The Metamorphism And Partial Melting Of Migmatitic Pelitic Granulite And The Origin Of The Leucogranites From The Central Himalayan Orogen

The Himalayan orogen was produced by the Cenozoic collision between the Indian and Asian plates. The Higher Himalayan Crystalline Sequences (HHS), located in the core of the Himalayan orogen, represents the deeply subducted and subsequent exhumed Indian continental crust. The HHS underwent high-grade metamorphism and partial melting, and records the history of formation and evolution of the Himalayan orogen. Despite a comprehensive study have been carried out for the HHS, there still exist debates on the petrogenesis of the high-grade metamorphic rocks and the Himalayan leucogranites. In this PhD thesis, we present a petrological, geochemical and geochronological study of the migmatitic pelitic granulites, in-source leucosomes and leucogranites of the HHS in the Yadong region. The obtained results reveal the P-T conditions, the initial time and duration of the metamorphism and partial melting of the HHS, the degree and mechanism of the partial melting, the geochemical characteristics and origin of the in-source leucosomes and leucogranites. These provide new insights into the tectonic evolution of the Himalayan orogen.A combined study of petrology, phase equilibria modeling and zircon U-Pb geochronology was carried out for the migmatitic pelitic granulites. The results indicate that the migmatitic pelitic granulites underwent complex metamorphic evolution and associated partial melting, with two stages of mineral assemblage, including the early peak metamorphic assemblage of garnet+kyanite+biotite+ quartz+plagioclase+K-feldspar, and the late retrograde assemblage of garnet+ plagioclase+K-feldspar+sillimanite/cordierite+biotite+muscovite+quartz. Phase equilibria modeling shows that the migmatitic pelitic granulites underwent high-temperature (HT) and high-pressure (HP) metamorphism under peak P-T conditions of ca.800-835± and 12.8-14 Kbar, followed by nearly isothermal decompression and isobaric cooling process, with a clockwise P-T-t path. The anatexis of the pelitic granulites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde and peak-metamorphic process. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.%of the rocks, and the extracted significant amount of melt provides potential resource for the formation of Himalayan leucogranites. The zircon U-Pb dating data shows that the migmatitic pelitic granulites probably witnessed a prolonged HT metamorphism and melting episode that began at ca.31 Ma and lasted to ca.20 Ma.A comprehensive study of petrology, geochronology and geochemistry was performed for the in-source leucosomes derived from the pelitic granulites in the Yadong region. The results show that the whole-rock compositions of the in-source leucosomes are as follows: SiO2= 71.19-74.70 wt.%, TiO2= 0.18-0.24 wt.%, Al2O3-12.61-15.37 wt.%, TFe2O3= 0.51-1.48 wt.%, MnO= 0.50-0.67 wt.%, MgO= 0.87-2.24 wt.%, CaO = 1.68-3.04 wt.%, Na2O = 6.99-9.56 wt.% and K2O = 0.09-0.26 wt.%. These rocks have high alumina saturation index (A/CNK), belonging to peraluminous rocks. On the chondrite-normalized REE pattern diagram, in-source leucosomes are enriched in LREE, with pronounced positive Eu anomalies. On the primitive mantle-normalized trace element diagram, all the rocks are enriched in Rb, Th, U, K, Pb, Zr, Hf and Sm, and have negative Ba, Nb, Ta, Sr and Ti anomalies. LA-ICP-MS U-Pb zircon dating on the in-source leucosomes yields three groups of age, namely 32.4-24.2 Ma, 20.9-14.3 Ma and 14.2-12.6 Ma, representing the times of the metamorphism and partial melting of the migmatitic pelitic granulites, early and late melt crystallization, respectively. The present study reveals that the compositions of the in-source leucosomes are distinctly different from the Himalayan ieucogranites, indicating that the melts derived from the partial melting of the migmatitic pelitic granulites experienced fractional crystallization, and the in-source leucosomes formed as the plagioclase-rich cumulate rocks.An integrated geochemical and zircon U-Pb dating study of the Yadong leucogranites shows that these rocks have varying but high SiO2 (69.77-75.32 wt.%), Al2O3 (13.24-16.74 wt.%), Na2O (2.62-4.22 wt.%), K2O (3.99-5.67 wt.%) contents and A/CNK, and low CaO (0.53-1.09 wt.%), TFe2O3 (1.33-2.71 wt.%), MgO (0.12-0.57 wt.%) and TiO2 (0.05-0.22 wt.%) contents, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and LREE, 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 eHf(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 TiO2, MgO, CaO, Sr, Ba and Zr contents, and lower Rb/Sr ratios than the tourmaline-muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681-784℃) of the TML are 20-80℃ higher than those (663-705℃) of the TML. Combining with previous results, we propose that the Himalayan leucogranites are derived from a two-component mixing of melts produced by partial melting of the pelitic and felsic granulites of the HHS, and the TML were derived from the muscovite-dehydration melting under HT conditions, whereas the 2ML dominantly resulted from the biotite-dehydration melting under higher temperature and pressure conditions.Based on the studies above, we propose a tectonic model for the high-grade metamorphism and partial melting of the HHS and the formation of the leucogranites in the core of the Himalayan orogen. Following the collision of Indian and Asian plates, the Indian continental crust was gradually subducted beneath the Lhasa block, the pelitic and felsic rocks of the HHS experienced the amphibolite-to granulite-facies metamorphism and associated muscovite-dehydration melting, which resulted in the formation of melts that have similar chemical compositions to the TML. With the further subduction of the continental crust, the pelitic and felsic rocks underwent higher temperature and pressure granulite-facies metamorphism and associated biotite-dehydration melting, which generated the melts with similar compositions to the 2ML. We propose that the partial melting of the HHS occurred dominantly during the subduction of the Indian continent, and the generated melts (leucogranites) experienced a prolonged crystallization process during the exhumation of the HHS.