南极拉斯曼丘陵长英质变质岩中磷灰石-斜方辉石组合的岩石学意义

Petrological implications of the apatite-orthopyroxene association in the high-grade felsic gneiss of the Larsemann Hills,East Antarctica

南极拉斯曼丘陵高级长英质变质岩中出现的磷灰石-斜方辉石(Ap-Opx)组合表明岩石经历了麻粒岩相条件下的变质作用,长英质片麻岩发生了强烈的脱水深熔作用。随着熔体的迁出,长英质片麻岩中的残留或者镁铁质岩石中的分异产物,均可形成Ap-Opx组合。残留矿物如磷灰石、金属氧化物等具有丰富的稀土元素,强烈影响着岩石中的稀土元素配分型式。磷灰石-斜方辉石组合的存在说明体系中有适量但不是很充足的有效钙质组分。由磷灰石、磁铁矿-钛铁矿、石榴石,直至斜方辉石,一方面反映了活动组分-残留组分的不断变化,另一方面,也说明高级变质作用过程中体系的失水程度逐渐增加;变质作用中富含该组合的岩石是经变质过程形成的特有岩石,没有固有的原岩可对应。

The existence of the special apatite-orthopyroxene association in the felsic gneiss of the Larsemann Hills,east Antarctica,suggests the granulite facies metamorphism of the gneiss.The concomitant fluid-absent partial melting in the felsic gneiss was responsible for the melt segregation and residues which were favorable for the formation of Ap-Opx.The residual minerals like apatite and ilmenite are enriched in REE and have strongly affected the REE patterns of the rocks.The presence of the Ap-Opx assemblage implies the existence of appropriate but insufficient effective CaO component in the system.The crystallization of minerals in order of apatite,magnetite-ilmenite,garnet and orthopyroxene has demonstrated the changing mobile-residual components and progressive dehydration in the high-grade metamorphism.The metamorphic felsic rock rich in Ap-Opx is the unique rock derived in metamorphism,with no corresponding protolith.