Geochemical constraints on Cenozoic intraplate magmatism and their relation to Jurassic dolerites in Tasmania, using Sr-Nd-Pb isotopes

Abstract

Tasmania hosts two prominent Meso-Cenozoic mafic magmatic provinces: abundant Jurassic dolerites that belong to the Ferrar Magmatic Province that formed during Gondwana break-up; and Cenozoic, mainly basaltic volcanics, which represent intraplate magmatism, post-Gondwana break-up. The origin of the Cenozoic volcanics is still controversial, in that the detailed role of competing asthenospheric and lithospheric sources remains unclear. This study presents new geochemical and isotopic data which is used to identify and contrast the magmatic sources and their relationship to Jurassic and Cenozoic magmas. The compositions of the Cenozoic lavas span a wide range from highly silica-undersaturated olivine melilitites and nephelinites to basanites, ne-hawaiites, olivine basalts and quartz-tholeiites. New Sr-Nd-Pb isotopic data combined with major and trace element geochemistry show that the most silica-undersaturated rocks are most enriched in incompatible elements and also have the least radiogenic Sr and most radiogenic Nd. Sr-Nd-Pb isotope ratios for the most silica-rich lavas trend towards values for the Ferrar Jurassic dolerite source. More generally, Sr, Nd and Pb isotopes in the Cenozoic lavas can be modelled by a three component mixture of Pacific Mid-Ocean Ridge Basalts (MORB), HIMU-like (characterized by high time-integrated μ (²³⁸U/²⁰⁴Pb)) and Jurassic dolerite sources. Pb isotope values confirm the HIMU-like component in the array. Various degrees of melting during decompression melting has also contributed to the wide range of compositions observed, with most silica-undersaturated rocks originating at the highest pressures (>20 kbar) and by the smallest percentages of melting. We suggest that this process, combined with variable interaction of MORB and HIMU-like asthenospheric sources with lithospheric mantle containing remnant Jurassic signatures, resulted in the wide range of Cenozoic lava compositions observed today.