甘肃北山牛圈子地区志留纪骆驼圈西岩体的岩石成因及构造意义

摘要

北山牛圈子地区的骆驼圈西埃达克岩为过铝质钙碱性岩浆系列,具有高的SiO2(>57.94%)、Al2 O3(>16.66%)和低的MgO(<4.32%)含量,较高的Na2 O/K2 O比值(>3.13),显示富钠的特征;而在微量元素上,高Sr(589×10-6~1170×10-6),低Y(4.60×10-6~13.90×10-6)和Yb(0.43×10-6~1.32×10-6);轻重稀土分异明显(La/Dy)N=4.60~13.87、(La/Yb)N=7.17~20.58,Eu异常不明显(Eu/Eu?=0.85~1.32);富集轻稀土和大离子亲石元素,而亏损高场强元素Nb,Ta,Ti和重稀土HREE,与世界上典型的俯冲洋壳熔融形成的埃达克岩特征相似.结合区域上埃达克岩的研究成果和岀露的构造位置,以及与牛圈子蛇绿岩的关系,认为骆驼圈西埃达克岩是志留纪(U-Pb年龄为428.9 Ma)时期热的洋壳向公婆泉—东七一山早古生代活动陆缘带俯冲过程中部分熔融的产物.对该岩体的研究,为进一步研究北山造山带早古生代北山洋盆闭合和晚古生代北山洋盆重新裂开提供新的依据.%West-Luotuojuan Adakitesin Beishan Niujuanzi Area belongs to Peraluminous calcium alkaline magma series with a high content of SiO2 ( >57. 94%) , Al2 O3 ( >16. 66%) , a low content of MgO ( <4. 32%) , a relatively high Na2 O/K2 O ratio, displaying Na-rich feature. In respect of trace elements, it has a high content of Sr (589 ×10 -6 ~1170 ×10 -6), a low content of Y (4. 60 ×10 -6 ~13. 90 ×10 -6) and Yb (0. 43 ×10 -6 ~1. 32 × 10 -6). The differentiation of light and heavy rare earth is obvious ((La/Dy)N=4. 60~13. 87, (La/Yb)N=7. 17~20. 58 ) , and there is not obvious anomaly of Eu ( Eu/Eu? =0. 85~1. 32 ) . Rich in light rare earth and large ionlithophile elements, and the loss of high field-strength elements Nb, Ta, Ti and heavy rare earth HREE, are similar with the typical Adakite rocks which formed by subducted oceanic crust melting. It is believed that West-Luotuojuan Adakite is a partial melting product which warm oceanic crust in Silurian ( U-Pb age of 428. 9 Ma ) period moving to Gongpo spring-east Qiyi mountain early Paleozoic active continental margin in the process of subduction. The study of the rock mass provides evidence for the view that Beishan orogenic belt closed in the late early Paleozoic and split anew in the early late Paleozoic.