Causal relationship between mafic magma underplating and migmatization of arc crust: Evidence from the Madras block of Southern Granulite terrane, India

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 ɛNd 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 ɛNd 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. 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 ɛNd (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. 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 ɛNd (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.