Petrochemistry of the Mt. Jope Volcanics and a mafic-ultramafic sill in the Fortescue Group near Mt. Turner, Hamersley Ranges, W.A.

Abstract

Petrological and geochemical studies were carried out on eighty samples collected from the Mt. Jope Volcanics and a mafic-ultramafic sill complex in the underlying Hardey Sandstone, in the Fortescue Group of the Hamersley Ranges, Western Australia. The samples include basalts, dolerites, various pyroclastics, some cumulate rocks, cherts and cherty tuffs. Thin section descriptions and whole-rock and trace element data are presented for many of the samples. Using various discrimination and variation plots, the basalts were characterized as continental tholeiites and were probably basaltic andesites on the basis of their chemistry and texture. The dolerites are probably members of a single differentiation sequence which can be combined with that of the basalts in a proposal that all the igneous rocks are genetically related. Metamorphism has reached lower greenschist grade in the field area, with the development of actinolite, albite, chlorite and clinozoisite­epidote as typical minerals. Mineralogical and textural equilibrium is only reached where solutions had free access, as in the foliated amygdalar volcanics. The conditions for the metamorphism are probably 2-4kb and 340° - 440°C with the CO2 component of the metamorphic fluids being very low. Principal component and cluster analysis was used to help characterize the chemistry of the samples and to compare these with the average compositions and some suites from other areas.