Decoding the nature of interaction between felsic clasts and mafic magma in a subvolcanic magma chamber from amphibole–titanite transformation and chemistry

The Ghansura Rhyolite Dome of Bathani volcano-sedimentary sequence, eastern India, represents a subvolcanic felsic magma chamber that was invaded by crystal-rich mafic magma during its cooling phase to form an assortment of hybrid rocks. A prominent solidified portion of the magma reservoir was embedded in the intruding mafic magma as fragments or clasts that produced mafic rocks with felsic clasts. Two distinct compositional zones could be identified in the mafic rocks containing felsic clasts- (a) medium-grained mafic zones that are dominated by amphiboles, and (b) fine-grained felsic zones consisting primarily of quartz and feldspar. Amphiboles occur in most of the felsic clasts suggesting the mechanical transfer of crystals from the mafic to the felsic zones. Compositions of amphiboles were determined from both the mafic and felsic zones that show linear compositional variation from actinolite to ferro-hornblende through magnesio-hornblende, suggesting the interplay of complex substitutions in individual amphibole sites. Cationic schemes have confirmed the role of pargasite (Pg)-type substitution, which is a combination of edenite (Ed)- and tschermakite (Ts)-type substitutions. Moreover, amphibole has been extensively replaced by titanite in the studied rock. Titanite produced in the mafic zones due to the destabilization of amphiboles was observed migrating from the mafic to the felsic zones through mineral-transporting veins. Compositions of titanite were determined from grains that occur in association with amphiboles and those which are present as individual entities in the felsic zones. Similar to amphiboles, titanite also displays cationic substitutions in the studied rock. From the results presented in this work, we infer that extensive replacement of amphibole by titanite and cationic substitutions in amphiboles, and also titanite, may be considered important petrogenetic indicators to decipher magma mixing events.