Terminal tectono-magmatic phase of the New England Orogen driven by lithospheric delamination

Abstract

The recognition of lithospheric delamination as a mechanism for magmatism and uplift is under-recognized in the geological record. A pertinent example is the terminal phase of the New England Orogen in eastern Australia, where current explanations of slab roll-back-driven extension are incompatible with plate motions in the Late Triassic. Although abundant mafic rocks are present, almost all Late Triassic temporal information is from felsic rocks. To investigate potential Late Triassic mafic magmatism in the New England Orogen, we date a series of tholeiitic and alkaline mafic products in its back-arc (Sydney Basin) using plagioclase 40Ar/39Ar and apatite U-Pb geochronology. We obtained a plagioclase 40Ar/39Ar plateau age of 202.77 ± 0.68 Ma (2σ) from tholeiitic magmatic products and an apatite U-Pb age of 202 ± 7 Ma (2σ) from a proximal alkaline sill some 70 m deeper, both of which overlap within uncertainty. Complementary trace element geochemistry shows that the tholeiitic and alkaline magmatic products were derived from a similar deep, garnet-bearing source, which we attribute to upwelling asthenosphere underneath a thickened lithosphere. Our data suggest that extension occurred ∼10 m.y. later in the back-arc basin than along its arc, further supporting the notion that slab roll-back could not have caused this terminal phase of Late Triassic extension in the New England Orogen. The Late Triassic magmatism in the New England Orogen is best explained by lithospheric delamination as it accounts for the orogenic architecture, chemical signature of the ca. 200 Ma products and spatio-temporal distribution of Late Triassic magmatic products.