Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Jagatbikas Nanda ¹, Saibal Gupta ¹, Manish A. Mamtani ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302, IN

Abstract

The Koraput Alkaline Complex (KAC) lies on the NE-SW trending Sileru Shear Zone (SSZ) separating the Proterozoic Eastern Ghats Province from the Archaean Indian craton. The core of the KAC is made of hornblende gabbro, which is rimmed by a band of nepheline syenite in the east and syenodiorite in the west. The timing of magmatism with respect to the SSZ is disputed. The KAC was deformed during emplacement, and a magmatic foliation related to the syn-emplacement deformation, D₁, is present in the gabbroic core. The dominant D₂-related field fabric strikes NE-SW and is penetrative in parts of the gabbro and marginal lithologies. E-W trending D₃ shear zones cut across the complex. Distinct textural domains resulted from strain partitioning during deformation. Parts of the complex with magmatic textures constitute Domain-1, while D₂ and D₃ fabric zones comprise Domains-2 and 3 respectively. Temperatures in the KAC initially decreased following D₁, but increased through D₂ and D₃. Anisotropy of magnetic susceptibility (AMS) studies show that the magnetic fabric generally follows S₁ in Domain-1. While the magnetic fabric in Domain-2 is dominantly parallel to S₂, some of it parallels S₁. The latter is a relict D₁ fabric that is recognized from AMS analysis but is obliterated in the field, which confirms that the KAC pre-dates the SSZ. The response of magnetic fabrics to temperature and implications of the study for Indo-Antarctica amalgamation are discussed.

Keywords

Anisotropy of Magnetic Susceptibility, Alkaline Complex, Fabric, Eastern Ghats Belt, Indo-Antarctica Amalgamation.

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Saibal Gupta ¹, Aditi Das ¹, Sudipta Goswami ², Ananda Modak ¹, Suman Mondal ¹

Affiliations
1 Department of Geology and Geophysics, I.I.T., Kharagpur, Kharagpur - 721 302, IN
2 School of Environmental Sciences, University of East Anglia Norwich, NR4 7TJ, GB

Abstract

Inverted metamorphism in the Himalayas is closely associated with the Main Central Thrust (MCT). In the western Himalayas, the Main Central Thrust conventionally separates high grade metamorphic rocks of the Higher Himalayan Crystalline Sequence (HHCS) from unmetamorphosed rocks of the Inner sedimentary Belt. In the eastern Himalayas, the Inner sedimentary Belt is absent, and the HHCS and meta-sedimentary Lesser Himalayan Sequence (LHS) apparently form a continuous Barrovian metamorphic sequence, leading to confusion about the precise location of the MCT. In this study, it is demonstrated that migmatitic gneisses of the sillimanite zone in the higher structural levels of the HHCS are multiply deformed, with two phases of penetrative fabric formation (S_1HHCS and S_2HHCS) followed by third folding event associated with a spaced, NW-SE trending, north-east dipping foliation (S_3HHCS). The underlying LHS schists (kyanite zone and lower) are also multiply deformed, with the bedding S₀ being isoclinally folded (F_1LHS), and subsequently refolded (F_2LHS and F_3LHS). The contact zone between the HHCS and LHS is characterized by ductile, top-to-the southwest shearing and stabilization of a pervasive foliation that is consistently oriented NW-SE and dips northeast. This foliation is parallel to the S_3HHCS foliation in the HHCS, and the S_2LHS in the LHS. Early lineations in the HHCS and LHS also show different dispersions across the contact shear zone, implying that pre-thrusting orientations of the two units were distinct. The contact shear zone is therefore interpreted to be a plane of structural discordance, shows a shear sense consistent with thrust movement and is associated with mineral growth during Barrovian metamorphism. It may well be considered to represent the MCT in this region.

Keywords

Inverted Metamorphism, Main Central Thrust, Higher Himalayan Crystalline Sequence, North Sikkim.

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