幕阜山地区新元古代花岗岩地球化学特征及成因探讨

Geochemical characteristics and genesis of the Neoproterozoic granite in Mufushan area

位于湘鄂赣三省交界、江南造山带中段的幕阜山地区是我国重要的稀有金属矿集区,其南部的新元古代三墩花岗岩体为中细粒弱片麻状二云母斜长花岗岩。全岩地球化学分析显示,三墩花岗岩具有高钠、铝等特点,A/CNK=1. 15～1. 39,A/NK=1. 39～1. 69,为过铝质花岗岩;∑REE在(70. 45～84. 36)×10^(-6),LaN/YbN在10. 82～11. 23,轻稀土元素中度富集,Eu弱亏损(δEu=0. 99～0. 77);富集大离子亲石元素,Nb亏损。综合分析,该岩体为I型花岗岩,形成于大陆火山弧构造背景,推测为江南造山带形成过程中格林威尔造山运动板块碰撞形成,为聚合板块边缘俯冲带岩浆作用的产物。

The Mufushan area in the middle part of the Jiangnan Orogen,which is located at the junction of Hunan,Hubei and Jiangxi,is an important rare metal ore concentration area in China.The Neoproterozoic Sandun granite in the southern part of Mufushan area is mainly composed of weakly-gneissic medium-fine grained two-mica plagioclase granite.Whole rock geochemical analysis of samples show that Sandun Granite belongs to peraluminous calc alkaline granite and is characterized by high Na,Al,and LILE contents,low Eu and Nb,with A/CNK=1.15-1.39,A/NK=1.39-1.69,ΣREE=70.45×10-6-84.36×10-6,La N/Yb N=10.82-11.23.Geochemical features and diagrams of the granite samples show similarity to I-type granite,and indicate that the Sandun granite was formed under a tectonic setting of continental island arc.Taking into account the regional geology,we consider that the Neoproterozoic granite in Mufushan area was formed by plate collision of Grenville orogeny during the formation of Jiangnan Orogen,and is the product of magmatism for the converging plate marginal subduction zone.