David C. Champion

Chapter 4.3 Geochemistry of Paleoarchean Granites of the East Pilbara Terrane, Pilbara Craton, Western Australia: Implications for Early Archean Crustal Growth

Publisher Summary This chapter presents the geochemistry of paleoarchean granites of the East Pilbara terrane, Pilbara craton, Western Australia, and the implications for early archean crustal growth. The Pilbara Craton is divided into the 3.53–3.07 Ga East Pilbara Terrane (EPT), which includes the Pilbara Supergroup, the 3.27–3.11 Ga West Pilbara Superterrane (WPS), and the 3.2 Ga Kurrana Terrane (KT). Each of these is distinguished by unique lithostratigraphy, structural map patterns, geochemistry, and tectonic histories and they are separated by late tectonic, dominantly clastic sedimentary rocks of the De Grey Supergroup deposited in the Mallina and Mosquito Creek basins. The exposure of preserved granite totals an area of ∼24,000 km 2 . Most of the known periods of granite intrusion in the EPT either broadly correspond to the periods of greenstone development or postdate greenstone formation. The geological and Sm–Nd data further indicate that post-3.2 Ga crustal growth extended to the western part of the EPT, and the post 3.2 Ga granites in that region are, accordingly, petrogenetically grouped with those found in the Mallina Basin. It is found that the 3.32–3.24 Ga granites have an expanded silica range, but include much more silica-rich members than the older granites.

Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Publisher Summary This chapter describes the lithostratigraphy, geochemistry, and structural and metamorphic geology of the ancient, eastern nucleus of the Pilbara Craton. The 3.53–2.83 Ga Pilbara Craton of Western Australia is one of only two areas on the Earth that contain large, well-exposed areas of little deformed, low-grade Paleoarchean rocks and the other being the Kaapvaal Craton in southern Africa—and as such is important for understanding the early Earth and the processes involved in the formation of continental crust. The Pilbara Craton is famous for its well preserved Paleoarchean volcano-sedimentary succession that includes evidence of the oldest life on the Earth, and for its classic dome-and-keel geometry of ovoid, domical granitic complexes, and flanking arcuate, synclinal greenstone belts. The 3.53–2.83 Ga Pilbara Craton is a nearly circular piece of crust in the northwestern part of Western Australia whose boundaries are defined by aeromagnetic and gravity anomalies, and by orogenic belts. A significant result of the extensive SHRIMP geochronology of volcanic and sedimentary rocks from the Pilbara Supergroup is the discovery of abundant inherited zircons that are older than the oldest dated supracrustal rocks.

Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons

Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.

Chapter 4.2 The Oldest Well-Preserved Felsic Volcanic Rocks on Earth: Geochemical Clues to the Early Evolution of the Pilbara Supergroup and Implications for the Growth of a Paleoarchean Protocontinent

Publisher Summary This chapter examines the geochemical clues to the early evolution of the Pilbara Supergroup and implications for the growth of a paleoarchean protocontinent. Geochemical studies of mafic rocks can potentially provide direct clues on mantle evolution, mantle conditions, and juvenile crust formation, whereas geochemical studies of felsic rocks can provide clues as to how the crust differentiated. The Warrawoona Group, in the Pilbara Craton of northwestern Australia, is a basalt-dominated volcano-sedimentary sequence formed through essentially continuous volcanism from 3.515 to 3.426 Ga. The Pilbara Craton is divided into the 3.53–3.17 Ga East Pilbara Terrane, the 3.27–3.11 Ga West Pilbara Superterrane, and ∼ 3.2 Ga Kurrana Terrane, distinguished by unique lithostratigraphy, structural map patterns, geochemistry, and tectonic histories. The early volcano-sedimentary history of greenstones within the East Pilbara Terrane is described by the Pilbara Supergroup, which is composed of four demonstrably autochthonous groups, of which the 3.53 to 3.426 Ga Warrawoona Group is the stratigraphically lowest group. The petrogenesis of felsic volcanic rocks is also elaborated in the chapter.

Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating

ABSTRACT The advent of chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-IDTIMS) has revolutionised U–Pb dating of zircon, and the enhanced precision of eruption ages determined on volcanic layers within basin successions permits an improved calibration of biostratigraphic schemes to the numerical time-scale. The Guadalupian and Lopingian (Permian) successions in the Sydney, Gunnedah, Bowen and Canning basins are mostly non-marine and include numerous airfall tuff units, many of which contain zircon. The eastern Australian palynostratigraphic scheme provides the basis for much of the local correlation, but the present calibration of this scheme against the numerical time-scale depends on a correlation to Western Australia, using rare ammonoids and conodonts in that succession to link to the standard global marine biostratigraphic scheme. High-precision U–Pb zircon dating of tuff layers via CA-IDTIMS allows this tenuous correlation to be circumvented—the resulting direct calibration of the palynostratigraphy to the numerical time-scale highlights significant inaccuracies in the previous indirect correlation. The new data show: the top of the Praecolpatites sinuosus Zone (APP3.2) lies in the early Roadian, not the middle Kungurian; the top of the Microbaculispora villosa Zone (APP3.3) lies in the middle Roadian, not the early Roadian; the top of the Dulhuntyispora granulata Zone (APP4.1) lies in the Wordian, not in the latest Roadian; the top of the Didecitriletes ericianus Zone (APP4.2) lies in the first half of the Wuchiapingian, not the latest Wordian; the Dulhuntyispora dulhuntyi Zone (APP4.3) is exceptionally short and lies within the Wuchiapingian, not the early Capitanian; and the top of the Dulhuntyispora parvithola Zone (APP5) lies at or near the Permo-Triassic boundary, not in the latest Wuchiapingian.

Crustal evolution constraints on the metallogeny of the Yilgarn Craton

Improved understanding of the evolution of the Archean Yilgarn Craton, Western Australia, indicates a collage of crustal fragments formed through both autochthonous and allochthonous processes. Komatiite-associated nickel sulfide (KANS), volcanichosted massive sulfide (VHMS) Zn-Cu and orogenic Au deposits are associated with specific crustal domains. KANS deposits are concentrated in terranes with evidence for pre-existing crust. In contrast, VHMS deposits are associated with rift-like zones of isotopically primitive crust. Gold mineralization predominates in domains with evidence for komatiite magmatism, melts sourced from subduction-modified mantle, and lithospheric-scale orogeny. Development of the mineral systems likely reflects interaction of specific geotectonic processes with the pre-existing crustal and mantle reservoirs.

Preservation of a fragmented late Neoproterozoic–earliest Cambrian hyper-extended continental-margin sequence in the Australian Delamerian Orogen

Abstract Mafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica. Supplementary material: Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821

Characteristics and geodynamic setting of the 2.7 Ga Yilgarn heterogeneous plume and its interaction with continental lithosphere: evidence from komatiitic basalt and basalt geochemistry of the Eastern Goldfields Superterrane

A database of 1075 high-precision geochemical analyses of least-altered ultramafic–mafic units, predominantly flows, was compiled for the Eastern Goldfields Superterrane. Samples are divided into a high-Mg population at MgO≥10–24 wt% and a basaltic population where 4≤MgO<10 wt%. There are eight groups based on (La/Sm)N and Nb/Th ratios. Five magma series are identified. Uncontaminated komatiitic basalts have MgO ∼11–23 wt% and Nb/Th≥8, whereas contaminated counterparts have Nb/Th<8 corresponding to silicious high-Mg basalts (SHMB). A distinct second magma series with MgO ∼5–18 wt% MgO has a narrow range of Nb/Th at 0.5–≤2 over a range of (La/Sm)N from 0.7–5.5, unlike contaminated suites where (La/Sm)N and Nb/Th are correlated; this series corresponds to the enriched Paringa Basalt representing shallow melts of heterogeneous domains of the plume with recycled ancient continental lithosphere, or an independent plume. Prevalent, crustally uncontaminated, tholeiitic basalt magma series three all have Nb/Th≥8, span Mg-rich to fractionated Fe-rich counterparts, and range from LREE-depleted to mildly LREE-enriched where high Nb/Th ratios stem from eclogite streaks in the asthenosphere plume; contaminated equivalents have Nb/Th<8. A fourth alkaline, high-Mg magma series has a narrow range of MgO at ∼13–16 wt%, extends to elevated TiO2 and Ni contents relative to komatiitic basalts at that MgO range, and features (La/Sm)N ≥2. Two additional uncontaminated tholeiitic basaltic groups are defined respectively by high-Nb to 20 ppm akin to alkaline ocean island basalts, and high-ΣREE relative to the other basaltic groups. The former, a fifth magma series, reflect melts of an eclogite-rich domain of the plume. Contamination of all groups, when present, was dominantly by interaction with continental mantle lithosphere with a minor crustal component. Komatiitic basalts are fractionation products of komatiites erupted from the hot axis of a mantle plume whereas prevalent tholeitic basalts are liquids derived from the cooler plume annulus. In all cases melting was in anhydrous peridotite. Ratios of Nb/Th in uncontaminated samples span 8–24 signifying that the Neoarchean mantle was as heterogeneous in terms of this ratio as Phanerozoic asthenosphere. In contrast, the fourth alkaline magma series stems from decompressional melting of metasomatised, hydrous, continental mantle lithosphere at >90km. Komatiites and komatiitic basalts are most abundant proximal to terrane boundaries because mantle plumes are ‘steered’ to the margins of thin, rifted, continental lithosphere. Given that mantle plumes melt on impingement at the base of the lithosphere, (Gd/Yb)N ratios are used as a proxy to ‘map’ the thickness of the contemporaneous lithosphere.

Tectonic controls on the endowment of Archean cratons in VHMS deposits: Evidence from PB and Nd isotopes

Comparison of Pb and Nd isotopes of the well-endowed Abitibi-Wawa Subprovince with the poorly endowed Eastern Goldfields Province implies that volcanic-hosted massive sulfide (VHMS) endowment in Archean terrains is controlled by crustal character. The Abitibi-Wawa Subprovince contains mostly primitive crust formed in a wide extensional environment. The high heat flow, promoted by thin crust and high level intrusions, and the extensional structures that characterize such an environment encouraged the formation of extensive VHMS deposits. In contrast, extensional environments are limited to a relatively narrow zone in the Eastern Goldfields Province. Although this zone contains VHMS deposits, they are not abundant enough to create high endowment. Provinciality of Pb isotopes in lode Au deposits from near Leonora in the Eastern Goldfields Province supports the concept of a rift-like zone.

Insights into the evolution of the Thomson Orogen from geochronology, geochemistry, and zircon isotopic studies of magmatic rocks

Abstract Zircon U–Pb ages, εHf(t), and δ18O isotopic data together with geochemistry and limited Sm–Nd results from magmatic rocks sampled in deep-basement drill cores from undercover parts of the Thomson Orogen provide strong temporal links with outcropping regions of the orogen and important clues to its evolution and relationship with the Lachlan Orogen. SHRIMP U–Pb zircon ages show that magmatism of Early Ordovician age is widespread across the central, undercover regions of the Thomson Orogen and occurred in a narrow time-window between 480 and 470 Ma. These rocks have evolved εHf(t)zrn (−12.18 to −6.26) and εNd (−11.3 to −7.1), and supracrustal δ18Ozrn (7.01–8.50‰), which is in stark contrast to Early Ordovician magmatic rocks in the Lachlan Orogen that are isotopically juvenile. Two samples have late Silurian ages (425–420 Ma), and four have Devonian ages (408–382 Ma). The late Silurian rocks have evolved εHf(t)zrn (−6.42 to −4.62) and supracrustal δ18Ozrn (9.26–10.29‰) values, while the younger Devonian rocks show a shift toward more juvenile εHf(t)zrn, a trend that is also seen in rocks of this age in the Lachlan Orogen. Interestingly, two early Late Devonian samples have juvenile εHf(t)zrn (0.01–1.92) but supracrustal δ18Ozrn (7.45–8.77‰) indicating rapid recycling of juvenile material. Two distinct Hf–O isotopic mixing trends are observed for magmatic rocks of the Thomson Orogen. One trend appears to have incorporated a more evolved supracrustal component and is defined by samples from the northern two-thirds of the Thomson Orogen, while the other trend is generally less evolved and from samples in the southern third of the Thomson Orogen and matches the isotopic character of rocks from the Lachlan Orogen. The spatial association of the Early Ordovician magmatism with the more evolved metasedimentary signature suggests that at least the northern part of the Thomson Orogen is underlain by older pre-Delamerian metasedimentary rocks.