Category Archives: Geology education

The Giles Complex intrusions, central Australia

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)

Brief overview

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.

image001

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.

Ongoing research

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).

Publications & theses from the Giles Complex team 1964 – 2007

Publications

Collerson, K.D., Oliver, R.L. & Rutland R.W.R. (1972).  An example of structural and metamorphic relationships in the Musgrave Orogenic Belt, central Australia.  J. geol. Soc. Aust. 18, 379-394.
Compston, W. & Nesbitt, R.W. (1967).  Isotopic age of the Tollu Volcanics, W.A.   J. geol. Soc. Aust. 14, 235-238.
Facer, R.A. (1967).  A preliminary study of the magnetic properties of rocks from the Giles Complex, central Australia.  Australian J. Science 30, 237-238.
Facer, R.A. (1970).  Magnetic properties of the Giles Complex, central Australia. Search 1, 76-77.
Facer R.A. (1971).  Magnetic properties of rocks from the Giles Complex, central Australia.  Royal Society of NSW Journal and Proceedings 104, 45-61.
Facer, R.A. (1971).  Intrusion and magnetization of the Giles Complex, central Australia. Geophysical Journal of the Royal Astronomical Society 22(5), 517-520.
Goode, A.D.T. & Krieg G.W. (1967).  The geology of the Ewarara Intrusion, Giles Complex, central Australia. J. geol. Soc. Aust. 14, 185-194.
Goode, A.D.T. & Nesbitt, R.W. (1969).  Granulites and basic intrusions of part of the Eastern Tomkinson Ranges, central Australia.  Spec. Pub. Geol. Soc. Aust. 2, 279-281.
Goode, A.D.T & Moore A.C. (1975).  High pressure crystallisation of the Ewarara, Kalka and Gosse          Pile intrusions, Giles Complex, central Australia.  Contr. Mineral. Petrol. 51, 77-97.
Goode A.D.T. (1975).  A transgressive picrite suite from the western Musgrave Block, central Australia.  J. geol. Soc. Aust. 22, 187-194.
Goode, A.D.T. (1976a).  Small scale primary igneous cumulus igneous layering in the Kalka layered intrusion, Giles complex, central Australia.  J. Petrol. 17, 379-397.
Goode, A.D.T. (1976b).  Sedimentary structures and magma current velocities in the Kalka layered intrusion, central Australia.  J. Petrol. 17, 546-558.
Goode A.D.T. (1976c).  Vertical igneous layering in the Ewarara layered intrusion, central Australia.  Geol. Mag, 114, 365-374.
Goode, A.D.T. (1977).  Flotation and remelting of plagioclase in the Kalka intrusion, central Australia: petrological implications for anorthosite genesis.  Earth & Planetary Science Letters 34 (3), 375-380.
Goode, A.D.T. (1978).  High temperature, high strain rate deformation in the lower crustal Kalka intrusion, Central Australia.  Contr.Mineral. Petrol. 66, 137-148.
Gray, C.M (1977).  The geochemistry of central Australian granulites in relation to the chemical and isotopic effects of granulite facies metamorphism.  Contr. Mineral. Petrol. 65, 79-89.
Gray, C M. (1978).  Geochronology of granulite-facies gneisses in the Western Musgrave Block, Central Australia.  J. Geol. Soc. Aust. 25, 403-414.
Gray, C.M. (1987).  Strontium isotopic constraints on the origin of Proterozoic anorthosites.  Precambrian Research 37, 173-189.
Gray C.M. & Compston W. (1978). A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites.  Geochim. Cosmochim. Acta 42, 1735-1747.
Gray, C. M. & Goode, A.D.T. (1981).  Strontium isotopic resolution of magma dynamics in a layered intrusion.  Nature 294, 155-158.
Gray, C.M. & Goode, A.D.T. (1989).  The Kalka layered intrusion, Central Australia: a strontium isotopic history of contamination and magma dynamics.  Contr. Mineral. Petrol. 103, 35-43.
Gray, C.M., Cliff, R.A. & Goode, A.D.T. (1981).  Neodymium-strontium isotopic evidence for extreme contamination in a layered basic intrusion.  Earth Planet. Sci. Letts 56, 189-198
Kleeman, J.D. & Nesbitt, R.W. (1967).  X-ray measurements on some plagioclases from the Mt. Davies Intrusion, South Australia.  J. geol. Soc. Aust. 14, 39-42.
Moore, A.C. & Goode, A.D.T (2007).  Petrography and origin of granulite‐facies rocks in the Western Musgrave Block, Central Australia.  J. geol. Soc. Aust. 25, 341-358.
Moore, A.C. (1968).  Rutile exsolution in orthopyroxene.  Contr. Mineral. Petrol. 17, 233-236.
Moore, A.C. (1969).  Corona textures in granulites from the Tomkinson Ranges, central Australia.  Spec. Publ. Geol. Soc. Aust. 2, 361-366.
Moore, A.C. (1970).  Descriptive terminology for the textures of rocks in granulite facies terrains.  Lithos 3, 123-127.
Moore, A.C. (1971a).  Corundum-ilmenite and corundum-spinel associations in granulite facies rocks from central Australia.  J. geol. Soc. Aust. 17, 227-230.
Moore, A.C. (1971b).  Some aspects of the geology of the Gosse Pile Ultramafic intrusion.  J. geol. Soc. Aust. 18, 69-80.
Moore, A.C. (1971c).  Mineralogy of the Gosse Pile ultramafic intrusion, central Australia.  Plagioclase.  J. geol. Soc. Aust. 18, 115-126.
Moore, A.C. (1971d).  Mineralogy of the Gosse Pile ultramafic intrusion, central Australia.  Pyroxenes. J. geol. Soc. Aust. 18, 243-258.
Moore, A.C. (1973).  Studies of igneous and tectonic textures and layering in the rocks of the Gosse Pile intrusion, central Australia.  J. Petrol. 14, 49-80.
Nesbitt, R.W. & Kleeman, A.W. (1964).  Layered intrusions of the Giles Complex.  Nature 203, 391-393.
Nesbitt, R.W. & Talbot, J.L. (1966).  The layered ultrabasic and basic rocks of the Giles Complex, central Australia.  Contr. Mineral. Petrol. 13, 1-11.
Nesbitt, R.W. (1966).  The Giles Complex, an example of a deeply eroded volcanic zone.  Bull. Volcanogique 29, 271-282.
Nesbitt, R.W., Goode, A.D.T., Moore, A.C. & Hopwood, T.P. (1970).  The Giles Complex, central Australia; a stratified sequence of mafic and ultramafic intrusions.  Geol. Soc. S. Africa Spec. Publ. 1, 547-564.
Oliver, R.L., Collerson, K.D. & Nesbitt, R.W. (1969).  Precambrian geology of the Musgrave Block.  Excursion Guide No 13, ANZAS 1969, 37-40.

PhD theses

Bell, T.H. (1973).  Mylonite development in the Woodroffe Thrust, central Australia.  Unpubl. PhD thesis University of Adelaide.
Collerson K.D. (1972).  High grade metamorphic and structural relationships near Amata, Musgrave Ranges, central Australia.  Unpubl. PhD thesis University of Adelaide.
Facer, R.K. (1969).  Magnetic properties of the Giles Complex, central Australia. Unpubl. PhD thesis University of Sydney.
Goode A.D.T. (1970).  The petrology and structure of the Kalka and Ewarara layered basic intrusions, Giles Complex, central Australia.  Unpubl. PhD thesis University of Adelaide.
Gray, C.M. (1971).  Strontium isotopic studies in granulites.  Unpubl. PhD thesis Australian National University.
Moore A.C. (1970).  The geology of the Gosse Pile ultramafic intrusions and the surrounding granulites, Tomkinson Ranges, Central Australia.  Unpubl. PhD thesis University of Adelaide.

Honours theses

Barnes, L. (1968).  The petrography and geochemistry of some high grade metamorphic rocks from the Mt Davies-Giles region, central Australia.  Unpubl. Honours thesis University of Adelaide.
Blight D.F. (1969).  The geology, petrology and geochemistry of an area south of Tollu, W.A.  Unpubl. Honours thesis University of Adelaide.
Bowden, P.R. (1969).  Geology of the Tollu area Western Australia.  Unpubl. Honours thesis University of Adelaide.
Coin, C.D.A. (1970).  A study of the granulite facies terrain near Amata.  Unpubl. Honours thesis University of Adelaide.
Goode, A.D.T. & Kreig, G.W. (1965).  The geology of the Ewarara intrusion, Giles Complex, central Australia.  Unpubl. Honours thesis, University of Adelaide.
Gray, C.M. (1967).  The geology, petrology and geochemistry of the Teizi meta-anorthosite.  Unpubl. Honours thesis University of Adelaide.
Kleeman J.D. (1964).  Studies on the X-ray diffraction, analysis and geochemistry of plagioclase from the Mt Davies igneous intrusion.  Unpubl. Honours thesis University of Adelaide.
Miller, C. (1966).  A geochemical study of clinopyroxenes from the igneous intrusion South Davies, N.W. South Australia.  Unpubl. Honours thesis University of Adelaide.
Smith, P.C. (1970). The geology of the Hinckley Ranges, W.A.  Unpubl. Honours thesis University of Adelaide.
Steele, R.J. (1966).  Gravimetric investigation of the Mt Davies and Gosse Pile intrusions of the Giles Complex.  Unpubl. Honours thesis University of Adelaide.
Yong, S.K. (1964).  The distribution of trace elements Ni, Cu, Sr, Cr, and Mn in the Mt Davies basic intrusion of South Australia.  Unpubl. Honours thesis University of Adelaide.

A Lesson Learned

Field observations

As part of ongoing studies of the nature and distribution of Permian glacigene sediments on Fleurieu Peninsula, we were shown to a location just below the plateau surface near Spring Mount, west of ‘Minnawarra’ Homestead (Fig. 1).  Here, on a north-facing spur high in the landscape, adjacent to scattered outcrops of extensively weathered and ferruginised bedrock, there was a surface scatter of rounded cobbles and pebbles cascading down towards a small dam in the valley. We would normally be looking for just this type of geological occurrence as evidence for Permian glacigene sediments from which the cobbles and pebbles (erratics, outwash gravels) would have been eroded. This site was somewhat removed from the usual locations of Permian sediments that are common nearby at lower elevations in the Inman Valley to the south, and in the Hindmarsh Tiers and Myponga valleys to the north. Nevertheless, the scatter of rounded clasts was distinctive.

Figure 1:  Location map.  Site studied marked by red star.

Figure 1: Location map. Site studied marked by red star.

On closer examination, the clasts were commonly of quartzite and fine-grained gneissic rocks but we could not identify any granite cobbles: boulders and cobbles of Encounter Bay Granites are very common in the Permian deposits along the northern margins of the Inman valley, for example. In addition, many of the cobbles and pebbles here were somewhat oblate in shape.

We traced the scatter of clasts downhill towards a small dam. On the eastern side of the dam a shallow cutting had been made in the hillslope to provide vehicular access to the dam.

Figure 2:  Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 2: Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 3:  Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material.  Note the oblate character of many of the cobbles.

Figure 3: Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material. Note the oblate character of many of the cobbles.

Various exposures en route to the cutting, for example on the downslope side of a Xanthorrhea (grass-tree), revealed cobbles and pebbles weathering from a bleached, weathered and generally poorly consolidated sand-silt matrix (Fig. 2), which is very like the situation we have observed in Permian glacigene deposits.  First observations on approaching the cutting by the dam confirmed these relationships (Fig. 3).  At the cutting, however, the actual field relationships of the clasts and the matrix are more clearly seen and the clasts tend to have a well defined ‘imbricate’ orientation in the host matrix (Fig. 4).  Immediately to the east in the cutting the relationships become clear:  the clasts are actually contained within steeply dipping weathered bedrock.  Their oblate shape and orientation is a function  of deformation of the bedrock and they are aligned parallel to the steeply dipping layering represented by schistosity roughly parallel to bedding (Fig. 5).  The weathering of the bedrock, which is related to its occurrence in close proximity to the deeply weathered pre-Tertiary summit surface of Fleurieu Peninsula, and which is particularly characteristic of the area around Spring Mount, has extensively altered the rock matrix but apparently little affected most of the contained clasts.

Some outcrops in the small tributary immediately to the west of the dam are of weathered and ferruginised schist and gneiss, but there are no rock clasts evident.

Figure 4:  Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.  Again note the oblate character of many of the cobbles.

Figure 4: Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.

Figure 5:  Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Figure 5: Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Clearly, the geology is not reflective of Permian glacigene deposits but of weathered and altered bedrock most likely to be the basal Proterozoic conglomerate that unconformably overlies the Barossa Complex basement inlier in this region. Reference to the geological map (Fig. 6) confirms this possibility. The essentially unweathered conglomerate, which is also spectacularly deformed, is well-exposed in the Inman Valley at Grey Spur, just south of the Spring Mount locality. The same formation (assigned to the Aldgate Sandstone; SARIG mapping) is exposed on the coastline at Lady Bay, south of Normanville, but the deformation here has been very intense and the contained cobbles and pebbles are significantly deformed.

Conclusion

The ‘lesson learned’ is that all is not as it initially may seem in field geology, and jumping to conclusions is not recommended. A close examination of field relationships in any locality, together with questioning of initial conclusions and gathering of all available evidence, might actually uncover an interesting story that would otherwise be missed.

Figure 6:  Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows:  Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered  zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

Figure 6: Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows: Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

A R Milnes

R P Bourman