The Genesis Of The “Tetrad Effect” Of The Zhuogapu Highly Fractionated Granite In The Northern Lhasa Terrane

The“Tetrad Effect”,which often exists in highly evolved systems,its genesis has always been controversial.A large quantities number of studies indicate that highly evolution system is the most ideal object of study for studying the“Tetrad Effect”.Meanwhile,comparing to leucogranite in southern Tibet,there are a few relative reports about high silicic granite in the northern Lhasa Terrane.Thus,this paper selected Nyima as the study region,which located in the middle of the southern margin of the northern Lhasa Terrane.This study reports zircon U-Pb age,whole-rock major and trace element,and zircon Hf isotope data for the Zhuogapu granites.The Zhuogapu intrusion consists of monzogranites,syenogranite,and alkali granite,with high SiO₂,alkali,and K₂O content,characterized by its high-K calc-alkalic and weakly peraluminous,belong to highly fractionated I-type granitoids.They are divided into two groups based on the rare-earth elements feature.Group I shows normal REE patterns and were may originated from coeval granodiorite magmas through significant fractional crystallization of plagioclase,K-feldspar and biotite;Group II samples show various degree of“tetrad effect”patterns.In this work,“tetrad effect”is simulated by quantitative and consistent fractionated crystallization of monazite and magnetite,which means the differentiation of accessory mineral facies can produce the“tetrad effect”.Zircon?Hf?t??–0.8-+4.44?suggest the Zhuogapu granites may originated from the partial melting of ancient lower crust of central Lhasa subterrane with contribution from juvenile crust,which may from the northern Lhasa subterrane lower crust or emplacement of basaltic magma beneath the central Lhasa subterrane.Combination with coeval igneous rocks and previous research results,the northern Lhasa Terrane may have undergone crustal thickening at ca.100 Ma.