PARTIAL MELTING DURING MID- TO LOWER-CRUSTAL DEFORMATION: “IN SITU” MELTS, MELT CHANNEL-WAYS, AND TEMPORAL EVOLUTION OF MELT CHEMISTRY

Complex ductile deformation in granulite-facies migmatitic orthogneisses of the Grenville-aged Albany-Fraser Orogen in the locality of Bremer Bay, Western Australia, was associated with widespread partial melting during most of the deformation history and caused rheological weakening. Field structures suggest that most melts crystallized “in situ” (at or very near the location where they were generated) or traveled short distances along different pathways. However, evidence also indicates that transport of melt from and through these rocks increased as deformation progressed. The earliest structures include: (1) isoclinally folded cm-wide felsic leucosome bands; and (2) the dominant migmatitic foliation defined by a segregation of leucosome and restite bands, due to “in-situ” generation, mm- to cm-scale migration, and subsequent crystallization. Intermediate melt structures include: (3) leucosomes that disrupt the migmatitic layering, locally accommodate non-coaxial shear, and are associated with second generation cm- to m-scale folds; and (4) granitic to intermediate melts crystallized in the necks of three phases of cm-scale to decameter-scale bidirectional boudins, which vary from (a) irregular bodies with diffuse to discrete boundaries with the surrounding migmatitic orthogneiss to (b) regular, discrete bodies that form “pull-apart” veins that clearly disrupt the migmatitic foliation. The latest structures include: (5) irregular areas with diatexitic textures of diverse nature, where partially melted and unmelted portions of the rock are not segregated; (6) melt channel-ways which consist of cm- to decameter-scale diatexitic bands clearly disrupting the migmatitic fabric and early folds; and (7) late, intermediate or felsic, discrete, undeformed, few cm- to few dm-wide pegmatite bands that cut across all ductile structures in the field area. Preliminary geochemistry indicates that leucosome chemistry also evolved throughout the deformation history.