Volcanological, geomorphological, geophysical, petrographical, geochemical, geochronological and paleomagnetic data have been integrated in this study presenting the first 1:50,000 scale volcanological map of the Börzsöny Mts, North Hungary. From the Middle Miocene to the Quaternary, a calc-alkaline volcanism ocurred along the Carpathian arc. The volcanic rocks of the Börzsöny Mts, erupted in three stages (Karátson 1995), are among the first products of this magmatic activity. They are predominantly medium-K andesites and dacites; more mafic rocks (basaltic andesites) can only be rarely found. During the first stage of the volcanic activity (16-15 Ma), a prevailing explosive, partly submarine volcanism resulted in am ± bi ± garnet bearing dacitic-andesitic ignimbrites deposited on land and under water. Pumice-rich to pumice-poor, lithic-containing, mass flow-redeposited volcaniclastics are also frequent. Commonly at the periphery of the mountains, extensive, ungraded, unstratified, dacitic-andesitic breccias, occurring at reconstructed caldera rims, indicate pre-caldera cones that may have been collapsed by one or two large caldera-forming events. First-stage and post-caldera dacitic lava domes, sometimes revealing their subvolcanic level at present, are also bi ± garnet bearing, whereas second-stage, less silicic lava domes are coeval with an intense hydrothermal activity and related ore mineralization (15-14 Ma). The third-stage volcanism produced the most voluminous, well-preserved High Börzsöny lava dome complex (14-13.5 Ma). Individual domes/crater remnants of this edifice have been identified, and their products are classified as py ± am andesite lava flows, autobreccias and block and ash-flow deposits.
Karátson D, Acta Vulcanologica, 7(2), 110-117 (1995)
Basaltic volcanism of Kula, the youngest activity of Western Anatolian Quaternary Volcanism crops out in the Gediz Graben. The volcanism is represented by the cinder cones, maars and lava flows related to both cones and fissures, covering an area of 350 km2. Morphological features led us to distinguish three generations of basalts: First Generation, Second Generation and Third Generation.
Although, no ultramafic-mafic nodules were found in the first generation lavas, they are present within the ejectas of cinder cones and maars of the latest generations. Most of the nodules are rounded and reach 40 cm in diameter. Petrographical determinations show that the hornblendites are the most common lithologies, while lherzolites, pyroxenites and gabbros are present in minor amounts.
The hornblendites exhibit generally porphyritic texture and the volcanic glass constitutes a major interstitial phase.The gas vesicles are also observed. Modally 70% constituants of the samples is formed by the phenocrysts phases. They are essentially composed of long prismatic hornblendes and kaersutites, while tabular hornblende, augite, biotite, plagioclase and apatite are present in minor amounts. Textural characteristics of the hornblendites and similar mineralogical compositions between the hornblendites and the basalts led us to think that hornblendites and basalts were derived from the same parental magma. The hornblendites are probably the products of the early segregation event of the host basaltic magma in a magma chamber. Apart from the porphyritic texture, hornblendites represent occasionally adcumulate and/or mesocumulate texture with sporadically pegmatoidic aspect.
On the other hand, lherzolites, pyroxenites and gabbros show the cumulate textures and their mineralogical compositions are obviously different from the host basalts. The lherzolites have poikilitic texture and several olivines, rounded minerals of various size, are included in large clinopyroxenes and orthopyroxenes. Chromites are rather small euhedral minerals. The pyroxenites exhibit adcumulate texture and composed essentially of subhedral augite and rare interstitial hornblende. The gabbros, mesocumulates, are constituted principally by plagioclase, augite, biotite and hornblende. Calcite and titanite are occasional primary minerals in gabbros.
Lherzolites, pyroxenites and gabbros represent probably the different units of cumulates formed in a magma chamber previously emplaced than the hornblendite bearing basalt generations. The rounded shape and the existence of interstitial volcanic glass show that these cumulates were not completely solidified when they were included by the basalts.
Karadag is the smallest stratovolcano of Central Anatolian Volcanic Province (CAVP) and constitutes the southernmost end of the CAVP. The field works led us to distinguish four evolutionary stages;
First stage, the initial phase is characterized by the emplacement of andesitic-dacitic lava flows, lava domes and associated block and ash flows. They exposed at the southern and the eastern parts of volcano. Afterward, the volcanism gained an explosive character toward the end the stage and a vulcanian style eruption occurred. The products of this eruption crop out around Milizli and the exposure is composed of stratified monolithological dacitic fragments. The vesiculated fibrious pumices, bread-crust bombs and some fine grained ash beds representing the dune and anti-dune structures are occasionally present within the deposits. These deposits are overlain by pumiceous ash flows with two units which represent at its base plinian airfalls. This paroxysmal eruption probably caused a caldera collapse on the volcano and its somma (Kartallik-Sizak hills) is present in the eastern part of volcano.
Second stage witnessed an important phase of dome extrusions and associated block and ash flows. They are exposed at S, SE and SW of the volcano. The block and ash flows represent in general Pelean style deposits and they are sometimes reworked to lahars.
Third stage, the products are exposed at N, NE and NW of the volcano. The debris flow deposits are recognized. These deposits are probably related to the horse-shoe shaped morphology at yhe summit area. Numerous dykes and domes were observed within the caldera. The latest activity is represented by the phreatomagmatism. Eruption started with phreatic character including fine grained ash surges. Toward the end of the eruptions, the pumices appear. While the base surge deposits dominate the lower part of the sequence, phreatoplinian and plinian deposits occur at the top, before the emplacement of pumiceous ash flow deposits.
Fourth stage, the latest activities are represented by the dome extrusions and block and ash flows. The development of a rift zone on the southern flank permitted the alignement of five domes. They have relatively fresh morphology and represent abundant cognate inclusions and emulsified facies.
The west-european rift was associated with the occurence of a typical intraplate alcaline volcanism. In the Massif Central (MC) area, the spatial and temporal distribution of tertiary volcanism reveals two successive episodes in the west-european rift formation. During the late Eocene and lower Oligocene, crustal extension prevailed in the northern part of the Massif Central together with the filling of NS oriented grabens by lacustrine or marine sediments (up to 3000 m). This episode of crustal extension was followed during the upper Oligocene and lower Miocene by scattered volcanism still located in the northern part of the MC only. This first episode of crustal extension and sedimentation followed by volcanism is interpreted as resulting from passive rifting mainly located in the northern part of the MC.
In contrast, the oligocene event of sedimentation in the south of the MC was not associated with significant thinning of the crust and lacks of volcanism in the lower Miocene. There, volcanism started in the upper Miocene, 15 Ma after the end of the sedimentation, and is associated with the building of huge strato-volcanoes (Cantal and Mont Dore) and large magmatic provinces (Aubrac, Devès and Velay). This major magmatic event in the MC is well correlated with the thermal anomaly deduced from tomographic studies, which suggest a WNW-ESE oriented lithospheric thinning. We interpret this episode, which took place mainly in the south of the MC several millions years after the end of the sedimentation and which is associated with the uplift of the whole area and the major magmatism event, as due to the thermo-mechanical erosion and thinning of the lithosphere without any lithospheric extension.
Such an evolution in two different episodes may be interpreted as resulting from the formation of the Alpine mountain chain. It has been shown already that the vertical forces induced by the deep lithospheric alpine root could originate extension in the adjacent lithosphere. This may be the primary cause of the passive rifting recorded in the north of the MC. At the same time, the formation of the lithospheric root in the Alps displaces asthenospheric material which flows underneath. The transfer of hot asthenospheric material below the southern part of the Massif Central lithosphere may result in thermal erosion leading to the second magmatism episode, which mimics active rifting.
An intensive research project on the Cantal stratovolcano, which covers almost 2500 km2 and is the largest such perialpine structure, has resulted in the first comprehensive synthesis of the volcano. The 1:100,000-scale digital synthesis integrates abundant unpublished data and new geological, geochemical, geophysical and geochronological data. The stratovolcano was formed between 13 and 3 Ma on an uplifted Hercynian basement associated with Oligocene sedimentary basins. The overall geometry of the stratovolcano is relatively simple, namely a central trachyandesitic volcano (15 km in diameter) surrounded by debris-avalanche and debris-flow deposits (70 km in diameter) sandwiched between an upper and a lower basaltic lava unit. Trachyandesitic material with minor trachyte and rhyolite erupted between 9.7 and 6.7 Ma. This episode led to the construction of a high stratovolcano which collapsed several times around 7.1 Ma and produced the gigantic debris-avalanche deposits and associated debris flows that can be found all around and up to 35 km from the volcano's center. Tentative reconstructions of the stratovolcano before collapse lead to pre-failure altitudes above 3000 meters and debris avalanche material up to 200 km3. Geophysical and geological data indicate the absence of a major caldera beneath the stratovolcano.
The mio-pliocene Cantal stratovolcano in southern France consists essentially of breccias (80%). They are derived, partly, from pyroclastic eruptions in the central volcano and from several sectorial collapses of the volcanic edifice (Fréour and al, 1998) and from lahars (Arnaud et al, 1998) whose products now surround the central stratovolcano and form a breccia piémont. In the ring plain, the lahars generally overly the debris avalanches products. The best criterias to discriminate between the different volcaniclastic deposits are the thickness of the units, their textures and internal structures, and their distal evolution. The geomorphological criterias (topography, hummocks, levees, etc) are extremely difficult to apply on paleovolcanoes.
Debris avalanche deposits, comprising block facies and matrix facies, are characterized by thick units : several ten meters. Superposition of successive units can reach up to 400 m. The block facies consists of relatively coherent fragments from the original stratovolcano. The blocks preserve their original stratifications but are commonly deformed and fractured. The matrix facies is a material texturally similar to debris flow deposits. Its thickness increases towards the distal end of the deposit. In some distal areas, a lahar marginal facies must derive from wet debris-avalanche because it contains many sedimentary clasts picked-up from the substratum.
Debris flow and hyperconcentrated flow deposits built a thick pile of superposed units (up to 450 m) which cover the debris avalanche deposits. With distance from the central volcano, there is a lateral evolution, with dilution from debris flow to hyperconcentrated flow. Then, towards the distal end, the thickness of each deposit and the dimensions of blocs decrease as the sorting improves. In the proximal area, two main types of lithostratigraphic evolution exist within the lahar deposits; they are named "normal" and "inverse". The "normal lithostratigraphic succession" corresponds to a progressive erosion of the strato-volcano because the dilute facies (elvolved debris flows and hyperconcentrated flows) are more and more abundant in the upper parts of the proximal sections. A second type of exceptional lithostratigraphic evolution is named "inverse lithostratigraphic succession" because the debris flows cover the hyperconcentrated facies as well as the lacustrine volcano-sedimentary deposits. This inverse succession could indicate the existence of a phase of reconstruction just after a debris avalanche.
Arnaud N, Leyrit H, Jamet A, Binet F & Vannier W, RST Abs Brest, 64, (1998).
Fréour G, Leyrit H & Nehlig P, RST Abs Brest, 116, (1998).
The Cantal stratovolcano (1855 m) is the largest "peri-alpine" european volcano with an area of 2500 km2 and a volume of approximately 1000 km3. The original trachyandesitic edifice is unknown due to plio-pleistocene erosion. The ring plain is surrounded by extensive fans of volcaniclastic deposits consisting of lahar deposits superimposed on debris avalanche (DAv) deposits which are geometrically and genetically associated. Lahar deposits are located within a radius of about 20 km from the presumed volcanic edifice between Puy Mary and Puy Griou. From 9 to 6.5 Ma, DAv and lahars were emplaced on a granito-gneissic basement, filled oligocene sedimentary grabens and covered miocene basaltic lava flows. Lahar and DAv deposits were fossilised by trachyandesitic and basaltic lava flows which have been erupted until 3 Ma.
At least, three lahar generations are associated with four DAv deposits. The first lahar generation is intercalated between two DAv deposits near the Col d'Aulac (Mars valley) about 12 km to the northwest of the presumed volcanic centre. The second generation is located near Thièzac (Cère Valley) about 10 km to the south, and the third generation nearby Auzolles (Lagnon valley) lies about 13 km to the east of the presumed volcanic centre.
In the proximal areas (as far as 10 km from the supposed volcanic edifice), lahar deposits are either clast-supported (Cère valley to the Southwest of the volcano) or matrix-supported (Mars valley to the Northwest of the volcano), ungraded or inversely to normally graded and contain mainly trachyandesitic clasts and a few basaltic clasts. These proximal lahar deposits include voluminous hyperconcentrated flow deposits (up to three meters in thickness) having parallel fabric and soft-deformation layers in the basal units. Other lahar deposits have a non-oriented fabric suggesting strong shear stress and turbulent motion. In the upper Mars valley, lahar deposits are separated by hardened sandy layer (<1 cm). These lahar deposits are clast-supported and ungraded or inversely to normally graded. Parallel fabrics suggest low shear stress and laminar motion.In the distal areas (from 10 km to the present-day volcanic margins), lahar deposits no longer exceed one meter in thickness and contain epiclastic deposits with rounded elements, contrasting with the angular aspect of the other clasts. Most of the deposits remain clast-supported. Fabrics lack specific orientation, pointing to turbulent emplacement (strong shear stress).
Emplacement, location and extent of the lahar deposits are likely related to the generations and emplacements of the DAv deposits. Inset lahar deposits are observed in the DAv deposits of the former upper Cère valley. It suggests a re-excavation of this valley and the inheritance of some sections of the current drainage system as old as upper Miocene period.
The mio-pliocene Cantal stratovolcano, is the result of two main edification periods : ankaramite and silica over-saturated series (from basaltic trachyandesites to rhyolites) between 10 and 7 Ma, silica-undersaturated series (basanites to phonolites) from 7 to 3 Ma.
Mineralogical and geochemical studies of over-saturated series of rocks older than 8 Ma, sampled in the area of the Elancèze Massif show that the major mechanism of differenciation is the fractional crystallization of olivine, clinopyroxene, plagioclase and amphibole. In more detail, the fractionation of the olivine (Fo84-63) and of the clinopyroxene (Wo52-39 - En42-39 - Fs6-22) dominates the evolution within the basalts. The basaltic trachyandesites contain abundant plagioclase (An60-40), clinopyroxene (Wo50-39 - En44-41 - Fs9-17) and amphibole (kaersutite and magnesian hastingsite). Rare orthopyroxene (Wo3 - En68 - Fs29) appears in some evolved basaltic trachyandesites. In trachyandesites, the plagioclase (An46-29) fractionates abundantly, with clinopyroxene (Wo47-42 - En46-42 - Fs11-12) and biotite (TiO2 5,58-5,83). This evolution seems to characterize all the rocks during the first period (Baubron et Demange, 1977; Wilson et al, 1995; this study).
New geochemical data (trace elements) were obtained on most of dated samples. Compared to the older basalts, the recent basanitic rocks have higher contents of incompatible elements and higher LREE/HREE ratio. For a similar source, this could suggest a smaller degree of partial melting. Indeed, basanites have lower partial melting rate (7 - 8%) than basalts (15 - 20%). However, in more details, in a (La/Yb) vs La diagram, no clear relation exists between the basaltic magma, the oversaturated series and their age. This implies that a more complex evolution, involving contamination or assimilation, must be taken into account. In the diagram Th/Yb vs Ta/Yb, ancient basalts show an interaction with a "continental" component with a high Th/Ta.
Baubron J-C & Demange J, document BRGM, (1977).
Wisldon M, Downes H & Cebria JM, J. Petrol, 36, 1729-1753, (1995).
The open quarry of Montaigu-le-Blin is located in the Limagne graben, on the uplift block of Saint-germain. This quarry presents a lot of varieties of stromatolites, sediments and fauna. This shoal shows the duality between the limestone building and the terrigeneous sediments. Moreover, with the fauna, we can observe some climatic variations which occured during Aquitanian. The sediments have been accumulated in the hollows, in depositional sequences: green marls, formed in reducing environment, brown marls and oncolites, formed in oxidizing environment, and pedogenesis surfaces, formed during emersion. These sequences are related to the fluctuations of the lacustrine basin. We can also notice this fluctuations in the limestone building: they depend on the variations of their environment. The cyanobacterials which composed them are numerous, and some of them can be identified. So it's possible to relate some ecological variations within an edifice. Three separated areas can be identified: (1) limestone building with a bacterial veil,which results of a deepening of the area; (2) a massive construction, only presents in one front of the quarry, formed by Plaziatella sp. and Sarfatigerella sp. (among other cyanobacterials), which altern with layers of Phryganea tubes; (3) more or less regular constructions with Cladophorites, at the top of all the fronts. These three areas seem to have different ecological contexts. We can also meet isolated limestones like stromatolites "en chou-fleur", which seem to be related to chanels areas. The fauna is diversified, especially the birds. There are 60 different species. We can also find a lot of Phryganea tubes. These accumulations, in association with cyanobacterials (more precisely Broutinella sp.), can reach 8 meters high. This quarry presents exceptional qualities of outcrops. The geometrical relations of deposits (horizontal and vertical variations of sediments), the wealth of fauna (Phryganea, molluscas, birds, mammals, reptiles...) and cyanobacterials enable the reconstruction of a model of carbonated sedimentation and there are few exemples elsewhere.
The Upper Rhinegraben can be classified as a low-volcanicity rift. Nevertheless, Tertiary graben tectonics is accompanied during more than 50 Ma by a typical alkaline rift-valley volcanism. Primary magma compositions in this province, as revealed by chemistry (Mg#, Ni, Cr) and mantle xenoliths, are olivine nephelinites and olivine melilitites. There is a gradual transition between melilite-free olivine nephelinites and larnite-normative, melilite-bearing olivine nephelinites to olivine melilitites. Trace element signatures and isotopic compositions define a plume-enriched asthenospheric source. Enrichment factors for the most incompatible elements are up to 100 times compared to primitive mantle. Constantly present negative K anomalies point to residual phlogopite in the asthenospheric source. LREE/HREE fractionation is indicative of residual garnet. Carbonated primary silicate magmas of this type give way to the separation of carbonatites during fractionation and ascent. Mantle samples as xenoliths in the primary magmas are nodules of spinel lherzolites, amphibole lherzolites and garnet-spinel lherzolites enclosed in olivine nephelimites and melilitites and in associated diatreme breccias. These samples constrain the lithospheric conditions underneath the rift.
Sigmund, J & Keller, J, Min. Mag, 58A, 840-841, (1994).
Keller, J, Sigmund, J Müller-Sigmund, H & Czirjak, A, Terra Nova 9, Abstr. Suppl. 1. EUG 97 Strasbourg, 1, 56, (1997).
Patterson, R Wilson, M & Keller, J, J. Conf. Abstr, 1, 448, (1996).
Keller, J, Brey, G, Lorenz, V & Sachs, P, IAVCEI Int. Volc. Congress Mainz 1990. Field Guide, 60pp, (1990).
Wilson, M Rosenbaum, JM & Dunworth, EA, Contr. Mineral. Petrol, 119, 181-196, (1995).
In the southern part of the Upper Rhine Graben the most prominent example of rift related magmatic activity is represented by the partly eroded Miocene Kaiserstuhl volcanic complex. Subvolcanic intrusions of carbonatites are now exposed in the center of the Kaiserstuhl. A preliminary conventional K/Ar determination on mica from an altered carbonatite yielded 14.2 Ma (Lippolt et al. 1963; recalculated with new constants). Three lines of evidence can be followed in order to evaluate the timing of the carbonatite emplacement within the magmatic Kaiserstuhl activity:
1) bracketing between sediments of known biostratigraphic age 2) numeric age determinations on rocks of known field relations to the carbonatite 3) numeric age determinations on phlogopite of the carbonatite itself
Chattian sediments underlying the Kaiserstuhl complex and late Burdigalian sediments in the upper part of the volcanic succession define the time frame for the carbonatitic intrusion.
An upper age limit of 16.2 ± 0.2 Ma (1<sigma>) could be obtained by laser incremental heating experiments on single sanidine crystals from the uppermost phonolitic t3 tephra, which contains carbonatite xenoliths. For a further approximation of the carbonatite intrusion age we have dated crosscutting dikes of known relative succession. A Mondhaldeite dike yielded 16.6 ± 0.2 Ma (1<sigma>) on amphibole (laser total fusion experiments) and one dike phonolite from the central part of the Kaiserstuhl yielded a plateau date of 17.2 ± 0.1 Ma (1<sigma>) on sanidine (incremental heating experiment).
The emplacement of ultramafic diatreme breccias is directly related to the carbonatite intrusion in space and time. Unaltered phlogopite crystals from the carbonatite and the diatreme breccia could be obtained from two research drill cores (KB3 and KB2). Laser Ar/Ar and conventional K/Ar analyses of these phlogopites yielded identical results (17.2 ± 0.3 Ma (1<sigma>) for the diatreme breccia and 17.3 ± 0.1 Ma (1<sigma>) for the carbonatite) reflecting their synchronous emplacement.
With these dates the timing of the carbonatite intrusion within the 2 Ma duration of the magmatic Kaiserstuhl activity is now well constraint.
Lippolt HJ et al., Jh. geol. Landesamt Bad.-Württ., 6, 507-538, (1963).
Petrographical, geochemical and geochronological data on two selected stratigraphic sections from the volcanic clast-rich turbidites of the Taveyanne Formation provide a basic contribution for hypotheses on Tertiary alpine volcanism. No primary traces of volcanism are known throughout the Alpine belt, even though several scattered evidence of a magmatic activity are recorded in Periadriatic calc-alkaline/shoshonitic dikes and plutons. Hypotheses on volcanic sources of the clasts and spatial/temporal relationships of the "Taveyanne" volcanism with the collisional belt are still object of debate. The data here presented provide more constraints for the volcanic scenario in the Alpine collisional belt.
Taveyanne turbidites are composed of volcanic fragments (65 to 92% by volume), petrographically grouped into three main categories: basic (basaltic andesites: 1-10% by vol.), intermediate (andesites: about 85%) and acid (dacites and rhyolites 5-15%). Non volcanic components include plutonic/gneissic rock fragments and intrabasinal and extrabasinal carbonates. Modal petrographical investigations shows different mafic/acid volcanic lithic ratios that suggest that the Taveyanne sandstones were fed by different volcanic edifices mainly composed of calc-alkaline rock series.
Whole-rock analyses coupled with microprobe analyses on the main mineral phases (clinopyroxene, plagioclase, amphibole, biotite) were performed on selected samples containing > 90% clasts by volume of volcanoclastic material. The minor-, trace- and REE-element geochemical features closely resemble those for orogenic, medium to high-K, calc-alkaline rock series.
Geochronological 40Ar/39Ar datings on four volcanic amphiboles yielded plateau ages of 32.5 +- 0.2 Ma (i.e. Lower Oligocene), unequivocally proving that the emplacement of this volcanism was collisional. Biostratigraphical data on calcareous nannofossil associations on the Taveyanne Formation referred it to the upper portion of the Lower Oligocene (ca 32 - 29 Ma).
The high-K serial affinity of the lavas and the lower Oligocene age fits well with the magmatic activity in the internal sector of the Western Alps (andesitic to ultrapotassic dikes, plutons and rare volcanic flows).
All these data suggest that the orogenic magmatism was located on the upper plate of the colliding system, in an internal position with respect to the Penninic thrust front. A very short time span between the volcanism and the deposition of the volcanic clasts occur. Tectonic transport from the volcanic sources to the front of the active "fold and thrust" Alpine belt appear the only reasonable mechanism to account the assumed distance between the original position of the volcanic edifices and the position of the Taveyanne basin in the foreland.
The borehole slotter relief technique has been used for the determination of the recent tectonic stress field throughout the Jura Mountains. Measurements were carried out at 33 new test sites. When taken together with the already published stress data, a detailed picture of stress states in the Jura Mountains can be presented. Five stress provinces can be distinguished : (1) the East Province with an E-W to NW-SE orientation of the maximum horizontal stress (SH), developed in the Lägeren chain but which is also common in the Molasse Basin (to the south) and the Tabular Jura and the Black Forest (to the north), (2) the Central Province with a NNW-SSE to NNE-SSW orientation of SH throughout the central and northern Jura Mountains, also extending southwards to the Molasse Basin and northwards into the Upper Rhine Graben, (3) the Northwest Province around Besançon with an E-W to NW-SE orientation of SH, (4) the Southwest Province with a N-S to NE-SW orientation of SH, which is also developed in the easternmost Bresse Depression and the Ile Crémieu and, (5) the Southeast Province with an E-W to NW-SE orientation of SH.
The most important features are: (a) stress provinces are not always restricted to the tectonic units of the Jura Mountains but extend well into northern foreland areas, (b) narrow transition zones between the stress provinces cut across the major tectonic boundaries of the Jura Mountains, and, (c) fold-derived palaeostress directions of SH show little agreement with the contemporary stress field.
It is concluded that, with the possible exception of the Southeast Province, the contemporary Jura stress field is the result of a regional tectonic stress reorientation in the Alpine foreland rather than controlled by active decollement tectonics in a foreland fold and thrust belt.
One peculiar feature of the internal part of the SE Carpathians is the synchronous occurence on the same site of mafic alkaline and calcalkaline volcanics. Eruptions occur in a late-collision, and not syn-subduction, environment. Besides the volcanic site crosscuts the major suture between the upper (Africa) and the lower (Europe) plates such that alkaline and calcalkaline magmas necessarily originated from the mantle below the lower, and not upper, plate. For understanding this unusual sheme, we postulate that there is necessarily a genetical link between deep and shallow processes, and that only an integrated study enables to find an acceptable solution. The surface data are as follows: the SE Carpathians are in uplift since the plate convergence stopped (around 10 Ma) and the uplift rate is accelerated since 4 Ma; the calcalkaline volcanism is active since 9.4 Ma with a progressive shifting of the active craters towards SE; since 2.35 Ma, alkaline eruptions occur synchronously at 40 km southwestward of the calcalkaline sites; since 4 Ma, intramontaneous basins form at the E-SE front of volcanoes; both volcanoes and basins formed in an uplift-induced NE-SW extension context, by gravity spreading towards SE above a detachment horizon within the crust; the primitive feature of mantle magmas and the alignement of craters reflect the existence of very deep NW-SE crustal fractures. The depth data plumb to the volcanic site are as follows: the lithospheric mantle thickness is reduced (40 km against 100 km elsewhere); the heat flux is very high; the seismicity is confined to a narrow vertical slab hanging between 70 and 200 km depth at 130 km southeastward far from the Africa-Europe suture; all of that is explained with the delamination of the lower mantle lithosphere followed by asthenosheric rise inside the newly created space; besides, a low velocity zone exists within the mantle just below the Moho; this subcontinental mantle, found as xenoliths in basalts, is an harzburgitic mantle metasomatized by ultramafic melts either interstitially (secondary lherzolites), either through fracture systems (monomineral veins); the trace element patterns and isotopic ratios of different mantle lithologies compared to those of calcalkaline and alkaline volcanics evidence that the latter comes from partial melting of the former; the LVZ represents a partially molten zone underlining a crust-mantle decoupling in an uplift context. The chronological order of events is assumed to have been: asthenospheric rise -> density decrease and uplift of the lithosphere -> crust-mantle decoupling. Mantle deformed in a ductile way, decompressed and partially melt. Crust bent and locally fractured enabling rising and eruption of magmas. At surface uplift induced extension, fracturation and basin formation by gravity spreading above a intracrustal detachment horizon.
Tertiary history of the Carpathian-Pannonian Region (CPR) can be characterised by complex geodynamic and associated magmatic events. During this about 20 Ma long period various magmatic activities took place (potassic-ultrapotassic, calc-alkaline, alkaline volcanics). Among them, genesis of the calc-alkaline volcanic products is a subject of long debate, partially due to the lack of precise trace element and isotope data. For the last few years, detailed geochemical study has been carried out in the eastern segment of the Carpathian arc suggesting a direct relationship between volcanism and subduction events (Mason et al., 1996). On the contrary, genesis of the western segment of the Carpathian arc is more controversial (Konecny et al., 1995; Downes et al., 1995).Based on new major and trace element and Sr-Nd-Pb isotope data we propose the following main conclusions: 1. The Miocene calc-alkaline volcanism in the Northern Pannonian Basin, Inner Western Carpathian Arc could be attributed primarily to extensional tectonic phase (i.e. thinning of the lithosphere resulted in partial melting of the metasomatized lithospheric mantle and partly of the lower part of the crust) and cannot be directly connected to subduction and/or slab breakoff mechanisms. Primary magmas of the calc-alkaline volcanics could have generated in enriched mantle sources (lithospheric mantle) previously metasomatized by subduction-related fluids. Thus it appears, that different petrogenetical models can be applied for the calc-alkaline volcanism in the western and eastern segment of the Carpathian arc. 2. Rapid ascent of garnet-bearing magmas characterised the first stage of the volcanism. High pressure garnets are interpreted as primary ones, i.e. they were crystallised from the magmas. Two main groups of garnets (S- and I-type) and garnet-bearing volcanics (metasedimentary lower crustal melts and mantle-derived melts contaminated by lower crustal rocks) can be distinguished based on primarily the 87Sr/86Sr isotope ratios, Y-HREE concentration of the host rocks and major element composition and 18O data of garnets. Eruption of garnet-bearing volcanic rocks may indicate a change in the tectonic regime of the area, i.e. transition from compressive to extensional phase. 3. Crustal contamination was more characteristic in the early stages of the calc-alkaline volcanism (Early Miocene and Earliest Badenian) and then decreased with time. 4. Petrogenesis of the volcanic series of Börzsöny-Visegrád Mts. can be described by combined assimilation and fractional crystallisation (AFC) processes, whereas in the Central Slovakian Volcanic Field (CSVF) and in Bükkalja both AFC and simple fractional crystallisation could have occurred. 5. From the Sarmatian, increased influence of EAR (common European Asthenospheric Reservoir)-like asthenospheric mantle can be recognised in the CSVF volcanic products. Tertiary magmatic activity of this region terminated by pure asthenosphere-derived alkaline basaltic volcanism.
Downes, H, Pantó, Gy, Póka, T, Mattey, D & Greenwood, PB, Acta Vulcanologica, 7, 29-41, (1995).
Konecny, V, Lexa, J & Hojstricova, Acta Vulcanologica, 7, 63-78, (1995).
Mason, PRD, Downes, H, Thirlwall, M, Seghedi, I, Szakács, A, Lowry, D & Mattey, D, J. Petrology, 37, 927-959, (1996).
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