The lower crust in eastern Australia: xenolith evidence

Hundreds of Mesozoic to Recent xenolith-bearing basalt flows, diatremes, cinder cones and maars have erupted through the low-grade Palaeozoic metamorphic rocks of the Tasman Fold Belt, which makes up the eastern third of the Australian continent. Granulite-facies xenoliths, interpreted as lower-crustal material, have been found at more than 40 of these localities. A palaeogeotherm, based on garnet pyroxenite xenoliths from Victoria, also fits available data from New South Wales and Queensland. A crustal thickness of 25 - 30 km, and lower-crustal temperatures of 700-850°C, have been derived by referring T estimates for spinel lherzolite xenoliths to this geotherm. The lower-crustal xenolith suites are dominated by basic pyroxene granulites and garnet granulites; more silicic xenoliths are very rarely reported. The granulites cover a range in composition typical of intraplate basaltic magmas and cumulates. Granoblastic microstructures are typical, but common relict igneous features suggest that all the mafic granulites originated as igneous rocks. The lower crust in this area probably formed through multiple intrusion of basic magmas near the crust/mantle interface over a long time span. The strongly layered 'lower crust' seen on seismic reflection studies in eastern Australia probably represents this thick crust-mantle transition zone. The seismic 'Moho' reflects the depth at which the proportion of basic to ultrabasic rocks drops below a critical value, and probably lies well below the petrographically defined base of the crust.