湖南紫云山岩体的地质地球化学特征及其成因意义

Geological and Geochemical Characteristics and Genetic Significance of the Ziyunshan Pluton in Hunan Province

紫云山岩体由花岗闪长岩和黑云母花岗岩两种岩石类型组成，前者为岩体的主体，呈环状分布于岩体的周边，发育同时代的暗色微粒包体；后者为补充侵入体位于岩体的中央部位，两者呈明显的侵入接触关系，高精度LA-ICP—MS锆石U-Pb年龄分别为222．5±1Ma和222．3±1．8Ma，都为印支晚期的产物。两种岩石类型的SiO2含量存在明显间断，A／CNK变化范围分别为0．99～1．05和1．08～1．15，花岗闪长岩为准铝质-弱过铝质Ⅰ型花岗岩，黑云母花岗岩为弱过铝-强过铝质S型花岗岩。两类岩石都具有高的Rb、Cs含量和Rb／Sr比值，低的Sr、Ba、Eu含量，强-中等负铕异常，以及高的（87Sr／86-Sr）：值和低的ε-Nd（t）值，显示了前寒武纪变质基底起源的花岗岩的地球化学特征。两种岩石类型形成的温压条件和成岩深度有明显差别，暗示两者的源区不同。紫云山岩体两类岩石都是在印支陆块与华南陆块碰撞挤压一松驰构造环境下形成，岩浆底侵作用形成了少量有幔源组分加入的花岗闪长岩，其后上地壳泥砂质沉积物部分熔融形成了黑云母花岗岩。华南印支期主要花岗岩体的空间分布和高精度年龄数据显示，在华南内陆带和武夷-云开山脉带，由南而北，岩体形成时代逐渐变新，是印支陆块与华南陆块碰撞拼合所引起的挤压应力由南向北传递的结果。

The Ziyunshan pluton consists of granodiorites and biotite granites, the former granites ring distribute in its marginal belt, are the main rock type with zircon LA-ICP-MS age of 222. 5±1Ma, which are randomly distributed mafic micogranular enclaves, and the latter granites in its interior part were intruded into the former granites, with zircon LA-ICP-MS age of 222.3±1.8Ma. The both types rocks are Late Indosinian granites. The SiOz content of two types of granitic rocks have obvious discontinuity, and A/ CNK range respectively 0.99 - 0. 105 and 1.08 - 1.15, the granodiorites are metaluminous to weakly peraluminous I-type granites, and the biotite granites are weakly peraluminous to stongly peraluminous S-type granites. The two types granites both have high Rb, Cs, Rb/Sr, and （SZSr/S6Sr）i values, and low Sr, Ba, Eu, eNa（t）values, slightly to moderately negative Eu anomalies. These geochemical characteristics indicate that they derived from Precambrian metamorphic basement. The different P-T conditions and depths of their formation indicate that they have different regions. The two-stage granitic rocks of Ziyunshan pluton were formed by the collisional compression and decay between Southern China and Indo- China. The ganodiorites with a little mantle component were formed by magmas underplating, then the partial meltling of mud-sandy sediments in upper crust produced the biotite granites. The spatial distribution and high-precision age data of the Indonian granites in South China show that the rock-age of granitic rocks gradually turn new from south to north in Southern China inland belt and Wuyi-Yunkai mountains belt, along with the compressive stress transfered from south to north caused by the collision between Indo-China and Southern China.