GEMOC - Department of EPS - Faculty of Science

AUSTRALIAN PROTEROZOIC GRANITES - CHARACTERISTICS, SOURCES AND POSSIBLE MECHANISMS FOR DERIVATION AND EMPLACEMENT.

L.A.I. Wyborn^1,2, I.V. Bastrakova¹ and Anthony R. Budd¹

1. Australian Geological Survey Organisation
2. GEMOC

Granites are a prominent component of almost every Australian Proterozoic orogenic domain. Exposed outcrops of granites and their comagmatic volcanics cover at least 145 000 km2, and based on gravity and aeromagnetic data, the subsurface extents of granite plutons are likely to be at least 3 times greater. As part of a collaborative project between AGSO and the State and Territory Geological Surveys, and using techniques developed by Bruce Chappell and his coworkers in the Lachlan Fold Belt, some 10 000 chemical analyses were compiled of Proterozoic granites and felsic volcanics, as well as information on their age, mineralogy, host rocks, and associated mineralisation. Using this database and the 'suite' concept (Chappell, 1984), it is possible to identify 9 major types, most of which have analogues in lower Palaeozoic granite suites of the Lachlan Fold Belt.

Most Australian Proterozoic felsic melts were emplaced between 1880 to 1500 Ma, with minor episodes occurring between 1200 to 1050 Ma and at 600-700 Ma. I-type granites predominate. S-types are a minor component and comprise <3 % of the total area of granite exposed. Although I-type granites show distinct compositional changes with time, there are 3 characteristics common to most suites:

1) The majority are I-(granodioritic) type in character with a SiO2 range of 60 to 77 wt % and there are no significant suites of I-(tonalitic) type or M-types as defined by Chappell and Stevens (1988).

2) Most Australian Proterozoic granites have high K2O/Na2O which manifests itself as ubiquitous K-feldspar phenocrysts commonly up to 4 cm in diameter. This high ratio is unique in Australian granites: Archaean granites generally are higher in Na2O contents, whilst Palaeozoic granites have lower K2O values.

3) Proterozoic mantle normalised trace element patterns are characteristically Sr-depleted, Y-undepleted and imply derivation from source regions in which plagioclase was stable. This also infers that the granites were derived from depths of <35 km and required geothermal gradients of >30°km-1. These high gradients are compatible with the High Temperature Low Pressure (HTLP) metamorphism that is endemic to Australian Proterozoic terrains. The dominance of Sr-depleted types is also in common with lower Palaeozoic granites of the Lachlan Fold Belt. Sr-undepleted, Y-depleted granites, implying a garnet residual source, comprise <4.0% of Australian Proterozoic granites. This contrasts granites from subduction environments from mid Palaeozoic to recent times which have a far greater abundance of Sr-undepleted, Y-depleted compositions. Australian Archaean granites contain roughly 50% of each type (D.C. Champion, pers. comm, 1998).

Within the period 1880 to 1500 Ma Wyborn et al. (1997) have shown the dominant Sr-depleted, Y-undepleted I-(granodioritic) types can be further divided into three groups which show a time progression in geochemistry. The oldest group (Group 1) at 1870-1850 Ma (~ 31%), consists of restite-rich granite suites which are characterised by phenocryst-rich volcanics. On Harker variation diagrams the volcanics and granites are chemically indistinguishable, and with increasing SiO2 most major and trace elements show a linear pattern. Group 2, emplaced at 1840-1800 Ma (~ 30%), is a low-Ca type that shows evidence of magmatic fractionation. There is increasing heterogeneity between individual plutons and leucogranites can clearly be identified. On Harker variation diagrams major and trace element patterns increase exponentially for Rb, U, and Rb/Sr with increasing SiO2. The youngest group, Group 3 (~24%) is the most enriched in incompatible elements and comprises three subgroups: Subgroup 31, dated at around 1800-1780 Ma, has very high values of Zr, Nb and Y; Subgroup 32, usually emplaced between 1760 and 1650 Ma, is enriched in F and has variable amounts of Y, Zr, and Nb; and Subgroup 33, emplaced from 1640 to 1500 Ma, is more oxidised with a wide range in SiO2 values and higher CaO and Na2O contents. This group also has the lowest values of Y, Zr and Nb in Group 3.

Based on the argument that 'Granites are images of their source rocks' (Chappell, 1979) the chemical parameters above constrain source characteristics. The I-(granodioritic) character argues against a direct mantle derivation, and implies an I-(tonalitic) source (Chappell and Stephens, 1988). As the exposed Australian Archaean Crust is strongly bimodal, Proterozoic granites are unlikely to be sourced from Archaean crust as it is either too felsic or too mafic to form the vast quantities of Proterozoic I-(granodioritic) types. Age constraints on the source region are provided by Sm-Nd model ages which range from 2.0 to 2.6 Ma and it has been argued that the sources were underplated, evidence for which is seen in seismic refraction data, which also indicate a plagioclase bearing lower crust (Goncharov et al., 1998) as is required by the mantle-normalised trace element patterns of the granites. The high K2O contents also require the presence of K-rich minerals such as K-feldspar, biotite and amphibole in the source region. A simple explanation for the geochemical evolution from Groups 1 to 3 is that as the temperature in the source region increases, the magma production is dominated initially by minium melting of quartz, K-feldspar, albite and some biotite, with calcic plagioclase, amphibole and some biotite being restite phases (ie, restite-rich Group 1 granites). As the temperature increases in the source region, melting is initially dominated by biotite breakdown and is then followed by amphibole breakdown as source temperatures reach >1000°C, progressively producing Group 2 then Group 3 granites. In reality there is a continuum between the three Groups which simply reflects increasing temperature in the source region.

However, the inferred increase in lower crustal temperatures between 1880 to 1500 Ma based on the granite data clashes strongly with evidence from the mafic igneous rocks which infer a temperature decrease over the same period, with high Mg-tholeiites dominating before ~1850 Ma and continental tholeiites after ~1850 Ma (with the exception of high Fe-tholeiites at Broken Hill and Mount Isa at ~1690 to 1670 Ma). Most granite suites, particularly in the 1840-1880 Ma range, do show some evidence of coeval mafic intrusions, but these are never comagmatic: nor are they present in sufficient quantities to be the 'heat engine' for generating the required massive crustal melting. Further, recent modelling suggests that temperatures generated by emplacement of mafic intrusions are not likely to reach the high temperatures required for the Group 33 granites (1000°C) and that the time taken to generate sufficient crustal melting could actually be >30 Ma (Wyborn et al., 1997). In reality, the tectonic setting in terrains where many of these granites are emplaced are actually characterised by thermal subsidence phases, inferring that the mantle lithosphere is cooling and thickening (e.g., Sandiford et al, in press).

Several researchers (e.g., Chamberlain and Sonder, 1990; Sandiford and Hand, 1998; Hobbs et al. 1998) have investigated the consequences of high contents of heat producing elements (K, Th and U) within the crust to generate abnormally high geothermal gradients, and ultimately HTLP metamorphism and anatexis. Their work is highly relevant given that Australian Proterozoic granites are more enriched in K, Th, U than at almost any other time with the exception of some late Archaean granites. Independent validation of how high these high K, Th and U values are comes from present day heat flow measurements in the Australian Proterozoic which average 85 mWm-2 with values locally in excess of 100 mWm-2 (Sandiford and Hand, 1998 based on Cull, 1982). As modelled by Sandiford et al. (in press) and Hobbs et al.(1998), the end result of these high heat values are high mid-crustal temperatures that do not necessarily cause melting within the lower crust, but that they are capable of it, and perhaps even able to cause minor mantle melting at shallow levels.

What is clear is that modelling by these researchers show that it is possible to generate high temperatures at relatively low pressures without the need for 'active' mantle-driven processes e.g., mantle plumes, mantle underplating or subduction. The conditions for melting come from within the crust, and as each successive granite event in the Proterozoic becomes more enriched in the heat producing elements it may help to explain why the temperatures of formation of the granites are increasing with time, whilst the mantle is cooling - in fact it is paradoxically the mantle cooling that is indirectly causing the crustal melting as the more radiogenic heat sources are progressively buried to deeper levels within the crust by the addition of sediments on top of the crust during thermal subsidence. The efficiency of the heating process is in part controlled by the absolute contents of radiogenic elements in the felsic igneous rocks and in sediments derived from them. The thermal conductivity of the 'burying' sediments also plays an important role in determining the temperatures that are ultimately reached in the crust. It is significant that those Proterozoic terrains containing sequences dominated by quartz sandstones do not have the younger, high temperature granites of Subgroup 33. Given the correct conditions, it is possible to generate widespread granitic melting events, instead of linear belts of granite that are commonly associated with subduction or extensional environments. In the Proterozoic many granite suites are large 'amorphous blobs'.

Having created the melts without invoking subduction or mantle plumes, a mechanism is probably still required to allow the melts to intrude into the upper crust. The shape of the apparent polar wander path (APWP) between 1800 to 1500 Ma, confirms that the Australian plate was reasonably mobile at the time of major felsic magma generation. Magma emplacement was also coincident in time with inflection points on the APWP. These inflection points are recognised as significant interplate tectonic events with associated intraplate effects that cause major episodic migration of basinal fluids. Similar intraplate tectonic responses distal to plate boundary tectonic effects may have also allowed granitic melts to migrate into the upper crust (Wyborn et al., 1998)

It is proposed that crustal heating as a result of high K, Th and U contents within the crust, was possibly responsible for the generation of Sr-depleted, I-(granodioritic) type magmas which dominate the Australian crust from the late Archaean to the Siluro-Devonian. However, similar Sr-depleted I-(granodioritic) types from each major era are distinct in composition, with the radiogenic and incompatible elements decreasing in abundance in each type with time. As the I-(granodioritic) types are ultimately derived from distinct major underplating events, then each successive event must be of a different composition, which is possibly controlled by mantle characteristics changing with time in response to a cooling earth. As the abundance of radiogenic elements clearly decreases with time in I-(granodioritic) types after an initial late Archaean peak, then the ability for radiogenic crustal heating processes to generate significant magma volumes would also diminish with time. This is reflected in the decrease in dominance of Sr-depleted, I-(granodioritic) types after the lower Palaeozoic. Subduction-related processes then appear to become a major granite-generating mechanism resulting in the greater prominence of I-(tonalitic) types (ie. Cordilleran granites) in Australia from the mid Palaeozoic onwards.

REFERENCES

Chamberlain, C P, and Sonder, L J, 1990. Heat-producing elements and the thermal and baric patterns of metamorphic belts. Science 250, 763-769.

Chappell, B W, 1979. Granites as images of their source rocks. Geological Society of America, Abstracts with Programs 11, 400.

Chappell, B W, 1984. Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. Philosophical Transactions of the Royal Society of London, Series A 310, 693-707

Chappell, B W, and Stephens, W E, 1988. Origin of infracrustal (I-type) granite magmas. Transactions of the Royal Society of Edinburgh, Earth Sciences 79, 71-86.

Cull, J, 1982. An appraisal of heat Australian heat-flow data. BMR Journal of Australian Geology and Geophysics 7, 11-21.

Goncharov, A, Drummond, B J, Tripolsky, A, and Wyborn, L A I, 1998. Average composition of the crust in the Australian, Fennoscandian, and Ukrainian shields from refraction seismic studies and petrophysical modelling. AGSO Research Newsletter 28, 20-28.

Hobbs, B E, Ord, A, and Walshe, J L, 1998. The concept of coupled geodynamic modelling with special reference to the Yilgarn. In: Abstracts for 'Geodynamics and gold exploration in the Yilgarn', Australian Geodynamics Cooperative Research Centre, Nedlands, 36-39.

Sandiford, M, and Hand, M, 1998. Australian Proterozoic high-temperature, low-pressure metamorphism in the conductive limit. In: Treloar, P.J., and O'Brien, P.J., (eds), What drives metamorphism and Metamorphic reactions? Geological Society of London, Special Publications, 138, 109-120.

Sandiford, M, Hand, M, and McLaren, S, in press. High Geothermal Gradient metamorphism during thermal subsidence. Earth and Planetary Science Letters.

Wyborn, L A I, Ord, A, Hobbs, B, and Idnurm, I, 1997. Episodic crustal magmatism in the Proterozoic of Northern Australia - a continuum crustal heating model for magma generation. Australian Geological Survey Organisation, Record 1997/44, 131-134.

Wyborn, L A I, Idnurm, M, Budd, A R, Bastrakova, I V, Hazell, M S, and Edgecombe, S M, 1998. What do ~10 000 whole rock geochemical analyses tell us about Australian Proterozoic Intraplate Igneous Activity. Geological Society of Australia, Abstracts 49, 484.