Petrology, Geochronology And Geochemistry Of Granitic Rocks In Rutog At Northern Lhasa Terrane, Tibetan Plateau

There are many Early Cretaceous volcanic rocks in the northern and centralLhasa area, their petrogenesis and geological setting has long been a debate up to now.This thesis reports the data of stematic petrology, geochemistry, zircons Hf isotopesand trace element of the magmatic rocks in the Rutog region of northern LhasaTerrane, Tibet, aiming to provide more information for the solving of these question..The major rock types of the granitoids in Rutog occur as stocks, includegranodiorite, granite, quartz-diorite, as well as a number of dioritic enclaves. Theresult of zircon U-Pb LA-ICP-MS dating indicate: The granitoids rocks in Rutogformed during Early Cretaceous（116~105Ma）and Late Cretaceous（83~70Ma）.Themain diagenetic stage of the host rocks are in accordance with the dioritic enclaveswhich belong to the same period.The geochemistry feature of the Rutog granitic rocks reveal: Early Cretaceousgranitic rocks and the dioritic enclaves are metaluminous, medium-high-Kcalc-alkaline rocks. All these samples are enriched in light REE and depleted in heavyREE，and most samples show well-developed to weak negative Eu anomalies; LILEand Pb are enriched and HFSE are depleted. Late Cretaceous granitic rocks and thedioritic enclaves are metaluminous, medium-high-K calc-alkaline rocks. They areenriched in light REE and depleted in heavy REE; LILE are enriched and HFSE aredepleted. Both of them are I-type granite. All the zircon Hf isotopic of samplesindicate that the Early Cretaceous granitic rocks display negative Hf(t) values and oldcrustal model ages; the Late Cretaceous granitic rocks display positive Hf(t) valuesand relative young crustal model ages, relatively. The bulk-rock of zircon saturatedtemperature demonstrate the mantle-derived materials most likely involved in thepetrogenesis of the Rutog Pluton.The petrogenesis of Early Cretacesous granitoid rocks in Rutog maybe formedin the setting of south subducton of Bangong-Nujiang sea floor and the subductionslab break-off which caused the riseing asthenosphere materials and melting thelithosphere mantle and crust, and the basic magma and acidic magma formed. After the basic magma and acidic magma mixing, the Rutog pluton in Early Cretacesouswere formed. The Late Cretaceous granitic rocks formed in the setting ofpost-collisional of the Qiangtang block-Lhasa block, and the underplating ofmantle-derived magmas caused partial melting of the lower crust and mixed.Fractional crystallization occurred during the mixed magma invaded the upper crust,and the Late Cretaceous granitic rocks in Rutog area were formed.