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 (Fo75-55) and a high temperature (1200-1000 °C), low pressure (∼ 1 kbar), continental tholeiitic fractionation trend under moderate fO2 ( ∼ 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 aSiO2 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/86Sri 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.
|Number of pages||39|
|Journal||Mineralogy and Petrology|
|Publication status||Published - 1996|