Viscous dilation as a mechanism of magma mixing in the Ghansura Rhyolite Dome of Bathani volcano-sedimentary sequence, Eastern India

Abstract

The comingled mafic-felsic rocks of the Ghansura Rhyolite Dome of Bathani volcano-sedimentary sequence, eastern India, have preserved veins of iron oxide. These veins are transporting iron oxide from the mafic zones into the adjacent felsic zones and enriching the latter with iron oxide. An interesting phenomenon displayed by the iron oxide veins is that as they travel from the mafic zones into the felsic zones, the veins undergo viscous dilation. Results presented in this work suggest that when crystal-rich mafic magma came in contact with felsic magma, diffusion of elements occurred from the felsic phase to the mafic phase. The diffusion of elements facilitated conversion of amphibole in the mafic phase into biotite, iron oxide, calcite and quartz. The newly produced iron oxide in the mafic phase then ventured into the adjacent felsic magma as veins. As these veins ventured into the felsic phase, the similarity in viscosities between the two fluids caused the iron oxide veins to undergo viscous dilation. The onset of viscous dilation increased the interfacial area between the two fluids and enhanced mixing by facilitating chemical diffusion between them.