The Alpine geodynamic evolution of Penninic nappes in the eastern Central Alps
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Abstract:
The eastern Central Alps consist of several Pennine nappes with different tectonometamorphic histories. The tectonically uppermost units (oceanic Avers Bündnerschiefer, continental Suretta and Tambo nappes, oceanic Vals Bündnerschiefer) show Cretaceous/early Tertiary W‐directed thrusting with associated blueschist facies metamorphism related to subduction of the Pennine units beneath the Austroalpine continental crust. This event caused eclogite facies metamorphism in the underlying continental Adula nappe. The gross effect was crustal thickening. The tectonically lower, continental Simano nappe is devoid of any imprint from this event. In the course of continent‐continent collision, high‐ T metamorphism and N‐directed movements occurred. Both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions. Crustal thinning commenced during the regional temperature peak. A final phase is related to differential uplift under retrograde P–T conditions. Further thinning of the crust was accommodated by E‐ to NE‐directed extensional deformation.Keywords:
Blueschist
Greenschist
continental collision
The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia occupies the central region of the Central Asian Orogenic Belt, the largest Phanerozoic orogenic belt in the world. The complex consists mainly of orthogneisses intercalated with eclogites and micaschists in a mélange zone. Most of eclogites are strongly amphibolitized. In this study, we examined petrography and mineral chemistry of eclogites and amphibolitized eclogites, respectively. The result of our research shows that Chandman eclogites experienced multiple events of metamorphism in throughout their subduction and subsequent collision history. We revealed that eclogites were subjected to blueschist facies metamorphism before the peak eclogite facies stage. In addition, we have studied amphibolitized eclogite, and revealed that another distinct progressive medium pressure (MP) epidote-amphibolite facies metamorphic event took place in the eclogite, consistent with collision process. The multiple events of metamorphism in eclogites have been revealed by zonation textures of HP amphiboles zoned with glaucophane→barroisite→Mg-hornblende and MP amphiboles zoned with actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite. These amphiboles with different zonation textures reflect their metamorphic history of subduction to collision events.
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Amphibole
Glaucophane
Hornblende
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Abstract Documentation of pressure–temperature ( P–T ) histories across an epidote‐amphibolite facies culmination provides new insight into the tectono‐thermal evolution of the Brooks Range collisional orogen. Thermobarometry reveals that the highest grade rocks formed at peak temperatures of 560–600 °C and at pressures of 8–9.5 kbar. The thermal culmination coincides with the apex of a structural dome defined by oppositely dipping S2 crenulation cleavages suggesting post‐metamorphic doming. South of the thermal culmination, greenschist facies and lowermost epidote‐amphibolite facies rocks preserve widespread evidence for an early blueschist facies metamorphism. In contrast, no evidence for an early blueschist facies metamorphism was found in similar grade rocks of the northern flank, indicating that the southern flank underwent initial deeper burial during southward underthrusting of the continental margin. Thus, while the dome shows a symmetric distribution of peak temperatures, the P–T paths followed by the two flanks must have varied. This variation suggests that final thermal re‐equilibration to greenschist and epidote–amphibolite facies conditions did not result from a simple process of southward underthrusting followed by thermal re‐equilibration from the bottom upward. The new data are inconsistent with a previous model that invokes such re‐equilibration, along with northward thrusting of epidote–amphibolite facies rocks over lower grade rocks presently on the southern flank of the culmination, to produce an inverted metamorphic field gradient. Instead, it is suggested that following blueschist facies metamorphism, rocks of the southern and northern flanks were juxtaposed, during which time the more deeply buried south flank was partially emplaced above rocks to the north, where they escaped Albian epidote–amphibolite facies overprinting. Porphyroblast growth, which post‐dates the main fabric on the north flank of the culmination may be the result of Albian thermal re‐equilibration following this deformation. Post‐metamorphic doming resulted from a combination of Albian‐Cenomanian extension and Tertiary deformation.
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Abstract 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 (M 1 –M 2 ). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M 1 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 2 σ values) and subsequent cooling and partial exhumation to greenschist facies. The M 2 metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M 1 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 M 1 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 M 2 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.
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Abstract The Qinling–Dabie accretionary fold belt in east‐central China represents the E–W trending suture zone between the Sino‐Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high‐pressure/low‐temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote–amphibolite/eclogite facies. The ‘Hongan metamorphic belt’can be divided into three metamorphic zones, based on progressive changes in mineral assemblages: Zone I, in the south, is characterized by transitional blueschist–greenschist facies; Zone II is characterized by greenschist facies; Zone III, in the northernmost portion of the belt, is characterized by eclogite and epidote–amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P–T conditions during formation of this belt. Preliminary P–T estimates for Zone I metamorphism are 5–7 kbar, 350–450°C; estimates for Zone III eclogites are 10–22 kbar, 500 ± 50°C. The petrographic, chemical and structural characteristics of this metamorphic belt indicate its evolution in a northward‐dipping subduction zone and subsequent uplift prior to and during the final collision between the Sino‐Korean and Yangtze cratons.
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The high- to ultrahigh-pressure metamorphic rocks of the Atbashy complex were petrologically investigated. The eclogites of the Choloktor Formation show a prograde evolution from epidote-blueschist facies (P = 17–21 kbar and T = 450–515 °C) to peak eclogite-UHP conditions (P = 26–29 kbar and T = 545–615 °C) with a subsequent epidote-amphibolite and greenschist facies overprint. The mica-schists of the Choloktor Formation also show a clockwise P-T path from blueschist/epidote-blueschist facies conditions through peak eclogite facies conditions (P = 21–23 kbar and T = 530–580 °C) to retrograde epidote-amphibolite and greenschist facies stages. A comparison of the P-T paths in the eclogites and mica-schists of Choloktor Formation reveal that they may have shared their P-T history from peak to retrograde stages. The mica-schists of the Atbashy Formation record peak metamorphism of P = 10–12 kbar and T = 515–565 °C, which indicates that the highest grade of regional metamorphism in the Atbashy Ridge was epidote-amphibolite facies. The newly obtained P-T conditions for the mica-schists of Choloktor Formation indicate that sheets of sedimentary rocks were brought to great depths along the subduction zone and they metamorphosed under eclogite facies HP conditions. The eclogite blocks were amalgamated with mica-schists of Choloktor Formation in the eclogite facies HP conditions and together they experienced isothermal decompression to ∼40 km. During this path, the eclogites and mica-schists of Choloktor Formation docked with mica-schists of Atbashy Formation at 10–12 kbar and 515–565 °C, and from this depth (∼40 km) the whole sequence was exhumed together. These new results improve our understanding of high-pressure metamorphism in subduction-related accretionary prism zones and the exhumation processes of deeply-seated rocks in the Atbashy HP-UHP complex.
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Glaucophane
Lawsonite
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Blueschist
Coesite
Omphacite
Amphibole
Phengite
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Documentation of pressure-temperature (P-T) histories across an epidote-amphibolite facies culmin- ation provides new insight into the tectono-thermal evolution of the Brooks Range collisional orogen. Thermobarometry reveals that the highest grade rocks formed at peak temperatures of 560-600 � C and at pressures of 8-9.5kbar. The thermal culmination coincides with the apex of a structural dome defined by oppositely dipping S2 crenulation cleavages suggesting post-metamorphic doming. South of the thermal culmination, greenschist facies and lowermost epidote-amphibolite facies rocks preserve widespread evidence for an early blueschist facies metamorphism. In contrast, no evidence for an early blueschist facies metamorphism was found in similar grade rocks of the northern flank, indicating that the southern flank underwent initial deeper burial during southward underthrusting of the continental margin. Thus, while the dome shows a symmetric distribution of peak temperatures, the P-T paths followed by the two flanks must have varied. This variation suggests that final thermal re-equilibration to greenschist and epidote-amphibolite facies conditions did not result from a simple process of southward underthrusting followed by thermal re-equilibration from the bottom upward. The new data are inconsistent with a previous model that invokes such re-equilibration, along with northward thrusting of epidote-amphibolite facies rocks over lower grade rocks presently on the southern flank of the culmination, to produce an inverted metamorphic field gradient. Instead, it is suggested that following blueschist facies metamorphism, rocks of the southern and northern flanks were juxtaposed, during which time the more deeply buried south flank was partially emplaced above rocks to the north, where they escaped Albian epidote-amphibolite facies overprinting. Porphyroblast growth, which post- dates the main fabric on the north flank of the culmination may be the result of Albian thermal re-equilibration following this deformation. Post-metamorphic doming resulted from a combination of Albian-Cenomanian extension and Tertiary deformation.
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