北山地区照壁山南A型花岗岩地球化学特征及构造意义

侵入到红柳河蛇绿岩中的照壁山南黑云母二长花岗岩,其SiO2含量为68.86% ~ 73.27%,具高碱略富K贫Na,(K2O+Na2O=7.03 ~ 7.98,K2O/Na2O=1.16 ~ 1.36),AI含量中等(Al2O3=13.75%~14.1%)、弱过铝质(A/NKC=1.03 ~ 1.07)的特点,属高钾钙碱性系列弱过铝质花岗岩类.岩石具较高的Ga×104/Al(5.22 ~ 6.48)和Y/Nb (2.71 ~ 3.43) 值,在原始地幔标准化图解上显示出K、La、Nd、Zr、Hf、Sm富集, Nb、Ta、Sr、P、Ti强烈亏损,轻、重稀土分异明显,(La/Yb)N=7.19 ~13.15,负Eu异常(δEu=0.82 ~ 0.84),REE配分曲线呈略右倾"海鸥型"型的特点.研究结果表明黑云母二长花岗岩是在早泥盆世后碰撞伸展环境下,早期俯冲的与岛弧有关的物质受到底侵幔源玄武岩浆的加热部分熔融后,而形成的A(A2)花岗岩.红柳河-牛圈子-洗肠井蛇绿岩带西段所代表的古洋壳在晚奥陶世—早泥盆世间闭合(446.4~404.8Ma),之后出现一个以挤压结束伸展开始为特征的动力学演化阶段,其转化时间可能在415~404.8Ma.

Geochemical Characteristics and Tectonic Significance of A-type Granite in the South Margin of Zhaobishan,Beishan Area

The biotite monzogranite, located in the south margin of Zhaobishan, was intruded in Hongliuhe ophiolite, which are characterized by high-K and calc-alkaline composition with SiO2 68.86%~73.27%, high alkali (K2O+Na2O=7.03~7.98), coupled with midium aluminum (Al2O3=13.75%~14.1%) and weakly peraluminous (A/NKC=1.03~1.07). These rocks have high ratios of Ga×104/Al(5.22 ~ 6.48)and Y/Nb (2.71 ~ 3.43) in trace elements, which also show evidently loss in Nb, Ta, Sr, P, Ti as well as the enrichments of K, La, Nd, Zr, Hf and Sm. They are also rich in LREE with (La/Yb)N=7.19 ~ 13.15, and negative Eu anomalies(δEu=0.82 ~ 0.84). The results indicate that this biotite monzogranite belongs to A(A2) granite, which was formed by partial melting of young oceanic crust and island arc after had being used to be heated by basaltic magma in the extensional environment of early Devonian. Represented by the west part of Hongliuhe-Niujuanzi-Xichangjing ophiolitic belts, the paleo-ocean was closed between the late Ordovician and early Devonian(446.4~404.8Ma). After that, there was a dynamical evolution which was characterized by the end of the squeeze and the beginning of stretch, with the conversion time may be around 415~404.8 Ma.