湖南白石峰岩体的年代学、地球化学特征及其构造背景

Geochronology, Geochemical Characteristics and Tectonic Significance of the Baishifeng Pluton in Hunan

白石峰岩体位于湖南衡山,是衡山复式岩体的组成部分,侵入到新元古代板溪群及泥盆—二叠纪地层中,岩性为二云母花岗岩。LA-ICP-MS锆石U-Pb年龄为157 ±2 Ma与151 ±2 Ma。白石峰岩体为强过铝质(A/CNK >1.1)、镁质、高钾钙碱性系列花岗岩。微量元素含量较低(∑REE = 50.4 ×10?6~197.6 ×10?6),轻稀土富集,Ba、Nb、Sr和Ti相对亏损,表现出明显的负Eu异常(δ Eu为0.32~0.62)。岩体的εHf(t)值分别在?13.14~?11.06与?18.76^+0.51之间,显示源区为壳源。综合分析表明,白石峰花岗岩形成时代为晚侏罗世,花岗岩类型属于S型花岗岩,可能为前寒武纪基底部分熔融形成。白石峰花岗岩形成于后碰撞环境,与华南地区晚中生代整体呈伸展的构造环境有关。

The Baishifeng pluton,located in Hengshan,Hunan Province,is a component of the Hengshan complex.On the outcrops,the pluton invades into the Neoproterozoic Banxi Group and the Devonian-Permian strata.Petrographically,the pluton displays the characteristics of the two-mica granite.Two samples are analyzed by LA-ICP MS and yield the weighted mean 206Pb/238U age of 157±2 Ma and 151±2 Ma,respectively,suggesting Late Jurassic magmatic emplacement.The Baishifeng pluton is characterized by a strongly peraluminous(A/CNK>1.1),magnesia,high-potassium calc-alkaline series.The content of trace elements is low(∑REE=50.4×10?6-197.6×10?6),but shows high LREE with obvious negative Eu anomalies(δEu is 0.32-0.62).In the primitive mantle normalized spidergram,the samples exhibit relative depletion in Ba,Nb,Sr and Ti.The two samples have notably negativeεHf(t)values(?13.14-?11.06 and?18.76-+0.51,respectively),indicating generation in crust.The granite samples show S-type granite affinity.Our results,combined with other geological evidence,indicate that the Baishifeng pluton may be formed by the melting of the Precambrian basement in the post-collision setting.We infer that the Baishifeng granitic pluton is more likely to be related to the overall extensional setting during Late Mesozoic in South China.