超高压变质作用不同阶段的流体变化：碧溪岭榴辉岩的氢、硼同位素分析

大别山碧溪岭榴辉岩中有三种含水矿物:多硅白云母、角闪石和黑云母,它们分别是超高压(UHP)阶段(即柯石英榴辉岩相阶段)或者石英榴辉岩相阶段、退变质后成合晶阶段和角闪岩相退变质阶段的产物,本文利用离子探针技术对它们进行了氢同位素和硼同位素的分析。三种矿物内部的同位素组成都是均一的,多硅白云母的δD为-105‰±9‰,δ~(11)B为-25.9‰±2.0‰;角闪石的δD为-100‰±9‰,δ~(11)B为-24.4‰±0.9‰;黑云母的δD为-65‰±4‰,δ~(11)B为-19.3‰±1.3‰。多硅白云母和角闪石的氢-硼同位素组成在误差范围内是相同的,而和黑云母则有明显的差别,这表明,从UHP阶段或者石英榴辉岩相阶段到随后的后成合晶阶段,变质流体是内部缓冲的,而在角闪岩相变质阶段,则有了外来流体的加入,这个流体是相对富集D和~(11)B的。碧溪岭榴辉岩矿物相对于其地壳原岩表现出低δ~(11)B的特征,说明俯冲过程中板块经历了强烈的脱硼。     

Hydrogen Isotopic Compositions of Pyroxenes in Nushan Peridotite Xenoliths,SE China

Hydrogen isotopic compositions of pyroxenes in peridotite xenohths from the Nushan volcano, Anhui Province, SE China have been obtained using an ion probe (Cameca 1270). D/H ratios are constant within a single grain and among the different grains from the same sample. The lack of correlation between the D/H ratios and the hydrogen contents indicates that the hydrogen isotopic compositions are inherited from their mantle source. Combining with the δD values of coexisting amphiboles, it is inferred that the Nushan mantle experienced at least a two-stage metasomatic event.One was responsible for formation of amphiboles with extremely variable δD values, and the other was probably responsible for the high δD values (up to -20‰) of some clinopyroxenes from peridotites, clinopyroxene and mica megacrysts. High δD values point to a subduction-related fluid being involved in one metasomatic event. The primary δD values (-90‰ to -140‰) of the Nushan pyroxenes, together with data gleaned from the literature, suggest that the D/H ratios of the nominally anhydrous mineral reservoir might have differed from that of the other mantle hydrogen in being relatively depleted in D.     

大别山北部黄土岭片麻岩的锆石U-Pb年龄和氧同位素组成：古老的原岩和多阶段历史

利用阴极发光和离子探针技术，对大别山北部黄土岭片麻岩的锆石进行了内部结构、U-Pb年龄和氧同位素组成的系统分析。黄土岭片麻岩的锆石具有核-幔-边三层结构，核部是原岩的岩浆锆石，形成时代为-2800Ma；幔部是变质重结晶的产物，变质作用的时代为-2000Ma；边部则是在-110Ma从部分熔融体里结晶出来的增生锆石，是受大别山燕山期构造-岩浆事件影响的反映。黄土岭片麻岩和麻粒岩中-2800Ma和-2000Ma的年龄，以及麻粒岩中约3400Ma的残留锆石年龄均可以与扬子崆岭群变质岩对比。由此可以推断，北大别在构造属性上属于扬子板决，扬子与华北板决的缝合线应在北大别以北。锆石的核-幔-边具有不同的氧同位素组成，氧同位素数据表明黄土岭片麻岩的原岩是S型花岗岩，在-2000Ma和-110Ma的事件中都有外来物质(流体或熔体)的加入。