湖南沩山岩体多期云母的Rb-Sr同位素年龄和矿物化学组成及其成岩成矿指示意义

Single-grain mica Rb-Sr isochron ages and mineral chemistry for the Weishan pluton in Hunan Province and implications on petrogenesis and mineralization of Mesozoic composite granite in South China

华南分布了大量的印支期-燕山期复式花岗岩体,其成岩及相关成矿过程一直存有争议。本文选择湖南典型的花岗质复式大岩基——沩山岩体为例,通过细致研究其中多期云母的单颗粒Rb-Sr年龄及地球化学组成,探讨了它们对相关成岩成矿过程的指示意义。单颗粒云母Rb-Sr等时线定年结果显示,沩山复式花岗岩主体唐市黑云母二长花岗岩中黑云母结晶年龄为221.9±5.8Ma(MSWD=3.8),主体边部的新浦高演化花岗岩中黑云母结晶年龄为227.0±13Ma(MSWD=1.6),主体内部的巷子口二云母花岗岩中白云母结晶年龄为210.1±3.3Ma(MSWD=0.21)。电子探针主量元素及激光探针LA-ICM-MS微量元素测定结果表明,沩山复式花岗岩中黑云母或白云母的成分存在明显差异,由此指示该岩体多期岩浆的温度、氧逸度以及源岩浆成分的差异。本文研究还表明,巷子口二云母花岗岩中白云母属于次生白云母,不能用来限定岩浆源区深度及岩石成因。鉴于多期云母的成分分异趋势及指示意义,我们认为沩山复式花岗岩多期岩浆具有同源关系,均来自于同一深部大岩浆房,是在不同时期不同构造背景下经历分离结晶和/或同化混染作用侵位而成。其中,印支晚期及之后的二云母花岗岩可能最具铀矿找矿前景。

Petrogenesis and mineralization on numerous Mesozoic composite granites in South China is the subject of an ongoing debate.This paper reports biotite and muscovite single-grain mineral Rb-Sr ages and geochemical results for the Weishan granite,a typical Mesozoic composite granitic pluton in South China that includes Xinpu highly-evolved granite,Tangshi biotite monzogranite and Xiangzikou two-mica granite,probing into multistage magmatisms and implications for the petrogenetic and metallogenic processes.The results show that（1） biotite that crystallized at 227.0±13Ma（MSWD=1.6） from the Xinpu highly-evolved granite is low in Al,Mg,Rb and ACNK ratios,but high in Fe and Fe/（Fe＋Mg） ratios;（2） biotite that crystallized at 221.9±5.8Ma（MSWD=3.8） from the Weishan＇s main body,the Tangshi biotite monzogranite,is high in Mg and Sr,but low Fe/（Fe＋Mg） and Rb/Sr ratios;and（3） biotite from the Xiangzikou two-mica granite,formed at 210.1±3.3Ma（MSWD=0.21） that were determined by single-grain muscovite Rb-Sr isochron age,is low Sr and high in Al,Rb,ACNK and Rb/Sr ratio;and（4） muscovite from the Tangshi biotite monzogranite is formed during the hydrothermal process,and is characterized by very low concentrations for most major and trace elements and high Fe/（Fe＋Mg）,Cs/Nb,Rb/Sr ratios,whereas muscovite from the Xiangzikou two-mica granite is formed by metasomatisms of biotite and K-feldspar etc.,with higher Al,Ti and Na.The magma temperature and oxygen fugacity drop from the Tangshi biotite monzogranite to the Xiangzikou two-mica granite,finally to the Xinpu highly-evolved granite,while the magma for the Xiangzikou two-mica granite was involved in more pelitic sediments.Muscovite from the Xiangzikou two-mica granite is secondary in origin,and thus cannot be used as a pressure indicator to designate another magma source.The clear fractionation trends of biotite and muscovite compositions claim kindred among multistage magmas for the Weishan composite granite,which were mainly derived from a single psammite magma chamber,and intruded in batches under different tectonic regimes after extensive fractional crystallization and assimilation.Both of the Xinpu highly-evolved granite and the Xiangzikou two-mica granite are potential sources for uranium mineralization,but the latter shows a better perspective because of intense hydrothermal alternation.