安徽南部溪口岩群流纹岩地球化学特征及成因

Geochemical characteristics and petrogenesis of rhyolites of the Xikou Group in South Anhui Province

溪口岩群流纹岩出露于安徽南部休宁一带,位于扬子地块东南部、江南造山带东段。通过研究溪口岩群流纹岩岩相学、主量、微量及稀土元素特征,探讨流纹岩的成因及其形成的构造环境。主量元素地球化学特征表明:该流纹岩高硅,高钠低钾,高铝,低MgO、CaO、TiO2和MnO,属于钙碱性系列过铝质流纹岩。微量及稀土元素地球化学特征表明:该流纹岩富集大离子亲石元素K和Rb,Ba、Sr、Sm和Ce相对亏损,高场强元素U明显富集,Nb、P、Zr和Hf相对亏损,Ti明显亏损。稀土元素总量∑REE为(25.11~54.51)×10^-6,轻稀土元素相对富集,重稀土元素相对亏损,轻、重稀土元素分异明显,球粒陨石标准化稀土元素配分曲线明显右倾,Eu负异常明显。安徽南部溪口岩群流纹岩是洋壳局部重融的产物,形成于与同碰撞有关的火山弧构造环境。

Rhyolites of the Xikou Group are tectonically located at the southeastern part of the Yangtze Plate and the eastern Jiangnan Orogen, with outcrop mainly in the Xiuning area, South Anhui Province.This study investigated the petrogenesis and tectonic setting of Xikou Group rhyrolites through petrographic and geochemical analysis.Geochemical characteristics of major elements indicate that the rhyolites are characterized by high SiO2,high Na and low K,high Al2O3,low MgO,low CaO,low TiO2,and low MnO,suggesting a typical calc- alkaline series with peraluminous features.Geochemical characteristics of trace elements show that rhyrolites are enrichment of large- ionlithophile elements K and Rb, relative depletion of Ba, Sr, Sm and Ce;obvious enrichment of high field strength element U, relatively depletion of Nb, P, Zr and Hf, obviously depletion of Ti.The total REE content is (25.11~54.51)×10 -6 ,with slightly enriched LREE, relatively depleted HREE and significant LREE-HREE differentiation.Chondrite-normalized REE distribution patterns show obviously right-dipping and distinct Eu negative anomaly.The above study suggests that the Xikou Group rhyolites in South Anhui were product of partial melting of the oceanic crust and formed in a volcanic arc tectonic environment related to syncollision.