Detrital zircon geochronology of Neoproterozoic to Devonian sediments from the Georgina and Amadeus basins of the former Centralian Superbasin reveals marked changes in provenance as a result of changing tectonic and eustatic conditions. Through much of the Neoproterozoic, sediments appear to have been sourced predominantly from the Arunta and Musgrave inliers. A close similarity in detrital age signatures of late Neoproterozoic sedimentary rocks in the Amadeus and Georgina basins suggests that the two basins were still contiguous at this time. A dominant population of Grenville-age zircons in Early Cambrian sediments of the Amadeus Basin reflects the uplift of the Musgrave Inlier during the Petermann Orogeny at 560-520 Ma, which shed large amounts of detritus into the Amadeus Basin. Early Cambrian sediments in the Georgina Basin have a much smaller proportion of Grenvillean ages, possibly due to the formation of sub-basins along the northern margin of the Amadeus Basin, which might have acted as a barrier to sediment transfer. An influx of ~0.5-0.6 Ga zircon towards the end of the Cambrian marks the formation of the Larapintine seaway across central Australia, possibly as a result of intracratonic rifting. Detrital zircon spectra from sedimentary rocks deposited within the seaway are almost identical to those of Ordovician sediments from the Pacific Gondwana margin, implying that sediment was transported across central Australia via the seaway from southeastern Australia. The remarkably consistent ‘Pacific Gondwana’ signature of Cambro-Ordovician sediments in central and eastern Australia reflects a distal source, possibly from as far afield as the Mozambique Belt of eastern Africa. The peak development of the seaway in the early to mid Ordovician coincides with granulite-facies metamorphism at mid-crustal depths (the Larapinta Event). The presence of the seaway, the lack of a local basement zircon component in Cambro-Ordovician sediments and the relative maturity of these sediments suggests that no mountain building accompanied metamorphism, consistent with an extensional or trans-tensional tectonic setting. Syn-orogenic sediments deposited at ~435-405 and ~365 Ma during subsequent basin inversion as a result of the Alice Springs Orogeny have detrital signatures similar to those of Cambro-Ordovician sedimentary rocks, indicating that they were derived from reworking of the ~500-460 Ma sequence.
3.2 Introduction
The depositional record of sedimentary basins provides valuable information about the effects of tectonism, complementing structural and metamorphic studies in more highly
Chapter 3 Amadeus and Georgina basins
deformed basement terrains (e.g. Blair & Bilodeau, 1988; Haines et al. 2001; McCann & Saintot, 2003). The type, distribution and sequence of sedimentation reveals much about the development of accommodation space in response to basin forming processes, and in fossiliferous sequences can place valuable timing constraints on tectonic processes.
Dating of detrital zircon is an effective method for determining provenance of sedimentary sequences, and when used in conjunction with sedimentological studies is a powerful tool for analysing the timing and nature of orogenic events (Ross & Bowring, 1990; Mahoney et al., 1999; Sircombe & Freeman, 1999; DeGraaff-Surpless et al., 2002; Goodge et al., 2004a, b). The distribution of ages of detrital zircon in a sediment reflects zircon ages in the source region which, if sufficiently characteristic, can be used to identify the provenance. Sediment input into a basin is strongly influenced by uplift or subsidence of local basement regions, but may also be derived from sources many hundreds or thousands of kilometres distant. The varying significance of basement sources in a sediment can thus be used to track tectonic and eustatic events in the basin and surrounding regions.
The Amadeus and Georgina basins, which are remnants of the former Centralian Superbasin of central Australia (Walter et al., 1995), contain a depositional record spanning the Neoproterozoic to Devonian, and have been influenced by a number of intracratonic tectonic events. The most recently discovered of these is the early Ordovician Larapinta Event, which resulted in the formation of granulites in the Harts Range region, between the two basins (Mawby et al., 1999; Hand et al., 1999). Limited previous detrital zircon data from the granulites indicate that the protoliths to the high- grade metamorphics were deposited at a similar time to sediments in the Centralian Superbasin, and rapidly buried and metamorphosed beneath a marine seaway (Buick et al., 2001).
A systematic detrital zircon study of sediments from the Amadeus and Georgina basins has been carried out to track tectonic changes in the central Australian region and to provide a reference against which detrital zircon populations in the high-grade metasediments from the Harts Range region can be compared. If the metasediments and adjacent basin sequences can be correlated, then the relatively well-understood
Chapter 3 Amadeus and Georgina basins
sedimentary history of the Amadeus and Georgina Basins can be used to provide a context in which to understand the tectonism associated with the Larapinta Event. The results show that changes in provenance of the Centralian Superbasin sediments closely reflect tectonism within the superbasin and surrounding regions, and imply that the Larapinta Event took place beneath an extensionally generated depocentre.