Investigation of tourmaline, apatite and monazite from a low-pressure high-temperature regional aureole, Mount Stafford, Northen Territory, Australia

The interbedded metapelites and metapsammites of the Mount Stafford Beds preserve a suite of metamorphic isograds and progressive evolution of migmatite morphology as a result of a steep lateral geothermal gradient of 75°C / km across a 10 km wide regional aureole. Migmatite banding at Mount Stafford is controlled by the interbedded turbidite protolith, with differentiation due to leucosome/melanosome segregation. The melting history at Mount Stafford has been found to be controlled by a series of biotite breakdown reactions, at different P/T conditions and contrasting whole rock. Melt mobilisation is limited across wide areas, which is confirmed by the constant bulk rock composition of metapelites and metapsammites across the area. Textural observation from the in-situ relationships between apatite and micro-monazite ≈10um at low (greenschist) grades indicates the formation of metamorphic monazite at temperatures 25°C less than previously published for the Mount Stafford area, although this is in within temperature range of the observed new growth of monazite from a comparative the Cooma Complex. Within metapelites, monazite size and abundance increases with increasing metamorphic grade whilst an inverse decrease is observed with apatite mode and size. In metapsammites, apatite/monazite relationships are stable with monazite located as single grains in the matrix, and on rims of larger <250μm apatite and ilmenite grains, up to upper amphibolite grade. Xenotime is observed in samples from lower amphibolite to upper amphibolite, however mode decreases in the higher grades. Tourmaline formation in the lower (greenschist) grades at Mount Stafford has been found to be a result of closed system behaviour of fluids due to the breakdown of B-host minerals (clay minerals,mica) and formation of metamorphic tourmaline rims on detrital cores. The formation of up to 3mm unzoned euhedral tourmaline grains in metapelite is considered to be related to the rapid consumption of muscovite. Mode and form of tourmaline reduces up-grade, as form changes to subhedral grains along mineral boundaries. At highest grade, tourmaline is absent from metapsammite, and evident as small grains within cordierite in metapelite which indicates B loss via fluid movement at higher metamorphic grades.