藏东贡觉地区早—中三叠世马拉松多组火山岩锆石U-Pb年龄、地球化学特征及其对金沙江古特提斯碰撞时间的约束

金沙江结合带中保留有大量晚古生代—中生代的岩石与构造信息，是研究金沙江古大洋洋陆俯冲-陆陆碰撞演化过程的主要窗口。对贡觉地区早—中三叠世英安岩-流纹岩的锆石U-Pb年龄、全岩地球化学特征进行报道，并探讨其岩石成因与岩浆源区。获得4组长英质火山岩LA-ICP-MS锆石U-Pb年龄，分别为246.2±1.7 Ma、245.5±0.7 Ma、244.7±0.6 Ma与246.2±0.5 Ma，一致指示贡觉地区马拉松多组火山岩的形成时代为245~246 Ma。地球化学分析显示，马拉松多组长英质火山岩具有高Si（SiO2=67.16%~74.62%），高K（K2O=3.44%~5.94%），贫Na（Na2O=1.75%~2.53%），贫Mg（MgO=0.28%~1.04%）的特点。A/CNK值范围为1.07~1.41，总体属过铝质高钾钙碱性岩石。稀土元素配分曲线呈现出轻稀土元素明显右倾，重稀土元素相对平缓的"V"字形；大离子亲石元素Rb、K、Th相对富集，高场强元素Nb、Ta、Sr、Ti相对亏损，指示岩浆来源于古老地壳。研究认为贡觉地区马拉松多组长英质火山岩为金沙江古洋壳西向俯冲造陆背景下，幔源基性岩浆底侵下地壳部分熔融的产物，贡觉地区此时正处于俯冲到碰撞的转换阶段。结合区域地质资料与前人研究，认为金沙江结合带贡觉地区大致经历了二叠纪的俯冲消减、早三叠世俯冲-碰撞转换、中三叠世的陆陆碰撞与晚三叠世的后碰撞造山阶段，直到晚三叠世后期造山运动结束。

Zircon U-Pb age,geochemical characteristics and constraints on the Jinshajiang Paleo-Tethys collision of Early-Middle Triassic Malasongduo Formation volcanic rocks from the Gongjue area,Eastern Tibet

There is a large number of rock and structural information of the Late Paleozoic-Early Mesozoic retained in the Jinshajiang binding belt, which is the main window to study the evolution of ancient land subduction-land collision in the Jinshajiang.This paper reports the zircon U-Pb age and whole rock geochemical characteristics of the early Middle Triassic dacite-rhyolite in the Gongjue area, and discusses its petrogenesis and magma source area.Based on LA-ICP-MS zircon U-Pb dating, the weighted average U-Pb age of four felsic volcanic rocks is 246.2±1.7 Ma, 245.5±0.7 Ma, 244.7±6 Ma and 246.2±0.5 Ma, respectively, indicating the volcanic rocks were formed at 245~246 Ma.Geochemical analysis shows that felsic volcanic rocks in the study area are rich in Si (SiO2=67.16%~74.62%), rich in K (K2O=3.44%~5.94%), poor in Na (Na2O=1.75%~2.53%) and poor in Mg (MgO=0.28%~1.04%).The A/CNK values range from 1.07 to 1.41, indicating peraluminous high-K calc-alkaline characteristics.The REE distribution curve shows a "V" shape.Large ion lithophile elements and high field strength elements Rb, K, and Th are relatively enriched, while Nb、Ta、Sr and Ti are relatively depleted, indicating that the magma was originated from the ancient crust.It is suggested that the felsic volcanic rocks in the Gongjue area are the products of partial melting of the lower crust caused by the underplating of mantle-derived basic magma under the background of westward subduction of the Jinshajiang ancient oceanic crust, which is in the transition stage of subduction to collision.Combined with regional geological data and previous studies, it is considered that the Gongjue area of Jinshajiang junction belt experienced Permian subduction, Early Triassic subduction-collision transition, Middle Triassic continent- continent collision and Late Triassic post-collision extension, until the end of orogeny in late Late Triassic.