December 2007 LIP of the Month

The high Fe-Ti mafic magmatism in the Paleoproterozoic Broken Hill Block of the Curnamona Province, southeastern Australia, as part of a Large Igneous Province?

Massimo Raveggi
Australian Crustal Research Centre
School of Geosciences
Monash University
3800 Victoria, Australia

David Giles
South Australian Centre for Mineral Exploration Under Cover
School of Earth & Environmental Sciences
University of Adelaide
5005 South Australia, Australia

John Foden
Continental Evolution Research Group
School of Earth & Environmental Sciences
University of Adelaide
5005 South Australia, Australia

Mike Raetz
Australian Crustal Research Centre
School of Geosciences
Monash University
3800 Victoria, Australia

Contributed by Massimo Raveggi

Abstract

The metasedimentary sequences of the Willyama Supergroup in the Broken Hill Block of the Curnamona Province in western NSW (Fig. 1), Australia were intruded at ca. 1685 Ma by high Fe-Ti mafic rocks.  These unusual lithologies occur throughout the lower Willyama Supergroup stratigraphy (Fig. 2) and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600-1580 Ma). Our data indicate that the high Fe-Ti mafic meta-igneous rocks originated as a result of variable degrees of partial melting of a depleted mantle source, but slightly enriched with incompatible elements compared to present day N-MORB. This was followed by simple crystal fractionation or by an AFC (assimilation-fractional crystallisation) process involving only small degrees of crustal assimilation (rate of assimilation to rate of crystal fractionation, r = 0.05-0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt % of total iron as Fe₂O₃ and 4.2 wt % of TiO₂. Volumetrically, minor intermediate rocks evolved from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Rasp Ridge and Hores/Potosi Gneisses) are not linked by fractional crystallisation to the mafic melt that produced the meta-igneous high Fe-Ti mafic rocks and are products of the anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bimodal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Block (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Domain, in contrast, imply smaller degrees of partial melting, higher degrees of crustal contamination and the presence of a subcontinental lithospheric mantle source, which suggests relatively thicker lithosphere and places the Olary Domain on the rift margin (Raveggi et al., 2007).

Bryan and Ernst (2008) defined a Large Igneous Province as follows: area extents >1 Mkm²; igneous volumes > 0.1 Mkm³ with igneous pulse(s) of short duration (~1 to 5 Ma) during which > 75% of the total igneous volume has been emplaced; a maximum lifespan of ~50 Ma; intraplate tectonic settings. The Curnamona Province loosely fits this definition of Large Igneous Province. The extension of the Curnamona Province is not comparable to the one of Large Igneous Provinces and the high Fe-Ti mafic rocks are volumetrically subordinate in the stratigraphic sequence and manly occur in the Broken Hill Block. Nevertheless the magmatic activity occurred in an intra-continental rift setting and the bulk of the high Fe-Ti mafic rocks intruded in a relatively short time period of time at ca. 1685 Ma (see Raveggi et al., 2007 for a review of the geochronology of these rocks). This is indeed comparable to other Large Igneous Provinces.

Several authors have pointed out the lithological, geochronological, metamorphic and metallogenic similarities between the Broken Hill Block, the Eastern Fold Belt of the southeastern margin of the Mount Isa Inlier and the Georgetown Inlier (Fig. 3a) of North Queensland (Black et al., 1998; Page and Sun 1998; Williams, 1998; Giles and Nutman, 2003; Hills, 2003; Giles et al., 2004; Page et al., 2005). Importantly, the three terranes are characterised by the presence of broadly coeval high Fe-Ti mafic rocks  (Withnall, 1985; Williams, 1998; Baker and Crawford, 2006; Raveggi et al., in prep). High Fe-Ti mafic rocks are rare in the geologic record (Raveggi et al., 2007) and their occurrence in the Willyama Supergroup, the Eastern Fold Belt and the Georgetown Inlier provides compelling evidence for a genetic link between magmatism in these terranes. These similarities have lead to paleogeographic reconstructions suggesting that these terranes were much closer in the Paleo- to Mesoproterozoic than what they are now (Fig. 3a and 3b; Giles et al., 2004; Betts and Giles 2006). In this view the Curnamona Province, together with the North Queensland’s Eastern Fold Belt and Georgetown Inlier could possibly represent the erosional remnants or the pieces of a Large Igneous Province fragmented by subsequent plate tectonic break-up.

References