PUSHING DETRITAL AGE INFORMATION INTO THE MULTI-MINERAL ARENA: HOW RUTILE HELPS PROVENANCE INTERPRETATION BASED ON ZIRCON, AN AUSTRALIAN EXAMPLE

Zircons from eastern Australian sands and sandstones generally have 2 dominant age peaks which are Pan African (700-510 Ma) and Grenvillian (1250-950 Ma) in varying proportion. Exposed rocks of these ages are rare in Australia. Squire et al. (2006) argued that this dominant “two-peak” signature is “extra Australian”. They suggested that the signature is a result of the east-west Gondwanan (African) collision and supermountain formation whose erosion provided this 2-age-peak zircon age signature that has become tremendously widespread in a superfan. Veevers et al. (2004) interpret the on-continent re-erosion and dispersal of sands from the eastern seaboard inland. They interpret the dominant Pan African signature as from alkaline magmas (based on trace element composition), and assert an Antarctic origin.

We examined zircon from a Murray Basin sand from between Adelaide and Melbourne (n=110). It is a ~ 4 Ma strand line from the Murray River, Australia’s largest river whose basin covers the 1/8 of the continent. Instead of the age pattern one would suppose for Murray River sand, a blend of the common “two peak” ages plus samplings of the Mesozoic sedimentary rocks exposed in the drainage basin, this sample contained 77 zircons of Pan African age with 20 younger and 13 older ages. Just 2 of the zircons are Grenville-aged. The main age peak is 660-510 Ma, its mean is 567+/-8 Ma (2 s.e.) and 64% of these zircons have low Lu (< 22 ppm). Squire’s superfan zircons sampled 100+ km E are 50 Ma younger on average and are not dominated by the low Lu type. Because of the one age peak dominance, we interpret the Murray sample as derived primarily from a local source, in this case a re-eroded Paleozoic sediment, itself locally derived. We dated the rutile from the sample and found it is unimodal, with an average age of 530+/-20 Ma. Although rutile is inherently a metamorphic mineral, and most zircon igneous, this particular association of unimodal age peaks indicates that the 2 minerals are tied to a single rock package which we interpret as high-grade metamorphic; the low Lu feature of the zircons is interpreted metamorphic. Probably its local source is Antarctic. Analyses of rutile from Australian dune sands show this Pan African-aged peak is a dominant one among rutiles.