Coeval Felsic and Mafic Magmas in Neoarchean Calc-alkaline Magmatic Arcs, Dharwar Craton, Southern India: Field and Petrographic Evidence from Mafic to Hybrid Magmatic Enclaves and Synplutonic Mafic Dykes

摘要

We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30-40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.