川西甲基卡地区西康群砂岩地球化学特征及对物源区、构造背景的指示

Geochemical characteristics of sandstones from the Xikang Group in Jiajika area, western Sichuan, and their constraints on provenance and tectonic setting

松潘-甘孜造山带中部大面积分布西康群浊积岩,为一套厚度巨大(2000~3000 m)的泥质碎屑复理石建造,以深海-次深海海底扇沉积为特征。本文通过对甲基卡稀有金属矿集区分布的西康群砂岩进行主量元素、微量元素及粒度分析,从而揭示该区砂岩构造背景和物源属性。研究结果表明,砂岩样品SiO_(2)、CaO、MgO、Al_(2)O_(3)、Fe_(2)O_(3)、K_(2)O、Na_(2)O、MnO等含量在侏倭组、新都桥组和两河口组中各有差异,但总体上含量与大陆上地壳成分相当。稀土元素REE配分模式曲线一致,LREE/HREE=6.26~18.07,平均10.38,(La/Yb)_(N)=6.64~33.66,平均13.27,δEu=0.37~0.91,平均0.67,轻稀土元素相对重稀土元素明显富集,稀土元素分馏明显。成分变异指数ICV=1.40~5.23,平均2.43,为构造活动背景下的首次沉积物,受源区岩石控制明显,反映了碎屑岩岩源所处的环境为寒冷干燥的气候;Th/U=2.80~9.40,平均5.32,化学蚀变指数CIA=50.32~86.93,平均62.80,表明其经历了微弱的风化作用,后期(两河口组沉积期)可能由于区域构造趋于稳定而风化作用加强。物源判别及构造判别图解综合显示其物源主要来自长英质物源区,物源区以活动大陆边缘构造环境为主,具有构造活动强烈的构造背景。

In the middle part of the Songpan-Ganzi orogenic belt, there is a set of turbidity rocks of the Xikang Group, which is a set of sedimentary shale with huge thickness about 2000~3000 meters, and is characterized by deep-subdeep sea fan deposits. Based on the analytical content of major elements, trace elements and grain size analysis from the Xikang Group’s sandstones, the tectonic settings and provenance were studied. The results show that major elements SiO_(2), CaO, MgO, Al_(2)O_(3), Fe_(2)O_(3), K_(2)O, Na_(2)O and MnO from the Zhuwo, the Xinduqiao and the Lianghekou Formations of the Xikang Group are different from each other, but the overall content is comparable to that of the continental crust. The sandstones have a consistent REE pattern, enrichment of LREE relative to HREE, with LREE/HREE of 6.26~18.07(averaging 10.38),(La/Yb)_(N)of 6.64~33.66(averaging 13.27) and δEu of 0.37~0.91(averaging 0.67). Index of the chemical variation(ICV) of 1.40~5.23, with an average of 2.43, indicating that the sandstone is the first sediment against the background of tectonic activity which is obviously controlled by source rocks, and reflect a chemical weathering environment of cold and dry climate. Furthermore, the Th/U values of 2.80~9.40(with an average of 5.32) and chemical index alteration(CIA) of 50.32~86.93(with an average of 62.80) reflect that it has undergone a weak weathering that might have strengthened in the later period(the sedimentary period of the Lianghekou Formation) due to the stability of regional structure. The discriminatory plots of provenance and the tectonic discrimination diagrams reflect that the provenance is mainly from the quartz-feldspathic provenance area, which is dominated by the active continental margin tectonic environment and has a strong background of tectonic activity.