The Role of Subcontinental Lithospheric Mantle in the Generation of Continental Basalts: Geochemical Evidence from Lava-Field Provinces in New South Wales

Ming Zhang and Suzanne Y. O'Reilly

Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC)

School of Earth Sciences, Macquarie University, New South Wales 2109

Basaltic rocks from the lava-field provinces in New South Wales during the Cenozoic times are widespread and their Sr-Nd-Pb isotope ratios generally fall in the compositional space defined by oceanic island basalts (OIBs). This indicates a predominant role of sublithospheric mantle sources in the magma genesis and causes difficulties in recognising the role of subcontinental lithospheric mantle (SCLM) as one of the potential mantle source components for these basalts. However, detailed geochemical studies based on a recently established comprehensive dataset, in combination with analysis of tectonic histories, reveals that the lower SCLM (dominantly in the garnet peridotite stability field) has played a significant role in the genesis of lava-field basalts in New South Wales. The nature of the SCLM inferred from these basalts is consistent with the tectonic evolution in the Lachlan Foldbelt and the New England Foldbelt and corresponds to the geochemical characteristics of the entrained mantle xenoliths.

In the Dubbo area, the coexisting tholeiites and alkaline basalts erupted contemporaneous with the nearby plume-derived central-volcano provinces. The olivine tholeiites show an OIB-type incompatible element pattern and Sr-Nd isotope ratios similar to those of the inferred plume source for primitive basalts of the central-volcano provinces. Therefore, the enrichment in Sr-Nd isotope ratios, the positive correlation between 143Nd/144Nd and Sm/Nd, and the strong depletions of K and Rb exhibited by the alkaline basalts indicate that an old metasomatised SCLM source must have made substantial contributions in the magmas. The lack of correlations between 87Sr/86Sr and Rb/Sr in basaltic rocks from Dubbo basalts may be attributed to either partial melting events shortly before the Dubbo volcanism which change the Rb/Sr ratios in the source or the retention of Rb and K in the source during partial melting by phlogopite. A minor proportion of alkaline basalts from the Southern Highlands province to the south with relatively enriched Sr-Nd isotope compositions similar to the Dubbo basanites may also have been generated in the same way.

The primitive alkaline basalts from the Kandos province were erupted in two episodes separated by about 150 Ma: Jurassic (190 - 170 Ma) and Tertiary (50 - 45 Ma). Basalts from both volcanic episodes have similar OIB-like Sr-Nd isotope compositions (87Sr/86Sr=0.7032 - 0.7036 and eNd=+5.0 - +3.5) and incompatible element patterns (except for the prominent depletions in Zr and Hf). However, the similarity in both elemental and isotopic chemistry of the Kandos basalts over a time span of about 140 Ma implies that geochemical signatures of these basalts should be derived from a SCLM source (even through the resultant basalts are geochemically indistinguishable from those plume-derived basalts) because horizontal plate movement would have carried this lithosphere away far from the plume source region.

Decomposition of volatile-bearing minerals such as amphibole, apatite, and phlogopite in the metasomatised uppermost lithospheric mantle and their interaction with ascending basaltic magma are also an important factor in modifying the composition of magmas from sublithospheric sources. In Barrington, the primitive nephelinites and basanites have Sr-Nd isotope ratios similar to the other New South Wales lava-field basalts. However, they have distinctive incompatible element patterns that display strong depletions in Rb, K, Zr, and Hf and enrichments in Th, Nb and Ta. These patterns can be modelled by interaction between a parental magma with OIB-like trace element characteristics and an amphibole- and apatite-bearing peridotitic mantle. Both amphibole and apatite broke down at the early stage of the process, modifying incompatible element abundances and patterns, but without changing significantly their Sr-Nd isotope ratios as the late Paleozoic tectonism in the New England Fold Belt may not been able to create time-integrated changes in Sr-Nd isotope systematics within the upper SCLM.