西藏唐古拉岩浆岩带夏玛日花岗岩体的岩石学、岩石地球化学、锆石 U-Pb 测年及 Hf 同位素组成

摘要

唐古拉岩浆岩带位于青藏高原中央隆起带附近，构成南北羌塘的构造界限，特殊的构造位置使得有必要对该套岩体进行深入的研究工作。本文对唐古拉岩浆岩带西段夏玛日晚三叠世花岗岩体进行了系统的野外工作和室内分析测试，获得2件二长花岗岩样品中锆石边部振荡环带的 U-Pb 年龄为231．0±1．4Ma、227．2±1．6Ma，可以确定岩浆侵位结晶时代为晚三叠世；Hf 同位素测试结果显示，岩体具有负的 Hf 同位素初始比值εHf （t）且变化范围较小，两阶段地壳 Hf 模式年龄确定其源岩时代为元古代。核部继承锆石的 U-Pb 年龄较为分散，Hf同位素组成显示其成因复杂，且时代较老（＞1．2Ga，最高达3．0Ga），表明研究区存在前寒武纪结晶基底。岩石地球化学数据显示，岩体全碱含量（Na2 O＋K2 O）在4．86～7．45之间，铝饱和指数 A／CNK 在1．15～1．82之间（＞1．1，为强过铝质），CIPW 标准矿物组合全部含刚玉 C 分子（含量＞1％）。稀土配分曲线呈右倾缓倾斜型的特征，轻稀土富集而重稀土亏损；稀土元素总体分馏程度较高，且轻稀土分馏相对较强。大离子亲石元素、高场强元素含量均出现分化，K、Ti 等元素强烈亏损，Nb、Ba、Ce 及 Sr 等元素轻微亏损，而 Ta、Pb 等元素富集。夏玛日晚三叠世花岗岩应为壳源物质重熔的强过铝质 S 型花岗岩，其成因应与龙木错-双湖结合带的碰撞造山演化有关，在相对高温（800℃以上）且富含流体的条件下，由前寒武纪碎屑沉积物中硬砂岩成分部分熔融产生的熔浆在后期构造控制下上升侵位。%Tanggula Magmatic Belt located in the central uplift of the Qinghai-Tibet Plateau and separated the North and South Qiangtang basin,and was necessary to do further studies for its special tectonic location. Two samples were selected from the Xiamari granites in the western part of Tanggula magmatic belt,and obtained zircon U-Pb ages 231.0 ± 1.4Ma,227.2 ± 1.6Ma repectively by LA-ICP-MS in its oscillatory zones,which indicated that the magma emplaced and crystallized in Late Triassic.Two samples all showed negative Hf isotopic initial ratio,limited variation and very old two stage crust model ages,indicated that its primitive magma were old than Proterozoic.Inherited zircon had large range in its U-Pb ages,complex Hf isotopic composition and elder ages of Hf model ages (elder than 1.2Ga,up to 3.0Ga max),implied there were Prechambrian metamomorphic basement in study area.Geochemical data indicated that Xiamari granite chould be strongly peraluminous S-type granitoids which had the A/CNK ranging from 1.15 to 1.82 (>1.1 )and only contained C member in CIPW composition.All samples showed rightly-deviation from LREE to HREE,enriched in LREE,Ta and Pb,but deficited in Nb,Ba,Ce and Sr,espically in K and Ti.Late Triassic Xiamari granite should be strongly peraluminous S-type granitoids,and its primitive magma was formed by partial melting by greywacke member in Precambrian crustal clastic rocks,in the condition of relative high temperature (above 800℃)and fluid-enriched,as the magmatic response to Longmucuo-Shuanghu Tethys evolution.