柴北缘东坪地区片麻岩的地球化学特征及构造意义

Geochemical Characteristics of Gneiss in the Dongping Area,Northern Margin of Qaidam Basin and Tectonic Significance

东坪地区片麻岩可分为花岗片麻岩、斜长片麻岩和角闪片麻岩三类,SiO_2含量在57.47%~75.32%,全碱含量(Na_2O+K_2O)介于2.62%~7.40%,Al_2O_3含量在11.08%~16.29%,A/CNK范围0.84~1.78,属于中-酸性、高钾钙碱性、过铝质岩石;稀土元素配分模式表现为轻稀土元素富集、重稀土元素相对亏损的右倾型曲线,大部分样品具有负铕异常;微量元素表现为Rb、Ba等大离子亲石元素富集,Nb、Ta、Sr等元素相对亏损。综合岩石学及元素地球化学特征,东坪地区斜长片麻岩和角闪片麻岩的原岩为高钾钙碱性I型花岗岩,可能为壳源与幔源物质混合成因;花岗片麻岩的原岩属于陆源S型花岗岩,是地壳部分熔融后冷凝结晶的结果。通过本文研究结果并结合区域构造演化背景,认为东坪片麻岩形成于早古生代活动大陆边缘的岛弧环境,反映了柴北缘、北阿尔金断裂、北祁连地体经历了原特提斯洋闭合后的板块汇聚和碰撞作用。

The gneiss in the Dongping area was formed by medium-high grade regional metamorphism of Dakendaban group,which can be divided into granitic,plagioclase and calcareous gneisses. The major elements show the contents of SiO2（ 57. 47% ～ 75. 32%）,（ Na_2O ＋ K_2O）（ 2. 62% ～7. 40%）,Al_2O_3（ 11. 08% ～ 16. 29%） and A/CNK（ 0. 84 ～ 1. 78）,indicating the Dongping gneiss is of intermediate-acid,high calc-alkaline and peraluminous series. Analysis of REEs demonstrates the curves of LREEs enrichment and HREEs losses with right deviation. Most of samples have weak negative europium anomalies. The trace elements suggest enriched Rb,Ba,and relatively depleted Nb,Ta,and Sr. According to petrologic and geochemical characteristics,the original rock type of plagioclase gneiss and calcareous gneiss is I-type granite with the feature of high potassium calc-alkaline,formed by the mixture of crust and mantle source materials. Differently,the original rock type of granitic gneiss belongs to the terrigenous S-type granite,formed by the condensation and crystallization of partially melting crust material. In combination with tectonic setting,this study suggests that the Dongping gneiss formed in an island-arc environment of an Early Paleozoic active continental margin. It reflects that the Dongping area experienced the plate convergence and collision of the northern Qaidam basin,northern Altyn fault and northern Qilian terrane after the Proto-Tethys closure.