The geochemical and oxygen-isotope affinities of Proterozoic mafic granulites from the Einasleigh Metamorphics, northern Queensland

Abstract

1.6 Ga-old layered mafic rocks from the Einasleigh Metamorphics underwent two medium-high grade metamorphisms to produce a hornblende-stable granulite facies assemblage. The concentration of the immobile elements (Ti, P, Zr, Y, Nb) in 13 selected samples consistently indicates that these mafic granulites are indistinguishable from modern MORB. Furthermore, their K contents, K/Rb ratios, FeO ∗MgOTiO 2 signature and δ 18O values are similar to modern unaltered low-K tholeiites to ocean floor basalts which show some affinities to ‘plume’-type MORB. The only chemical features which separate the mafic rocks from pristine MORB are an increase in H 2O +, which predates the high grade event, a lower Fe 3+ ΣFe ratio, and a slight decrease in Na during the waning of the granulite facies metamorphism. These data are interpreted to indicate that the mafic rocks at Turpentine Hill underwent essentially isochemical granulite facies metamorphism, and their igneous precursors were derived from a mantle source similar to that which now underlies the mid-ocean ridges.