A Pilbara perspective on the generation of Archaean continental crust

Abstract

Models for the generation and evolution of continental crust are informed by a detailed understanding of regional geology and geochemistry. This contribution therefore evaluates the increasing amounts of high quality geochemical data, on well-characterized samples, now available from the magmatic rocks of the Pilbara Craton, Western Australia. The East Pilbara Terrane is characterized by vertical dome and basin tectonics, and the younger (Mesoarchean) rocks to the west are associated with lateral tectonics. The craton stabilized at ~2.95 Ga (Hickman, 2012), and it was overlain by voluminous, dominantly mafic lavas of the Fortescue Group in an intra-cratonic setting at ~2.7 Ga. The magmatic rocks of the Pilbara Craton are bimodal in SiO2, with the volcanic rocks typically having <60% SiO2, and the plutonic rocks >60% SiO2, and such differences are in part linked to different depths of magma emplacement.The volcanic rocks are divided into two groups, with Dy/La > and < 0.6. The geochemistry of the former group is consistent with broadly closed system differentiation, whereas the low Dy/La rocks largely reflect open system magmatic processes, and contributions from high silica magmas. The high Dy/La rocks have restricted Th/Nb ratios similar to those of primitive mantle, whereas the low Dy/La rocks range to higher Th/Nb values. In most cases these higher Th/Nb values reflect open system magma differentiation, but in some of the younger volcanic suites (the Bookingarra Group and Cooneeina Basalt) these values appear to be associated with subduction. There is no evidence for garnet or amphibole in the generation of the volcanic rocks, however the composition of the plutonic rocks reflects the involvement of both garnet and plagioclase, during melt generation and differentiation.The high Dy/La rocks have Sm/Nd ratios similar to CHUR, but their Lu/Hf ratios are significantly sub-chondritic. Many of the Pilbara rocks from 3.5–3.0 Ga were generated from mantle sources with broadly chondritic Hf isotope ratios, and so in the preferred model the mean composition of the high Dy/La group was generated by ~20% melting of chondritic mantle. Apart from the high Th/Nb magmas at 3.05–2.97 Ga (Smithies et al., 2005), there are surprisingly few changes in the minor and trace element features of the magmas associated with the generation of the craton (3.5–3.0 Ga), and after it stabilized at ~2.95 Ga. The 3.05–2.97 Ga plutonic rocks in the granitic complexes also have elevated Th/Nb, suggesting that such ratios were inherited from their mafic source rocks. This observation implies that the mafic sources and the granitic rocks were of similar age, and that there was rapid conversion of young mantle-derived basaltic crust into felsic continental crust, with little contribution from older basement material. The volcanic rocks are characterized by relatively low water contents (~<2%) on the basis of their low maximum Al2O3 values, a mean Fe3+/ΣFe of 0.19 ± 0.01, and models in which the high Dy/La rocks were derived from primitive mantle. Such low water contents relative to modern subduction-related magmas are a feature of the magmatic rocks of the Pilbara Craton, and the development of ‘granitic crust’ appears to be linked to the development of higher water contents associated with subduction, locally commencing at ca. 3.1 Ga, and to globally dominant plate tectonics.