Long-term Research Program initiated by Professor Bob Nesbitt between 1963 – 1970 in the Department of Geology & Mineralogy, The University of Adelaide
R.W. Nesbitt, Emeritus Professor, University of Southampton, UK (Nov 2017)
The Giles Complex is an iconic geological province straddling the junction of South Australia, Western Australia and Northern Territory. It was explored by Reg Sprigg and his colleagues in the 1950s as part of a mining company (Southwestern Mining) evaluation of its mineral potential and the SA sector was later mapped by the South Australian Geological Survey in the late 1950s. These early geological studies were essentially exploratory, setting out the distribution of the major rock types, but they provided little detail of the geological evolution and origin of these ancient rocks. In 1963, the area, being a remote and scientifically challenging geological province, provided an exciting challenge to a small University of Adelaide group. An important consideration at the time was the fact that as a University-based group we were not inhibited by State boundaries which allowed us to examine the whole igneous province on both sides of the WA-SA border. Several years of field studies, petrological, mineralogical and geochemical research were undertaken by me and my colleagues and post-graduate students in the Department of Geology & Mineralogy. The results were summarized in post-graduate research theses, reported at National and International conferences, and published widely in scientific journals (list attached). The theses and rock samples collected over the many field seasons, together with the respective thin and polished sections for petrographic study, are archived in the Mawson Collection in the Mawson Building.
This comprehensive suite of studies was largely completed in the early 1970s with later follow-up isotopic studies by Chris Gray when based at the ANU and later at La Trobe. The area was re-surveyed by the Australian Geological Survey Organisation (AGSO, Commonwealth Government) in 1987 and 1990 (AGSO Bulletin 239, 1996) which built on the work of Adelaide University.
Later work by the Geological Survey of Western Australia (http://www.dmp.wa.gov.au/Geological-Survey/West-Musgrave-Province-21418.aspx) was restricted to the Western Australian sector of the Complex. In South Australia, further studies have been significantly restricted because access is controlled by the local indigenous population.
Simplified geological map showing the location and distribution of the Giles Complex Intrusions (after Nesbitt et al, 1970)
Scientific significance & outcomes
The major outcomes of the work carried out by the Adelaide University group can be summarized as follows:
- The intrusive rocks of the Giles Complex were emplaced as a series of individual mafic-dominated sheets of varying dimensions, some as large as 25km in length and 4km thickness. The present outcrop area occurs over an area of about 2,500 sq.km.
- The intrusions were emplaced at varying depths in the crust with those in the east of the Complex being at deep crustal depths progressing to shallow depths in the west. The mapping and subsequent laboratory studies demonstrated that the Giles Complex rocks present an east to west vertical section of continental crust with the volcanics (at Tollu in Western Australia) representing the final extrusive sequence. Petrographic studies by Goode and Moore demonstrated that the layered intrusions in South Australia were emplaced at pressures equivalent to 30 to 40 km depth. Such pressures indicate that the intrusions were emplaced near the base of the continental crust with subsequent geological events bringing them to their present surface position.
- The Adelaide group, working with isotope geochemists at the Australian National University (Compston & Nesbitt 1967) were the first to determine the age of the Giles Complex rocks as 1060 Ma. This age has been subsequently verified by The Geological Survey of Western Australia using the latest zircon dating techniques (1040 to 1090 Ma) and AGSO in Canberra (1080 Ma)
- The intrusions were emplaced into already deformed high-grade gneisses and granulites representing at least one previous major tectonic event and, after emplacement, were subsequently deformed into varying orientations with some (e.g. Mt Davies) being overturned.
- Studies by Moore (1973) and Goode (1978) confirm that shortly after consolidation, magma chambers in the east suffered high temperature-high pressure strain in localised areas. These zones (sometimes more than 100 metres across) point to major deformation events deep in the crust which were responsible for the disruption of the original intrusions. Such zones are marked by spectacular gneissic deformation structures where most of the original minerals have been totally recrystallized leaving residual highly deformed crystals or augen within a fine-grained groundmass.
- Field studies demonstrate that during cooling, the magma-crystal mix behaved like aqueous sediments producing characteristic structures such as cross-bedding, slumping, load structures and ripple marks. This phenomenon was modelled by Goode in a series of important papers (1967a, b, c). Using this model allowed us to determine the original orientation of the magma bodies prior to the deformation event. Laboratory studies on fractionation trends in mineral groups also confirmed this interpretation (e.g. Kleeman & Nesbitt 1967).
- In several areas, the contacts of the intrusions, particularly in the east, are well exposed. Given that the intrusions crystallised from high temperature magmas (> 1100°C) one would expect a strong cooling reaction where the magma reacted to the host country rock. The fact that this reaction is surprisingly muted indicates that the temperature difference was small and this in turn indicates the host rocks were at high pressure at emplacement. Field and petrographic studies at the margins of Mt Davies has revealed the presence of incipient melting producing granophyre veins and inclusions. On-going research using laser ICPMS isotopic techniques is aimed at understanding the degree of involvement of the host granulite rocks.
The next stage of research is to understand how these intrusions fit into the evolution of continental Australia. The presence of such large quantities of magma in the continental crust is indicative of a major mantle melting event and may provide a model for the Large Igneous Provinces (LIPS) which mark major tectonic events in several continents (e.g. the Deccan and Siberian Traps).