Subgrain Boundaries as Sites of Melt Generation and Pathways for Melt Flow: Implications for Rheology From Combined Chemical and Quantitative Orientation Analysis

Abstract

AbstractWe report a detailed petrographic, compositional, structural, and crystallographic investigation into the partial melting process. Migmatitic samples that record biotite‐dehydration melting in the Wet Mountains of central Colorado preserve textural evidence for partial melting locations and an incipient former melt network. Detailed analysis of partial melting locations was conducted using a combined energy dispersive X‐ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) approach. Cuspate‐shaped films of former melt, now plagioclase, with low apparent dihedral angles and adjacent to rounded and corroded reactant minerals are interpreted as former locations of partial melting. We identified former melt films infiltrating and melting along quartz grains using optical microscopy and EDS and determined crystallographic misorientation across these transects using EBSD. Forty‐two percent of these transects are associated with subgrain boundaries, and the remaining transects are between adjacent quartz grains with various misorientations. Although previous work has emphasized grain boundaries and triple junctions as sites for melting, the presence of over 40% of the former melt films developed along subgrain boundaries suggests that these are favorable and potentially preferred locations for partial melting. Consequently, melting along subgrain boundaries must be integral to the evolution of partial melt networks and will increase melt network interconnectedness and generate a finer grain size, enhancing permeability and leading to focused melt flow. Given that these factors exert a fundamental control on crustal rheology, the role of melting and migration along subgrain boundaries should be investigated within diverse crustal and mantle lithologies and incorporated into models of partial melting.