Petrogenesis of the late-Delamerian gabbroic complex at Black Hill, South Australia: Implications for convective thinning of the lithospheric mantle

A group of funnel-shaped gabbroic plutons at Black Hill, South Australia, consist of a lower series of layered peridotite, troctolite and olivine gabbro cumulates overlain by gabbronorites and potassic gabbronorites, the latter yielding a Sm-Nd isochron of 489 + 39 Ma. Mineral assemblages in the gabbros record an olivine compositional hiatus (Fo_75-55) and a high temperature (1200-1000 °C), low pressure (∼ 1 kbar), continental tholeiitic fractionation trend under moderate f_O2 ( ∼ QFM) conditions. The liquid line of descent involved complex open system processes including recharge and crustal assimilation. In one pluton, fine-grained norites may reflect assimilation which resulted in an increased a_SiO2 ^liquid causing orthopyroxene to crystallize prior to plagioclase. All the gabbros, including the most primitive peridotites, are LREE and incompatible element enriched. Moreover, the calculated parental magma composition in equilibrium with the most primitive troctolite has high La/Yb, La/Nb, Ti/Y and low Rb/Ba, similar to that of basaltic dykes which cut the gabbroic complex. Such compositions are untypical of melts derived from the asthenosphere suggesting that the incompatible element enrichment is not simply due to small degrees of melting. Given the isotopic constraints (εNd; 3.4 to -4.6, ⁸⁷Sr/⁸⁶Sr_i 0.7038-0.7065), this enrichment is not easily reconciled by crustal contamination either, and instead it is inferred to reflect an enriched lithospheric mantle source. Published data on mantle xenoliths from local Tertiary volcanoes overlap the isotopic and geochemical array of the gabbros and dykes, supporting this hypothesis.