THE IMPORTANCE OF DEFORMATION IN SEGREGATION OF MELT FROM QUARTZO-FELDSPATHIC ROCKS

Petrographic studies of partially melted rocks in contact aureoles provide a means of observing essentially unmodified melt distributions. Given the almost complete absence of pervasive deformation in shallow aureoles and the possibility of constraining the timescale of melting, contact aureoles in the shallow crust also provide an opportunity to establish melt segregation rates during static anatexis. Several contact aureoles in the British Tertiary Volcanic Province were examined, with particular emphasis on quartzo-feldspathic lithologies. (i) Traigh Bhan na Sgurra, Isle of Mull – gabbroic magma flowing in a major feeder sill at 600 bars formed a 3 m wide aureole in garnet-grade psammites. Melting in the Qtz-Ab-Or±H2O system occurred together with metastable melting of muscovite. The positive volume change of reaction caused the formation of a completely interconnected array of melt-filled cracks. Despite the high permeability little or no segregation occurred during the 5-month event. (ii) Rum Igneous Centre – the intrusion of a 7 km radius body of mafic and ultramafic melt resulted in the melting of arkosic sediments and felsic gneisses. Melting also occurred around 50 m diameter gabbro feeder conduits. Apart from intrusion breccias developed along the margins of the pluton, evidence of migration of the melt phase (which reached up to 25 vol %) is limited to sparse veinlets. Melting around the gabbro conduits reached up to 95 vol % without disruption of mm-scale sedimentary layering. Extensive migmatitic disruption only occurs in the region of the intermittently active Main Ring Fault. Melt distribution is dominated by reaction, with little sign of movement from reaction sites. Similarly little evidence is seen for grain edge melt channels predicted from textural equilibrium constraints. Textural equilibrium is probably only important at the longer timescales involved in regional melting. The lack of melt migration, despite sufficiently high permeabilities, is attributed to the absence of pervasive deformation.