GEMOC ARC National Key CentreIan S. Williams1 and Bruce W. Chappell2
1. Research School
of Earth Sciences, ANU
2. GEMOC ANU
The Lachlan Fold Belt (LFB) is just part of a broad early Palaeozoic mobile zone that extended along the eastern Gondwana margin at least from the present Trans-Antarctic Mountains about 4000 km to north Queensland. It consists mainly of Ordovician turbidite intruded by Ordovician to Carboniferous granites. Some basic questions to be answered are 1) what is the protosource of the turbidites, 2) are they underlain by older turbidites, oceanic crust, or in part by Precambrian continental crust, and 3) what are the source rocks for the granites?
The LFB S-type granites contain large amounts of inherited zircon, but the I-types have little or none. The inherited zircon can be dated by U-Pb, but have those ages have been affected by the processes of partial melting and magma genesis? At Cooma, an ultrametamorphic S-type granite is preserved at the centre of a regional, low pressure metamorphic aureole within Ordovician turbidite. Detrital zircons from a low grade turbidite remote from the granite cluster in age between 460 and 600 Ma, 1000 and 1200 Ma, and at ~1.8 Ga, ~2.2 Ga and ~2.7 Ga. The granite inherited zircon age groupings are almost identical. Igneous and metamorphic zircon overgrowths and monazite give ~430 Ma, recording simultaneous metamorphism and magma genesis. The inherited zircon preserves an accurate, unbiased record of the ages of the zircons in the granite's sedimentary source rock. Other turbidites, and large, intrusive, S-type granite bodies nearby yield very similar results. In general, the more mafic the granite, the smaller the relative proportion of new zircon growth.
Inherited zircon is much less abundant in the Bega Batholith I-types, but the age groups are the same as in the turbidites and S-types. East to west, across the granite suites, both the abundance of inherited zircon and its mean age increase. This does not simply reflect the progressive evolution of a single magma system; the major element compositions of the different granite suites are similar, and individual suites are time transgressive. There are probably two main sources for the inherited zircon, one contributing just the 500-600 Ma component, the other contributing all components. The granite compositions preclude major host rock contamination during emplacement, so both inherited components most likely come from the granites' source. That is probably mainly 500-600 Ma igneous rock, plus a small sediment component which increases towards the continent. Inherited zircon in the I-type granites' mafic enclaves supports this suggestion. Independent of the range of inheritance ages in the host granite, the inheritance in the enclaves is mainly 500-600 Ma old. Assuming the enclaves to be refractory residues from the igneous source rock, then that rock is dominantly 500-600 Ma old.
The exposed Ordovician turbidites are too chemically mature to be the source of the batholithic S-types; they could be the source of the Cooma granite. Is their zircon signature distinctive? Exploration of detrital zircon ages in Ordovician sediments across the LFB shows that the signature varies very little either regionally or with sediment age. The same components recur, the only differences being small changes in their relative abundances. The same is true of Ordovician sandstone in the Amadeus Basin, and others have traced the signature as far as the Ordovician sediments of New Zealand and, more recently, Antarctica and South Africa. The Ordovician turbidites are part of a large volume of sediment that is very uniform in its protosource over its entire range and is therefore almost certainly not of local origin. Two samples of basal Ordovician turbidite are strongly depleted in the 500-600 Ma component, one Cambrian turbidite from western NSW resembles the Ordovician but lacks zircon younger than ~600 Ma, and a Cambrian sandstone from the Amadeus Basin contains no 500-600 Ma zircon at all. Possibly the characteristic eastern Gondwana zircon age signature was introduced to the area by the Ordovician turbidites. If so, this would limit the source of the S-type granites to being an immature portion of the Ordovician pile. This is consistent with the youngest inherited zircons in some S-type granites and enclaves being ~480 Ma old, implying (if those zircons are not isotopically disturbed) that the source sediments are no older than early Ordovician. There is no compelling evidence that the source (as distinct from the zircons it contains) is Precambrian. Absence of zircon ages reflecting the Australian hinterland make a remote protosource for the sediments, possibly the southern Mozambique Belt, the most likely.
The Palaeozoic granites and sedimentary
rocks of eastern Australia share a characteristic zircon age signature
that not only demonstrates a petrogenetic link between them, but also is
a distinctive fingerprint for eastern Gondwana.
