广西昆仑关A型花岗岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of Kunlunguan A-Type Granite,Guangxi

昆仑关岩体位于广西南丹—昆仑关燕山期花岗岩带西南端,岩石类型为黑云母花岗岩.岩相学及化学成分显示其属高钾钙碱性岩系,具高硅、富碱和铝、低磷和钛的特点,SiO2含量为68.13～72.61,K2O/Na2O比值介于1.28～1.69之间,铝指数A/CNK=0.72～1.28,属准铝质一强过铝质岩石.富集Ga、Rb、Th、U、Pb等元素,而强烈亏损13a、Nb、Sr、P和Ti.稀土元素具明显负Eu异常,总体呈现轻稀土富集的右倾"V"形配分模式.昆仑关古民单元锆石LA-ICP-MSU-Pb定年结果为93±1Ma,形成于晚白垩世.矿物组成及地球化学特征确定了南丹—昆仑关花岗岩带是华南西南端一条重要的铝质A型花岗岩带,控岩断裂构造为NW向南丹一昆仑关深大断裂.岩石学及主量和微量元素(含稀土)特征表明岩体侵位于后碰撞阶段的张性构造环境.结合研究范围在晚白垩世的构造背景及区域应力场匹配组合关系,认为其与印度板块在该时期北移所引起的古太平洋板块高角度俯冲所导致的弧后拉张有关,与浙闽沿海A型花岗岩或碱性岩分别代表了匹配构造体制下的两条A型花岗岩带.

Kunlunguan pluton, located in the southwest part of the Nandan-Kunlunguan granite belt, Guangxi, is mainly composed of biotite granite. Petrography and chemical compositions show that it belongs to high-K calc-alkaline series characterized by high silicon, is enriched in alkali and aluminum but depleted in phosphorus and titanium. Its SiO2 contents range from 68. 13% to 72. 61%, while K2O/Na2O ratios and A/CNK values vary from 1.28 to 1.69 and 0. 72 to 1.28 respectively, ascribed to a suite of metaluminous to intensively peraluminous rock. Ga, Rb, Th, U and Pb elements are enriched in the intrusion while Ba, Nb, Sr, P and Ti depleted. REE is characterized by obvious negative anomalies with Eu/Eu 0. 53-0. 73 and exhibits right-dipping ＂V＂ patterns with LREE enrichment. LA-ICP-MS zircon U-Pb dating yields a weighted mean 206pb/238u age of 93±1 Ma （MSDW=1. 7） for the Gumin unit of the pluton, implying its Late Cretaceous intrusion time. Mineralogy and geochemistry confirm that the Nandan-Kuntunguan granite belt is an important aluminous A-type granite belt in the South China, controlled by northwestward Nandan-Kunlunguan deep fault. Petrology, major and trace el- ement features display that the pluton intruded in a post-collisional extensive tectonic setting. These results, combined with Late Cretaceous tectonic environment and regional stress field matching relationship in the studied area, indicate the A-type granite action is due to back-arc extension related to high angle subduction of the ancient Pacific plate which is caused by Indian plate northward movement. This granite belt and those granite or alkaline rocks in the coastal areas of Zhejiang-Fujian provinces represent the two A-type granite belts under a matching tectonic system.