A geochemical and isotopic study of mafic and intermediate rocks in the Olary Province of South Australia – magma series discrimination and geochronological framework

Abstract

Sampling and analysis of the mafic and intermediate igneous rocks from the Olary Block in South Australia has revealed eight geochemically distinct rock types. The Outalpa Amphibolite is characterised by low concentrations of Fe(total), Ti, P, LREE and HFSE relative to the Cathedral Rock samples of Pierini (1994). The Antro and Poodla granitoids have intermediate compositions and exhibit remarkable geochemical similarity except for alkali abundances. Three types of apparently later greenschist facies dolerites can be distinguished by geochemical means. The HPT (high phosphorous & titanium) dolerites have higher concentrations of LREE and HFSE than the LPT (low phosphorous & titanium) dolerites. The Rainy Day dolerite has low phosphorous and high titanium concentrations, and has HFSE and LREE concentrations intermediate between the HPT and LPT dolerites. The Maldorky Lamprophyre that crops out south of the Olary township has lamproitic affinities and is geochemically similar to the post-Delamerian Ordovician lamprophyres near Truro and Anabama Hill. £Nd(T) values are generally higher for the Outalpa amphibolite, LPT dolerites and Rainy Day dolerites, indicating derivation from a more depleted source or greater crustal interaction. The Poodla Granitoid has significantly lower £Nd(T) than the Antro Granitoid: this is consistent with petrographic and geochemical evidence that suggests a greater level of crustal contamination of the former. A Pb/Pb zircon date for the Antro Granitoid was obtained using the evaporation ('Kober') method. A magmatic age of 1679±13Ma is comparable to SHRIMP ages from the Broken Hill Block (e.g. Page and Laing, 1992). Significantly, this age may constrain the intrusion of the Outalpa Amphibolite to post-1700Ma and pre-1680Ma