Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Abstract

Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F1 folding, whereas andalusite crystallized on a regional scale during the F1 folding and in the post-F1 and pre-F2 static phase. Rb-Sr isotope data for total-rock, feldspar, and muscovite samples of the metasediment-contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515±8 m.y. and 506±6 m.y. ago in the Late Cambrian epoch. Rb-Sr and K-Ar data for biotite from the granites record variable radiogenic Sr loss until about 469 m.y. ago and comparatively uniform radiogenic Ar loss until 460-475 m.y. ago. Rb-Sr data for Kanmantoo Group metasediments and a metamorphic pegmatite indicate crystallization ages between 459-463 m.y. ago. Thus the regional andalusite-grade temperatures and pressures, which appear responsible for the leakage of radiogenic Sr and Ar from biotite in the granites and the redistribution of Rb and Sr in the metasediments, seem to have persisted for some 50 m.y. after emplacement of the granites until the Early Ordovician epoch. There is evidence for further leakage of Sr and Ar from biotite in deformed granites from the margins of the intrusion more than 50 m.y. afterwards in the Late Silurian or Early Devonian, possibly during the F2 folding. Geological observations and radiometric data for other granitic rocks in south-eastern South Australia, including the Palmer Granite, are consistent with this structural and metamorphic history of the Encounter Bay region.