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The leucogranites in the Higher Himalayan Sequence (HHS) provide a probe to elucidate the crustal melting of continental collisional orogen. An integrated geochemical and geochronological study of the Yadong leucogranites, southern Himalaya, shows that these rocks have relatively high SiO 2 contents of 69.77 to 75.32 wt.% and alumina saturation index (A/CNK) of 1.09–1.40, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and depleted HFSE (Zr, Hf, Nb and Ta). LA–ICP–MS U–Pb zircon dating of ten samples yields crystallization ages ranging from 21.0 to 11.7 Ma. The zircons have variable ε Hf ( t ) values of − 26.3 to − 3.5 and corresponding Hf two-stage model ages of 2.77–1.33 Ga. The present study reveals that the muscovite–biotite leucogranites (2ML) have higher TiO 2 , MgO, CaO, Sr, Ba and Zr contents, lower Rb/Sr ratios than the tourmaline–muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681–784 °C) of the 2ML are 20–80 °C higher than those (663–705 °C) of the TML. Combining with previous results, we propose that the TML were derived from the muscovite-dehydration melting, whereas the 2ML dominantly resulted from the biotite-dehydration melting during the prograde metamorphism of the pelitic and felsic granulites of the HHS. Therefore, the Himalayan leucogranites were probably formed during the subduction of the Indian crust following the India and Asia collision. [ABSTRACT FROM AUTHOR]

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 10–12 kbar and 550–700 °C for the prograde stage, 13–16 kbar and 840–880 °C for the peak-metamorphic stage, and 5–6 kbar 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. 40 Ma, and lasted to ca. 8 Ma. 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. [ABSTRACT FROM AUTHOR]

Abstract: Charnockites, occurring in the well-studied Gangdese batholith in the southern Lhasa terrane, eastern Himalayan orogen, consist of orthopyroxene, clinopyroxene, plagioclase, and minor quartz and K-feldspar, and show adakitic affinity. Here we present a systematic study on the fluid inclusions in Gangdese charnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed and as trails along intragranular fractures in quartz, and that the secondary ones occur along healed mircofractures and coexist commonly with mineral inclusions of calcite, magnetite and hematite within the host plagioclase and quartz. The fluid inclusions contain dominantly near-pure CO2 with traces of N2. Most of the primary fluid inclusions have low CO2 homogenization temperatures and with densities of 1.138–1.013g/cm3 suggesting trapping pressures of 0.7–1.0GPa at temperatures of 850–950°C. We propose a model in which the southward subduction of Neo-Tethyan mid-oceanic ridge beneath the Lhasa terrane resulted in the release of heat and CO2 from the upwelling asthenosphere through a slab window, providing high-temperatures (HT) and dry CO2-rich fluids for the formation and stabilization of the adakitic charnockite as well as the associated HT granulite-facies metamorphic rocks in the Late Cretaceous, prior to the final collision of the Indian plate with the Asian continent. Our study provides new insights into the geodynamics of Andean-type orogeny in the southern Tibetan Plateau during the Late Mesozoic. [Copyright &y& Elsevier]