幕阜山复式花岗岩体多期次演化与白垩纪稀有金属成矿高峰:年代学依据

Multistage Magmatic Evolution and Cretaceous Peak Metallogenic Epochs of Mufushan Composite Granite Mass:Constrains from Geochronological Evidence

花岗岩浆的分异过程是制约稀有金属成矿的重要因素,造山过程中多期次岩浆活动的叠加作用易导致伟晶岩熔体的大量聚集成矿.华南幕阜山复式花岗岩体由多期次多阶段的花岗岩侵入体构成,在区域持续而频繁的多期次岩浆活动作用下形成了华南地区重要的稀有金属矿集区.对幕阜山复式花岗岩体边部的断峰山含铌钽铁矿白云母钠长石伟晶岩以及岩体中部大兴含绿柱石白云母钠长石伟晶岩进行了^(40)Ar/^(39)Ar同位素定年研究,其白云母^(40)Ar/^(39)Ar坪年龄分别为127.7±0.9 Ma和130.5±0.9Ma.结合野外观察基础及区域已有的同位素年代学数据,推断出在燕山早期至中期该地区经历了多期岩浆演化,且持续时间较长,而伟晶岩的稀有金属矿化发生在岩浆活动末期的白垩纪,体现了区域岩浆多期次的分异演化作用导致稀有金属逐渐富集成矿的过程.这些地质现象说明,幕阜山区域在印支期经历了广泛的陆陆碰撞造山作用,进入燕山期后构造背景开始由陆内碰撞挤压向伸展减薄转变,在岩石圈伸展过程中经由玄武质岩浆底侵作用的影响,下地壳发生熔融,多期次岩浆活动导致了最终的稀有金属成矿.

The differentiation process of granitic magma is an important factor constraining the metallogenic mechanism of rare metal.The superimposition of multistage magmatic activities in orogenic process could easily lead to the accumulation of metallogenic pegmatite melts.Mufushan composite granite mass,which is composed of multistage granite intrusions,formed an important rare metal ore concentration area in South China.In this paper we report muscovite ^40Ar/^39Ar dating results from tantalite-bearing muscovite albite pegmatite of Duanfengshan on the margin of granite mass,and beryl-bearing muscovite albite pegmatite of Daxing in its central region.The ^40Ar/^39Ar ages of muscovite are 127.7±0.9 Ma and 130.5±0.9 Ma.Based on thedetailed field work and existing regional isotope chronological data,it is suggested that this area experienced long-lasting multistage magmatic evolution from early to middle Yanshanian.Rare metal mineralization of pegmatite occurred at the end of magmatic activities in Cretaceous,which showed the process of gradual enrichment and mineralization of rare metal led by regional multistage magmatic differentiation and evolution.These geological phenomena showed that Mufushan area has undergone a widely distributed continental collision orogeny in Indosinian and its tectonic setting shifted from continental collision to extensional thinning at Yanshanian.The multistage magmatic activities associated with regional lower crustal melting,which caused by underplating of basaltic magma during the process of lithospheric extension,led to the final rare metal mineralization.