Abstract

Neoarchean migmatized granodioritic gneisses and mafic enclaves from the Madras block of the Southern Granulite Terrain (SGT) were studied to understand their genetic relationship. The gneisses show calc-alkaline trend, more magnesian than tonalites, enrichment of LILE and LREE with HFSE depletion, and zero to slightly negative ɛ_Nd values (t=2600 Ma) which indicate their precursors fractionated from sanukitoid magma generated by partial melting of hybridized mantle sources. Gabbroic magmas representing mafic enclaves with ɛ_Nd values, –1.68 to +0.45, formed by partial melting of̄ fluid metasomatised mantle wedge and hybridized by interaction with granite magma. Underplating of these mafic magmas provided heat to trigger anatexis of the granodioritic arc-crust in the presence of H₂O and formation of granite melts (leucosomes). The leucosomes with peritectic amphiboles have higher REE with prominent negative Eu anomaly, while quartzo-feldspathic leucosomes have lower REE, concave upward HREE and positive Eu anomaly. Fractionation and/or entrainment of amphibole, apatite, allanite, titanite and zircon controlled REE and other trace element abundances of the leucosomes. Thus, underplating of mafic magma caused migmatization, magma mixing and differentiation and transformation of the arc crust in the NE part of the Madras block which represents deeper parts of the eastern Dharwar craton.

Research highlights

• Neoarchean migmatitic gneisses in the Madras block of southern granulite terrain represent granodioritic magmas derived from metasomatised mantle wedge in arc setting.

• Fluid present melting of the granodiorite crust resulted in formation of leucosomes and small granite plutons.

• Source–melt relationship between them is confirmed by overlapping ɛ_Nd (t=2600 Ma) values (–1.42 to +1.25) for gneisses and leucosomes.

• Gabbroic or dioritic mafic microgranular enclaves represent magmas hybridized with the crustal melts.

• Underplating of mafic magmas triggered migmatization of the arc crust and mingling of mafic and granite magmas.

中文翻译：

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镁铁质岩浆底侵作用与弧形地壳混合作用的因果关系——来自印度南部花岗岩地块马德拉斯地块的证据

摘要

研究了来自南部花岗岩地块 (SGT) 的马德拉斯地块的新太古代混合花岗闪长片麻岩和镁铁质飞地，以了解它们的遗传关系。片麻岩显示钙碱性趋势，镁质比方长岩多，LILE 和 LREE 富集，HFSE 耗尽，以及零到略微负的 ɛ _Nd值（t = 2600 Ma），这表明它们的前体从杂化部分熔融产生的 sanukitoid 岩浆中分馏地幔来源。代表镁铁质飞地的辉长岩浆，其 ɛ _Nd值为 –1.68 至 +0.45，由流体交代地幔楔的部分熔融形成，并通过与花岗岩岩浆相互作用而混合。在 H 存在下，这些基性岩浆的底镀提供了热量来触发花岗闪长弧地壳的熔蚀₂ O 和花岗岩熔体（白体）的形成。包晶角闪石的白体具有较高的 REE 和显着的 Eu 负异常，而石英长石白体具有较低的 REE，HREE 上凹且 Eu 正异常。角闪石、磷灰石、allanite、钛石和锆石的分馏和/或夹带控制了白体的 REE 和其他微量元素丰度。因此，基性岩浆的底侵引起了代表东达尔瓦克拉通较深部分的马德拉斯地块东北部弧壳的混合、岩浆混合和分异转化。

研究亮点

• 南部麻粒岩地块的马德拉斯地块中的新太古代混合片麻岩代表了从交代地幔楔在弧形环境中衍生的花岗闪长岩浆。

• 花岗闪长岩地壳的流体存在熔融导致白体和小花岗岩岩体的形成。

• 片麻岩和白质体的ɛ _Nd (t=2600 Ma) 值（–1.42 至 +1.25）重叠，证实了它们之间的来源-熔融关系。

• 辉长岩或闪长岩的镁铁质微粒包体代表与地壳熔体混合的岩浆。

• 基性岩浆的底侵引发了弧壳的混合作用以及基性岩浆和花岗岩岩浆的混合。