GEOCHEMICAL CHARACTERISTICS OF LAVA-FIELD BASALTS FROM EASTERN AUSTRALIA AND INFERRED SOURCES - CONNECTIONS WITH THE SUBCONTINENTAL LITHOSPHERIC MANTLE

A Large new database of major, trace elements and Sr-Nd isotopic ratios from 11 Lava-field provinces in New South Wales and Queensland, eastern Australia allows detailed interpretation of the origin of these basaltic magmas. Isotopic signatures and trace element patterns identify an OIB-type (oceanic island basalt) source as a dominant component for most of these and some provinces appear to have additional significant components derived from the subcontinental lithospheric mantle (SCLM). The SCLM components have geochemical characteristics that overlap those observed in spinel Iherzolite xenoliths (samples of shallow lithospheric mantle) from eastern Australia. These SCLM components show geochemical provinciality that indicates the occurrence of distinct geochemical lithospheric domains reflecting the timing and style of tectonic evolution of different regions. One component reflects modification by subduction-related processes during the late Paleozoic and Mesozoic, one records enrichment by fluids during old metasomatic events and another suggests a metasomatic event involving a distinctive amphibole and apatite-style enrichment. The composition and age distribution of volcanic lava-field provinces older than 10 Ma are consistent with a model involving a regional upwelling (elongated N-S along eastern Australia) of deep hot mantle related to marginal rifting and with OIB-type source geochemical characteristics. Thermal inhomogeneities within this plume swath resulted in small diapirs which may have undergone melt segregation at about 100 km and incorporated varying amounts of SCLM components there or from higher levels of the SCLM during ascent. Subsequent hot-spot generated central volcanoes overprinted this lava-field volcanism, tapped a similar OIB-type source component and truncated the thermal events.