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1.
Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.  相似文献   

2.
The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.  相似文献   

3.
Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.  相似文献   

4.
The P–T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid‐greenstone contact is characterized by a contact‐parallel high‐strain zone that separates the generally low‐grade, greenschist facies greenstone belt from mid‐crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600–700 °C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of ～30–40 °C km?1. The most likely setting for these conditions was a mid‐ or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid‐crustal levels, followed by crustal exhumation related to the late‐ to post‐collisional extension of the granitoid‐greenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P–T paths indicate near‐isothermal decompression to conditions of ～500–650 °C and 1–3 kbar, followed by near‐isobaric cooling to temperatures below ～500 °C. In metabasic rock types, this retrograde P–T evolution resulted in the formation of coronitic Ep‐Qtz and Act‐Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solid‐state doming of the footwall gneisses and strain localization in contact‐parallel greenschist‐facies mylonites that overprint the decompressed basement rocks.  相似文献   

5.
Numerous lenses of eclogite occur in a belt of augen orthogneisses in the Gubaoquan area in the southern Beishan orogen, an eastern extension of the Tianshan orogen. With detailed petrological data and phase relations, modelled in the system NCFMASHTO with thermocalc , a quantitative P–T path was estimated and defined a clockwise P–T path that showed a near isothermal decompression from eclogite facies (>15.5 kbar, 700–800 °C, omphacite + garnet) to high‐pressure granulite facies (12–14 kbar, 700–750 °C, clinopyroxene + sodic plagioclase symplectitic intergrowths around omphacite), low‐pressure granulite facies (8–9.5 kbar, ～700 °C, orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding garnet) and amphibolite facies (5–7 kbar, 600–700 °C, hornblende + plagioclase symplectites). The major and trace elements and Sm–Nd isotopic data suggest that most of the Beishan eclogite samples had a protolith of oceanic crust with geochemical characteristics of an enriched or normal mid‐ocean ridge basalt. The U–Pb dating of the Beishan eclogites indicates an Ordovician age of c. 467 Ma for the eclogite facies metamorphism. An 39Ar/40Ar age of c. 430 Ma for biotite from the augen gneiss corresponds to the time of retrograde metamorphism. The combined data from geological setting, bulk composition, clockwise P–T path and geochronology support a model in which the Beishan eclogites started as oceanic crust in the Palaeoasian Ocean, which was subducted to eclogite depths in the Ordovician and exhumed in the Silurian. The eclogite‐bearing gneiss belt marks the position of a high‐pressure Ordovician suture zone, and the calculated clockwise P–T path defines the progression from subduction to exhumation.  相似文献   

6.
Thermobarometry suggests that ultrahigh‐pressure (UHP) to high‐pressure (HP) rocks across the Western Gneiss Region ponded at the Moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust. Eclogite across the c. 22 000 km2 study area records minimum pressures of c. 8–18 kbar and temperatures of c. 650–780 °C. One orthopyroxene eclogite yields an UHP of c. 28.5 kbar, and evidence of former coesite has been found c. 50 km farther east than previously known. Despite this widespread evidence of UHP to HP, thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform ‘supra‐Barrovian’ amphibolite‐facies overprint at c. 11 kbar and c. 650–750 °C across the entire area. Chemical zoning analysis suggests that garnet in these samples grew during heating and decompression, presumably during the amphibolite‐facies event. These data indicate that the Norwegian UHP/HP province was exhumed from mantle depths of c. 150 km to lower crustal depths, where it stalled and underwent a profound high‐temperature overprint. The ubiquity of late‐stage supra‐Barrovian metamorphic overprints suggests that large‐scale, collisional UHP terranes routinely stall at the continental Moho where diminishing body forces are exceeded by boundary forces. Significant portions of the middle or lower crust worldwide may be formed from UHP terranes that were arrested at the Moho and never underwent their final stage of exhumation.  相似文献   

7.
High‐P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M1–M2). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M1 metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M2 metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M1 cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M1 metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M2 are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.  相似文献   

8.
Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T ≈550–600 °C and P≈18–20 kbar) structurally beneath greenschist facies rocks (T ≈400 °C and P≈9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (the Gressoney Shear Zone, GSZ) dominated by top-SE movement related to crustal extension. Rb–Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion. Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic.  相似文献   

9.
The so‐called Plankogel detachment is an east‐west trending, south‐dipping low‐angle structure that juxtaposes the high‐P rocks of the eclogite type locality of the eastern European Alps against amphibolite facies rocks to the south. It occurs in both the Saualpe and Koralpe Complex in eastern Austria. During Cretaceous intracontinental subduction, the footwall and the hangingwall units of the Plankogel detachment were buried to different crustal levels as inferred by pseudosection modelling and conventional thermobarometry: ~23–24 kbar and 640–690 °C for the eclogite facies units in the footwall of the detachment and ~12–14 kbar and 550–580 °C for the amphibolite facies metapelites in the hangingwall. Despite the different peak metamorphic conditions, both sides of the detachment display a common overprint at conditions of ~10 kbar and 580–650 °C. From this, we infer a two‐stage exhumation process and suggest that this two‐stage process is best interpreted tectonically in terms of slab extraction during Eoalpine subduction. The first stage of exhumation occurred due to the downward (southward) extraction of a lithospheric slab that was localized in the trace of the Plankogel detachment. The later stage, however, is attributed to more regional erosion‐ or extension‐driven processes. Since the Plankogel detachment is geometrically related to a crustal‐scale shear zone further north (the Plattengneiss shear zone), we suggest that both structures are part of the same extraction fault system along which the syn‐collisional exhumation of the Eoalpine high‐P units of the Eastern Alps occurred. The suggested model is consistent with both the mylonitic texture of the Plattengneiss shear zone and the overall ambiguous shear sense indicators present in the entire region.  相似文献   

10.
The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.  相似文献   

11.
The youngest known ultrahigh‐pressure (UHP) rocks in the world occur in the Woodlark Rift of southeastern Papua New Guinea. Since their crystallization in the Late Miocene to Early Pliocene, these eclogite facies rocks have been rapidly exhumed from mantle depths to the surface and today they remain in the still‐active geodynamic setting that caused this exhumation. For this reason, the rocks provide an excellent opportunity to study rates and processes of (U)HP exhumation. We present New Rb–Sr results from 12 rock samples from eclogite‐bearing gneiss domes in the D'Entrecasteaux Islands, and use those results to examine the time lag between (U)HP metamorphism and later ductile thinning, penetrative fabric development and accompanying metamorphic retrogression at amphibolite facies conditions during their exhumation. A Rb–Sr age for a sample of mafic eclogite (with no preserved coesite) from the core zone of the Mailolo gneiss dome (Fergusson Island) provides a new estimate of the timing of HP metamorphism (5.6 ± 1.6 Ma). The strongly deformed quartzofeldspathic and granitic gneisses (90–95% by volume) that enclose variably retrogressed relict blocks of mafic eclogite (5–10% by volume) yield Rb–Sr isochron ages from 4.4 to 2.4 Ma. For the UHP‐bearing gneisses of Mailolo dome, previously published U–Pb ages on zircon and our Rb–Sr isochron ages are consistent with a mean time lag of 2.2 ± 1.5 Ma (~95% c.i.) for passage of the rock between eclogite and amphibolite facies conditions. New thermobarometric data indicate that the main syn‐exhumational foliation developed at amphibolite facies conditions of 630–665 °C and 12.1–14.4 kbar. These pressure estimates indicate that the lower crust of the Woodlark Rift was unusually thick (>40 km) at the time of the amphibolite facies overprint, possibly as a result of accumulation and underplating of UHP‐derived material from below. Our data imply a minimum unroofing rate of 10 ± 7 mm year?1 (~95% c.i.) for the (U)HP body from minimum HP depths (73 ± 7 km) to lower crustal depths. This minimum unroofing rate reinforces previous inferences that the exhumation from the mantle to the surface of the gneiss domes in the D'Entrecasteaux Islands took place at plate tectonic rates. On the basis of previous structural studies and the new thermobarometry, we attribute the high (cm year?1) exhumation to diapiric ascent of the partially molten terrane from mantle depths, with a secondary contribution from pure shear thinning of the terrane after its arrival in the crust.  相似文献   

12.
The late Palaeozoic western Tianshan high‐pressure /low‐temperature belt extends for about 200 km along the south‐central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high‐pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca‐amphibole and chlorite. The post‐eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post‐eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high‐pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine‐type’ tectonics. This contradicts previous models which favour ‘Franciscan‐type’ tectonics for the southern Tianshan high‐pressure rocks.  相似文献   

13.
The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO2 or other chemical factors.  相似文献   

14.
Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.  相似文献   

15.
Metabasic rocks from the Adula Nappe in the Central Alps record a regional high‐pressure metamorphic event during the Eocene, and display a regional variation in high‐pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High‐pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high‐pressure amphibolite or eclogite facies conditions followed by peak‐pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc ) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km?1 and 0.20 ± 0.05 kbar km?1. These results constrain the peak‐pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.  相似文献   

16.
Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as <10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K & BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P >28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.  相似文献   

17.
Metabasites from the northern Adula Nappe Complex (ANC) display a complex microstructural evolution recording episodes of deformation and metamorphic re‐equilibration that were obliterated in the surrounding phengite‐bearing schists. Pre‐D1 and D1 deformation episodes are preserved as mineral inclusions within garnet cores of some amphibole‐bearing eclogites and record high‐temperature greenschist‐/amphibolite‐facies conditions. D2 produced an eclogite‐facies foliation which developed at 580 ± 70°C and 19 ± 3 kbar. D3 was a composite deformation episode which can be divided into three sub‐episodes D3m, D3a and D3b which occurred as the metamorphism evolved from post‐eclogitic high‐pressure and low‐temperature conditions through to amphibolite‐facies conditions at 590 ± 30°C and 11.7 ± 1.3 kbar. The D3 deformation episode was responsible for the development of the S3 regional‐scale foliation in the surrounding schists, whilst D4 caused the development of an S4 greenschist foliation. The composite nature of the D3 episode indicates that rocks of the northern ANC experienced a protracted post‐eclogitic structural reworking and that the current structure of this part of the Alps is a late‐Alpine feature. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.  相似文献   

19.
New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/39Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. 40Ar/39Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr?1), perhaps because of continued tectonic unroofing.  相似文献   

20.
The eclogite type locality in the Eastern Alps (the Koralpe and Saualpe region) is the largest region in the Eastern Alps that preserves high‐pressure metamorphic rocks from the Eo‐Alpine orogenic event of the Cretaceous age. Thermobarometric data from the metapelitic gneisses in the region indicate that a metamorphic field gradient across the region can be divided into three parts. The northern part shows continuously increasing P–T from 10 ± 1.5 to 14 ± 1.5 kbar and 500 ± 68 to 700 ± 68 °C over a distance of 40 km. The continuous increase in P–T indicates that no major tectonic boundaries were active in this part during the Eo‐Alpine orogeny. Small discontinuities in the pressure gradient of the northern part can be correlated with more localized deformation. The central part exposes amphibolite–eclogite facies rocks with 15 ± 1.5 kbar and 700 ± 68 °C over about 20 km length. The southern part shows decreasing P–T conditions from 15 ± 1.5 to 10 ± 1.5 kbar and 700 ± 68 to 600 ± 63 °C over a distance of 10 km beyond which conditions remain roughly constant for the remainder of the profile. Overall, the field gradient is characterized by: (i) an increase in age with decreasing metamorphic grade and (ii) a T/P ratio that is lower than common metamorphic geotherms. The age–grade relationship is consistent with the timing relationship along piezothermal arrays predicted by simple models for regional metamorphism. However, the T/P ratio of the field gradient is inconsistent with such an interpretation. These inconsistencies indicate that the profile is not simply an obliquely exposed crustal section. We suggest that the exhumation of the transect is best explained with a two dimensional model of an extruding wedge, as has recently been suggested as a typical scenario for other large scale compressional orogens.  相似文献   

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