Rapid synconvergent exhumation of Miocene-aged lower orogenic crust in the eastern Himalaya
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Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr–1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.Keywords:
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Research Article| January 01, 2011 Contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet Jess King; Jess King † 1Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China †E-mail: jessking@hkucc.hku.hk Search for other works by this author on: GSW Google Scholar Nigel Harris; Nigel Harris 2Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar Tom Argles; Tom Argles 2Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Search for other works by this author on: GSW Google Scholar Randall Parrish; Randall Parrish 3Department of Geology, University of Leicester, LE17, UK, and Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Notts NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Hongfei Zhang Hongfei Zhang 4State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Search for other works by this author on: GSW Google Scholar Author and Article Information Jess King † 1Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China Nigel Harris 2Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Tom Argles 2Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Randall Parrish 3Department of Geology, University of Leicester, LE17, UK, and Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Notts NG12 5GG, UK Hongfei Zhang 4State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China †E-mail: jessking@hkucc.hku.hk Publisher: Geological Society of America Received: 09 May 2009 Revision Received: 02 Dec 2009 Accepted: 07 Dec 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (1-2): 218–239. https://doi.org/10.1130/B30085.1 Article history Received: 09 May 2009 Revision Received: 02 Dec 2009 Accepted: 07 Dec 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Jess King, Nigel Harris, Tom Argles, Randall Parrish, Hongfei Zhang; Contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet. GSA Bulletin 2011;; 123 (1-2): 218–239. doi: https://doi.org/10.1130/B30085.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This geochemical, geochronological, and structural study of intrusive rocks in the Sakya dome of southern Tibet has identified two distinct suites of anatectic granites that carry contrasting implications for the tectonic evolution of the India-Asia collision zone. The northern margin of the dome core was intruded by anastomosing, equigranular two-mica garnet granites between 28.1 ± 0.4 Ma and 22.6 ± 0.4 Ma, coeval with top-to-the-south shear. Trace-element and isotopic (Sr-Nd) characteristics indicate an origin from partial melting of a biotite-bearing source in the Indian crust under conditions of high-fluid-phase activity. These granites thus provide evidence for the melt weakening required by some thermo-mechanical models that predict the southward extrusion of a low-viscosity channel during the Oligocene. Evidence for subsequent shear-sense reversal may document initiation of this process. However, a younger suite of porphyritic two-mica granite plutons, emplaced between 14.5 ± 0.9 Ma and 8.81 ± 0.22 Ma, is derived from anatexis of muscovite-bearing metasediments of the High Himalayan Series under fluid-absent conditions. Ar-Ar cooling ages of 14.4–8.0 Ma from the Sakya dome postdate crystallization of the Oligocene granite suite by ∼10 m.y. but are coincident with mid-Miocene granite emplacement, suggesting uplift to depths of <10 km by the mid-Miocene. We propose that plate flexural response to Miocene slab steepening was a likely cause of dome uplift, and that this exhumation of midcrustal rocks triggered decompression melting at 15–9 Ma and emplacement of discrete granite plutons into the upper crust under brittle conditions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr−1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.
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Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. In general, many kilometers of oceanic lithosphere are ultimately consumed prior to the subsequent continental slab subduction and collision. The exposure of the eclogites derived from oceanic subduction and continental subduction at the surface of Earth today record provide different P-T-t records of the subduction process. The Huwan shear zone in the Hong’an orogenic belt, marking a former ocean-continent transition zone, has been the focus of many studies on subduction-related high-pressure metamorphism. In this study, Lu-Hf garnet, U-Pb zircon, and Ar- Ar mica ages are combined with geochemical data to understand the origin of two coexisting eclogite bodies exposed along the Xuehe River in the Huwan Shear zone. In total, the results indicate that the two eclogites have different protoliths but experienced a similar metamorphic history. This observation requires new tectonic model for the coupled subduction of oceanic and continental crust in subduction zones. Combined geochemistry and zircon U-Pb geochronology suggest distinct oceanic and continental affinities for the eclogite protoliths. The Lu-Hf dates of 261.5 ± 2.4 Ma of the continental-type eclogite and 262.7 ± 1.7 Ma of the oceanic-type eclogite reflect garnet growth and are interpreted to closely approximate the age of eclogite-facies metamorphism. Therefore, both the geochemically oceanic- and continental-type eclogites underwent the same episode of Permian eclogite-facies metamorphism. The Permian Lu-Hf ages of ca. 262 Ma and the obtained Triassic Ar-Ar ages (~240 Ma) of the oceanic-type and continental-type eclogites imply coupled subduction and exhumation of oceanic and continental crustal materials in the Hong’an orogenic belt during the Permian and the Triassic. Though limited, the geochemical and geochronological results of this study, together with the discrepant Carboniferous dates for the nearby eclogites of previous studies, apparently suggest that the Huwan shear zone was not always a single coherent unit but instead comprises different tectonic slices that were metamorphosed at different times before final assembly. Some slices of the oceanic and continental crust underwent two subduction cycles during the Carboniferous and the Permian, whereas some eclogites registered only a single subduction-exhumation loop during the convergence between the South China Block and the North China Block in the Huwan shear zone. The consistent ages of the oceanic- and continental-type eclogites disfavor the traditional mélange model that high-pressure rocks are dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels, forming a tectonic mélange.
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Research Article| December 01, 1995 Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania Andreas Möller; Andreas Möller 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany2Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Peter Appel; Peter Appel 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany Search for other works by this author on: GSW Google Scholar Klaus Mezger; Klaus Mezger 2Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Volker Schenk Volker Schenk 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Andreas Möller 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany2Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany Peter Appel 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany Klaus Mezger 2Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany Volker Schenk 1Mineralogisch-Petrographisches Institut, Universität Kiel, 24098 Kiel, Germany Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (12): 1067–1070. https://doi.org/10.1130/0091-7613(1995)023<1067:EFAGSZ>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Andreas Möller, Peter Appel, Klaus Mezger, Volker Schenk; Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania. Geology 1995;; 23 (12): 1067–1070. doi: https://doi.org/10.1130/0091-7613(1995)023<1067:EFAGSZ>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth's history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.
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The Braccia gabbro of Val Malenco, Italian Alps, intruded 275 My ago during Early Permian lithospheric extension. The intrusion took place along the crust–mantle transition zone and caused granulite metamorphism of lower-crustal and upper-mantle rocks. The magmatic crystallization of the gabbro was outlasted by ductile deformation, which is also observed in the other rocks of the crust–mantle transition. Two stages of retrograde metamorphism followed. Mineral parageneses in garnet–kyanite gneiss, metagabbro, and metaperidotite record a first stage of near-isobaric cooling under anhydrous conditions. The stabilized crust–mantle transition then persisted over a period of about 50 My into the Late Triassic. Exhumation of the crust–mantle complex began with the onset of continental rifting during Early Jurassic. This stage of retrograde metamorphism is recorded by near-isothermal decompression and partial hydration of the granulitic mineral assemblages. The whole crust-to-mantle complex was then exposed in the Tethyan ocean near its Adriatic margin. The magmatic assemblage of the Braccia gabbro formed at 1–1·2 GPa and 1150–1250°C. Microstructures show that the gabbroic rocks evolved from olivine gabbros through spinel to garnet granulite whereas the peridotites recrystallized within the spinel peridotite field and the pelitic granulites remained in the stability field of kyanite. Such an evolution is characteristic of isobaric cooling after magmatic underplating. Granulitic mineral assemblages record cooling from 850°C to 650°C with decompression to 0·8 ± 0·1 GPa, and dP/dT < ∼0·15 GPa/100°C. During later hydration, Cl-rich amphibole and biotite + plagioclase formed in the gabbros, clinozoisite + phengite + paragonite ± staurolite ± chloritoid in the metapelites and olivine + tremolite + chlorite ± talc in the ultramafic rocks at metamorphic conditions of 0·9 ± 0·1 GPa and 600 ± 50°C. Subsequent retrograde metamorphism involved decompression of ∼0·3 GPa and cooling to ∼500°C, consistent with the preservation of the olivine + tremolite + talc assemblage in ultramafic rocks. Estimated uplift rates of 1–2 mm/year indicate a 15–30 My exhumation related to Jurassic rifting. The two-stage retrograde path of the Malenco granulites separated by >50 My suggests that Permian extension and Jurassic rifting are two independent tectonic processes. The presence of hydrous, Cl-rich minerals at 600 ± 50°C and 0·8 ± 0·1 GPa requires input of externally derived fluids at the base of 30 km thick continental crust into previously dry granulites at the onset of Jurassic rifting. These fluids were generated by dehydration of continental crust juxtaposed during rifting with the hot, exhuming granulite complex along a active shear zone.
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