川西龙门山彭灌杂岩地球化学特征:岩石成因与构造意义  

GEOCHEMISTRY OF PENGGUAN COMPLEX IN THE LONGMENSHAN REGION,WESTERN SICHUAN PROVINCE,SW CHINA:PETROGENESIS AND TECTONIC IMPLICATIONS

作　　者：张沛[1] 周祖翼[1] 许长海[1] 张青林[1] 

机构地区：[1]同济大学海洋地质国家重点实验室,上海200092

出　　处：《大地构造与成矿学》2008年第1期105-116,共12页Geotectonica et Metallogenia

基　　金：国家自然科学基金项目(40621063)资助

摘　　要：对位于扬子地块西缘龙门山逆冲推覆构造带中的彭灌杂岩进行了岩石地球化学研究，重点讨论了岩石成因、成岩物质来源及其构造意义。彭灌杂岩为一新元古代侵入的杂岩体，主要由中酸性侵入岩组成，其地球化学特征表现为SiO2含量变化范围较大（57.70％-71.90％），Na2O〉K2O（K2O／Na2O=0.31-0.85），里特曼指数δ=1.33-2.28，A／CNK=0.93-1.08，彭灌杂岩主体属于准铝质到弱过铝质的钙碱性花岗岩类；稀土总量不高（∑REE=76.25×10^-6-139.80×10^-6），轻重稀土分异程度差别较大（Lan／Ybn=3.86-35.23），弱负到正Eu异常（δEu=0.70-1.86）；富集大离子亲石元素（LILE）Rb、Sr、Ba、K等，亏损高场强元素（HFSE）Nb、Ta、P和Y。与正常壳源钙碱性花岗岩相比，彭灌杂岩显示出更高的Mg#值（多数〉45）和Sr／Y比值（多数20.03-111.34）。通过与其毗邻的埃达克质雪隆包杂岩的比较，表明彭灌杂岩可能是底侵的幔源岩浆与下地壳熔融产物混合的结果。结合其它研究成果，认为扬子西缘新元古代大面积分布的岩浆岩形成于俯冲汇聚和火山弧构造环境。The Pengguan complex, located in the Longmenshan thrust belt, western margin of the Yangtze block, SW China, was formed during the Neoproterozoic. Major and trace element abundances are reported, and the source and tectonic significance of the complex are discussed in this paper. The complex is dominantly composed of acid-intermediate intrusive rocks. 12 samples from the Pengguan complex show variable major oxides with SiO2 ranging from 57.70% to 71.90%. The rocks have relative high Na2O contents （K2O/Na2O = 0.31-0.85 ） , 8 values vary from 1.33 to 2.28 and A/CNK ratios from 0.93 to 1.08, indicating calc-alkaline series metaluminous to weak peraluminous granitoids for the Pengguan complex. The rocks have relative low abundance of REE （∑REE=76.25×10^-6-139.80×10^-6） with variable differentiation between LREE and HREE （ LaN/YbN = 3.86-35.23） and weak negative to positive Eu anomalies （δEu = 0.70-1.86）. The rocks are enriched in Rb, Sr, Ba, K and depleted in Nb, Ta, P, Y. Most samples have relatively high Mg# values （ 〉 45） and Sr/Y ratios （20.03-111.34） compared to crust-derived calc-alkaline granitoids. Moreover, the Pengguan rocks show the affinity with the adjacent adakitic Xuelongbao plutonic complex. The geochemical features suggest that the Pengguan complex was generated by mixing of the underplated mantle-derived magma and the melting of lower crust. Combined with previous study, we consider the Neoproterozoic extensive igneous rocks along the western margin of the Yangtze block were formed in a subducted-related volcanic arc environment.

关 键 词：彭灌杂岩 地球化学 新元古代 龙门山 扬子西缘 

分 类 号：P588.121[天文地球—岩石学] P595[天文地球—地质学]