Constraints on Proterozoic crustal evolution from an isotopic and geochemical study of clastic sediments of the Gawler Craton, South Australia

Abstract

The Gawler Craton comprises tocks varying in age from Archaean to more recent Phanerozoic sediments. The rocks of greatest interest in defining processes of early crustal formation and evolution in the Australian continent, are the basement material older than approximately 1400 Ma (pre-cratonisation), comprising deformed and metamorphosed rocks suites of Archaean and Proterozoic metasediments and gneisses. These suites span an immense period of intense geological history, and as such are a topic of much past and present study. Detailed mapping in the Tumby Bay region of eastern Eyre Peninsula outlines stratigraphic and structural evolution of a sequence of Proterozoic rock suites, these are proposed to be related to other recognised deformation episodes elsewhere within the Gawler Craton, thus regional correlation is inferred. A new theory for development of two lineations within the map region is postulated by two movement directions along the Kalinjala Mylonite Zone. Geochemically the Proterozoic sediments of the Gawler Craton are similar to upper crustal average values of Taylor & McClennan (1985). However, characteristic depletions in Nb and Sr are recognised. Consistency in trace element compositions for Archaean and Proterozoic samples would suggest recycling of older Archaean crust into Proterozoic sediments and granitoids. Analysis of representative trace element ratios and indices of alteration and weathering suggest some change in geochemistry throughout the Proterozoic period. Selected Proterozoic elastic sedimentary suites were geochemicaly and isotopically (Sm-Nd) analysed, with the data being presented within this thesis. The most interesting of these being the Pandurra Formation, red-bed sediments deposited within the north-eastern Stuart Shelf region of the Gawler Craton. These sediments exhibit a change in measured isotopic values, with younger epsilon neodymium (ENd), and higher Sm/Nd ratios observed (ENd(O) = -14.67, Sm/Nd = 0.2441), than typical older Gawler Craton rocks (average Proterozoic sediments ENd(O) = -21.85, Sm/Nd = 0.1847). This isotopic shift is also recognised within the Adelaide Fold Belt to the east of the Gawler Craton (average shales ENd(O) = -16.20, Sm/Nd = 0.1942). A source for these younger signatures is not recognised within the Gawler Craton, and therefore more distal province sources, OR isotopic alteration in the originally considered 'robust' Sm-Nd isotopic system, are proposed.