CHARACTERIZING THE EARLIEST STAGES OF PARTIAL MELTING: A STUDY OF THE CONTACT AUREOLE OF THE MIOCENE MT. PERKINS PLUTON, NW ARIZONA

Partial melting is an important process in the evolution of continental crust; however, due to its progressive nature evidence of the early stages of partial melting is often overprinted by later, advanced stages of partial melting and/or deformation. Pyrometamorphic environments within the contact aureole of mafic intrusions provide natural laboratories for the study of the earliest stages of partial melt. Here we report on grain-scale melt pseudomorph microstructures from the pyrometamorphic aureole underlying an early gabbro phase of the Miocene Mount Perkins pluton. Pseudomelt structures are found in foliated Proterozoic-age hornblende-, biotite-bearing tonalitic gneiss within 2 meters of the pluton margin. Pseudomelt regions are filled with quartz + plagioclase + K-feldspar and represent crystallized in situ partial melt. Margins of the melt pseudomorphs are outlined by reactant (residual) grains that are dominantly quartz and plagioclase. Melt crystallization microstructures include a “string of beads” texture and thin films separating reactant grains forming melt-solid-solid junctions with low dihedral angles. Larger melt pseudomorph pockets (< 1 millimeter) contain plagioclase grains surrounded by either quartz or K-feldspar. Plagioclase textures in melt pockets are varied; euhedral, twinned igneous-like plagioclase crystals, some with optically-visible internal euhedral zoning, are interpreted as melt crystallization products. Rounded and embayed plagioclase grains in melt pockets are interpreted as residual reactant grains. Preliminary EMPA analyses show that the composition of residual plagioclase and igneous plagioclase in pseudomelt pockets are nearly identical (An38-49), suggesting kinetic barriers to plagioclase equilibrium melting behavior as observed experimentally. K-feldspar has only been observed in melt pockets and there is no evidence of dehydration melt reactions involving biotite, suggesting low temperature melting limited by K-feldspar and water availability. Continued work will investigate melt volumes and composition as a function of distance from the pluton.