DETRITAL ZIRCON PROVENANCE OF THE MESOPROTEROZOIC PANDURRA FORMATION, SOUTH AUSTRALIA: GAWLER CRATON ZIRCON POPULATION AND IMPLICATIONS FOR THE BELT SUPERGROUP

The companion continent rifted from the western margin of North America during the Meso- to Neoproterozoic has been the focus of considerable conjecture and debate. In a number of models, Australia and North America are juxtaposed at various times, with widely differing configurations. The non-North American origin for a unique 1580-1620 Ma detrital zircon component in sequences such as the Belt Supergroup has been used as evidence for proximity of the Australian continent. Whilst considerable data is available for the Belt, little is known of possible correlatives in Australia; for example the Mesoproterozoic Pandurra Formation from the Stuart Shelf, NE side of the Gawler Craton, South Australia. As a possible Belt correlative, the zircon provenance of the Pandurra should provide a template for a proximal Gawler Craton detrital zircon source.

The Pandurra Formation is a W to SW derived fluviatile sequence comprising medium to coarse-grained, poorly sorted quartz and lithic sandstones, plus conglomerates and shales. Volcanic horizons have not been recorded and the only radiometric age constraint is a crude syn-Belt whole-rock Rb-Sr isochron suggesting a maximum depositional age of 1424 ± 51 Ma (Fanning et al., 1983).

SHRIMP U-Pb ages of detrital zircons have been determined for 4 drillcore samples of the Pandurra Formation from the Stuart Shelf: a shale and sandstone each from Red Millers Creek 1 and CSR-PY1. In all four samples, the 1580-1620 Ma component forms between 40 to 60% of the detrital zircon population, with older groups between 1640-1780 Ma and a subordinate group >2440 Ma. Clearly the proximal 1585-1595 Ma Gawler Range Volcanics and Hiltaba Granite source dominate the detrital zircon population.

Within the Belt Supergroup of Montana and Idaho, the 1580-1600 Ma population is present in the lower-Belt Prichard Formation and overlying Ravalli Group, but is absent in upper-Belt Missoula Group sandstones (Link and Fanning, 2003). Thus in any supercontinent reconstruction, the Belt basin has to be relatively distal from the Gawler Craton on the Australian margin, as a direct provenance would record a more dominant 1580-1600 Ma component. Likely lower and middle Belt strata contain a mixture of a North American and exotic provenance, with dilution of any direct Gawler signature, if indeed the population in question is uniquely Australian.