福建云霄晶洞花岗岩及含石榴子石伟晶岩的硅氧同位素研究

Silicon and Oxygen Isotopic Composition of Miarolitic Cavity Granites and Garnet-bearing Pegmatites in Yunxiao,Fujian Province

在运用常规方法和激光烧蚀同位素分析方法分别对云霄晶洞花岗岩和伟晶岩(长石、石英和石榴子石)的硅、氧同位素组成进行研究的基础上,探讨了晶洞花岗岩及其含石榴子石伟晶岩的物质来源及形成条件。伟晶岩中的石英和长石的氧、硅同位素组成分别较之花岗岩中的石英和长石的氧、硅同位素组成,均未发生明显变化,表明二者岩浆来源一致。云霄县的乌山岩体主要由燕山期铝质A型花岗岩构成,实验得到的云霄晶洞花岗岩样品全岩的氧同位素组成符合A型花岗岩氧同位素组成特点。根据花岗岩中石英与长石矿物对的氧同位素分馏公式计算得到的温度为461℃,可能反映花岗岩结晶后曾受到水—岩交换作用的影响,这也与岩体中广泛发育伟晶岩晶洞的现象相一致。依据伟晶岩中石英—长石、石英—石榴子石矿物对的氧同位素分馏公式计算得到的温度分别为505℃、532℃,表明形成伟晶岩的流体温度达到500℃以上,高于前人得出的温度范围。伟晶岩中石榴子石硅、氧同位素比值均显著低于与其共生的石英和长石硅、氧同位素比值,表明石英与石榴子石之间、长石与石榴子石之间存在明显的硅、氧同位素分馏,且二者分馏趋势相一致,但其硅同位素分馏程度小于氧同位素分馏。

The oxygen and silicon isotopic compositions of minerals（quartz,feldspar and garnet） from the miarolitic cavity granites and pegmatites in Yunxiao county,Fujian province,are measured by using conventional method and laser probe analytic method for determining their material sources and forming conditions.The oxygen and silicon isotopic compositions of quartz and feldspar from the pegmatites are very similar to those of the granites,indicating that they have the same magma source.The Wushan granite in Yunxiao County mainly consists of Yanshanian aluminium A-type granite and its oxygen isotopic composition of the whole rock complies with the features of A-type granite.The temperature of 461℃ acquired for granite from the oxygen isotope fractionation between quartz and feldspar indicates that the granite might have experienced some extent water—rock interaction after crystallization,which is consistent with the extensive development of pegmatite miarolitic cavity in granite.Temperatures（ranging from 505℃ to 532℃） acquired from the oxygen isotopic fractionation formula of quartz—feldspar and quartz—garnet in the pegmatites suggest that the temperature of pegmatite-forming fluid should be about 500℃,higher than the temperature ranges by previous researchers.The silicon and oxygen isotopic ratios of garnet from the pegmatite were both significantly lower than those of coexisting quartz and feldspar in miarolitic cavity,indicating there are distinct fractionations of silicon and oxygen between garnet and quartz or feldspar,and both of which intend to share similar change pattern,but fractionation degree of silicon is smaller than that of oxygen.