Geochemistry and geochronology of the Rathjen Gneiss: implications for theearly tectonic evolution of the Delamerian Orogen

The Rathjen Gneiss is the oldest and structurally most complex of the grani tic intrusives in the southern Adelaide Fold-Thrust Belt and therefore prov ides an important constraint on the timing of the Delamerian Orogen. Zircon s in the Rathjen Gneiss show a complex growth history, reflecting inheritan ce, magmatic crystallisation and metamorphism. Both single zircon evaporati on ('Kober' technique) and SHRIMP analysis yield best estimates of igneous crystallisation of 514 +/- 5 Ma. substantially older than other known felsi c intrusive ages in the southern Adelaide Fold-Thrust Belt. This age places an older limit on the start of the Delamerian metamorphism and is compatib le with known stratigraphic constraints suggesting the Early Cambrian Kanma ntoo Group was deposited, buried and heated in less than 20 million years. High-U overgrowths on zircons were formed during subsequent metamorphism an d yield a Pb-206/U-238 age of 503 +/- 7 Ma. The Delamerian Orogeny lasted n o more than 35 million years, The emplacement of the Rathjen Gneiss as a pr e- or early syntectonic granite is emphasised by its geochemical characteri stics, which show affiliations with within-plate or anorogenic granites. In contrast, younger syntectonic granites in the southern Adelaide Fold-Thrus t Belt have geochemical characteristics more typical of granites in converg ent orogens, The Early Ordovician post-tectonic granites then mark a return to anorogenic compositions. The sensitivity of granite chemistry to change s in tectonic processes is remarkable and clearly reflects changes in the c ontribution of crust and mantle sources.