藏南冈底斯岩基日多地区花岗岩体形成时代和地球化学特征

Timing and geochemical characteristics of the Riduo granitic pluton within the Gangdese batholith,southern Tibet

冈底斯岩基广泛发育三叠纪-中新世的岩浆岩,是研究与新特提斯洋北向俯冲和印度-欧亚大陆碰撞相关的构造岩浆作用特征的天然实验室。日多地区花岗岩体位于藏南墨竹工卡县东侧日多乡附近,其主体为花岗岩,被花岗闪长玢岩脉侵入。锆石U-Pb地质年代学表明:主体花岗岩形成于62.7±0.5Ma,侵入其中的花岗闪长玢岩脉形成于59.5±1.5Ma,并捕获了大量的侏罗纪岩浆岩锆石(155.4±1.8Ma)。日多地区花岗岩体的全岩地球化学特征为:(1)高SiO_2、Na_2O、Al_2O_3,低FeO^T、MgO、TiO_2;(2)富集轻稀土(LREE),亏损重稀土(HREE)及高场强元素Nb、Ta、Ti、P元素;(3)具有Eu负异常,总体显示高钾钙碱性、过铝质花岗岩和岛弧型岩浆岩特征。锆石Hf同位素特征暗示其岩浆源区为基性下地壳物质。花岗闪长玢岩脉裹挟大量侏罗纪岩浆型锆石,表明冈底斯岩基拉萨以东地区可能经历了较广泛的晚侏罗世岩浆作用。

Gangdese batholith consists of different magmatic rocks with ages ranging from Late Triassic to Mid-Miocene,which provide important records to investigate the tectono-magmatic processes associated with the northward subduction of the Neo-Tethyan oceanic lithosphere as well as with the Indian-Eurasian continental collision.The Riduo pluton,near the Riduo town to the east of Maizhokunggar County,consists of coarse grained granites and granodoritic dikes.Zircon U-Pb dating reveals that the granite and the dikes were formed at 62.7±0.5Ma and 59.5±1.5Ma,respectively.In addition,the granodoritic dikes captured a number of zircon grains crystallized at 155.4±1.8Ma,probably from the Jurassic magmatic rocks.The Riduo granitic rocks are characterized by:1)high SiO2,Na2O and Al2O3,but low FeOT,MgO and TiO2;2)enrichment of LREE and LILE,but depletion of HREE and HFSE(Nb,Ta,and Ti);and 3)pronounced Eu negative abnormality.These features indicate that the Riduo granites belong to high-K,calc-alkaline and peraluminous granite with an island arc affinity.Zircon Hf isotopic compositions suggest that they are derived from melting of young lower crustal rocks.Jurassic zircon grains captured by the granodoritic dikes imply that the Gangdese batholith to the east of Lhasa could experience more widespread magmatism during the Jurassic time than those previously thought.