东南极拉斯曼丘陵长英质片麻岩的深熔作用与铁钛氧化物的聚集机制

Anatexis and enrichment mechanism of the Fe-Ti oxide minerals in the quartzofeldspathic gneisses from the Larsemann Hills,East Antarctica

东南极拉斯曼丘陵高级变质长英质岩石中铁钛氧化物的局部聚集与高级变质作用过程中的深熔作用有关,并非原岩富集这些组分。深熔作用造成惰性组分如铁钛氧化物滞留原地或略有聚集及活动性组分的迁移,而流体挥发组分优先聚集于熔体之中。当体系中水含量较低、处于不饱和状态时,深熔作用过程中形成局部“熔体”,其结晶所成的浅色体不具低共结组分,没有熔体结晶结构,不是真正的熔体,可能是(准)熔体。较粗粒的浅色体或伟晶岩也是与深熔作用有关的产物,其形成早于花岗岩脉或岩体,而与花岗质岩浆分异无关。伴随(准)熔体的出现,体系中组分的萃取、分异效果较为明显,即可造成组分分异,形成截然不同的异地、二相分异结构,分别形成固相残留物(组成可以不固定)和(准)熔体相。固相残留体中富铝、铁组分,形成矽线石和铁钛氧化物团块,其中少或无挥发分;与此对应,短距离迁移浅色体中往往贫铁钛组分,可见石榴子石、偶见铁钛氧化物矿物。这种挥发分不饱和状态下的深熔作用基本属于封闭体系,整体失水不显著,高级变质岩中的一些特征矿物如矽线石、石榴子石、堇青石、尖晶石的形成也与这种分异作用有关,但组分迁移范围有限,并可保存组分分异各阶段的产物。拉斯曼丘陵长英质岩系中大量铁钛氧化物和矽线石类矿物组合的形成,反映了临界状态下的局部或差异抬升,变形作用的非均匀性及相伴随的组分分异作用,很可能相当于早期格林维尔期构造的泛非期再活动。

Fe-Ti oxide minerals can be locally aggregated or enriched in the quartzofeldspathic gneisses from the Larsemann Hills,East Antarctica.The enrichment is related to anatexis and subsequent high-grade metamorphism,not inherited from the protolith.The partial melting process was responsible for the residues or local enrichment of the inert Fe-Ti-Al elements and the migration of mobile components,and the volatiles were preferentially incorporated in the melt.In the water-deficient system,the local“melt”in anatexis crystallized to form the leucosomes without melt texture or minimum eutectic composition,suggesting the meta-melt feature,not real melt.The coarse leucosome or pegmatite occurred in anataxis as pre-granitic vein or body and had nothing to do with the late stage differentiation of the granitic magma.Together with the presence of the meta-melt,the substantial differentiation of the components resulted in solid residues and corresponding meta-melt phases,with the former enriched in Al,Fe elements and sillimanite,Fe-Ti oxide minerals occurred.On the contrary,the leucosomes with possible short distance migration of components were poor in Fe,Ti compositions,and seldom formed garnet and Fe-Ti oxides.The volatile-unsaturated anatexis was an essentially closed system and the total dehydration effect was not obvious.The presence of some typical minerals in high-grade metamorphism of the quartzofeldspathic gneisses,such as sillimanite,garnet,cordierite and spinel,was derived from component differentiation in partial melting of the rocks.However,the components migration was limited,and the evolving stages of metamorphism and differentiation could be preserved.Voluminous Fe-Ti oxides and sillimanite in the quartzofeldspathic gneisses suggest the possible local or differential uplifting of the area,the heterogeneity of deformation and accompanying components differentiation,corresponding to the reactivation of the earlier structures in the Pan-African event.