Synplutonic mafic injections into crystallizing granite pluton from Gurgunta area,northern part of Eastern Dharwar Craton: Implications for magma chamber processes |
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Authors: | B C Prabhakar M Jayananda Mohamed Shareef T Kano |
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Institution: | 1.Department of Geology,Bangalore University,Bangalore,India;2.Department of Earth Sciences,Yamaguchi University,Yamaguchi,Japan |
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Abstract: | In this paper we document widespread coeval felsic-mafic magma interaction and progressive hybridization near Gurgunta in
the northern part of Eastern Dharwar Craton (EDC) where mafic magma pulses have injected into a 2.5 Ga granite pluton. The
pluton contains voluminous pink porphyritic facies with minor equigranular grey facies. The mafic body shows compositional
variation from diorite to meladiorite with hornblende as the chief mafic mineral with lesser clinopyroxene and biotite. The
observed variation on binary diagrams suggests that granite was evolved by fractional crystallization. Chemical characteristics
such as higher Al2O3 and moderate to high CaO, Mg#, Ni, Cr, Co and V are interpreted by slab-melting. Mafic bodies show lower SiO2, Na2O and K2O; but higher CaO, Mg#, FeO, Cr, Ni and V; higher LREE with moderate to higher HREE which suggest their derivation from mantle.
A major active shear zone has played an important role at the time of synplutonic mafic injection and hybridization process.
Field evidences suggest that the synplutonic mafic body has injected into the crystallizing felsic magma chamber in successive
stages. The first stage injection has resulted in extensive mixing and hybridization due to the liquidus state of resident
felsic magma to which hot mafic magma was injected. However, progressive mixing produced heterogeneity as the xenocrysts started
mechanically dispersed into hybrid magma. The second stage injection, after a time gap, encountered colder and viscous hybrid
magma in the magma chamber, which inhibited free injection. As a consequence, the mafic magma spread into magma chamber as
flows, producing massive mafic bodies. However, with the continued mafic pulses and the heat gradient, the viscosity contrasts
of mafic magma and felsic magma were again lowered resulting in second stage mixing. This episode was followed by mingling
when the granite was almost crystallized, but still viscous enough to accommodate lamellar and ribbon like mafic penetrations
to produce mingling. The successive mixing and mingling processes account for the observed heterogeneity in the granite pluton. |
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