Abstract

The Ghansura Felsic Dome of the Proterozoic Chotanagpur Granite Gneiss Complex in eastern India preserves three distinct varieties of hybrid rocks: (1) porphyritic hybrid rock, (2) non-porphyritic hybrid rock displaying emulsion texture, and (3) comingled mafic-felsic rock displaying viscous folding. These three hybrid rocks formed due to varying degrees of interaction of mafic magma with the host felsic magma. Geochemical signatures including linear arrays in binary diagrams and major-oxide mixing tests confirmed the hybrid nature of the rocks. Field and textural observations indicate that the felsic system was probably a subvolcanic zoned magma chamber containing individual magmatic layers of varying viscosities at the time of intrusion of the mafic magma.

The non-porphyritic hybrid rock displaying emulsion texture gives us a rare opportunity to understand the complex mechanisms facilitating magma mixing, as the exact processes by which mixing occurs remain unclear. Furthermore, this work attempts to explain the dynamics of a subvolcanic magma chamber involving the mixing of two disparate magmas. The emulsions are more or less spherical in shape and are dominantly composed of amphibole with biotite rims, and are set in a matrix of biotite, plagioclase, K-feldspar, and quartz. The amphibole constituting the emulsion rocks is actinolite in composition and commonly shows tschermakite (Ts) substitution. Biotite shows homogenous compositions and plagioclase occurring in the non-porphyritic hybrid rock is labradorite in composition and lacks compositional discrepancies indicating equilibrium growth. Pyroxene is present only in the mafic endmember of our mixing domain and is classified as augite and diopside.

Mineral-chemical data combined with recent developments in microfluidics were used to determine the complex processes governing magma mixing occurring at a micro-level. The work presented here uses selected results from microfluidic experiments with a view to understanding the formation mechanism of emulsions preserved in the hybrid rocks of the Ghansura Felsic Dome of the Chotanagpur Granite Gneiss Complex. From our findings, we infer that when mafic magma, containing clinopyroxene phenocrysts, came in contact with felsic magma, diffusion of volatiles and H+ ions occurred from the felsic system to the mafic system. These cations reacted with the clinopyroxene phenocrysts in the mafic magma to form amphibole (actinolite) crystals. The formation of amphibole crystals in the mafic system greatly increased the viscosity of the system, allowing the amphibole crystals to move into the adjacent felsic magma as veins. As these veins traversed the felsic medium, they were acted upon by compressive stress and underwent viscous swirling to produce rounded to sub-rounded emulsion features.

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