BASEMENT OF THE LACHLAN FOLD BELT: THE EVIDENCE FROM S-TYPE GRANITES

S-type granites, derived wholly or largely from sedimentary source rocks, comprise more than half of the granites that are so extensively developed in the Lachlan Fold Belt (LFB) (White & Chappell, 1988). Apart from granites and their related volcanic rocks, the other principal component of the LFB is the widespread Ordovician turbidites. Paradoxically, those sedimentary rocks have compositions that would not have been appropriate as source rocks of most of the S_type granites.

The turbidites have very distinctive compositions. They are very mature and range from clay-rich shales to quartz-rich greywackes. When unmetamorphosed they consist dominantly of clay minerals and quartz; other minerals, notably feldspars, are present in small amounts. This is reflected in their chemical compositions which are distinctively low in the feldspar components Na, Ca and Sr (Wyborn & Chappell, 1983), shown by the average composition LOT in Table 1.

One areally restricted group of S-type granites, the Cooma Supersuite, shares the distinctive chemical features of LOT (average CSS in Table 1). These granites, which may occur in association with high-grade metamorphic rocks, apparently formed from sedimentary rocks with compositions close to the least feldspar-poor of the Ordovician turbidites. This also accords with the isotopic data for CSS and LOT, shown in Table 1.

The much more abundant batholithic S-type granites of White & Chappell (1988) occur in contact-aureole to subvolcanic environments and have extensive volcanic equivalents. The great abundance of these rocks in the Kosciuszko region and the area immediately to the east, implies that prior to 430 Ma, voluminous sedimentary source rocks of appropriate composition were present at depth in that part of the crust. This is in contrast to the area further east where such granites are not seen. Those observations lead White et al. (1976) to propose an "I-S line" in the region of the Berridale Batholith, east of which S-type granites are not found. Those authors suggested that the I-S line corresponds to the eastern limit of crystalline basement, ìpossibly of Precambrian ageî. That line has subsequently been traced for a total distance of 600 km with only minor lateral discontinuities, from Bass Strait to the edge of the younger basin rocks north of Orange (Chappell et al., 1988), and it must mark a rather profound east-to-west discontinuity in the nature of the deeper crust.

The batholithic S-type granites range from very mafic to highly fractionated compositions. The more mafic rocks cannot be cumulates because of (i) the close correspondence in composition between some plutonic and volcanic rocks (Wyborn & Chappell, 1986), (ii) the fact that the Sr contents are transitional, without any hiatus, to those in granites of ternary minimum melt compositions, and (iii) because the variation in composition within individual suites is not consistent with their being cumulate rocks. The mafic rocks therefore were the parental magmas, which were either rather mafic melts (Gray, 1990; Collins, 1996), or mixtures of a more felsic melt and entrained restite (White & Chappell, 1988; Chappell, 1996). That the mafic magmas were restite-bearing magmas rather than melts, is evident, for example, from (i) the extensive zircon age inheritance (Williams, 1995), (ii) the fact that variation within the most mafic rocks of the Bullenbalong Suite has a component that is ìsedimentaryî, (iii) the gross isotopic heterogeneity within the one mafic S-type pluton for which that has been tested (87Sr/86Sr = 0.71115 to 0.71541 at 430 Ma for the Jillamatong Granodiorite), and (iv) variation within the more mafic rocks in all suites and more felsic rocks in some suites, is not consistent with melt compositions produced by fractional crystallisation. These observations elevate the compositions of the most mafic S-type granites to the status of source rocks that were mobilised by partial melting and which then moved bodily without any detectable fractionation of melt from restite. Such mafic granites form a strong image of their source rocks (Chappell, 1979). Since geobarometry on mineral assemblages in the Deddick Granodiorite (Maas et al., 1997) and volcanic rocks (Wyborn et al., 1981) gives pressures of equilibration of about 550 Mpa, it is inferred that those source rocks were located at depths of 15-20 km.

Inspection of the mafic Bullenbalong Suite composition MBS in Table 1 shows that the LOS composition is not an appropriate composition, at least on its own, to be the source material for the batholithic S-type granites. MBS contains substantially higher concentrations of Ca, Na and Sr, and is distinctly less radiogenic in Sr. This has long been recognised and has been accounted for in two contrasting ways. Wyborn (1977) suggested that the source of these S-type granites was a sediment, less mature and more feldspar-rich than the exposed Ordovician turbidites, which formed a basement to those younger rocks. The alternative view is that Ordovician sediments were mixed with mafic rocks (Gray, 1984), or that material of Cooma Granodiorite composition was mixed with mafic tonalite (Collins, 1996), to produce source rocks that were richer in Ca, Na and Sr, and isotopically less evolved than the Ordovician sediments.

A less mature sedimentary source for the batholithic S-type granites is favoured by several lines of evidence. The variation within suites of those S-type granites cannot be the direct result of mixing of the Ordovician rocks with more mafic igneous rocks, because the S_type granite suites become increasingly Al2O3-oversaturated with decreasing SiO2 content (White & Chappell, 1988); this is now recognised by all authors. For the same reason, the simple ìunmixingî of a felsic melt from a magma charged in some way with fragments or crystals of chemically and isotopically less evolved material, cannot have occurred. Any process of prior mixing would have to had produced source materials that were rather uniform in chemical but not isotopic composition, prior to separation of any crystals from melt. There are pelitic enclaves in the mafic S-type granites, which on the basis of the geobarometry of Maas et al. (1997) were derived from depths of 15-20 km. Such enclaves have compositions (CPE in Table 1) that are unlike any found among the Ordovician turbidites, and contain higher Ca, Na and Sr contents than those turbidites. Whether these enclaves represent accidentally incorporated material (Maas et al., 1997) or restite (Chappell et al., 1987) is not relevant to the present argument, and they clearly show that at least some more feldspar-rich sedimentary material was present in the mid-crust at 430 Ma. Other enclaves (microgranular or microgranitic) that occur in these granites have been cited as evidence for the presence of a mafic component in the source rocks, that may have contributed to melting, but it is highly probable that such enclaves are metamorphosed and partly melted fragments of marly sediments (D. Wyborn, pers. comm.). To increase Ca contents from 0.31% in LOT to 2.68% in MBS (Table 1) would require the addition of 25% basaltic material. It was pointed out by Wyborn (1977) that the addition of large amounts of basaltic material in that way would mean that the abundances of some other elements in the source rocks, such as K and Rb, would be too low to produce the observed compositions in the batholithic S-type granites. It is concluded that those granites were derived from the partial melting of sedimentary rocks that were less mature and more feldspar-rich than the exposed turbidites.

There is an extreme contrast between the iosotopic compositions of Sr and Nd for the Ordovician turbidites and all of the S-type granites at 430 Ma. The Nd values are extremely divergent from mantle values while in comparison the Sr compositions are not. (Nd values ~ -10 correspond to model ages of derivation from a depleted mantle ~1000 Ma older than the granites. Initial 87Sr/86Sr ratios and Rb and Sr contents of the batholithic S-type granites (MBS in Table 1) conform to a much younger model age. It is clear that the Sr isotopic compositions have been reset, while the Nd values probably have not. Resetting of the Sr isotopes would have occurred during sedimentation, from a high level down to a minimum level equal to sea-water values at that time. The MBS Sr isotopic compositions would have evolved from a Cambrian sea water Sr isotopic composition of 0.709 in 90 Ma, which is a maximum age of 520 Ma for deposition of the sedimentary rocks. The extent to which the isotopic composition in the sediments did not fully equilibrate with sea water, would correspond to a younger age of sedimentation. Hence the source rocks for the S_type granites cannot be Precambrian or early Cambrian in age. They must, however, have been different in composition from the widely exposed Ordovician turbidites, and if they were Ordovician in age, they were distinct in composition from those exposed rocks. The residuum from partial melting of those rocks must now be an important component at depth, of the crust of the LFB, in those areas where voluminous S-type granites are now exposed.

The question of source rocks for the I-type granites has not been addressed in this contribution. These generally had isotopic compositions when they formed which are not consistent with direct derivation from mantle materials. The proposal that those isotopic compositions reflect a component of Ordovician turbidite in the source rocks is not consistent with chemical data for the I-type granites (e.g. increasing Ca contents in I-type granites that are more isotopically evolved). It is highly likely that the source rocks of at least some of the I_type granites, correspond with an older component than the S-type source rocks, in the basement of the LFB.

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

Chappell, B.W., 1996. Compositional variation within granite suites of the Lachlan Fold Belt: its causes and implications for the physical state of granite magma. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 159-170.

Chappell, B.W., White, A.J.R. and Hine, R., 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 35, 505_521.

Chappell, B.W., White, A.J.R. and Wyborn, D., 1987. The importance of residual source material (restite) in granite petrogenesis. Journal of Petrology 28, 1111_1138.

Collins, W.J., 1996. Lachlan Fold Belt granitoids: products of three-component mixing. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 171-181.

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Maas, R., Nicholls, I.A. & Legg, C., 1997. Igneous and metamorphic enclaves in the S_type Deddick Granodiorite, Lachlan Fold Belt, SE Australia: petrographic, geochemical and Nd_Sr isotope evidence for crustal melting and magma mixing. Journal of Petrology 38, 815_841.

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Wyborn, L.A.I. (1977) Aspects of the geology of the Snowy Mountains region and their implications for the tectonic evolution of the Lachlan Fold Belt. PhD Thesis, Australian National University (unpubl.).

Wyborn, L.A.I. & Chappell, B.W., 1983. Chemistry of the Ordovician and Silurian greywackes of the Snowy Mountains, southeastern Australia: an example of chemical evolution of sediments with time. Chemical Geology 39, 81_92.

CPE: Average of 5 analyses of pelitic enclaves from the Cootralantra Granodiorite

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