GRAIN-SCALE MELT PSEUDOMORPH MICROSTRUCTURES FROM THE PYROMETAMORPHIC AUREOLE OF THE MIOCENE MOUNT PERKINS PLUTON: A POTENTIAL LINK TO OPEN SYSTEM PROCESSES DURING PLUTON CRYSTALLIZATION

The west-tilted 16 Ma Mt. Perkins pluton, Arizona, is a small (~6 km²) composite sill-like intrusive body emplaced into Proterozoic gneiss at a paleodepth of 7 km. Lithologies record a broad range of magma compositions (gabbro to granite); previous work demonstrated that the pluton was part of a magma conduit system. Here we report on recently discovered grain-scale melt pseudomorph microstructures from the pyrometamorphic aureole underlying an early gabbro phase (~1 km²) of the pluton. The described melt structures are found in foliated Proterozoic-age hornblende-, biotite-bearing quartzofeldspathic gneiss within 2 meters of the pluton margin. Former melt pseudomorphs are filled with quartz and feldspar and represent crystallized in situ partial melt. Margins of the melt pseudomorphs are outlined by reactant (residual) grains that are dominantly quartz. Thin feldspar melt pseudomorphs separating reactant grains form melt-solid-solid junctions with low dihedral angles. In larger melt pseudomorph pockets (<millimeter) isolated reactant grains are rounded and embayed. Melt crystallization microstructures include a “string of beads” texture formed of subhedral equant quartz + feldspar aggregates separating reactant grains, and in larger melt pockets numerous euhedral, twinned igneous-like plagioclase crystals, some with optically-visible internal euhedral zoning. The lack of melt pseudomorphs related to biotite indicates thermal conditions below dehydration-melting reactions and suggests H₂O-saturated melting. Melt pseudomorph structures are elongate in the plane of the inherited foliation. The adjacent gabbro preserves a meters-thick layered oliv-cpx-plag and hornblende-plag cumulate sequence. Systematic variation in ²⁰⁶Pb/²⁰⁴Pb (18.7 → 18.0) and Ɛ_Nd (-2 → -6) in the cumulates point to contamination by a crustal source. We hypothesis that partial melt from the subjacent gneiss was responsible for in situ contamination of the crystallizing mafic magma.