摘要
中国大陆科学钻探主孔岩芯190~320m榴辉岩中的富锶重晶石作为副矿物存在于超高压变质和退变质阶段,其中大部分的重晶石发现于绿辉石的退变质后成合晶中。具有不同Sr含量的重晶石在榴辉岩中至少具有3种产出状态,形成于3个变质阶段:Ⅰ超高压榴辉岩相阶段,存于石榴石包裹体中,SrSO4含量约在45mol%;Ⅱ早期退变质阶段,存于后成合晶(单斜辉石 钠长石)中,SrSO4含量变化范围很大,在0mol%~21mol%之间;Ⅲ晚期角闪岩相阶段,作为黄铁矿的氧化边存在,SrSO4占2mol%~5mol%。重晶石的形成与流体作用相关,富锶重晶石能反映原赋存流体的Sr/Ba值比较高。重晶石与其伴生矿物尤其硫化物的结构关系对岩体的相对氧化还原环境具有重要指示意义。本文通过榴辉岩中重晶石副矿物学研究,确定了榴辉岩曾经历超高压变质阶段的氧化环境、早期退变质阶段的还原环境和角闪岩相阶段的氧化环境。
Sr-rich barite exists as an accessory mineral at the ultrahigh-pressure(UHP)and retrograde metamorphic stage of eclogites from the 190~320m main-hole core of the Chinese Continental Scientific Drilling(CCSD),most of which is recognized in the symplectite of clinopyroxene(Cpx)and albite(Ab)formed by omphacite breakdown.It appears at least in three different occurrences at three metamophic stages:i.e.,stageⅠUHP barite which is preserved in garnet with homogeneously high Sr content(~45mol% SrSO4);stage Ⅱ early retrogressive barite which is dispersing in the symplectite(Cpx Ab)with greatly variable Sr-content(0mol%~21mol% SrSO4);stage Ⅲ amphibolite-facies barite occurs as the oxidized rim of pyrite with 2mol%~5mol% SrSO4.Barite is probably formed by the fluid activity and the Sr-rich barite may be precipitated from the fluid with high Sr/Ba.Barite together with the associated minerals especially the sulfide indicates the redox state of the eclogite at different metamorphic stages:Oxidation of the UHP eclogite-facies metamorphism,reduction of the early retrogression and oxidation of the amphibole-facies retrogression.