Volume 97, Issue 1 pp. 68-89
Original Article

Discovery of Early Tonian Calc-alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub-continental Lithospheric Mantle

Bapi GOSWAMI

Corresponding Author

Bapi GOSWAMI

University of Calcutta, 35 Ballygunge Circular Road, Kolkata, 700019 India

Corresponding author. E-mail: [email protected]Search for more papers by this author
Susmita DAS

Susmita DAS

University of Calcutta, 35 Ballygunge Circular Road, Kolkata, 700019 India

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Ankita BASAK

Ankita BASAK

University of Calcutta, 35 Ballygunge Circular Road, Kolkata, 700019 India

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Chitta BHATTACHARYYA

Chitta BHATTACHARYYA

University of Calcutta, 35 Ballygunge Circular Road, Kolkata, 700019 India

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First published: 07 October 2022

About the first and corresponding author:

Bapi GOSWAMI, male; born in 1969 in Kolkata, West Bengal Province; associate professor of Geology, University of Calcutta; he is currently interested in the study of the crustal evolution of the proterozoic Chhotanagpur Gneissic Complex of the Eastern Indian Shield. Email: [email protected].

Abstract

Reports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex (CGC) show calc-alkaline and shoshonitic characteristics. Relict porphyritic, sub-ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group III potassic and ultrapotassic rocks of Foley et al. (1987). The magma was derived from the metasomatized subcontinental lithospheric mantle (SCLM). Subduction-related sediment melts metasomatized the SCLM. Compositionally, the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite. 1%–5% batch melting of the SCLM could produce the parental magma of the mafic granulites. Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling. Peak metamorphic assemblage (M1: Grt-Cpx-Pl-Qz) in garnetiferous amphibolite equilibrated at 740°C and 8.7 kbar. The Cpx-Pl corona appeared around the garnet during decompression (M2: 655°C, 6 kbar). The Hbl-Pl symplectites around garnet formed during isobaric cooling (M3: 580°C and 5.9 kbar). The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision (∼990 Ma).

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