Abstracts Earth and Environmental Sciences

Migmatite delineates zones of melt flux through the upper crust, Wongwibinda, NSW

by Michael James Farmer

The small (<20km wide) high-temperature - low-pressure (HTLP) Wongwibinda MetamorphicComplex (WMC), southern New England Orogen, is characterised by an irregular elevatedmetamorphic field gradient (<100°C/km). An examination of the migmatite rocks shows they didnot experienced significant in-situ partial melting, suggesting the abundant leucocratic veins anddykes observed in outcrop are externally derived. Using a combined field and petrographic study,augmented by targeted electron backscatter diffraction (EBSD) and whole-rock geochemicalanalyses, two types of melt pathways through the migmatite have been identified: (i) veining anddyking, and (ii) grain-scale channelled porous flow. The pathways of porous flow are recognisedacross the field area as cm- to outcrop-scale channels, characterised by rare-earth element (REE)metasomatism, mineral mode changes, a randomisation of host crystallographic preferredorientation fabrics and the presence of cuspate K-feldspar pockets. The identification of collapsestructures, due to volume loss as melt-filled dykes and voids drained, demonstrates that some of theinjected melt escaped to structurally higher levels. The two types of melt pathways and theircharacteristics supports a melt flux hypothesis, which is interpreted to drive migmatisation and promote the formation of the elevated thermal field gradient.