•Garnet schists from the South Beishan Orogen experienced high-grade metamorphism at ca. 900Ma.•~900Ma metamorphism is coeval with extensive continental arc formation in the SBOB and CTA.•The SBOB ...and CTA occupied in the periphery of the Rodinia during the final assembly stage.
An early extensive Neoproterozoic (ca. 900Ma) continental magmatic arc system covering hundreds of kilometers has been reported to occur in the South Beishan Orogenic Belt (SBOB) and the Central Tianshan (CTA) in the southern Central Asian Orogenic Belt (CAOB). However, evidence for coeval high-grade metamorphism and thus the formation of an accretionary orogen in the framework of Rodinia is ambiguous or absent. This study provides new petrological, geochemical and geochronological data for garnet-bearing schists (quartz+garnet+biotite+plagioclase±muscovite) from the SBOB in order to constrain its Neoproterozoic metamorphic history. The metamorphic zircon rims are either unzoned or display sector zoning in CL-images and reveal REE patterns with flat HREE patterns and negative Eu anomalies, which are interpreted to be in chemical equilibrium with garnet and plagioclase. The zircon U-Pb dating yields concordant U-Pb ages of 900±3Ma, 897±2Ma and 898±4Ma for the metamorphic zircon rims. The inherited detrital zircon cores of one sample display a concordant U-Pb age of 1397±5Ma that is consistent with the timing of formation for the extensive Mesoproterozoic continental arc in the SBOB and CTA. Based on phase equilibrium geothermobarometry and average P-T thermobarometric calculations, minimum amphibolite-facies P-T conditions are estimated to be >600°C at pressure >0.6GPa, which is thought to have been overprinted by subsequent Paleozoic metamorphism. However, the Ti-in-zircon thermometer still reveals temperatures of up to 840°C using the composition of metamorphic zircon rims, suggesting former ca. 900Ma granulite-facies peak metamorphic temperatures. The combined petrological and geochronological evidence in conjunction with the continental affinity of the regional metamorphic rocks suggests that the SBOB and the eastern CTA experienced an early Neoproterozoic accretionary orogenesis during the final assembly stage of Rodinia.
Partial melting is thought to profoundly impact the rheology and deformation behavior of the middle crust. Consequently, investigations of the pressure-temperature conditions of metamorphism, rates ...of heating, and durations of anatexis can provide unique constraints on tectonic processes. The Greater Himalayan Sequence (GHS), in the metamorphic core of the Himalayan orogen, is commonly considered to represent exhumed, anatectic, mid-crust. Here, we present detailed petrological and geochronological analysis of anatectic pelitic schist and felsic paragneiss from the uppermost structural level of the GHS to understand the timing and conditions of Himalayan anatexis. Petrologic analysis indicates that these rocks experienced high-grade metamorphism and partial melting up to peak conditions of ca. 720–745°C and ca. 9.6–10 kbar. Melt volumes of ca. 3% increased slightly during exhumation with nearly constant or slightly decreasing temperature, then decreased as rocks cooled, ultimate crossing the solidus at ca. 5.5 kbar and 700°C. Well-correlated U–Th–Pb ages and trace element data (HREE, Y, and Eu/Eu*) for monazite and zircon require prograde metamorphism and initial partial melting of GHS rocks at ca. 50 and 42–40 Ma, respectively, and crystallization of melts at ca. 24–18 Ma. These data indicate a long-lived (ca. 22–24 Myr) partially molten mid-crust in the eastern Himalayan orogen that formed as much as 10 Myr earlier and lasted 10 Myr longer than numerical models of viscous flow have predicted. Thermal buffering and melt stagnation may reflect feedbacks between thermal structure and shear stress. The change from thermal and mechanical stasis to rapid exhumation and cooling at ca. 24 Ma corresponds with an orogen-wide shift in deformation patterns, and may reflect arrival of mainland India.
•The upper GHS in the eastern Himalaya retained anatectic melts for ca. 22–24 Myr.•Partial melting initiated at 42–40 Ma and melts crystallized by 18 Ma.•Melt stagnation may reflect feedbacks between thermal structure and shear stress.•Rapid exhumation starting ca. 24 Ma may reflect arrival of mainland India.
Despite several decades of investigations, the nature and timing of the India–Asia collision remain debated. In the western Himalaya, the leading edge of the Indian continent was deeply subducted to ...mantle depths and experienced ultrahigh-pressure metamorphism in the Eocene at c. 50 Ma. In this paper, however, we demonstrate that the North Himalayan metamorphic rocks in the eastern Himalaya underwent Early Eocene (48–45 Ma) medium-pressure (MP) metamorphism due to shallow subduction of the Indian continent beneath southeastern Tibet. The studied garnet–kyanite–staurolite schists occur in the core of the Yardoi gneiss dome, the easternmost North Himalayan Gneiss Dome, and represent the upper structural level of the Higher Himalayan Crystallines (HHC). Petrology and phase equilibria modeling show that these rocks have mineral assemblages of Grt + Pl + Bt + Qz ± Ky ± St ± Ms that were formed under conditions of 7–8 kbar and 630–660 °C. Zircon U–Pb chronology shows that these rocks have peak-metamorphic ages of 48–45 Ma and protracted zircon growth, indicating that the collision between Indian and Asian continents must have occurred at c. 50 Ma in southeastern Tibet. Combining with available data, we suggest that the HHC represents a crustal section of the subducted and subsequently exhumed Indian continent. Due to shallow subduction of the continent during the Eocene, the middle to lower crust of the continent was subducted into depths of 40–60 km and underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and intense anatexis, whereas the upper crust was buried to shallower depths of 20–30 km and witnessed MP metamorphism and intrusion of leucogranites derived from the lower structural level of the HHC.
•Early Eocene medium-pressure (MP) metamorphism in northeastern Himalayan is firstly reported.•The final collision of Indian and Asian continents in the eastern Himalaya was at c. 50 Ma.•Shallow subduction of the India continent resulted in the MP metamorphism of the upper crust of the continent.
The Nyingtri Group of the Lhasa terrane in southern Tibet consists dominantly of metasedimentary rocks and orthogneiss. These rocks have a similar mineral paragenesis of plagioclase + K-feldspar + ...biotite + quartz ± sillimanite ± garnet ± staurolite ± muscovite ± amphibole, indicating amphibolite-facies metamorphic conditions. Inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable206Pb/238U ages ranging from 3300 to 50 Ma. The data define two prominent age populations, 1200–1000 and 600–500 Ma, indicating that the source of the Nyingtri Group preserves the records of both Grenville and Pan-African magmatic-thermal events. Inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 Ma, limiting the depositional age of the metasedimentary sequence to Cambrian or older. Overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 Ma. On the basis of these results, together with the regional comparison, we infer that the Nyingtri Group was formed during or before the Cambrian, with a potential provenance from the Pinjarra Orogen of Western Australia–East Antarctica. This rock group, together with the Tethyan Himalayan Sedimentary Sequence, represents an early Paleozoic sedimentary cover of the northern margin of the Gondwana supercontinent that was intruded by Cambrian granites during the circum-Gondwana Andean-type orogeny. Along with published data, this study demonstrates that the Nyingtri Group was metamorphosed during Mesozoic and Cenozoic, as against the previous notion of a Precambrian metamorphic basement for the Lhasa terrane.
Revealing the timescales of metamorphic and anatectic processes is central to our understanding of tectonic evolution of collisional orogens. High‐temperature migmatites and leucogranites are well ...exposed in the Himalayan orogenic core, making it an ideal region to study the timing and duration of partial melting and melt crystallization of the orogen. Here, we report an integrated and comprehensive data set of petrography, U‐Pb age, and trace element data for zircon from a pelitic granulite and associated leucosomes of the Greater Himalayan Sequence (GHS) in the Yadong area, eastern Himalaya. Zircon grains with complex internal structure retain variable ages ranging from 32 Ma to 13 Ma that correlate systematically with changes in the concentrations of Y, Th, U, Hf, Nb, Ta, and HREE, and ratios of Th/U, Eu/Eu*, and Nb/Ta. Combined with petrologic analysis, we conclude that the granulite witnessed high‐temperature metamorphism, melting, and melt crystallization over ∼20 Myr. Prograde, simultaneous increases in pressure and temperature and associated dehydration melting began at least by ∼32 Ma and lasted until ∼24 Ma. Subsequent quasi‐isothermal decompression‐melting occurred between ∼22 and 19 Ma, and late melt crystallization spanned ∼19 to 13 Ma. Large volumes of melt generated during prograde metamorphism could have triggered exhumation of GHS rocks, increasing melt fraction through a positive feedback between exhumation and melting. More comprehensive analysis of different rock types led to more complete and different interpretations for the timing of exhumation and melt crystallization in the Yadong‐Sikkim region and might enable alternate interpretations elsewhere in the Himalayas.
Key Points
Zircon from a migmatitic pelitic granulite has variable ages, while Y, Th, U, Hf and HREE, Th/U and Eu/Eu* of zircon show systematic changes over time
The granulite experienced high‐temperature metamorphism, partial melting, and melt crystallization over ∼20 Myr
Intensive melting during burial triggered the exhumation of granulites and contributed to the formation of Himalayan leucogranites
The Namche Barwa Complex exposed in the Eastern Himalayan Syntaxis, south Tibet, underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and associated anatexis. The HP ...pelitic granulites contain garnet, kyanite, sillimanite, cordierite, biotite, quartz, plagioclase, K-feldspar, spinel, ilmenite and graphite. These minerals show composite reaction texture and varying chemical compositions and form four successive mineral assemblages. Phase equilibrium modeling constrains the P–T conditions of 10–12kbar and 550–700°C for the prograde stage, 13–16kbar and 840–880°C for the peak-metamorphic stage, and 5–6kbar and 830–870°C for the late retrograde stage, indicating that the HP granulites recorded a clockwise P–T path involving the early heating burial and anatexis through dehydration melting of both muscovite and biotite, and the late isothermal decompression and gradual melt crystallization under HT granulite-facies conditions. The zircon U–Pb dating reveals that the HT granulite-facies metamorphism probably initiated at ca. 40Ma, and lasted to ca. 8Ma. Therefore, the present study provides robust evidence for a long-lived HT metamorphism and associated anatexis in the deeply buried Indian continent and important constraints on the leucogranite generation and tectonic evolution of the Himalayan orogen.
•The HP granulite from the Eastern Himalayan orogen underwent a prolonged HT metamorphism.•The protracted HT metamorphism initiated at ca. 40Ma and lasted to ca. 8Ma.•The prolonged HT metamorphism reveals the tectonometamorphic evolution of tectonically thickened crust in large hot orogens.
Magmatic arcs are thought to be the primary sites of modern-day continental crustal growth, and arc crustal sections provide an exceptional opportunity to directly observe the geological processes ...that occur there, yet few deeply exposed arc sections are available for direct study. The Gangdese magmatic arc, southern Tibet, formed during the Mesozoic subduction of Neo-Tethyan oceanic lithosphere and Cenozoic collision between the Indian and Asian continents, and represent juvenile continental crust. However, the petrological components and compositions of the lower crust of the Gangdese arc remain unknown. Based on detailed geological mapping, we conducted a systemic geochemical, geochronological and zircon Hf isotopic study of well-exposed high-grade metamorphic and migmatitic rocks from the lower crust of the eastern Gangdese arc. The results obtained show that Late Cretaceous garnet amphibolites, dioritic and granitic gneisses, and Paleocene–Eocene garnet amphibolites and granitic gneisses are the main components of the Gangdese lower arc crust. These meta-intrusive rocks witnessed a long period of magmatic, and metamorphic and anatectic processes from the Middle Jurassic to the Late Eocene, and have chemical compositions that range from ultramafic to felsic, with an average SiO2 content of 57.61 wt% and Mg# value of 0.49. These new data indicate firstly that the Gangdese lower arc crust has an overall intermediate composition and typical feature of juvenile crusts, and therefore supports the recent proposition that continental lower crusts are relatively felsic in composition, instead of mafic. We consider that the downward transport of felsic intrusives and associated sedimentary rocks into the deep crustal levels and subsequent partial melting resulted in componential and compositional changes of the Gangdese arc lower crust over time. This is a potential key mechanism in transforming primary lower arc crust to mature continental lower crust for the magmatic arcs with a complete growth history.
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•The Gangdese arc lower crust consists of voluminously migmatitic garnet amphibolites and orthogneisses.•The Gangdese lower crust witnessed a long-lasting and episodic magmatism, metamorphism and associated anatexis.•The Gangdese lower crust has an overall intermediate composition.•The downward transport of felsic intrusives resulted in compositional changes of the lower arc crust over time.
•The Neoproterozoic (817–838Ma) granitoids occur in the eastern Himalayan orogen.•The granitoids show chemical affinity to arc volcanic rocks.•The granitoids were derived from the Paleoproterozoic ...crustal materials.•An extensive Neoproterozoic magmatic belt existed in the Himalayan orogen.•The granitoids were formed during the Andean-type orogeny along the northwestern margin of Rodinia supercontinent.
The Precambrian component and evolutionary history of the Greater Himalayan Sequence (GHS), forming the core of the Himalayan orogen, remain a subject of debate. Here, we report new geochronological and petrochemical data of the Neoproterozoic granitoids that occur as the orthogneisses of the GHS in Cona area, the eastern Himalaya orogen. The zircon U–Pb dating results reveal that these granitoids have crystallization ages of Neoproterozoic of 817–838Ma, and metamorphic ages of ca. 31Ma. The petrochemical data show that the granitoids are medium to high-K, peraluminous granodiorite and granite, display geochemical affinities with volcanic arc granitoids. The zircon Hf isotopic compositions show that the granitoids were probably derived from the partial melting of Paleoproterozoic crustal materials. These data, combined with previous results, indicate that an extensive Neoproterozoic magmatic belt may have existed in the Himalayan orogen, and that the Neoproterozoic granitoids resulted from the Andean-type orogeny that formed along the northwestern margin of Rodinia supercontinent, and experienced the Cenozoic high-grade metamorphism during the Himalayan orogeny.
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