越南西北部Pu Sam Cap高钾碱性岩LA-ICPMS锆石U-Pb年龄与Hf同位素组成及其区域构造意义  

Zircon U-Pb ages and Hf isotopic composition of the Pu Sam Cap high-potassic alkaline rocks in northwestern Vietnam and regional tectonic implication

作　　者：TRAN MyDung 刘俊来[1] NGUYEN QuangLuat 陈越[1] 纪沫[1] 唐渊[1] 

机构地区：[1]中国地质大学(北京)地质过程与矿产资源国家重点实验室,北京100083 [2]Hanoi University of Mining and Geology

出　　处：《岩石学报》2010年第6期1902-1914,共13页Acta Petrologica Sinica

基　　金：国家"973"项目(2009CB421001);国土资源部公益性行业科研专项经费项目(200811008);教育部111计划(B07011);科技支撑计划(2006BABO1A03)资助

摘　　要：越南西北部Pu Sam Cap高钾碱性杂岩位于哀牢山剪切带南段西侧,由粗面岩、煌斑岩、碱性正长岩与碱性花岗岩组成。其中粗面岩不整合于晚白垩世陆相沉积岩之上,碱性侵入岩侵入到三叠纪陆源沉积岩、侏罗纪流纹岩及40～35Ma含角闪石二长岩-花岗岩中。该杂岩的岩石具有低TiO2(<0.8%)、P2O5(<0.64%)和FeO*(<7.74%),高Na2O(1.41%～4.5%)和K2O(5.22%～9.4%)的地球化学属性,属于高钾碱性玄武质到高钾碱性长英质岩浆岩类型与钾质至超钾质岩浆系列。这套岩石富集大离子亲石元素LILE、轻稀土元素LREE和一组相容元素,具有明显的Ta、Nb和Ti异常,表明Pu Sam Cap高钾碱性杂岩形成于与俯冲带有关的大陆弧和后碰撞弧构造环境。锆石LA-ICPMS U-Pb年代学研究表明碱性正长岩与碱性花岗岩分别形成于32.70±0.24Ma、35.1±0.06Ma,上述年龄代表了Pu Sam Cap高钾碱性杂岩的侵入岩结晶年龄。两个样品中锆石的εHf(t)变化范围分别为-2.8～2.5和-4.9～0.1,在t-εHf(t)相关图上分析点落在下地壳和亏损地幔演化线之间,两阶段模式年龄变化范围为950～1286Ma和1107～1421Ma。该杂岩的εNd(t)变化范围-5.63～-3.26,具有低143Nd/144Nd比值(0.512336～0.512447)和高87Sr/86Sr比值(0.706254～0.707273),Sr-Nd同位素相关图上接近于EM2地幔端元趋势线,与大理地区新生代高钾岩浆岩十分相似,表明PuSamCap高钾碱性杂岩的岩浆是幔源岩浆和壳源岩浆的混合结果,同时可与大理地区新生代高钾岩浆岩对比,它们形成于哀牢山-红河左行走滑活动之前,墨江古特提斯洋盆的消减与印度-欧亚陆陆碰撞活动提供了宏观区域性构造背景。The Pu Sam Cap high-potassic alkaline rocks are located in western part of the southern segment of Ailao Shan-Red River shear zone,including high-potassic alkaline trachyte,lamprophyre,syenite and alkaline granite. There into,the trachyte lays unconformably upon the Upper Cretaceous sedimentary rocks. The syenite and alkaline granite intruded the Triassic terrigenous sedimentary rocks,Jurassic rhyolite and amphibole-bearing granitic intrusion of 40～35Ma. The Pu Sam Cap potassic alkaline rocks are geochemically characterized by relatively low TiO2 (<0.8%),P2O5 (<0.64%) and FeO* (<7.74%) and high Na2O (1.41%～4.5%) and K2O (5.22%～9.4%),coupled with high contents of incompatible trace elements. They are ultrapotassic or shoshonitic enriched in LILE,LREE and compatible elements,and have marked Nb,Ta and Ti depletion,which is similar to a subduction related component and has the characteristics of post-collisional arc type potassic igneous rocks. Zircon LA-ICPMS U-Pb dating gives the emplacement age of 32.70±0.24Ma for high-potassic syenite and 35.1±0.06Ma for alkaline granite. Their εHf(t) values are-2.8～2.5 and-4.9～0.1 with two-stage Hf model ages of 950～1286Ma and 1107～1421Ma,εNd(t) values of-5.63～-3.26,high 87Sr/86Sr(0.706254～0.707273) and low 143Nd/144Nd (0.512336～0.512447). It is suggested that the Pu Sam Cap high-potassic alkaline rocks were resulted from mixing between mantle and crust-derived magmas and may be derived from an EM2 mantle. The remnant of the subduction of Yangtze plate beneath the Indochina plate during the closure of the Mojiang Paleotethyan oceanic basin,and the Cenozoic continental collision between Indian and Eurasian plates provide a broad tectonic background for the Pu Sam Cap potassic alkaline magmatism.

关 键 词：Pu SAM Cap高钾碱性岩 锆石U-PB年龄 Hf同位素 地幔源区 哀牢山-红河剪切带 越南西北部 

分 类 号：P588.12[天文地球—岩石学]