I-TYPE GRANITES OF THE LACHLAN FOLD BELT AND THEIR SOURCE ROCKS

B.W. Chappell, GEMOC ANU

I-type granites comprise a little less than half of all the granites of the Lachlan Fold Belt. They occur in association with S-type granites throughout the central parts of the belt, but alone or with very minor amounts of such granites in the eastern and western parts. The I-type granites cover a wide range of compositions but their study is simplified by a subdivision into suites that share mineralogical, chemical, isotopic, and textural features. A broader and less precise subdivision is made into supersuites. Compositional differences between suites are thought to correspond to analogous differences between source rocks. Variations within suites resulted in some cases from fractional crystallisation, but more generally were the product of differing degrees of separation of a partial melt from the residual unmelted material (restite).

Rocks of the Boggy Plain Supersuite make up close to 10% of the I-type granites. These range from cumulate gabbros to highly fractionated monzogranites and that diversity was the result of fractional crystallisation. The higher temperatures of these granites and their high Sr and Ba contents have lead to the suggestion by D. Wyborn that their source rocks were related to plutonic equivalents of the Ordovician shoshonite volcanic rocks that are prominent in the northern part of the belt, and which share a similar geographic distribution.

Some other I-type suites of the belt, such as Jindabyne, are relatively mafic. However, more often there is a prominent felsic component, that was produced as a partial melt from quartz- and feldspar-bearing rocks in the crust. Progressively more mafic rocks of those suites contain increasing amounts of a restite component. There is at most a very minor component in these rocks of direct mantle derivation, so that chemical and isotopic compositions have the potential to provide valuable data on the nature of the deep crust.

Both the I- and S-type granites of the Lachlan belt have a range of compositions for radiogenic isotopes, with a small amount of overlap. McCulloch & Chappell (1982) interpreted such data in terms of distinct I- and S-type sources, noting that granites have other compositional features that preclude their formation from mixtures of isotopically more primitive and more evolved components. In contrast, Gray (1984) proposed a mixing model to account for the Sr isotopic compositions, and Keay et al. (1997) interpreted the isotopic data in terms of a three-component mixing model. However, such models are not consistent with chemical compositions of the granites in the Bega Batholith, the source of much of the isotopic data. From east to west, granites of that batholith are isotopically more evolved, which would be consistent with an increasing sedimentary component in the source rocks, and with the decreasing Na and Sr contents of the granites, and by inference their source materials. However, Ca increases in abundance westwards as the rocks become more isotopically evolved, which is not consistent with increasing amounts of a sedimentary component, particularly one that resembles the exposed Ordovician turbidites.

If mixing was an important process in producing the source materials of the I-type granites of the Bega Batholith, then it must have involved components, both igneous and sedimentary, that are not exposed at the surface. It is more likely that mixing was of secondary importance and that the different isotopic and chemical components, or suites, of the Bega Batholith, correspond to distinct source rock compositions, of various ages, in the deep crust. Further work should enable those deep components to be better defined and contribute to a better understanding of the remote lithosphere of the Lachlan Fold Belt.

REFERENCES

Gray, C.M., 1984. An isotopic mixing model for the origin of granitic rocks in southeastern Australia. Earth and Planetary Science Letters 70, 47_60.

Keay, S., Collins, W.J. & McCulloch, M.T., 1997. A three-component Sr-Nd isotopic mixing model for granitoid genesis, Lachlan fold belt, eastern Australia: Geology, 307-310.

McCulloch, M.T. & Chappell, B.W., 1982. Nd isotopic characteristics of S_ and I_type granites. Earth and Planetary Science Letters 58, 51_64.

FOR AGC Session 7C: Lower Crust and Mantle Studies

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