Proterozoic mafic dykes near Port Lincoln, South Australia: Composition, age and origin

Abstract

Igneous‐textured and recrystallized amphibolite or pyroxene granulite dykes cut Proterozoic granitoid gneisses in the Port Lincoln area of South Australia. The igneous‐textured dykes are largely Ol‐normative, and the amphibolites Qz‐normative, tholeiites. The igneous‐textured dykes comprise norites, gabbronorites and plagioclase‐phyric dolerites which may be readily distinguished by their different Hy/Di ratios. All three igneous‐textured groups are LREE enriched, but they are further distinguished by their different degrees of HREE fractionation, distinct Zr/Ti and Zr/Y ratios and different combinations of normalized negative Ti, Nb, P, Sr and V anomalies. The compositional variation within the three groups may be modelled by crystal fractionation of opx + cpx + plag and cpx + plag dominated assemblages. Rb‐Sr isotopic data from the norites and previously published Rb‐Sr and K‐Ar isotopic data from gabbros northwest of the Port Lincoln area collectively suggest mafic magmatism was widespread about 1600 Ma. The norites and gabbros define three apparently distinct isochrons with different initial 87Sr/86Sr ratios which show a crude positive correlation with the average Rb/Sr ratio. The Sr isotope and major element data suggest sial assimilation during fractional crystallization was not important in the norites. Some of the norites are highly magnesian and together with the plagioclase‐phyric dolerites share a boninite‐like geochemical signature. The gabbronorites resemble more closely within‐plate basalts. The association of the dykes with shelf‐like supracrustals overlying Archaean continental crust suggests a possible origin for the dykes at a rifted continental margin.