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1.
Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X A as well as P and T. The predicted mineral associations for the low- and high-X A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased  相似文献   

2.
The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.  相似文献   

3.
The metamorphic evolution of metapelites from the eastern partof the Monte Rosa nappe and the Camughera–Moncucco unit,both situated in the upper Penninic units SW of the Simplonline, were investigated using microstructural relationshipsand equilibrium phase diagrams. The units under considerationexperienced pre-Alpine amphibolite-facies conditions and underwenta complex metamorphic evolution during the Alpine orogeny. Peakpressures during an early Alpine high-pressure stage of 12·5–16kbar were similar in the Monte Rosa nappe and Camughera–Moncuccounit. A pronounced thermal gradient is indicated during decompressionleading to an amphibolite-facies overprint, as the decompressionpaths went through the chlorite, biotite and plagioclase stabilityfields in most of the Monte Rosa nappe, through the staurolitefield in the easternmost Monte Rosa nappe and in the Camughera–Moncuccounit, and through the sillimanite field in the easternmost Camughera–Moncuccounit. In high-Al metapelites the initial formation of stauroliteis related to continuous paragonite breakdown and associatedformation of biotite. In the course of this reaction phengitebecomes successively sodium enriched. In low-Al metapelites,in contrast, the initial staurolite formation occurs via thecontinuous breakdown of sodium-rich phengite. In both low- andhigh-Al metapelites the largest volume of staurolite is formedduring the continuous breakdown of sodium-rich phengite belowP–T conditions of about 9·5 kbar at 600–650°C.During this reaction phengite becomes successively potassiumenriched as sodium from phengite is used to form the albitecomponent in plagioclase. For ‘normal’ pelitic chemistries,phengite becomes Na enriched during decompression through thebreakdown of paragonite along a near-isothermal decompressionpath. The Na content in phengite reaches its maximum when paragoniteis entirely consumed. During further decompression the paragonitecomponent in phengite decreases again because Na is preferentiallyincorporated into the albite component of plagioclase. KEY WORDS: metapelites; white mica; high pressure; equilibrium diagrams; Western Alps  相似文献   

4.
Summary High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote ± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average-P–T calculations yield peak eclogite-facies P–T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P–T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P–T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, P–T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.  相似文献   

5.
Zr-in-rutile thermometry in HP/UHP eclogites from Western China   总被引：5，自引：0，他引：5  
Four Zr-in-rutile thermometry calibrations are applied to eclogites from Western China. Here, we show that if rutile grows in equilibrium with Qtz and Zrn, and is isolated inside garnet, it preserves its Zr composition and does not undergo compositional change due to cation exchange with the host garnet. It thus preserves the composition for the P–T conditions of its formation and the growth zoning of the host garnet. For the HP/UHP metamorphic temperature, the Tomkins et al. (J Metamorph Geol 25:703–713, 2007) calibration yields temperatures that agree well with previous studies, whereas the other three calibrations (Zack et al. in Contrib Mineral Petrol 148:471–488, 2004; Watson et al. in Contrib Mineral Petrol 151:413–433, 2006; Ferry and Watson in Contrib Mineral Petrol in 154:429–437, 2007), which do not include a pressure correction, give systematically lower temperatures. Zr contents of rutile inclusions within garnet show systematic decrease from garnet core to rim. The rutile inclusions in garnet rims contain the lowest Zr content, similar to that in the matrix. Analyses confirm that the pressure plays a significant role in modifying the primary temperature dependence of the Zr content of rutile. Rutiles trapped in garnets are unable to re-equilibrate easily during retrogression, but those in the matrix can do so, providing retrograde P–T path information.  相似文献   

6.
Extensive sampling along vertical and north-south cross-sections in the Gran Paradiso basement (internal crystalline massif, Western Alps) reveals an inverse gradient of zircon fission-track ages with respect to altitude, which is explained by an inverted metamorphism during Eocene time. Subsequent to the obduction of the Schistes Lustrés nappe (high pressure Eoalpine phase), the Gran Paradiso massif cooled down 90-80 Myrs ago. The Austro-Alpine nappe of Monte Emilius — Dent Blanche overthrust the Gran Paradiso massif 38–40 Myrs ago, inducing in its basement an inverted metamorphism or, more precisely, a nappe emplacement metamorphism (greenschist facies). The last cooling below 100° C occurred as early as 35—40 Myrs ago in the southern Gran Paradiso massif; in the northern part, this last cooling occurred 20 Myrs ago, accompanied by an important uplift phase (2 mm/year).  相似文献   

7.
Cretaceous melange of the Cordillera de la Costa belt, north–centralVenezuela, there are knockers of eclogite, barroisite-bearingeclogite, and pelitic glaucophane schist. These occur in a metamorphicmelange matrix that locally consists of marble, serpentinite,amphibolite, actinolite schist, feldspathic schist and gneiss,graphitic schist, chloritoid schist, and garnet-bearing micaschist. The protoliths for these various rock types exhibita wide age range (Cambrian to Early Cretaceous?). Recently discoveredknockers of pelitic glaucophane schist contain Mg-glaucophane+ paragonite + kyanite + garnet + talc + graphite + rutile +quartz. The coexistence of kyanite and Mg-glaucophane suggestsminimum P 2000 MPa at T > 600°C. Eclogite knockers fromthe same outcrop contain garnet and clinopyroxene which yield500°C for cores, 700°C for rims, and P 1200 MPa. Theassemblage garnet–biotite–phengite–albitewithin schists of the melange matrix of this locality indicatesmetamorphic conditions of T = 450–520°C at P = 1800MPa. Because all lithologies in this outcrop record high-P conditions,this metamorphic melange formed before or during peak metamorphismin a mid-Cretaceous subduction zone. KEY WORDS: geothermobarometry; high-P pelitic schist; eclogite; Puerto Cabello; Venezuela  相似文献   

8.
HP/UHP and LT metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material, most often exhumed prior to continental collision. The prograde pressure–temperature history of HP–UHP rocks strongly depends on the convergence rate and on the subduction zone geometry. The maximum pressure recorded provides a proxy for the depth of shearing off and stacking of HP metamorphic nappes. A 2-D thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached HP metamorphic units as a function of the slab dip angle and the convergence rate. Model results are applied to the metamorphic nappe pile of the inner Alps. A mean convergence rate of 1 cm/year during the subduction of the Briançonnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 Ma. On this basis, the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle. The slab dip angle is shown to increase, from the northeast to the southwest, along the Alpine arc with estimated values of 20° for Suretta, 30–45° for Monte Rosa and Gran Paradiso, and 60° for Dora Maira. From Eocene to Oligocene times, the increase in slab dip angle is controlled by changes of buoyancy, due to the spatial configuration of the Valaisan trough and the incoming of crustal material within the subduction zone.  相似文献   

9.
In northwest Turkey, high-pressure metamorphic rocks occur as exotic blocks within the Çetmi mélange located on the south of the Biga Peninsula. Rutile chemistry and rutile thermometry obtained from the eclogite and associated garnet-mica schist in the Çetmi mélange indicate significant trace element behaviour of subducted oceanic crust and source-rock lithology of detrital rutiles. Cr and Nb contents in detrital rutile from garnet-mica schist vary from 355 to 1026 μg/g and 323 and 3319 μg/g, respectively. According to the Cr-Nb discrimination diagram, the results show that 85% of the detrital rutiles derived from metapelitic and 15% from metamafic rocks. Temperatures calculated for detrital rutiles and rutiles in eclogite range from 540 °C to 624 °C with an average of 586 °C and 611 °C to 659 °C with an average of 630 °C at P = 2.3 GPa, respectively. The calculated formation temperatures suggest that detrital rutiles are derived from amphibolite- and eclogite-facies metamorphic rocks. Amphibolite-facies rocks of the Kazdağ Massif could be the primary source rocks for the rutiles in the garnet-mica schist from the Çetmi mélange. Nb/Ta ratios of metapelitic and metamafic rutiles fall between 7–24 and 11–25, respectively. Nb/Ta characteristics in detrital rutiles may reflect a change in source-rock lithology. However, Nb/Ta ratios of rutiles in eclogite vary from 9 to 22. The rutile grains from eclogites are dominated by subchondritic Nb/Ta ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.  相似文献   

10.
Petrology of eclogites from north of Shahrekord, Sanandaj-Sirjan Zone, Iran   总被引：1，自引：0，他引：1  
Summary Metabasic rocks were recently found within a ductile shear zone in the north of Shahrekord, being a part of the structural zone of Sanandaj-Sirjan, SW Iran. The rocks give evidence of a so far unrecognized eclogite facies metamorphic event and testify to high pressure metamorphism in the Sanandaj-Sirjan Zone, near the Main Zagros Reverse Fault, which is the assumed suture zone between the Arabian plate and the Iranian block. The eclogites occur as lenses or blocks within ortho- and paragneisses. The petrographic features and reaction textures display at least two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage. The eclogite facies metamorphism is indicated by omphacite + garnet + sodic-calcic amphiboles (barroisite, magnesiokatophorite and magnesiotaramite) + phengite + rutile + (clino-)zoisite + quartz ± dolomite. The garnets are mainly almandine-rich, which fits with the C-type eclogite classification. Calcic amphiboles (hornblende, tschermakite and pargasite) + plagioclase are secondary phases formed during the retrograde amphibolite-facies metamorphism. P-T estimates for the eclogite facies give pressures of 21–24 kbar and temperatures of 590–630 °C (geothermometry) and 470–520 °C (THERMOCALC), respectively. Geothermobarometry for the amphibolite-facies metamorphism yields 10–11 kbar and 650–700 °C. Author’s address: Ali Reza Davoudian, Department of Natural Resources, Shahrekord University, Shahrekord, Iran  相似文献   

11.
The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO2, total Fe, V, Co and Sr, and very low SiO2, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.  相似文献   

12.
AOYA  MUTSUKI 《Journal of Petrology》2001,42(7):1225-1248
Both structural and petrological data can be used to constrainthe P–T path of an eclogitic schist unit (the Seba basicschist) in the Sambagawa belt of SW Japan. The relationshipsbetween these two sets of data are well defined by porphyroblasticand other microstructures. The derived P–T path for theSeba basic schist has an overall clockwise trajectory with thedecompression, or exhumation-related, path taking place undera lower P/T gradient than the burial, or subduction-related,path. The clockwise nature of the P–T path is qualitativelysupported by chemical zoning of amphibole coexisting with eclogiticminerals. The significant feature of the P–T path is thepresence of two temperature maxima, the first in the eclogitefacies and the second in the epidote-amphibolite facies. Theexistence of two temperature maxima gives a simple explanationfor the observation that metamorphic zonal boundaries postdatingthe eclogite facies metamorphism cross-cut the distributionof the main eclogite bodies in the Sambagawa belt. Estimatesof metamorphic pressure using the jadeite content of clinopyroxenein the Seba area demonstrate the existence of a tectonic discontinuitybetween the eclogitic schist and surrounding non-eclogitic schist.Structural studies show that although these two units have experiencedvery different peak metamorphic conditions, they became juxtaposedduring a single ductile deformation affecting both units. Thisdeformation is related to exhumation of the eclogitic schistand subduction of the non-eclogitic schist, indicating thatboth were formed during the same subduction event. The presenceof a major tectonic boundary between two units with a similarorigin as subducted and accreted material, but contrasting metamorphichistories, can be interpreted in terms of nappe tectonics, andthe existence of an ‘eclogite nappe’, the thirdnappe of the Sambagawa belt, is proposed. KEY WORDS: deformation stage; dual thermal maxima; eclogite; P–T–D path; Sambagawa belt  相似文献   

13.
 The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated 40Ar–39Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The 40Ar–39Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their 40Ar–39Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge. Received: 12 October 1995/Accepted:25 June 1996  相似文献   

14.
The Austroalpine Sesia-Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier-Rafray, Etirol-Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt-Saas) tectonic elements, or within the latter. Rb-Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia-Lanzo inlier, is of Eocene age (49-40 Ma), like the underlying Zermatt-Saas ophiolite (45-42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar-39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt-Saas nappe (46-43 Ma) are consistent with Rb-Sr dating, whereas that on the Austroalpine Glacier-Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt-Saas nappe, 20-25 Ma later than the eclogitic Sesia-Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte-Ligurian ocean far from the Dent Blanche-Sesia domain and Adriatic margin.  相似文献   

15.
 The highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events. We illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb zircon ages from an anatectic granite at Verampio and migmatites at Corcapolo and Lavertezzo yield 280–290 Ma, i.e., Hercynian ages. These ages indicate that the highest grade of metamorphism in several crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar systematics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zone. The emplacement age of these pegmatites gives a minimum estimate for the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins represent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448±5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zone. Received: 23 May 1995/Accepted: 12 October 1995  相似文献   

16.
The Rodna Mountains afford the most internal structural window into the crystalline units of the Eastern Carpathians in Romania. The Rodna Mountains consist of Variscan metamorphic nappes that were restacked in the Alpine phase of Carpathian development forming the Subbucovinian and Infrabucovinian nappes. In order to evaluate age of deformation, ten samples were taken from the zone of greenschist facies mylonitic schist that marks the Alpine tectonic boundary between the Subbucovinian and Infrabucovinian nappes and 40Ar/39Ar laser single-grain ages determined for schistosity-forming muscovite. Microstructural assessment of quartz and muscovite distinguished two deformation events. Single-grain ages from the microstructurally most strongly reworked samples (four samples) give a tight clustering of ages at ca. 95 Ma. The least reworked schists have a broader clustering of ages spanning ca. 200–280 Ma with a late Permian peak and some samples showing outlier ages in the range 200–100 Ma. The relative development of the outliers, which correlates with evidence for increased microstructural reworking, is interpreted to mark progressive isotopic resetting. The ca. 95 Ma ages for the most reworked schists are estimates for the age of the Alpine nappe stacking. The ca. 200–280 Ma ages are similar to those of magmatism, metamorphism, and sedimentation thought to mark post-Variscan-pre-Alpine rifting and ocean basin formation in parts of the Alps and may be the thermal imprint of a related event in the Eastern Carpathians.  相似文献   

17.
Within the Mediterranean realm, the Rhodopes represent a nappe stack of oceanic and continental fragments assembled along the Eurasian continental margin during the Alpine orogeny. The timing of the high-pressure (HP) metamorphism has long been ambiguous, lacking detailed geochronological and geochemical control on subduction-exhumation and nappe stacking processes. Here we apply the Lu–Hf and Sm–Nd chronometers to a suite of representative eclogite samples covering two different key units of the Rhodopean nappe stack: (1) the Kimi Complex (Upper Allochthon) and (2) the Middle Allochthon. In addition to geochronology, we also determined whole rock Hf and Nd isotope compositions as well as major and trace element concentrations in order to constrain the nature of the eclogite protoliths. Two HP metamorphic events were revealed by Lu–Hf geochronology: (1) a Lower Cretaceous event in the Upper Allochthon (126.0 ± 1.7 Ma) and (2) an Eocene event in the Middle Allochthon (44.6 ± 0.7 Ma; 43.5 ± 0.4 Ma; 42.8 ± 0.5 Ma), at conditions of ca. 700°C/20–25 kbar. Our new data provide direct evidence for multiple subduction events in the Rhodopes. Exhumation and subsequent thrusting of the Middle Allochthon on the Lower Allochthon can be narrowed down to the time span between 42 and 34 Ma. In a broader tectonic context, the Eocene ages for the HP metamorphism support the view that the Rhodopes represent a large-scale tectonic window, exposing the deepest nappe units of the Hellenides.  相似文献   

18.
Identifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large‐scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso‐ and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18–20 kbar, 480–520 °C and of ~13–18 kbar, 500–530 °C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high‐P part of the albite stability field at P–T conditions of ~12.5–14.5 kbar and 530–560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na‐bearing phases (glaucophane, paragonite) have been overprinted by the low‐P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid‐solution model not incorporating a pyrophyllite component.  相似文献   

19.
The metamorphic sequences of the Saxonian Erzgebirge were thoroughly overprinted by a Variscan medium-pressure event under amphibolite facies conditions. However, eclogitic relics documenting an older high-pressure event are widespread. P-T conditions of the eclogite-facies metamorphism systematically decrease, over a distance of 50 km, from about >29 kbar/850°C, in the central part, to 20–24 kbar/650°C, in the westernmost part of the Erzgebirge crystalline complex. A distinct gap in P-T conditions exists between the central and the western Erzgebirge coinciding with the fault zone of the Flöha syncline. Therefore, the eclogitebearing sequences are assumed to represent at least two different nappe units. The lower-grade eclogite assemblages in the western Erzgebirge display a continuous metamorphic zonation with a gradual decrease of peak metamorphic temperatures towards the west. Assemblages formed in the stability field of coesite and thus indicating a regional ultra-high pressure metamorphism, are restricted to the central Erzgebirge, where they are widespread in the eclogites, but also present in metaacidic country rocks. The same high-temperature/high-pressure conditions, testifying to a burial of at least 100 km, were independently recorded for the ultramafic garnet pyroxenites associated with the eclogites of the central Erzgebirge. Mineral relics included in the eclogite phases and mineral assemblages formed by retrograde reactions permit reconstruction of the prograde and retrograde P-T paths in the different parts of the Erzgebirge crystalline complex.  相似文献   

20.
《Geodinamica Acta》2000,13(2-3):133-148
The Piemontese zone, at the western border of the Gran Paradiso massif (western Alps) consists of the oceanic schistes lustrés units and the continental Grand Paradiso (GP) unit. In complement to previous work, tectono-metamorphic investigations allow us to establish pressure gaps between the three studied units with: the upper schistes lustrés (LS) unit, metamorphosed under blueschist facies conditions (9.5 ± 2 kbar, 340 ± 30 °C). The lower schistes lustrés (LI) unit, metamorphosed under low temperature eclogite facies conditions (12.5 ± 3 kbar, 480 ± 50 °C). The Gran Paradiso (GP) unit, metamorphosed under higher pressure and temperature eclogite facies conditions (12 to 20 kbar, 500 ± 50 °C). The whole metamorphic pile recorded a decompression under amphibolite facies conditions for the LI unit and in the greenschist facies conditions for the LS unit and GP unit, i.e. with a slight and late temperature increase. Fluid inclusion studies on quartz in late cracks and shear planes (C’) show that the three units were juxtaposed and deformed together under metamorphic conditions of 4 ± 1 kbar, and 400 ± 50 °C. A finite strain field study reveals regional scale extensional tectonics beginning under greenschist facies conditions and ending under brittle conditions. Ductile greenschist deformation corresponds to dome and basin trajectories of the foliation surface. A partition is observed between domains where a pure shear regime prevails in the core of the domes and domains where a simple shear regime prevails, towards the WNW, at the rim of the domes, at the contact between units. Brittle deformation corresponds to a continuum of the ductile deformation. Meanwhile, metamorphic contrasts suggested by pressure estimates cannot be ruled by the relative extension-related displacements. This extension corresponds to the accommodation of the vertical indentation of the high pressure units by the stacking of deep crustal slices, in front of the rigid Apulian mantle back-stop, during continuous convergence.  相似文献   

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