Age and isotope geochemistry of magmatic rocks of the Lohit Plutonic Complex, eastern Himalaya: implications for the evolution of Transhimalayan arc magmatism

摘要

We present integrated in situ zircon U-Pb and Hf isotope data, along with whole-rock Sr-Nd isotope and geochemical data for plutonic rocks of the Lohit Plutonic Complex of eastern Himalaya to understand the age distribution and petrogenetic-tectonic evolution of the Transhimalayan batholiths in the eastern Himalaya. The plutonic rocks are gabbro, diorite and quartz-diorite in composition, have calc-alkaline signatures with metaluminous character and have an affinity with Gangdese I-type granitoids of southern Tibet. The enrichment of large ion lithophile elements with highly depleted negative Nb anomalies is consistent with their origin in a subduction-related environment. Our laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages suggest that two major stages of magmatic emplacement took place at c. 145 Ma (Early Cretaceous) for gabbro and c. 100-90 Ma (early Late Cretaceous) for diorite and quartz-diorite. The zircon Hf isotopes in both the gabbro and diorite samples show high positive epsilon(Hf) (t) values between +11 and +20, suggesting crystallization from juvenile crust or depleted mantle derived melts, as also constrained by the low initial Sr-87/Sr-86 ratios (0.7038-0.7045) and high positive epsilon(Nd)(t) values (+2.9 to +5.9). Results of REE modelling indicate that the source of these rocks is garnet lherzolite. Field observations and mineralogical and geochemical characteristics in conjunction with the isotopic database suggest that the Transhimalayan magmatic rocks in eastern Himalaya were generated by partial melting of the lithospheric mantlewedge and emplaced into the lower crust (c. 25-30 km) during at least two major magmatic events that took place in the Cretaceous period.