Grain-scale plastic deformation of chromite from podiform chromitite of the Naga-Manipur ophiolite belt, India: Implication to mantle dynamics

摘要

The mantle peridotites of the Naga-Manipur ophiolite belt, India host concordant podiform chromitite bodies that preserve various magmatic and deformation structures. Magmatic structures preserve evidence of a gradual transition from nodular to massive varieties. Nodular types are least affected by deformation. Deformation structures, restricted mainly in massive types include intracrystalline features of brittle as well as crystal plastic processes within chromite. Brittle deformation is preserved as grain fracturing and intense microbrecciation along narrow zones forming protocataclasite. Crystal plastic deformation has generated chromite subgrains and new grains in response to recovery and recrystallization processes. Subgrains are polygonal in shape and the boundaries between them are commonly straight forming a mosaic with most meeting at triple junctions. Subgrains are commonly internally zoned and the mosaics formed by them correlate well with the compositional maps. Subgrains show high Al at their cores and high Fe at their boundaries in response to differential, diffusive chemical re-equilibration. We discuss how this compositional resetting related to deformation is significant in various geologic studies related to tectonic setting and thermobarometric evolution of an area. Deformation mechanisms are likely to have started with crystal plastic processes and later switched over to the brittle regime. The preservation of all grain-scale deformation structures in the chromitite bodies of the Naga-Manipur ophiolite belt is suggestive of mantle dynamics, vertical accretion and horizontal movement in particular.