内蒙古乌拉山-大青山地区变泥质岩的地球化学特征及构造意义

Geochemistry and its tectonic implications of the Wulashan-Daqingshan metapelites in Inner Mongolia

乌拉山-大青山地区位于华北克拉通西北缘孔兹岩带的中段,该地区出露大面积的古元古代高级变质表壳岩系岩石,例如夕线（堇青）石榴二长片麻岩、石榴黑云二长片麻岩、黑云长英质片麻岩、石榴石英岩、大理岩、磁铁石英岩和钙硅酸盐岩等。其中该地区典型的变泥质岩石（夕线（堇青）石榴二长片麻岩和石榴黑云二长片麻岩）多以似层状产出,夕线（堇青）石榴二长片麻岩含有大量的石榴石、夕线石和/或堇青石,并保留典型的堇青石反应边结构。岩石地球化学研究结果表明上述变泥质岩石样品明显富Al2O_3,属于过铝质岩石,其平均化学成分与上地壳平均值较为接近。样品的稀土配分型式具有中等程度的轻重稀土元素分异,轻稀土明显富集和弱铕负异常的特征。微量元素蛛网图显示变泥质岩石样品富集大离子亲石元素如Rb和Ba,而高场强元素如Zr、Hf含量相对较低,且Nb、Ta、P和Ti呈明显亏损的特征。多种原岩判别图解显示其原岩为一套富铝粘土岩及杂砂岩,且成熟度较低,为近源沉积。多数样品的CIA值低于太古宙后澳大利亚平均页岩和北美页岩的CIA值,反映其源岩经历了中等程度的风化作用。物源以上地壳的长英质成分为主,可能有古老沉积物的加入。多种构造判别图解表明研究区样品形成于有演化岛弧发育的活动大陆边缘环境。结合前人对孔兹岩带出露的变泥质岩石的变质作用研究普遍得到近等温减压型顺时针P-T演化轨迹以及锆石同位素年代学研究结果,综合表明乌拉山-大青山变泥质岩石形成于有演化岛弧发育的活动大陆边缘环境,中元古代物质（2.1-2.0Ga）为其提供主要的沉积物源,随后卷入-1950Ma华北克拉通西部古老陆块之间的碰撞造山作用,经历了大规模麻粒岩相变质作用,并于1920-1850Ma碰撞后折返至近地表。

Paleoproterozoic high-grade supracrustal rocks involving sillimanite-（ cordierite-） garnet gneisses,garnet-biotite gneisses,quartzofeldspathic gneisses,garnet-bearing quartzites,marbles,magnetite quartzites and calc-silicate rocks are cropped out in the Wulashan-Daqingshan area in the central segment of the Khondalite Belt,North China Craton. Among which,typical metapelitic rocks including sillimanite-（ cordierite-） garnet gneisses and garnet-biotite gneisses occur as interlayers,moreover sillimanite-（ cordierite-）garnet gneisses are consist of garnet,sillimanite and/or cordierite,and preserve cordierite-bearing symplectites. Geochemical analyses reveal that the metapelitic rock samples are peraluminous with the enrichment of Al2O_3,and the average bulk composition is similar to that of the upper continental crust. The chondrite-normalized rare earth element（ REE） pattern for the samples displays characteristics of medium fractionation in light and heavy REE,strong enrichment in light REEs and weak negative Eu anomalies. The primitive mantle-normalized trace element pattern shows the enrichment of large ion lithophile elements such as Rb and Ba,the contents of higheld strength elements such as Zr and Hf are relatively low; furthermore Nb,Ta,P and Ti are distinctly depleted. The protoliths of the studied metapelitic rocks are discriminated as aluminum-rich claystone and graywacke with low maturity and proximal sedimentation.The Chemical Index of Alteration（ CIA） values of the samples are lower than those of post-Archean Australian shale and North American shale composite,indicating the source rocks of the samples underwent moderate chemical weathering. The uppercrust felsic sources with possible old sediment component make a contribution to their provenance. Tectonic discrimination diagrams show that the samples formed in the active continental margin with evolved arc setting. Combined with the reported clockwise P-T paths with nearisothermal decompressional stages and analyses of zircon dating for metapelitic rocks in the Khondalite Belt,it is therefore inferred that the metapelitic rocks in the Wulashan-Daqingshan area formed in the active continental margin with evolved arc setting,and midPaleoproterozoic materials（ 2. 1 - 2. 0Ga） provided the main sedimentary materials. The protoliths of the metapelitic rocks were involved in the Paleoproterozoic continent-continent collisional event along the Khondalite Belt in the North China Craton at - 1950 Ma followed by exhumation and cooling at 1920 - 1850 Ma accompanied with amphibolite-granulite-facies metamorphism.