Deformation-driven emplacement-differentiation in the Closepet Pluton, Dharwar Craton, South India: an alternative view

In the Late Archean N-trending Closepet Pluton, trains of euhedral K-feldspar phenocrysts and matrix-supported idiomorphic K-feldspar crystals in the central part of the pluton define oblique-to-pluton margin steep-dipping E/ENE-trending magmatic fabrics. The magmatic fabric is defined by phenocryst-rich and phenocryst-poor layers, with the euhedral porphyries continuous across the layers. The fabrics are near-orthogonal to the gently-dipping gneissic layers in the host gneisses. The fabrics curve adjacent to locally-developed N/NNE trending melt-hosted dislocations parallel to the axial planes of horizontal/gently-plunging N trending upright folds in the host gneisses. In the pluton interior, both fabrics in the intrusives formed at supra-solidus conditions, although the volume fraction of melts diminished drastically due to cooling/melt expulsion. At pluton margin, the N-trending fabric is penetrative and post dates magma solidification. Within the pluton, the major element oxides, ΣREE, anorthite contents in plagioclase, and (Mg/Fe+Mg) ratios in biotite decrease with increasing SiO2 from phenocryst-rich (up to 75 % by volume) granodiorite to phenocryst-poor (<15 volume %) granite that broadly correspond to minimum melt composition. The chemical-mineralogical variations in the pluton is attributed to deformation-driven ascent of magma with heterogeneous crystal content, ascending at variable velocities (highest in crystal-poor magma) along oblique-to-pluton margin E/ENE-trending extensional fractures induced by dextral shearing.