Archaean mafic magmatism in the Kalgoorlie area of the Yilgarn Craton, Western Australia: a geochemical and Nd isotopic study of the petrogenetic and tectonic evolution of a greenstone belt

The compositional variation in the komatiites, basalts and gabbros of the Kalgoorlie greenstones illumines the magmatic–tectonic evolution of this Archaean belt. Gabbro sills and basalts are the principal host for gold mineralization in the goldfields, yet have received relatively little attention. We present major, trace, rare earth element, and Nd isotope data for several suites of these rocks from around the Kalgoorlie gold mines. At the base of the Kalgoorlie sequence, the Hannan's Lake Serpentinite is an uncontaminated komatiite that fractionated by olivine-pyroxene fractionation. The geochemistry of the overlying high-Mg Devon Consols basalt was controlled by moderate levels of crustal contamination and low-pressure fractionation of olivine and orthopyroxene. The Paringa Basalt is an internally variable high-Mg basalt and it is strongly contaminated by crustal material. Zircon xenocrysts in the mafic lavas show that they were erupted through continental crust. All of the mafic/ultramafic lavas appear to have been derived from a mantle plume; early uncontaminated magmas evolved through to strongly contaminated magmas, all derived from shallow parts of the plume (residual plagioclase). Later magmas were less contaminated and were derived from deeper in the plume (residual garnet). Layered gabbro sills intruded 20–30 Ma after the eruption of the (ultra)mafic volcanic sequence. The uncontaminated Golden Mile–Aberdare gabbro is a highly fractionated tholeiitic rock. The Eureka-Federal gabbro is a thin, weakly differentiated sill. The Williamstown peridotite is intruded deeper in the stratigraphic pile. Both of these latter sills may be contaminated. The transition from high-Mg to tholeiitic magmatism coincided closely with the onset of compressional regional deformation and felsic magmatism. The Eastern Goldfields lack classic features of subduction — ophiolites, sheeted dykes, paired metamorphic belts, and regional granitoid zonation. Felsic volcanic rocks between 2710 and 2670 Ma do not show an evolution in composition as would be expected in a transition from plume to subduction tectonics. Regional compressive deformation may be related to stress in the crust with the impingement of neighboring plumes, and/or subsequent granitic plutonism. Crustal melting and generation of felsic melts may have been a response to this massive mafic volcanic event.