Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt
Introduction
The processes that formed the Archean tonalite-trondhjemite-granodiorite (TTG) series that volumetrically dominates Archean crust (Martin, 1986, Martin, 1994) provide insights into how, when, and at what rate early continental crust formed. TTG suites comprise rocks with a high silica content (averaging > 68 wt.% SiO2) that are sodic (Na2O ~ 4.65; K2O/Na2O ~ 0.4) and form part of a group showing little or no increase in K2O/Na2O with increasing SiO2. These rocks typically have high La/Yb (~ 48) and low Yb (~ 0.6 ppm) (Martin et al., 2005), reflecting a garnet and/or amphibole residue, and high Sr, Eu and Ba reflecting the absence of plagioclase in the source. Their formation has been attributed to melting of subducted basaltic slabs or of overthickened mafic crust (e.g., Barker and Arth, Barker and Arth, 1976, Martin, 1986, Drummond and Defant, 1990, Rapp et al., 1991, Atherton and Petford, 1993).
However, while experimental studies and trace element modelling show that the source for true TTG was generally basaltic (e.g. Sen and Dunn, 1994, Rapp and Watson, 1995, Moyen and Stevens, 2006), these studies also impose several caveats. Most notable – but largely hidden in previous discussions of TTG petrogenesis – is that even TTG at the LILE-poor end of the compositional spectrum are too enriched in K, LILE, Th and LREE to be simply a result of melting of either MORB-like basaltic crust or of the slightly LILE- and LREE-enriched low-K tholeiites generally thought typical of Archean basaltic crust (e.g. Tarney and Jones, 1994, Kemp and Hawkesworth, 2003, Clemens et al., 2006, Champion and Smithies, 2007). These enrichments in TTG are not a result of fractional crystallisation, because there appears to be no systematic correlation between (for example) SiO2 or MgO, and LILE-, LREE-enrichments in most TTG suites (e.g., Martin, 1994, Clemens et al., 2006, Champion and Smithies, 2007, Moyen et al., 2007). More complex models involving magma mixing, assimilation or mingling are not only inconsistent with field evidence that shows that microgranular (or other) enclaves are very rare (Collins, 1993), but generally do not solve the requirement for LILE-enrichment.
These constraints on source composition are particularly important for Paleoarchean TTG that form a major component of the East Pilbara Terrane in the Pilbara Craton of northwestern Australia (Appendix Fig. 1), because these rocks appear to have resulted through melting of thickened mafic crust in a non-subduction environment (Smithies, 2000, Smithies et al., 2005, Champion and Smithies, 2007, Van Kranendonk et al., 2007a, Van Kranendonk et al., 2007b). This requires that source rocks with compositions uncharacteristic of most preserved Archean crust must once have formed a fundamental component of the supracrustal sequence.
In this paper, we describe a package of LILE-, Th- and LREE-enriched basalts and basaltic-andesites from the 3.53–3.51 Ga Coucal Formation in the stratigraphically lowest parts of the Warrawoona Group, Pilbara Craton. We show that > 3.5 Ga supracrustal packages like this were possibly the ultimate source of nearly all of the granites that volumetrically dominate this region. Implicit in this hypothesis is that such packages were more typical of the pre-3.5 Ga supracrustal sequences than the relatively LILE- and LREE- poor basalts that now dominate the Archean supracrustal record. We examine the implications that this hypothesis may have on models for early crustal growth of the Pilbara Craton. The existence of LREE-enriched basalts within the > 3.51 Ga Theespruit Formation, of the Barberton greenstone belt, South Africa (e.g. Hofmann and Harris, 2008) suggests that this hypothesis might also be relevant to other tracts of Paleoarchean crust.
Section snippets
Geological setting
The East Pilbara Terrane (Appendix Fig. 1) is one of the world's best-preserved Paleo- to Mesoarchean granite–greenstone terranes. The stratigraphically lowest Warrawoona Group is a 3.53–3.42 Ga volcanosedimentary succession, at least 12 km thick, dominated by pillowed tholeiitic and komatiitic basalt (see Van Kranendonk et al., 2007a, Van Kranendonk et al., 2007b). However, Pb- and Nd-isotopic data and inherited zircon age data indicate more than 200 m.y. of crustal pre-history (e.g. Van
Constraints on the genesis of Pilbara Paleoarchean TTG
Following Barker and Arth (1976), Champion and Smithies (2007) divided Paleoarchean TTG of the Pilbara Craton into two broad subgroups; a high Al2O3, Na2O, Sr, low Fe, Y, HREE (high-Al) subgroup, which forms the majority of rocks, and a low Al2O3, Na2O, Sr, high Fe, Y, HREE (low-Al) subgroup. Both are sodic with K2O/Na2O varying from < 0.25 to close to 1, with evidence for only limited K-enrichment. Granites of both groups range from slightly to strongly LREE-enriched when normalised to
Trace-element models of potential TTG source regions
Trace-element modelling was undertaken to test whether the Coonterunah F2 basalts and basaltic-andesites of the c. 3.51 Ga Coucal Formation formed a compositionally more viable source to the Pilbara TTG than typical Archean mafic crust (see also Champion and Smithies, 2007). As an example of typical Archean basalt, we have used the average compositions of tholeiitic basalts from the c. 3.46 Ga Apex Basalt (Table 1), which stratigraphically overlies the Coonterunah Subgroup. Melt compositions
Discussion
The voluminous TTG and TTG-like rocks of the East Pilbara Terrane mainly formed between 3.5 and 3.45 Ga, from a source older than 3.5 Ga (McCulloch, 1987, Champion and Smithies, 2001, Smithies et al., 2003) (Fig. 3). In the case of the East Pilbara Terrane, the fact that this TTG is neither mantle-derived nor subduction-related (i.e. are not partly or wholly mantle- or slab-melts) indicates that their enriched basaltic source must have formed a major part of the pre-3.5 Ga supracrustal sequence.
An infracrustal model for Paleoarchean crustal evolution
From studies of the O- and Hf-isotopic compositions of zircons, Kemp et al. (2006) concluded that the processes controlling the growth of ancient continental growth were intrinsically different from those that controlled crustal differentiation. The initial formation of Pilbara TTG and subsequent, protracted reworking (differentiation) of the resulting Pilbara crust to form high-K granites forms a spectacular example of these decoupled processes.
Our model for the generation of early Pilbara
Conclusions
- •
The geochemistry of TTG requires that the source was more enriched than typical Archean basalt. In the case of the voluminous Paleoarchean (< 3.5 Ga) TTG of the Pilbara Craton, this source was older (> 3.5 Ga) than all but the oldest preserved fragment of the supracrustal sequence. These two important constraints are satisfied by the discovery that this oldest preserved fragment comprises 3.51 Ga basalts and andesites enriched in K, LILE, Th and LREE (Coonterunah F2-type rocks) to the extent that
Acknowledgements
We would like to thank Paul Morris and Chris Kirkland for comments on various drafts. Arthur Hickman is thanked for his support throughout our work in the Pilbara. Journal reviews by Tony Kemp and Chris Hawkesworth further significantly improved the manuscript. Suzanne Dowsett and Mike Prause are thanked for drafting the figures. Published with the permission of the Executive Director, Geological Survey of Western Australia and the Chief Executive Officer, Geoscience Australia.
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