Geochemical evidence from Lower Permian volcanic rocks of northeast New South Wales for asthenospheric upwelling following slab breakoff

Abstract

The ca 280 Ma Alum Mountain Volcanics and Werrie Basalt were erupted in northeast New South Wales, Australia, during Early Permian lithospheric extension that separated discrete episodes of subduction of Carboniferous and Late Permian — Triassic age. The Alum Mountain rocks, which are preserved in two major synclines in the southeast Tamworth Belt, are mostly basalt, but andesite and rhyolite are also present. The Werrie Basalt found further north in the Tamworth Belt and in the floor of the Gunnedah Basin is composed mainly of basalt, but includes more evolved rocks in the vicinity of several eruptive centres. The Alum Mountain rocks have REE abundances similar to N‐MORB, with flat REE patterns, (La/Sm)N ratios ranging from 0.54 to 1.07, and (La/Yb)N ratios from 0.94 to 2.78, suggesting an origin by large degrees of partial melting of asthenosphere at a depth <75km. The end values range from +5.61 to +8.73. The latter value corresponds to that of the depleted mantle at 0.2 Ga. Werrie Basalt samples have positive ϵNd values, ranging from +2.05 to +6.00, suggesting an asthenospheric origin for these rocks. Spider diagrams show a clear negative Nb anomaly, typical of subduction zones, but LREE/HREE [(La/Sm)N = 1.61 to 2.20; (La/Yb)N = 5.07 to 8.81], Ti/Zr, and Ti/P ratios are close to OIB values. The enriched character of the Werrie Basalt has resulted from either asthenospheric melts being progressively modified during ascent of fractionating magmas through the lithosphere, or by partial melting of a mantle metasomatised by subduction. The presence of a significant depleted‐mantle component in the signature of the Lower Permian volcanic rocks indicates rise of the local mantle geotherm to allow extensive melting. We therefore propose a model of asthenospheric upwelling and lateral flow following breakoff of the Carboniferous subducting slab. Our model of asthenospheric convection as derived from eastern Australia suggests a major role for the asthenosphere in subduction zones: not only is the asthenosphere the reservoir from which magmatic arc melts originate, but we surmise that the behaviour of asthenospheric mantle at subduction zones may have far‐reaching implications for the overall thermal state of the planet.