Radiohalos in the Cooma Metamorphic Complex, New South Wales, Australia: The Mode and Rate of Regional Metamorphism

The Cooma metamorphic complex in southeastern Australia is a classical example of regional metamorphic zones centered on a granodiorite generated by partial melting at the highest metamorphic grade. Samples collected along a traverse from the low-grade biotite and then andalusite zone schists through the high-grade K-feldspar and migmatite zone gneisses into the central granodiorite contain increasing numbers of Po radiohalos with increasing metamorphic grade. The highest Po radiohalo numbers are in the high-grade zones and the granodiorite. These radiohalo patterns correlate with the Po radiohalos being generated by the hydrothermal fluids flowing out of the central granodiorite as it crystallized and cooled, their numbers diminishing as the hydrothermal fluid flow decreased outwards. This is further evidence consistent with the hydrothermal transport model for Po radiohalo formation. Furthermore, generation of the regional metamorphic complex only required 12–20 days, based on the catastrophic granite formation of the adjacent Murrumbidgee Batholith whose heat and hydrothermal fluids generated the regional metamorphic zones of the complex from the mineral constituents of the original fossiliferous sediment layers, then the central granodiorite as a consequence. This sequence of outcomes is consistent with creationist models for catastrophic granite formation and regional metamorphism driven by catastrophic plate tectonics during the year-long biblical Flood