UPb and RbSr geochronology and geological evolution of the Harts Range ruby mine area of the Arunta Inlier, central Australia

Abstract

Supracrustal and meta-igneous rock units from the ruby mine area of the Harts Range, eastern Arunta Inlier, central Australia, have been dated by the zircon UPb and RbSr total-rock (TR) and mica methods. A well-defined zircon discordia for a weakly deformed specimen of the Bruna granite gneiss yields an age of emplacement of 1748 −4 +5 Ma, thereby constraining the minimum age of the Irindina supracrustal assemblage. Metapelitic gneiss within the supracrustals and a meta-igneous ultramafic boudin from the associated Harts Range meta-igneous complex yield highly discordant zircon data, revealing a strong early Palaeozoic overprint. RbSr TR data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed, also apparently during the lower Palaeozoic. However, RbSr model age calculations and the zircon UPb data suggest a maximum age of about 2000 Ma for the supracrustal and meta-igneous rocks, and argue for new Proterozoic crust formation. Zircon UPb data from a deformed pegmatite, emplaced in the meta-igneous complex, yield an emplacement age of 520 −4 +5 Ma, further pointing to Lower Palaeozoic magmatism and deformation. Correlations of U content and calculated 206 Pb 238 U age for the ultramafic boudin zircons suggest that new growth of low-U zircons occurred during retrogression associated with this event. The Sr-isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of Palaeozoic Sr. Our data suggest lower Palaeozoic (possibly Delamerian) tectonothermal activity to be more important in the evolution of the Harts Range area than previously recognised. On the other hand, RbSr mica ages for deformed and undeformed pegmatites, and TR isochrons for the latter, show that pervasive tectonothernal activity had ceased by about 315 Ma and that regional cooling occurred between about 345 and 325 Ma. Local shear-zone biotite resetting may have persisted to about 300 Ma, consistent with the previously recognised Alice Springs Orogeny. Possible dilational Pb loss in the Bruna zircons occurred at about 103 Ma.