东准噶尔卡拉麦里黄羊山花岗岩岩石成因探讨

黄羊山花岗岩体是卡拉麦里造山带典型的后碰撞花岗岩体, 发育大量闪长质微细粒包体。黄羊山花岗岩具有高硅(72.21%~77.36%)、低铝(9.00%~12.93%)、贫钙镁(CaO: 0.20%~1.21%; MgO: 0.03%~0.44%)、富碱(Na2O+K2O: 7.43%~9.02%)以及高分异(SI=0.28~3.47, DI=76.45~95.99)的特征。强烈富集LILE和HFSE (Zr+ Nb+Ce+Y=260.01 μg/g~797.83 μg/g), Ga含量高(10000×Ga/Al=3.95~5.69), 属于A型花岗岩类。岩石学和岩相学(包体细粒淬冷边, 反向脉, 复合包体以及寄主岩石和包体中斜长石斑晶在形态、成分、光性上的一致性等)、岩石地球化学(Y/Nb=2.77~6.82, La/Nb=0.91~4.33, Ba/Nb=0.13~37.86等)、Sr和Nd同位素(ISr多数在0.7031~0.7041, εNd(t)在5.2~7.1之间)以及LA-ICP-MS锆石U-Pb测年(寄主岩石为311±12 Ma, 包体为300±6 Ma)综合研究显示, 黄羊山花岗岩是壳-幔源岩浆混合成因。从晚石炭世到二叠世, 东准噶尔地区进入后碰撞构造演化阶段, 在后碰撞构造阶段, 早期的俯冲板片断离, 软流圈减压熔融, 玄武质岩浆底侵至下地壳底部, 底侵基性岩浆带来的巨大热量, 导致地壳物质熔融, 形成大规模的花岗质岩浆, 两种岩浆发生了不同程度的混合, 其中闪长质微细粒包体就是基性的幔源岩浆和酸性的壳源岩浆不同程度混合的产物。

A Tentative Discussion on the Genesis of Huangyangshan Granite Body in Kalamaili Orogen, East Junggar

The Huangyangshan pluton is a typical post-collision pluton in Kalamaili orogen of East Junggar,with many dioritic microgranular enclaves distributed in it.It is characterized by high Si(72.21%～77.36%1,low A1 (9.00%～12.93%),poor Ca and Mg(CaO 0.20%～1.21%,MgO 0.03%～0.44%),rich alkali(Na2O+K2O 7.43%～9.02%),high extent of differentiation(SI=0.28～3.47,DI=76.45～95.99),strong enrichment of LILE and HFSE (Zr+Nb+Ce+Y=260.01μg/g～797.83μg/g),and relatively high Ga content(10000xGa/A1=3.95～5.69),indicating an A type granite.The lithologic and petrologic characteristics(such as the fine-grained cold-quenching ring of the enclave,the reverse vein,the complex enclaves,the consistency of plagioclase phenocrysts from both host rocks and enclaves in crystal form,composition and optic character),the petrogeochemical data(Y/Nb=2.77～6.82.La/Nb=0.91～4-33,Ba/Nb=0.13～37.86),the Sr and Nd isotope data(most ISr range from 0.7031 to 0.7041,8Nd(t)=5.2～7.1)and the LA-ICP-MS zircon U-Pb dating(the age of the host rock is 311±12 Ma,and that of the enclaves iS 300±6 Ma)all suggest that the Huangyangshan pluton is of the crust-mantle hybrid genesis.From Late Carboniferous to Permian,Junggar region entered into the post-collision tectonic evolution stage,during which the early subduction slabs were ruptured and separated, the asthenosphere was decompressed and melted, the basalt magma experienced underplating clue to the underplating of the basaltic magma, the low crust was melted by the high temperature and formed acid crust-magma, and the mixing between the basic magma from the mantle and the acid magma from the crust took place and formed the Huangyangshan magma-mixed pluton. The dioritic microgranular enclaves thus serve as the recorder of the crust-mantle magmatic mixing process.