In transpressive orogens, syntectonic high-temperature (HT) metamorphism typically is kyanite-sillimanite or andalusite-sillimanite type, in which rocks record clockwise P-T paths. These features reflect strongly perturbed geotherms generated by: 1) heat conduction to the surface during syntectonic erosion of the thickening orogen; 2) dissipation of mechanical energy generated during deformation; and, 3) mass transfer within the orogen that advects hot material to shallow depths; where, initial distribution of heat production with depth, including anomalous heat-producing layers, and changes along the length of the orogen are important variables. The Acadian metamorphic belt of the northern Appalachians is characterized by elevated modern-day heat flow (~65 mWm-2) and high heat production (~3.5x10-6Wm-3). In western Maine, Acadian HT metamorphism culminated in anatectic migmatites and emplacement of granites derived from crustal sources, particularly the Silurian metasedimentary sequence of the Central Maine Belt. Metamorphic field gradients suggest high-to-moderate rates of temperature change, but reveal only small variations in pressure, features typical of HT metamorphism in transpressive orogens. The stratigraphic sequence includes formations with high heat production, a consequence of high U and Th contents fixed in strongly reduced sediments of the precursor anoxic basin. Oblique translation during contractional deformation thickened the stratigraphic sequence and displaced isotherms toward the surface creating a near-isothermal corridor in the orogenic crust. This thermal structure is imaged by the geometry of the 'migmatite front', which is essentially an isothermal surface given the dP/dT of the solidus for crustal melting. One consequence of the coupled mechanical and thermal evolution of transpressive orogens is melt migration to progressively shallower crustal levels by differential stress-induced processes as the thermal corridor is propagated upward, and advection of heat to the upper crust. The source of immigrant granite can be traced using isotopic fingerprinting, so that we can assess the volume of melt derived from different sources in the crust. We can use this information to determine whether the thermal perturbation associated with HT metamorphism extended to the Moho, reflected in voluminous granite magmatism from lower crustal or mixed (crust and mantle) sources, or was damped in the lower crust. In western Maine, the absence of a significant volume of granite with a geochemical signature indicating derivation from basement inferred to underlie the Central Maine Belt is consistent with calculated intermediate-to-low reduced heat flow from the lower crust and mantle, and implies low thermal gradients in the lower crust under inferred granulite facies conditions. Thus, the thermal structure of the deep crust contemporaneous with shallow crustal HT metamorphism can be constrained by inferences based on volume and source of granite. Furthermore, in western Maine, Acadian orogenesis and metamorphism involved redistribution of energy and mass within the crust, rather than addition of energy and mass by mantle processes.
The Neoproterozoic and Palaeozoic of central Australia preserves a remarkable record of intraplate basin formation and orogenesis. Whilst the mechanism by which strain is localised in an intraplate setting is poorly understood, it is likely to be intimately related to thermal weakening of the lithosphere. It is therefore useful to examine evidence for the thermal structure of the crust during such events. A major intraplate event in central Australia, the Petermann Orogeny, affected the Mesoproterozoic Musgrave Block and Neoproterozoic Amadeus Basin at c.560-520 Ma. In the Mann and Petermann Ranges in the northwestern Musgrave Block, south dipping thrusts that were active late in the Petermann Orogeny have juxtaposed domains which underwent peak metamorphism at differing crustal levels. The ability to examine different crustal levels within the same orogen provides an opportunity to gain insights into the thermal structure of the crust during this event.
In the Mann Ranges, Mesoproterozoic granulites, post-tectonic granites and mafic dykes have been reworked by pervasive mylonitic fabrics at conditions of ~12-13 kbar and 700-750°C. In outcrops 10-30 km to the north, high pressure garnet amphibolite facies assemblages developed at 9-10 kbar and c.700°C. A major north-directed structure, the Woodroffe Thrust, juxtaposes this domain against migmatised granites that were metamorphosed at c.650°C at 6-6.5 kbar. All of these estimates reflect peak metamorphism during the Petermann Orogeny, and the similarity of peak temperatures at significantly different crustal levels implies an unusual geotherm during this event. The exposure of rocks which were at 630-650°C at 6-7 kbar implies a thermal gradient of ~30-35°C km-1 in the upper 20-30 km of the crust, whilst the exposure of rocks metamorphosed at 700-750°C at 40-50 km depth suggests a much lower thermal gradient in the order of 5-10°C km-1 in the lower crust. This suggests that a heat source resided within the crust during the Petermann Orogeny, without a significantly elevated mantle heat flow.
In spite of the existence of significant partial melting within the mid crust, there is no evidence for magmatism associated with the Petermann Orogeny as would have been expected if the lower crust had reached elevated temperatures. However, at depths of 20-30 km, corresponding to the most thermally perturbed level in the crust, there are voluminous Mesoproterozoic granites which had anomalous radiogenic heat production of ~5-7 µWm-3 at the time of metamorphism. Recent models (eg Sandiford & Hand, 1998) suggest that burial of a body with high heat production beneath an insulating cover may lead to elevated temperatures in the mid-crust. The existence of high heat producing layer at the same depth as the greatest thermal perturbation during the Petermann Orogeny suggests such a mechanism may have significantly contributed to the thermal regime during this event.
Sandiford M & Hand M, What Drives Metamorphism and Metamorphic Reactions? Geol Soc, London, 103-114, (1998).
It is known from numerical modeling of wet crustal melting that thermal structure of the crust in the area of partial melting is characterized by vertically stabilized temperatures that range from 600 to 700°C. This temperature distribution in the middle and lower crust cause high temperature gradient in the upper crust above and resulted in significant curvature of the thermal gradient line. Alternatively, almost straight line-shaped thermal gradient is usually supposed for the crust under granulite metamorphic conditions.
Recently published results of petrologic mapping of granulite-gneiss assemblages make it possible to estimate real temperature distribution in lower crust during high-grade metamorphism. It was shown (Mints, Konilov, in press) on a basis of generalization of lateral and vertical distribution of P-T parameters in granulite-gneiss belts of various age that thermal gradient within lower crust during granulite metamorphism is rather low, ca. 5-10°C/km. Low thermal gradient inside the lower crust results in high temperatures and rather high T/P values in corresponding sections of upper crust.
The real nature of low thermal gradient in lower crust under granulite conditions is unclear yet. Variants of explanation might be: high thermal conductivity during granulite metamorphism due to fluid penetration and intensive deformations. It seems possible that increasing of thermal conductivity due to appearance of dry partial melts may be of special significance. Heat transfer with moving partial melts and penetrating fluids may have a role too.
Two extreme types of granulitic crustal sections may be distinguished. First type (GM1) is corresponded to normal and especially to thinned (40-25 km) crust. High and ultra-high temperatures (up to 1050°C) are characteristic for the deepest granulites of this type. Second type (GM2) is corresponded to thickened (50-55 km) crust. It is characterized by moderately high temperatures in lower crust (700-800°C at crust-mantle boundary). In modern geodynamic environments GM1 granulite metamorphism have to be suspected in the crust with extremely high heat flow (120-150 mW/m2) areas and "dry" fluids, such as back-arc seas and rift systems. These areas are generally related to asthenosphere upwelling accompanied by crustal extension and/or crustal thinning. Maximal heating is characteristic for the most thinned crust. GM2 assemblages are characteristic for collision orogens with moderately high heat flow (70-130 mW/m2). Metamorphism of GM2 type is to be related to compression conditions and collision crustal stacking. Metamorphism of lower crust of active continental margins and island arcs (heat flow around 80 mW/m2) must be in no higher amphibolite facies generally due to buffering influence of partial melting under wet fluid conditions resulted from dehydration of subducting plate.
Field and petrological studies permit constraint of P-T-t paths of Precambrian granulite gneiss terranes and estimation of thermal gradients in the crust. To apply this kind of approach, however, it is crucial to be certain that the studied mineral assemblages formed during a single phase high-grade event. Geochronology is important to test this commonly made assumption and to constrain the duration of metamorphism.
Although the Okiep area in western Namaqualand is part of a well studied low-P high-T terrane, the timing and duration and the single-phase versus poly-phase nature of high-grade metamorphism is still debated. According to some models, high-grade metamorphism was >1.06 (up to 1.2 Ga), coeval with the main D2 deformation and predating emplacement of major intrusive suites and associated W-Mo and Cu deposits (1.06-1.04 Ga); metamorphic fabrics of ores were seen as evidence for a second phase of high-grade metamorphism (Raith and Harley, 1998). In contrast, recent single zircon U-Pb ages of ca. 1.03 Ga (zircon overgrowths in orthogneisses) were interpreted to date the high-grade event. According to this new model, metamorphism is single phase, overlaps or postdates the emplacement of the major intrusive suites and is syn-D3 (Robb et al. in press).
To resolve this problem, EMP Th-U-total Pb model ages of monazite from high-grade pelites and high precision Re-Os ages of molybdenites from four W-(Mo) mines were obtained. Monazites gave the following model ages: Ratelpoort Formation (NA 326-7): 1042 ± 17 Ma; Wolfram Formation (NA6): 1038 ± 12 Ma; Springbok Formation (WHS14): 1047 ± 18 Ma. The Re-Os data for molybdenites hosted in foliation-parallel quartz (± feldspar, ± garnet) veins yield single mineral ages between 1000 and 1026 Ma and give a well defined 187Re-187Os isochron age of 1019 ± 5.6 Ma (2 sigma). Formation of W-Mo mineralisation thus overlaped with the retrograde stage of the Kibaran event. Local subhorizontal shearing, previously assigned to late D2, and metamorphic re-equilibration affecting W-Mo ores therefore must be younger than ca. 1.02 Ga and postdate peak metamorphism.
Some conclusions of our study are: (a) Th-U-total Pb ages for monazite are consistent with new zircon SHRIMP ages and directly date high-grade metamorphism in western Namaqualand. (b) Re-Os ages for molybdenite directly date the formation of W-Mo deposits; in contrast to previous concepts, these deposits are post-peak metamorphic. (c) The general correlation of subhorizontal "D2" structures throughout Namaqualand may be problematic. (d) Present geochronological evidence does not support concepts of high-grade metamorphism prior to 1.06 Ga followed by crustal cooling during the ca. 1.06 -1.03 period.
Raith JG & Harley SL, J. Met. Geol., 16, 281-305, (1998).
Robb LJ, Armstrong RA & Waters DJ, J. Petrol, in press
Recent geological and geophysical observations in the Himalaya and adjacent Tibetian Plateau imply that since the onset of collision about 50 Ma the thickness of the crust increased up to 70-80 km thick. Seismic data show that marked low velocity zone lies within the crust at depths of 15 to 20 km. That zone is assumed to be the molten granite layer (Nelson et al., 1996 ) according with the model of the thickening crust melting (England and Thompson, 1984).
In Caucasus the Trans-Caucasian plate underthrust northwards, beginning at 25 Ma, below the Scythian plate assumed to be an active Paleozoic continental margin of the East-European craton. The crust thickened here up to 60 km comprises the zone of low velocity and loss of reflection, probably containing granite melt (11 km depth, 10 km thick,Vp=5.8-5.9 km/s, d=2.5-2.67 g/cm3) (refs. in Rosen, 1998). The Late Pliocene, 2-3 Ma, collisional granites were melt out in the lowermost level at 30 km depth under 1000°C. Granite magma comes up and accumulates at the middle crust level (low seismic velocity zone, 10-20 km depth, probably 600°C) and arise to the uppermost level where granite liquid burst into the cold country rocks (Rosen, 1998). Thermal structure of the crust appears approximately as follows: 1) depth 0-10 km, thermal interval 0-600°C, thermal gradient 60°C /km, 2) 10-30 km, 600-900°C, 15°C/km, 3) 30-60 km, 900-1000°C, 3,3°C/km. Such temperature distribution may be the reason of low gradients calculated for granulite-grade areas.
The study is supported by RBRF Projects No. 97-05-64463, 96-05-64190.
England A E, and Thompson P C., Jour. Petrol., 25/4, 894, (1984).
Nelson K D, Zhao W, Brown L D et al., Science, 274, 1684, (1996).
Rosen OM, Goldschmidt Conf. Abs, 17, (1998).
In many settings, destructive plate margins involving continental crust appear to develop in a typical sequence of stages from subduction to collision to tectonic exhumation and erosion. Much insight into the tectonic evolution has come from linking observed orogen structures to increasingly sophisticated kinematic models, both analog and numerical. By contrast, our understanding of the thermal evolution has not, so far, benefited from comparable studies, linking numerical models to well-documented PTt data sets.
We present results of 2-D finite element models designed to investigate the thermal consequences of deformational scenarios proposed in recent years, e.g. for the Central Alps. The FEM code uses an adaptive grid approach which allows deformation and differential movement of subgrids to simulate tectonic mass flow. The geometry and velocities of plate convergence, large-scale strain partitioning (subduction channel, slab break-off, tectonic exhumation) and erosion are described explicitly as functions of time. At present, our kinematic model involves a down-going slab, tectonic fragments that are first subducted then obducted, and a convecting (subcontinental) mantle. The thermal anomaly developing in the subduction-collision cycle, its spatial-temporal evolution during and following the tectonic exhumation, and its decay during the backthrusting stage (which involved rapid uplift and erosion) have been modeled.
For the Central Alps, PTt-paths computed for different parts in the orogen profile are compared to those documented in the Lepontine metamorphic belt. Our results prove valuable in understanding (a) the links between its tectonic and metamorphic history, (b) the significance of thermobarometric data, and (c) the interpretation of a number of mineral chronometers.
Basement in Pointe Géologie area (66°40'S - 140°00'E) (Terre Adélie - East Antarctica) mainly consists in a Proterozoic metasedimentary complex. The metasediments underwent a 1.7 Ga thermal event leading to high-grade amphibolite facies assemblages biotite + cordierite + sillimanite ± garnet and to migmatites and anatexites by dehydration melting reactions. This event occurs at low pressure and high temperature conditions of 3.5-5.5 kbar and 650°-750°C, with the rocks experiencing a H2O = 0.9 (Monnier O.,1995). Later, retrogression of migmatites and anatexites led to cristallization of two different assemblages: (i) commonly the retrograde imprint is quite limited and expressed by an overall biotite + muscovite ± andalusite ± garnet equilibrium assemblage. (ii) locally, retrogression, which develops along East-West, gently deeping bands cross-cutting the sub-vertical North-South foliation, is more extended with parageneses involving muscovite + biotite ± garnet ± chlorite. Field relationships, thermobarometric calculations and bulk rock chemistry indicate that the hydration event responsible for the observed retrogression occured under greenschist facies and static conditions and upon a chemically homogeneous migmatitic material. Retrogression occurs at PT conditions of a maximum 3.5 kb and 500-600°C. Two pseudo sections has been calculated with the same bulk rock composition but for two different H2O contents. They show that muscovite + biotite ± garnet ± chlorite assemblages from the most retrogressed bands are consistent with 20% more of H2O. Therefore the occurrence of both retrogression assemblages could be explained neither by different P-T environment nor distinct protoliths but by an heterogeneous distribution of fluids within the migmatitic complex. Yet, available field data give no indication on what could control the occurrence of such heterogeneities in water content within the migmatitic complex.
Monnier O, Thesis, 319, (1995).
Within Belomorian-Laplandian metamorphic belt evolved between Kola and Karelian cratons at least four major tectonic cycles are recognised that may be supported by the geological and isotope-geochronological evidences (Miller et al., 1999).
The most ancient event related to development of the arc system near Late Archaean continental margin (ca 2.85 Ga) and accresional prism in connection with subduction of oceanic crust under continent. There is only poor information on PT condition of the metamorphism, particularly on high-temperature and low or moderate pressure were recognised in the Chupa unit. The following Archaean event in the lower part of the late Archaean Belomorian allochthone is identified as moderate-pressure granulite metamorphism which was coeval with plutonic activity led to formation of calc-alcaline assemblages of the igneous rocks (2.72 Ga) geochemically similar to those of the active continental margin of the Andian type. High-grade metamorphic rocks involved in the nappes and was cooled at the pressure increasing from 6 up to 12 kbars in connection with thickening of the crust within the collision zone (2.70 Ga). After that retrograded granulite rocks were subjected by decompression to 5-6 kbars during exhumation of the deep-sitting complex. The last event of the evolution, included the partial melting and polimigmatite formation, had the ca 2.6 Ga age. This anticlock-wise path, including the branch of compression cooling, is very characteristic for the lower part of the large nappes related to the collision stage of tectonic evolution.
High-pressure and high-temperature metamorphic processes which are correlated with early Palaeoproterozoic (2.45 Ga) extension tectonics (rift formation) and connected with mafic (peridotite-gabbro-norite and gabbro-anorthosite) and felsic (K rich granitoids) plutonic activity. Within the Belomorian belt only the lower crust metamorphic rock assemblages are exposed because the high pressure, up to 12-14 kbars could be estimated. During retrograde evolution the metamorphic rocks in the shear zones were subjected by isobaric cooling and then decompression. This anticlock-wise path, including the branch of isobaric cooling, connected with the heat source die off. It means the crystallisation of the magmas emplaced into the lower crust as result of rifting.
Development of the late Palaeoproterozoic (Laplandian) nappe system was accompanied by high-grade (up to granulate facies), high-pressure metamorphism. In the lower parts of the thick nappes inverted zonality and anticlock-wise PTt paths of evolution were established that reflects doubling the crust and followed it exhumation of the lower crust complexes because of thrusting.
Metagabbro bodies are found within mantle derived ultramafics and metapelitic-arenaceous granulites and migmatites of the Calabride nappe from the northern sector of the Calabrian Coastal Chain, Calabrian Arc. Metagabbros outcrop as centimetric veins and up to metric dykes intruded within harzburgitic ultramafics and as small banded-textured bodies between ultramafics and granulites with sharp contacts with both the country-rocks. The metagabbros show tholeiitic fractionation trend. Incompatible elements spider diagrams normalized against MORB show more or less levelled patterns around 1, except for the conspicuous Th negative anomaly. The REE normalized patterns are upward convex with slight Eu negative anomaly and LaN/YbN ratio ranges from 0.07 to 2.39. The geochemical data suggest that the gabbros represent the products of partial melting of partly depleted mantle source at Spl-lherzolite facies conditions. Considering that: i) the metagabbros are intruded within ultramafics and between ultramafics and crustal granulites, ii) the metagabbros experienced the same tectonometamorphic evolution of the country-metapelites according to a tectonic model of asymmetric extention) (Piluso and Morten,1997), iii) the calculated physical conditions of the climax granulitic event are of 750° - 800°C, T for the gabbros and of 0.9 - 1.1 Gpa for the granulitic metapelites (Piluso and Morten, 1997), it is possible to hypothesize an intrusion of gabbroic melts at the crust-mantle boundary. Thus the presence of metagabbros within the crystalline basement from Catena Costiera allows to suggest that: i) a thermal anomaly developed nearby the crust-mantle boundary due to astenospheric upwelling; ii) the lithosphere suffered a mechanical destabilisation owing to viscosity decrease caused by gabbroic melts intrusion and by the melting of crustal country-rocks. Unfortunately there are no age determinations of the gabbro magmatism, while the granulitic metamorphism has been dated at about 295 Ma in the Serre massif, southern sector of the Calabria Arc (Schenk, 1989). Then, an older age for the gabbro magmatism may beenvisaged. Taking into account the above and the tectonometamorphic evolution of the crystalline basement rocks from Catena Costiera, the underplating magmatism in the northern sector of the Calabrian Arc is likely linked to a lithospheric thinning precursor of the Tethyan mesozoic rifting.
Piluso E, Morten L, Quaderni di Geodinamica Alpina e Quaternaria, 4, 208-209, (1997).
Schenk V, Geol. Soc. Special Publ, 43, 337-342, (1989).
The concept on parabolic temperature-pressure dependence (paleogeotherm) in the South India granulites has been initially emitted by Harris et al. (1982). It was based on T-P data from Nilgiri, Madurai and Nagercoil at assuming that these blocks are the parts of single craton and that metamorphism in each block was synchronous. However, recent geochronological data evidence that polymetamorphism in these blocks varies in age from the early Paleoproterozoic to Pan-African (Bartlett et al., 1998). We consider additional data from Nilgiri supporting concept on nonlinear character of geothermal line (convex towards the T-axis). The research of geothermal gradients needs high precision of geothermobarometers and their coordination for various mineral assemblages. We used the consistent systems of geothermometers and geobarometers of TPF data base. The additional testing of both systems on a basis more than 800 independent experiments has confirmed their reliability.
The Nilgiri granulites were formed at two early stages of metamorphism. "Cores of minerals" and "matrix grains" correspond to the first stage and second one is unequivocally characterized by coronitic structures only (Srikantappa, 1996). Our processing of analytical data gave that temperature change in dependence on depth is low. The inferred value of "internal" geothermal gradient is ~6°C/km for the first event and ~4°C/km for the second one. This results in high temperatures and rather high T/P values in corresponding sections of upper crust. According to published isotope and our chemical Th-U-total Pb geochronological data the time span between events was not less than 100 Ma. Our observations allow to suggest that significant cooling possibly up to stable continental geotherm could be real between noted high-grade events.
Obtained data support the main conclusion of Harris and co-authors (1982) on the character of T-P dependence during granulite metamorphism. However, the geothermal curve obtained by above authors is in reality the superposition of fragments of different time geotherms. The nature of low geothermal gradient in the lower crust under granulite conditions is unclear. One explanation is high thermal conductivity due to fluid penetration, appearance of partial melts and intensive deformations. At any case it seems evident that the lower crust under granulite conditions could be an effective heat conductor. Hence, cooling after mantle heat flux cessation is to be rather rapid. This may make a significant hidden cooling between consecutive high-grade events similar to suggested for metamorphic evolution of Nilgiri or noticed for East Antarctica (Hand et al., 1994; Fitzsimons, 1997).
Harris NBW, Holt RW & Drury SA, J. Geol., 90, 509-527, (1982).
Bartlett JM, Dougherty-Page JS, Harris NBW, and others, Contrib. Mineral. Petrol., 131, 181-195, (1998).
On the basis of the gravity field interpretation by the method of gravitational sounding with the use of the apparatus of numerical differentiation the mantle blocks having high and low meanings of density are revealed. As the high density upper mantle rocks are impenetrable for the deep hot mantle flows, hence, it is possible to consider them a rather cold. And as the lower density upper mantle rocks are penetrable for the deep mantle flows it is possible to consider them a rather hot. So it is possible to use the materials of gravity field interpretation when the mantle temperature regime is determined. The summary schematic maps of the temperature regime of the consolidated crust and sedimentary cover reflect the presence of cold and hot regions at the territory of Pechora plate with the stabilized thermal regime during the basic stages of the history of the earth crust formation. The temperature regimes of the earth crust and upper mantle comparison is marked their unequivocal conformity to each other. The stabilized cold territories of the sedimentary cover and consolidated crust are situated above the cold mantle blocks, and the stabilized hot regions of the earth crust - above the rather warmer mantle blocks. Hence, the thermal regime of the earth crust is unequivocally determined by the thermal regime of the upper mantle.
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