enthalpy of dissolution in lead borate (2PbO*B2O3)x at T = 973.17 K of this sample was measured. Additionally, similar measurements were performed for brucite (Mg(OH)2), quartz (SiO2), and corundum (Al2O3).
These quantities as well as fH0(enstatite) (Berman, 1988) were then utilized to obtain the enthalpy of formation from the elements for MgMgAl-pumpellyite: -13794.9 ± 16.7 kJ/mol.
This value is very similar to a value obtained by regression analysis of the reaction 6 MgMgAl-pumpellyite = 10 pyrope + 5 kyanite + coesite + 21 H2O (Fockenberg, 1998). Using Berman's internally consistent data base (Berman, 1988) for the product phases and Massonne's (1995) estimates for the heat-capacity and P-V-T behavior of pumpellyite, the following values could be calculated:
fH0(MgMgAl-pumpellyite) = -13797.1 kJ/mol,
S0(MgMgAl-pumpellyite) = 711.7 J/(mol*K)
On the other hand the standard enthalpy of formation of this phase estimated by Massonne (1995), -13893.5 kJ/mol, is approximately 100 kJ/mol different to our values.
Additionally, P-V-T data of MgMgAl-pumpellyite will be determined using powder X-ray diffraction methods at high temperatures and pressures (MAX-80, HASYLAB, Hamburg).
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Fockenberg T, Am Mineral, 83, 220-227, (1998).
Massonne H-J, Ultrahigh Pressure Metamorphism (Coleman RG & Wang X, Eds. ), Cambridge University Press, U. K, 33-95, (1995).
Navrotsky A, Phys Chem Minerals, 24, 222-241, (1997).
Schreyer W, Maresch WV & Baller T, Progress in Metamorphic and Magmatic Petrology (Perchuk LL, Ed.), Cambridge University Press, U. K, 47-64, (1991).
At Lago di Cignana (Western Alps, Italy) metabasic rocks crop out that experienced ultra high pressure (UHP) metamorphism. The UHP mineral assemblage in these rocks is at least partly overprinted by minerals stable at lower PT conditions (omphacite is partly transformed to a symplectite of blue green amphibole and albite, garnet is rimmed by a different blue green amphibole, glaucophane is rimmed by yet another blue green amphibole, rutile reacted to ilmenite and titanite and paragonite is overgrown by new clinozoisite). The rocks with the smallest amounts of replacement products (15 to 20 vol.%) have very similar amounts of all replacement products. Similar amounts of some of these replacement products are also observed in more completely overprinted rocks. This indicates that a first stage of replacement reactions occurred in all rocks and later replacement stages were localised. Only this first stage of lower PT overprint will be discussed here.
In the rocks that experienced only the first stage of overprinting the replacement minerals are restricted to domains with a maximum size of one square mm in thin section. These domains are not connected, but all have similar relative amounts of the replacement products. This suggests that in all domains the same reaction was responsible for the formation of the replacement products. Assuming that during this reaction the domain was a closedsystem, a balanced reaction must have existed between product and educt phases that produced the observed relative amounts of product phases.
Decomposing the matrix consisting of the compositions of product and educt phases in the system SiO2, TiO2, Al2O3, FeO, MgO, MnO, CaO, K2O, Na2O, H2O, using singular value decomposition, gives two linear independent reactions between the phases. A combination of these reactions can however not produce the observed relative amounts of product phases. Inclusion of other phases in the matrix (e.g. water or quartz) or exclusion of components present in minor amounts shows that balanced reactions with the observed amounts of reaction products can only be obtained with quartz as additional phase.
This indicates that the initial replacement of the UHP mineral assemblage started in domains where quartz was available and ceased when quartz was no longer available. This inference is supported by the observation that quartz is rare in the metabasic rocks and if present always surrounded by a rim of lower PT replacement products.
Although quartz is no longer available in most reaction domains, the necessity of quartz in a balanced reaction between product and educt phases shows that it must have been present. The inferred presence of quartz allows the use of geothermo- and geobarometers that require the presence of quartz to estimate PT-conditions for the onset of this lower PT-replacement.
Retrogressive fluid infiltration in the Loch Tay Limestone, SW Scottish Highlands, resulted in dolomitisation of calcite-rich marble layers and 18O enrichment in dolomite (Fein et al., 1994). Coincident dolomite reaction fronts and oxygen isotopic fronts propagated into and along calcite-rich marble layers in wall rock away from fault/veins suggesting that oxygen isotopic exchange between fluid and the host marble was facilitated by the replacement of calcite by dolomite. Retrogression under lower greenschist facies conditions was driven by the infiltration of high 18O, low 13C brines (Fein et al., 1994). Textures at reaction fronts revealed by cathodoluminescence imaging include mottled interiors of calcite grains and zoning of calcite grains adjacent to grain boundaries. Electron probe analyses of these zones shows small (0.8 wt.% FeCO3) increases in Fe content compared to the centres of calcite grains. Initial formation of dolomite occurred principally at calcite-calcite grain boundaries and calcite triple junctions; dolomite is also found forming in the interiors of calcite grains apparently by dissolution-reprecipitation. This suggests that grain boundaries were a pathway for dolomitising fluid to migrate away from faults and veins, and that this fluid migration altered the chemical composition of the calcite grains and generated porosity. In addition, there was alteration of the interiors of calcite grains prior to or during dolomitisation. The dolomite that replaces the calcite exhibits oscillatory zoning from an Fe:Mg ratio of 0.09 to ankerite with an Fe:Mg of 0.8 at the rim. The fluid infiltration may therefore have been pulsed. The centres of some dolomitised areas show patches of ankerite with comparable Fe-compositions to ankerite at the interface with calcite. These ankeritic patches appear to have replaced initial dolomites with lower Fe-contents. This suggests late permeability enhancement by dolomite dissolution. In phyllitic layers, which alternate with marble layers, fluid has apparently exploited deformed zones, which are now filled with dolomite. Dolomitisation appears to have occurred further from the fault/vein in these phyllitic layers than the adjacent marble layers. The phyllitic layers therefore provided a pathway for fluid to interact with the marble beyond the dolomite reaction front in the adjacent marble layers. This indicates that some mass transfer has occurred across the layers in addition to layer parallel mass transfer. It may be that phyllitic layers acted as foci for fluid flow in addition to fault zones. This study shows that permeability was controlled by a dissolution reprecipitation mechanism that operated both along grain boundaries and within the interiors of calcite grains and along discrete fractures. Chemical reaction can be interpreted in terms of a balance between fluid supply and dolomitisation reactions.
Fein JB, Graham CM, Holness MB, Fallick AE & Skelton ADL, J. metamorphic Geol, 12, 249-260, (1994).
The Yuzhno-Chuyskii range is a collisional complex exposed at the northern margin of the Altai-Mongolian microcontinent and formed during the Middle-Paleozoic accretionary events. It is an E-W trending massif (~20x100 km) that consists of several tectono-metamorphic zones, surrounded by sedimentary rocks of Vendian-Cambrian age. Five metamorphic zones have been recognized that are interpreted to record the succession of two main tectonometamorphic events. During an earlier stage of accretion and collision with the Siberian continent, a kyanite-sillimanite type of metamorphism (P=5-7 kbar, T=580-670°C) developed contemporaneously with a strong ductile deformation marked by N-S trending mineral lineations and sublatitude folding. This early stage is mainly preserved in the western part of the Yuzhno-Chuyskii complex. It has been overprinted by an andalusite-sillimanite HT/LP type of metamorphism (P=1.5-3.5 kbar, T=500-680°C) in the eastern part of the ridge, associated with E-W dextral strike-slipfaulting.Geochronological investigations have been initiated using laser-probe 40Ar/39Ar, Rb-Sr and U-Pb methods. Amphiboles from the northeastern metasedimentary Katun suite give discordant results (450-610 Ma) that could be interpreted to reflect an early accretional-collisional event or to indicate excess argon contamination. The age of low-pressure M2 metamorphism is constrained by Rb-Sr whole-rock dates of migmatites (380 ± 19 Ma) from the central and late tourmaline granite (357 ± 21 Ma) from the eastern parts of the ridge. 40Ar/39Ar mica ages are in agreement with these dates. In the western part of the ridge, hornblende from M1 crystalline schists displays a saddle-shaped spectrum with an intermediate plateau date of 407 ± 4 Ma related to the collisional event. Hornblende from the Karasu granodiorite gives a well-defined plateau date of 210 ± 2 Ma which is consistent with Rb-Sr ages from Li-ore bearing granitic plutons to the south of the ridge. The emplacement of these late granitoïds resulted in a partial argon resetting in the micas from country rocks. These dates and forthcoming results are used to reconstruct P-T-t paths in the studied area of Altai-Mongolian range.
Amphiboles from high-grade rock types in and near the Palala shear zone (a Proterozoic suture zone between the Central and Southern Marginal zones of the Limpopo Belt) were analysed by 39Ar/40Ar. One special case from a Cpx+Opx+Hbl gabbro gave a discordant age spectrum. Several step ages are older than the most probable age of peak metamorphism at 2.02 Ga, as determined by U/Pb on titanite.
Rather than dismissing this sample as "useless due to excess Ar", we applied Cl/K vs. Ca/K correlation obtained from Ar isotopes (cfr. Villa, this session). By this approach we identified mixing trends between 4 different end-members. However, chemical analyses performed by a WDS electron microprobe gave homogeneous compositions for all analysed amphiboles.
In order to look for different mineralogical compositions at a scale smaller than the 5 µm resolution of the WDS data, we analysed the amphibole grains by transmission electron microscope (TEM) equipped with a EDS microanalysis. With this investigation technique different amphiboles intergrown at a scale 0.5 µm were detected: Tschermakite, Mg-horneblende/edenite, and actinolite. Furthermore fine (0.2-0.3 µm) layers of chlorite overgrowing the originary amphibole were revealed.
Extrapolating the mixing trends to pure end-member amphiboles, the influence of foreign phases can be modelled away. Minor chlorite alteration and late actinolite overgrowths account for higher and lower outliers, respectively. The ages pertaining to tschermakite and Mg-hornblende are indistinguishable at 2.02 Ga. The accurate age of the metamorphic peak is thus unravelled.
The Champtoceaux nappe is a crustal-scale thrust located in the South-Armorican Massif between the Nort-sur-Erdre Fault to the north and the southern branch of the South Armorican Shear Zone to the south. The nappe consists of several imbricated units, the lower one (Cellier Unit) being characterised by well-preserved eclogite-facies rocks. We report here new petrological and Sm-Nd data in order to constrain the P-T conditions and the timing of the high-pressure event.
The studied sample is a fine-grained eclogite similar to the one described by Godard et al (1981). The primary assemblage consists of garnet, clinopyroxene, phengite, glaucophane, rutile and minor pyrrothite. Garnet grains are either idioblastic or atoll-like with a phengite core and preserve growth zoning (increasing Prp and decreasing Grs from core to rim). The jadeite content of the clinopyroxene varies between 45 and 55 mole per cent. Glaucophane occurs as poeciloblastic, slightly zoned (increasing Fe content from core to rim) crystals in equilibrium with garnet and omphacite. P-T estimates are 15-20 kbar at about 600°C, the growth of glaucophane idioblasts recording the earliest stages of the cooling history.
Zircons extracted from this sample have been studied by Paquette (1987), who obtained an U-Pb age of 358 ± 2 Ma (upper intercept). This age has been interpreted as (i) a minimun age for the eclogite-facies event (Paquette, 1987), (ii) a possible age for the HP event (Ballevre et al, 1994), or (iii) a resetting of the U-Pb clock at 450-500°C (Faure et al, 1997). A new Sm-Nd analysis of this sample gives an isochron age (whole rock-garnet) of 359 ± 17 Ma, which is consistent with the U-Pb age. The preservation of growth zoning in garnet and glaucophane, and the lack of amphibolite-or greenschist-facies overprint indicates that resetting of both the U-Pb and Sm-Nd systems is highly unlikely. Moreover, 39Ar-40Ar data in a large variety of samples from different lithologies (eclogites, garnet-kyanite micaschists, leucocratic gneisses) from the same unit show that closure of the argon clock took place at about 350-360 Ma.
The above data show that the high-pressure event in the basal unit of the Champtoceaux nappe took place in the uppermost Devonian (about 360 Ma), i.e. is much younger than expected, and that cooling occurred shortly after the pressure peak.
The polymetamorphic basement of the Tisza Unit, SE Transdanubia, Hungary forms a detached fragment of the Variscan European foreland of the Neotethyan realm (Kovács, 1982; Szederkényi, 1996). The prograde evolution path of a gneiss-amphibolite complex characteristic of the main part of the Tisza Unit was reconstructed, investigating the polymetamorphic rocks of the borehole Baksa-2. The results obtained by detailed microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric calculations and the shape of the P-T loop suggest a complex Variscan polyphase polymetamorphic model (Árkai et al., 1985) rather than a polycyclic (pre-Variscan-Varican) one (Lelkes-Felvári and Sassi, 1981). On the basis of preserved relic, large garnet porphyroblasts and their inclusions, as well as adjacent (matrix-forming) mineral assemblages, the physical conditions were estimated. The early part of the prograde path with kyanite as index mineral, is characterized by T-P conditions of 480°C and 460 MPa, respectively. The metamorphism reached its peak conditions of 660 ± 25°C, and 750 ± 50 MPa, when both kyanite and staurolite were stable. The estimated metamorphic thermal gradient of the prograde part might be between 24 and 28°C/km. This metamorphic clymax was followed by a nearly isothermal decompression at 650 ± 40°C, which resulted in a significant decrease of pressure down to 440 ± 20 MPa. This event is marked by the presence of sillimanite and the second generation of garnet, and was closely related to the synkinematic post-collisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit (Buda, 1981). The thermal gradient of this phase proved to be of ca. 37°C/km. In amphibolites intercalated with gneisses, only this last event was preserved, providing less precise T-P estimates (ca. 650-690°C/400-500 MPa). The present paper demonstrates the first, almost complete, continous, clockwise P-T-relative time path in the Variscan metamorphic basement of the Hungarian part of the Tisza Unit.
Árkai P, Nagy G & Dobosi G, Acta Geol. Hung, 28, 165-190, (1985).
Buda Gy, Acta Geol. Acad. Sci. Hung, 24, 309-318, (1981).
Kovács S, Geol. Rundschau, 71, 617-640, (1982).
Lelkes-Felvári Gy, Sassi FP, IGCP Project No. 5 Newsletter, 3, 89-99, (1981).
Szederkényi T, Acta Miner. Petr. Szeged, 37, 143-160, (1996).
Draper and Nagle (1991) mapped and described the Rio San Juan Complex, the largest exposed area of Cretaceous basement rocks in the North Coast Zone of Hispaniola. These authors suggest that the predominantly mafic schists of the northern part of the complex were metamorphosed in a Late Cretaceous subduction zone and later protruded at depth by serpentinite melanges entraining tectonic blocks of various rock types. We are carrying out petrographic and microanalytical studies of samples from the mafic schists (Hicotea schist, Puerca Gorda schist) and from blocks of metamorphic rock in the melanges (Jagua Clara melange, Arroyo Sabana melange) to gain more insight into subduction zone processes. Classical thermobarometers and petrogenetic grids, in conjunction with information on relative timing from compositional zoning in porphyroblasts and inclusion relationships, allow P-T-conditions and preliminary P-T-paths to be delineated.
The mafic schists have been subjected to a pervasive greenschist overprint, but a lawsonite-blueschist mineralogy is still preserved. Maximum P-T-conditions are estimated to have been between 5 and 10 kbar, at temperatures below 400°C. Blocks from the Arroyo Sabana melange show distinctly heterogeneous P-T histories. A granitic igneous rock exhibits lawsonite growth at the expense of feldspar and muscovite. An eclogite yields peak P-T-conditions of 780°C and a minimum of 17 kbar and an exhumation path to 500°C, with P still higher than 6.5 kbar. An epidote-amphibolite shows no evidence of high-pressure metamorphism. Metamorphic blocks from the Jagua Clara melange yield an intriguing and relatively uniform P-T-development. The following P-T-path is entirely recorded in a single specimen of eclogite; segments of this path are observed in three further samples. An original amphibolite-facies mineralogy (600-650°C/6 kbar) is overprinted by a low-grade blueschist assemblage (400-450°C/6 kbar). These blocks then followed a typical subduction-zone prograde path to maximum conditions of 16-20 kbar and as high as 680°C. Exhumation essentially retraces the prograde path at only slightly higher temperatures and can be followed to below 400°C.
Draper G, Nagle F, Geol. Soc. Am. Spec. Pap, 262, 77-95, (1991).
Upper cretaceous up to paleocene volcano-sedimentary and turbiditic sandstones and siltstones of the Kumroch Range and the Kamchatka Mys Peninsula in Eastern Kamchatka have been investigated in order to determine the p-T-conditions during the accretion of the Ozernovsky-Valaginsky Terrane and the Vetlovsky Terrane in the Kumroch Range and of the Kamchatka Mys Terrane. The first results show impressive trends allowing to retrace the accretionary events. We use different analytic approaches in order to determine the p-T-conditions of the low grade metamorphic and higher diagenetic units. Data of chlorite cristallinity mesurement show a variation of 0.357-0.439° 2<theta> (0.7nm-peak) and can be placed into the field of higher diagenesis (Árkai et al., 1995). In this intervall of data the formation conditions of the chlorite authigenesis are ¾200°C (Frey, 1986). Cristallinities increase as a function of stratigraphic age. Illite is found in several samples but difficult to use for mesuring cristallinities due to the superposition with the laumontite-leonhardite (110) peak at 0.95nm. Zeolites could successfully be separated by using a flotation method and will be investigated by spectroscopic and geochemical methods. Corresponding to the results of clay mineralogy we got data of coalification measurements of organic matter in the pelites. The values varie between 1.2 and 0.7% Rmoil. We can also observe an increasing of the coalification values in function of the stratigraphic age of the rocks. The vitrinite reflection data of a large sample set will be used to calculate the maximum temperatures depending on the effective coalification time using the EASY%-Ro model (Sweeney & Burnham, 1990).The fraction <63 µ m of the volcano-clastic rocks shows obviously the presence of partially dehydrated laumontite-leonhardite. The paragenesis of these rocks is low quartz, albite, chlorite, laumontite, ± illite, ± montmorillonite, ± hornblende (edenite, pargasite), ± augite and therefore they can be plotted into the zeolite zones IIIb and IVa defined by Iijima (1980). Prehnite and pympellyite as facies critical minerals for a low grade metamorphism (subgreenschist facies) are not found. As these minerals will be formed by alteration of the zeolites when pressure and temperature of the described paragenesis increase, we are able to suggest a upper temperature limit of maximum 200-250°C (Frey, 1986). The deformation structures of quartz minerals in the psammites indicate similar conditions. All described parameters of the early pilot project indicate conditions from 100 to ca. 230°C and 0.5 up to 2 kbar. The sample set of now 70 specimen and the use of several analytical methods complementing each other should give us more differentiated data to the points mentioned above, to contribute some new aspects for this accretionary history in the collision zone of Kamchatka.
Árkai P, Sassi FP & Sassi R, Eur. J. Mineral, 7, 1115-1128, (1995).
Frey M, Schweiz Mineral Petrogr. Mitt, 66, 13-27, (1986).
Iijima A, Proceedings of the fifth international conference on zeolites, Rees LV, 103-118, (1980).
Sweeney JJ & Burnham AK, Bull. Amer. Assoc. Petrol Geol, 74(10), 1559-1570, (1990).
The interpretation of the behavior of a mineral subjected to a range of petrologic and geological conditions in a fashion that integrates all aspects of its crystal chemistry, including its composition, crystal structure and elastic properties, can have relevant implications in deciphering metamorphic processes. Herein we consider the interrelationships among the muscovite crystal-chemical features with respect to P, T of formation and recrystallisation under the influence of a differential stress. The starting point for our considerations has been the recently published compressibility data of three compositionally well constrained white micas and related crystal structure refinements obtained over a range of different pressures. Considering the chemistry of these three crystals, variations in the compressibilities measured and structural responses to increasing P can be ascribed solely to the Na/(Na+K) ratio. This enables one to better understand the effects of isochemical P increase on a white mica structure. Key results are: (1) compressibilities and structural adjustments are clearly a function of Na/(Na+K) ratio; (2) structural adjustments (e.g. <alpha> angle, dimensions of IV and VI sheets, etc.) can destabilize the structure; (3) most of the compression involves shortening of the c cell dimension. The main implications of such results are: a): it is now well recognized that significant increase of P causes the K- and phengite-content of muscovite to increase. This is true even if the mineral assemblage is Al-saturated. The increasing the K- and phengite-content of muscovite also changes the mica structurally in terms of cell dimensions, (<alpha> angle, etc. The key point is that these changes are exactly those needed to mitigate the destabilizing effects caused by isochemical increase of P on muscovite. Hence, we suggest that the well-known increase of K- and phengite-content in natural muscovite in response to P increase can be understood in terms of the structural adjustments needed to maintain stability of the mineral. b): It is generally believed that, during the formation of slates, the platy minerals like muscovite and chlorite become aligned by some combination of rotation and recrystallisation such that their (001) planes are perpendicular to the direction of maximum compression. In several cases, TEM-SEM studies have shown that the oriented muscovite is more K- and phengitic-rich than the unoriented flakes. This is clearly related to the recrystallisation which accompanies the formation of the preferred orientation. Such composition changes cause the muscovite c dimension to shrink, but the a and b cell dimensions increase. Hence the dimensional changes of such muscovite mirror the strain experienced by the rock as a slate forms. Moreover, the above mentioned compressibility data show that as muscovite K-content increases, its intrinsic compressibility increases. Differential stresses are thought to be small during formation of slates. Nonetheless, we suggest that when low-grade metamorphism occurs in association with a differential stress, the above noted crystallochemical aspects of muscovite readily facilitate and favor its recrystallisation into the preferred orientation that typifies slates. In summary, the above two cases provide two examples for which it appears that the petrologic and geologic processes can be intimately related to the crystallochemical response of an affected mineral.
The Oonagalabi domain in the eastern Arunta Block, Central Australia, comprises a succession of orthogneisses and subordinate supracrustal units that were metamorphosed under transitional amphibolite to granulite facies conditions. Metapelites show evidence for partial melting and segregation of melt, leaving a restitic residue. Metabasites display orthopyroxene ± garnet-bearing segregation pods, in which orthopyroxene and garnet are commonly rimmed by calcic amphibole. Hence, high-grade partial melting was followed by cooling and minor back reaction.
Aluminous lenses in a concordant orthoamphibole-dominated unit show early orthoamphibole + spinel ± corundum, overgrown by sapphirine and garnet. The earliest recorded assemblage in K-poor, silica-saturated rocks is orthoamphibole + sillimanite + quartz, overprinted by garnet and cordierite. In sillimanite-absent domains, breakdown of the anthophyllite component of orthoamphibole has left behind Na-enriched (~2 wt% Na2O) gedrite enclosed by large orthopyroxene grains. This was followed by back reaction of orthopyroxene with the fluid to produce pure anthophyllite. The combined evidence suggests a prograde evolution culminating at ca. 750-800 oC, 8-9 kbar, followed by minor cooling.
The gradation from mono- or bimineralic orthoamphibole (gedrite + anthophyllite) rocks to orthoamphibole + spinel ± garnet and orthoamphibole + spinel + corundum + sapphirine + garnet assemblages in aluminous lenses, coupled with the constant KD of garnet-orthoamphibole Fe-Mg exchange in all rock types, suggests that the entire sequence witnessed a single, transitional amphibolite to granulite facies event. Hence, the formation of the orthoamphibole-dominated unit and its associated Cu-Zn(-Pb) mineralisation probably reflects a metasomatic event prior to high-grade metamorphism.
The high fluorine content of orthoamphibole and phlogopite, and the presence of fluorite in marble layers and of chondrodite in some orthoamphibole-dominated lenses, suggest a high F content of the metasomatic fluids. We argue that F and the Cu-Zn(-Pb) sulfides were derived from the Oonagalabi bimodal igneous suite, which was emplaced in an early extensional setting (Sivell, 1986). Fluorine probably played an important role in stabilising orthoamphibole and phlogopite during subsequent transitional granulite metamorphism.
Sivell WJ, Contributions to Mineralogy and Petrology, 93, 381-394, (1986).
We investigate the chemical composition of coexisting pyroxenes from basic granulites with related enderbites and regressively altereted granulites. The content of Y, Yb, Sc in clinopyroxenes increase with a rise of temperature. The content of Mn, Y, Yb, Sc in orthopyroxenes increase as the result of the drop in pressure in the act of enderbite forming. The lowering of V, Cu, Cr in orthopyroxenes is a consequence of the temperature rise. The distribution of Cu, Yb, Sc between coexisting pyroxenes is close to ideal for enderbites. The mineral equilibrium was established during enderbite forming with drop in pressure and approximately fixed temperature.
The new data on petrology of the granulites from Lapland Belt testifies that granulitic metamorphism was repeated doubly. The low pressure metamorphism was most conspicuous in the north-east (upper) part of the granulite complex, the high pressure metamorphism appeared in the south-west (lower) part. The area of high pressure granulites declines from the east to the west. Coarse-grained migmatizated rocks with cordierite are the result of the early low pressure granulitic metamorphism. The new data on composition of zoned garnets allows to determine metamorphic conditions as 750-950°C, 5-6 kbar for low pressure stage and 750-800°C, 8-9.5 kbar for high pressure stage of granulitic metamorphism. Conceivably the low pressure granulites represent the mineral forming conditions of Archean rocks in place of Lapland granulites. This conditions are similar with the metamorphism of Late Archean metasedimentary Cola series.
Metapelites metamorphosed at low-temperature (<600°C) and medium to high-pressure (up to 20 kbar) conditions are widespread in the peri Mediterranean Tethysian orogens resulting from subduction. When these rocks are devoid of the common blueschist facies index minerals such as carpholite ± chloritoid, glaucophane or jadeite, they are classically considered to have been metamorphosed in greenschist facies conditions. In consequence, only minimum or maximum pressure conditions corresponding to blueschist-greenschist facies transition are estimated from metapelite mineralogy, but a more continuous assessment of the shape of metamorphic P-T path is generally difficult or impossible. A possible way to improve this situation is to use the composition of chlorites and micas, which is very sensitive to pressure and temperature. Both these phases systematically occur in metapelites, whatever the metamorphic conditions.
This is the case for metapelites that occur in Tinos island (Cyclades Archipelago, Greece). In these rocks, the degree of the late greenschist overprint is correlated with the gradient of deformation, along a N-S transect crossing a major extensive shear zone. It is noteworthy that blueschist facies metapelites are preserved far from the shear zone, whereas the retrogression is complete in the vicinity of the contact. The Si-content variations in chlorites and micas are correlated together and related to the distance to the detachment. This compositional variation is interpreted in term of progressive increase of retromorphosis towards the shear zone. These chemical variations can also be correlated to a local increase of deformation and probably of the availability of fluids.
The compositional variation of chlorites and micas has been used to constrain the exhumation path. We first localize good indicators of equilibrium in the schistosity and in the porphyroblasts. Analysis of texture and microstructures in the queues of crystallization of albite provides information to discriminate the different generations of chlorites and micas and on their order of crystallization. On the contrary, the systematic study of sodic amphiboles-bearing schists was less successful. For example, it is unknown which part of the amphibole is in equilibrium with the chlorite-micas couples. About 200 chlorites-micas couples were used to compute the evolution of P-T conditions during retromorphosis. The obtained P-T paths allow us to discuss the modalities of exhumation of the high-pressure rocks of Tinos Island.
Phengites with compositions on the pseudo-binary muscovite-celadonite
KAl2[AlSi3]O10(OH)2-KAlMg[Si4]O10(OH)2)
join were synthesized experimentally at conditions of 700-1025°C and 1.5-9.2 Gpa. Syntheses at lower temperatures and higher pressures did not result in a mica phase. Syntheses were considered as successful if (i) phengites were large enough to be measured by electron microprobe analysis, (ii) compositions were close to the pseudobinary above, deviations were accepted in the range 0.95<K<1.0 and 1.98-2.02 octahedra per formula unit, (iii) yields were >90% mica. Successful syntheses yielded phengites between cel(26) and cel(99). Grain sizes were typically 10-30 micron allowing for microprobe analysis with a defocused electron beam. After microprobe analysis, run products were ground, mixed with an internal Si-standard, and measured by X-ray powder diffractometry. Cell parameters were determined by full-profile Rietveld analysis.
Molar volumes as a function of composition were then fit on a data set including our samples having pseudo-binary compositions, a few Na- and Fe-poor natural phengites from the literature, and the end member muscovite volume as reported by Guidotti et al. (1992; V(Musc)= 14.078(6) J/bar). The excess volume deviates positively from the ideal volume of mixing; the molar volume has small variatons in the range cel(26)-cel(40) and then drastically drops, reaching a minimum at the celadonite endmember. The volume decrease is related to the decrease of all cell parameters, with c having the largest and a the smallest variation. A fit with an asymmetric Margules model yielded V(Cel)=13.954, W(MuscCel)=0.244, and W(CelMusc)=0.203 J/bar; r2=0.948 . A fit with a symmetric model yielded V(Cel)=13.956(39), W= 0.222(28) J/bar, r2=0.948 . As both fits were of identical quality we of course retain the simpler symmetric formulation for the excess volume of the muscovite-celadonite solid solution. Our volume data of the phengite solid solution, based on specimen whose composition and cell parameters were independently determined, is largely different from that reported by Massone and Szpurka (1997).
We used the resulting volume data together with experimental data on the assemblage garnet-cpx-phengite to adjust the thermodynamic properties of the muscovite-celadonite solid solution. Preliminary applications on well studied metamorphic terrains, such as the Franciscan belt, yield pressures in accordance to phengite-independent estimates.
Guidotti et al, Eur. J. Min, 4, 283-297, (1992).
Massone & Szpurka, Lithos, 41, 229-250, (1997).
The Cono and Monastero metagabbros are exposed at the southern boundary of the Sesia-Lanzo Zone, the most internal slice of eclogitised continental crust in the Alpine nappe belt. They are almost completely transformed into glaucophane, epidote, garnet, chloritoid rocks, during Alpine times, and are heterogeneously and pervasively deformed and metamorphosed. Some anastomosing shear bands separate less deformed domains that preserve pre-Alpine textural and/or mineralogical relics.
The igneous precursors consist of gabbronorites and Fe-gabbronorites with plagioclase (An50-70), clinopyroxene, orthopyroxene, Ti-rich amphibole, + apatite, + rutile, + oxides (magnetite and ilmenite). Whole-rock and clinopyroxene trace elements composition suggest that the parental liquids of the gabbros were crustally contaminated MOR basalts.
During the polyphase pre-Alpine evolution the gabbros re-equilibrated under granulite facies conditions (P=5-7 kb e T=800-900°C), with a successive retrograde P-T path through amphibolite facies (P=4-5 kb, T=500-600°C) up to greenscbist-facies conditions (T<450°C & P<4 kb) that was assisted by different fluid phases. Pre-Alpine syn-metamorphic structures consist of narrow amphibolite- to greenschist-facies shear zones. The shallower depths reached by these gabbros are compatible with the intrusion of porphyritic andesitic-rhyolitic dykes with chilled margins.
The P-prograde Alpine evolution is documented by the development of progressively pressure-increasing parageneses: - epidote and paragonite are followed by garnet, glaucophane and chloritoid on plagioclase microdomains; - glaucophane, garnet, + chloritoid replace mafic minerals. The pre-Alpine transformation rate and the bulk composition discriminate the type of high pressure assemblage.
The Alpine peak conditions were evaluated at pressures of about 20 Kb and temperatures of 400-500°C on the basis of the eclogitic garnet-glaucophane-chloritoid parageneses, that are in places associated with omphacite-garnet eclogitic assemblage. In the porphyritic andesitic-rhyolitic dykes jadeite + quartz assemblage is widespread.
The tectono-metamorphic evolution of the Corio and Monastero gabbros was recorded during two main geodynamic episodes. The high T/P ratio characterising the pre-Alpine retrograde evolution suggests that it took place in an abnormally high thermal regime, that can be induced by astenosphere upwelling during lithospheric thinning. Afterwards, typical subduction-related environment 15 testified by Alpine high-pressure, low temperature metamorphism. This event is correlated with the subduction of the Ligure-Piemontese Basin, whose remnants are preserved in the nearby eclogitised ophiolitic complexes. Metagabbros exhumation partly occurred under greenschist facies conditions mainly developed along narrow high-strain rate, chlorite-albite-rich bands cutting through the gabbro bodies and the country rocks.
Here we present results of our investigation devoted to intracrystalline zoning in metamorphic garnets grown at fluid flux. We have studied, firstly, the effect of fluid composition on behaviour of fractionation coefficient KFe-Mg between aqueous solution and garnet solid solution, which describes the distribution of components between reservoir and growing crystal. The second direction of our study was numerical modeling of zoning profiles at garnet crystallization. The models used have been constructed on the base of kinetic equations for crystal growth from aqueous fluid and mass balance in the system.
When studying the effect of fluid composition on garnet fractionation coefficient we have examined such parameters of aqueous fluid as salinity, acidity and concentration of MgCl2 and FeCl2. In the case of fresh water (fluid with low ionic strength) KFe-Mg<1, and garnet is enriched by Fe while the fluid is enriched by Mg (Rayleigh fractionation). This leads to Mg enrichment in the rim of garnet grains (normal zoning). Salinity has weak influence on behaviour of the component distribution between garnet and fluid. At high concentration of MgCl2 and FeCl2 KFe-Mg is near unity and weakly depends on temperature. Therefore zoning in such case should be only slightly manifested. The influence of pH is weak in acid solutions while arising of alkalinity leads to inversion of fractionation coefficient (i.e. KFe-Mg becomes more than 1). It means that fractionation in alkaline solution may form the reverse zoning. Such zoning would be contrast enough.
The modeling of zoning profiles in metamorphic garnet has manifested that garnet growth at fluid flow can form non-monotonic profiles (so called complex zoning) or profiles typical for high-temperature garnets with reverse zoning (homogeneous core and narrow rims with sharp increasing of Fe content). Such profiles can be complicated by small inflexions. Analysis of numerous data on geological position of reverse zoned garnets shows that such zoning is common for diaphthorites or narrow zones affected by fluid alteration during retrograde metamorphism. This does not contradict to assumption that the reverse zoning in garnet grains can originate at crystallization under fluid flow through metamorphic rock.
Thus, we can summarize that a possible growth mechanism producing reverse and complex zoning is garnet crystallization in the open system, under fluid flow. Zoning trend is determined by fluid composition (salinity, pH and solute concentration). Combination of the Rayleigh fractionation and fluid flux in the system can create various types of zoning profiles, including non-monotonic ones.
The work has been supported by the RFBR grants 97-05-65518, 96-05-66060 and 96-15-98427.
Hydrothermal tube-in-tube experiments involving strong thermal gradients were initially designed to determine if mass transfer between solids and solution and mass transport induced by thermal gradients within closed systems could account for the spatial distribution of naturally occurring hydrothermal sequences of crystallisation.New experiments involving a variation of temperature with time or a strong thermal gradient were conducted in order to study the transport and transfer of aluminium in a closed medium along with dilute water. Results show that in contrast to the common assumption, aluminium is highly mobile at 3 kbar, 300-550°C in a water saturated medium with alkali (K/NaASH). The solubility and the transport of aluminium are controlled by the availability of alkali. Starting from a mixture of kyanite + quartz + muscovite at the hot extremity of a thermal gradient, aluminium is transported toward the cold extremity in form of a complex with an Al/K stoichiometry close to unity. However, since more aluminium than alkali are released by the dissolution of muscovite, an Al-rich phase (kyanite) forms close to the starting minerals undergoing dissolution, although aluminium is mobile in our system. Then, the variation of the solubility of Al-K complex with temperature leads to the formation of muscovite (+quartz) at the cold extremity of the thermal gradient. A quantitative interpretation of the experimental results was done using literature data on aluminium speciation in dilute water. Extrapolation of the laboratory results to natural rocks suggests that 1) the diffusion of aluminium in water is an efficient process of transport under medium-grade, low- to medium-pressure conditions. This conclusion is consistent with the occurrence of Al-bearing phases in synmetamorphic veins. 2) mass-transfer estimates based on mass-balance analyses postulating a fixed aluminium reference frame should be considered with caution. 3) the high fluid/rock ratio calculated from the amount of aluminosilicates occurring in veins of medium-grade metapelites are questionable because such calculations neglect the importance of the transport of aluminium by diffusion.
Predazzite in the Debnik anticline was formed during the contact metamorphism and hydrothermal alteration of uniform series of Givetian dolomites. Metamorphism caused by the late Palaeozoic ryodacite shallow intrusion resulted in the formation of ca. 300 m thick contact aureole.
In the inner part of the contact aureole altered rocks occur as white calcite-brucite marble (predazzite). The process of predazzite formation can be explained by dolomite thermal decomposition and hydration in the following reaction:
CaMg(CO3)2+H2O = Mg(OH)2+CaCO3+CO2 (1)
In the outer part of the contact aureole white predazzite dominates along fractures or layers whereas the remaining rock is black, and composed of dolomite, calcite and brucite (Haranczyk & Lewandowska, 1994). Temperature, due to the constant distance from the intrusion, must be ruled out as a factor controlling the observed variation of mineral assemblages in a range of centimetres. In the field, metamorphism is marked by a distinct change in the rock colour. Unmetamorphosed dolomites were brown due to the presence of organic matter (0.5 wt.%). Metamorphic fluids removed organic matter causing predazzite whitening. In the black altered parts, however, the organic matter underwent coalification and constitute up to 0.3 wt.% TOC (Lewandowska & Rospondek, 1998). The differences in mineralogy of the white and black rock can be explained by the influence of the reaction (1) on the composition of metamorphic fluids. In the white zones, fluid composition was externally buffered and CO2 derived in reaction (1) was removed. The reaction (1) was completed, and predazzite exclusively composed of calcite and brucite formed. Penetrating fluids removed organic matter causing whitening. However, the efficiency of the external buffering decreased away from the fissures. In the black zones reaction (1) internally buffered composition of metamorphic fluids. This process increased XCO2, dolomite decomposition was restrained, and this mineral is still an important component of the rock. The amount of rock penetrating water declined rapidly and the organic matter was preserved in a coalified form.
Emplacement of ryodacite intrusion at shallow depths caused the thermal expansion of country rock fluid and hydraulic fracturing of the dolomites in the contact aureole. Along fractures composition of metamorphic fluids was externally buffered and predazzite was formed. Away from fissures fluid composition was internally buffered and dolomite decomposition was restrained. Processes of external and internal buffering well explain coexistence of predazzite and dolomite in the zones which underwent metamorphism at the same temperature.
Haranczyk C, & Lewandowska A, Przew LXVZj Pol Tow Geol, 112-114, (1994).
Lewandowska A, & Rospondek M, Pr Spec Pol Tow Min, 11, 133-135, (1998).
The Serra of Freita region (Central Iberian Zone) is dominated by pre-Ordovician metasedimentary rocks of the Slate-Greywacke Complex. These formations were intruded by syn, late and post-D3 granitoid plutons during the Variscan orogeny. Data from detailed structural, metamorphic cartography, petrography and mineral chemistry shows that the Variscan regional metamorphism: (a) occurred mainly, during the final period of the D2 deformation and syn-late D3 times; (b) is of low pressure high temperature type (LPHT) with mineral parageneses containing andalusite in medium and high grade metamorphic zones; (c) is prograde and (d) practically isobaric.
Locally, in staurolite-andalusite and sillimanite zones, the regional metamorphic sequence, marked by prograde dehydration reactions, is interrupted. These interruptions correspon