【摘要】 近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素,以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期（163Ma和160Ma）的年龄,属于晚侏罗世;南侧岩体花岗岩年龄为142～147Ma,属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩,富集大离子亲石元素和轻稀土元素,亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分(ε_Hf（t）=-16.8～-13.6),可能来源于古老下地壳的部分熔融;包体具有相似的Hf同位素成分(ε_Hf（t）=-15.7～-13.6),结合晚侏罗世寄主岩与包体较高的MgO（4.13%～6.90%）、Cr(146×10^-6～370×10^-6)和Ni(31×10^-6～113×10^-6)含量,说明包体代表的中基性岩浆可能源于古老富集地幔的熔融,并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的ε_Hf（t）值（-8.2～-4.8和-3.8～+0.2）,指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入,并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合,经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶,最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。更多还原

【Abstract】 The research on the petrogenesis,magma source region and evolution of the Mesozoic magmatism in the western Central Lhasa Terrane is still ambiguous and further geological,geochemical and geochronological work is needed.This work mainly focuses on petrography,geochemistry,zircon U-Pb geochronology,trace elements and Hf isotope analysis,in order to better reveal the nature of source region and geodynamic processes of the magmatism in Zuozuo,Gar County,western of the Central Lhasa Terrane.The granodioritic and dioritic batholith located in the north of Zuozuo were formed at Late Jurassic（163Ma and 160Ma）,whereas the ages of granodiorite and diorite in the south side of Zuozuo are ranging from 142Ma to 147Ma.The host granodiorite and diorite of Late Jurassic in the north and Early Cretaceous batholiths in the south side of Zuozuo are geochemically metaluminous to weak peraluminous,high potassium calcium-alkaline I-type granite.They are enriched in large ion lithophile elements（LILEs）,and light rare earth elements（LREEs）and depleted in high field-strength elements（HFSEs）.The host rocks from the Late Jurassic batholith have negative and relatively uniformε_Hf（t）value（-16.8～-13.6）,which indicates its origin from melting of ancient lower crust material.Enclaves from the Late Jurassic batholith have similar relatively negative and uniformε_Hf（t）value（-15.7～-13.6）.Combined with the higher contents of MgO,Cr and Ni in the Late Jurassic host rocks and enclaves,we may infer that they should be melting of ancient enriched mantle,and there is sufficient magma mixing between the two magma represented by the Late Jurassic host granodiorite and the diorite enclaves.Granitoids from the Early Cretaceous batholith has relative negative and wide range ofε_Hf（t）values（-8.2～-4.8 and-3.8～+0.2）,which suggests much depleted mantle substances in the Early Cretaceous magmatism,and the depleted mantle material is magma mixed with the acidic magma formed by the melting of the ancient crustal material of the Lhasa Terrane.After the magma mixing,the amphibole and feldspar were first separated and crystallized,and a series of medium grained acidic rocks of the Early Cretaceous batholith in the south were crystallized.The increase in the amount of the depleted mantle materials in the Early Cretaceous is probably due to the roll-back of the southward subduction of Bangong-Nujiang Neo-Tethyan oceanic sea floor.更多还原