Australia: The Land Where Time Began

A biography of the Australian continent 

Pilbara OPS

The Pilbara Craton in northwestern Australia, which dates to 3.53-2.83 Ga, contains several low-grade greenstone belts dating to the Paleoarchaean that are remarkably well preserved, with little deformation (Van Kranendonk et al., 2007). Broadly the Pilbara is devisable into the western and eastern blocks.

In the western block there are several greenstone belts, the most prominent of which is the Cleaverville Belt that dates to 3.3-3.2 Ga, which consists of basaltic greenstones on the coast with pillow lavas, breccias and hyaloclastites, that are overlain by bedded cherts and banded iron formation (BIF), ferruginous and siliceous mudstones, and sandstone/mudstone/conglomerate turbidite at the top (Kato et al., `998). Thrusts into well-defined duplexes have duplicated the rocks (Ohta et al., 1996); the original oceanic plate stratigraphy provides the unravelling. The basalts are low-K tholeiites that are richer in FeO though are otherwise similar to modern MORB (Ohta et al., 1996). The metamorphic grade of the basalts increases downwards, which corresponds to the thermal structure of the metamorphism of the ocean floor at a mid-ocean ridge (Shibuya et al., 2007). It is demonstrated by the detailed trace element chemistry through the sediments (Kato et al., 1998) that Eu anomalies decrease up-section as REE contents and LREE/HREE ratios increase, which shows that the REE signatures are remarkably similar to those of modern hydrothermal sediments that are precipitated near a mid-ocean ridge, and there is significant terrigenous material in the upper clastic sediments. It is suggested by these relations that sediments shifted from proximal hydrothermal, through distal hydrothermal to terrigenous, which in turn suggests deposition during horizontal ridge-trench transition (Kato et al., 1998).

The Cleaverville Group on Dixon Island comprises imbricated cyclic packages of pillow lavas that are cut by dolerite dykes and sills (feeders to higher basalts), rhyolite flows (up to 900 m), pyroclastic breccias and felsic ashfall tuffs Rhyolite tuffs (150 m) that are cut by veins of black hydrothermal chert up to 2 m wide, as well as white silica dykes, bedded black cherts that are up to 100 m thick, and clastic sediments that are unconformable. The lithostratigraphy, that includes the overall OPS, is very similar to that of a modern immature island arc like Izu-Bonin in Japan (Kikokawa & Taira, 1998). This mode of evolution is consistent with the Pacific-type accretion that was envisaged by (Krapez & Eisenlohr, 1998).

There are 4 greenstone belts/Groups that wrap around granitic domes in the East Pilbara Block (3.16-3.53Ga); though the origin of the East Pilbara rocks has proven to be very controversial. The Warrawoona is the main Group, which is suggested by Van Kranendonk et al. (2007) to consist largely of mafic lavas that are at least 12 km thick. The main Groups and localities in East Pilbara that have OPS are:

The lower part of the Warrawoona Group in the North Pole region is well exposed. It has been demonstrated (Kitajima et al., 2001) from structural mapping at a scale of 1:5,000, which covers an area of 10 x 30 km, that the OPS succession consists of a pile of pillowed basaltic greenstones, that is ~6 km thick, with local komatiites, bedded tuffaceous cherts (>30 m), local felsic volcanics, and conglomerates and sandstones at the top of one of the latest units, and not of a single stratigraphical unit (Van Kranendonk et al., 2007; and earlier papers). They defined 5 units that were each capped by a bedded chert, and each unit is separated by thrusts that are layer-parallel and clear, small duplexes (on first inspection the thrusts appear to be bedding planes). Basaltic greenstones (MORB chemistry) are present in the 2 lower units with mid-ocean hydrothermal metamorphism of ridge type, that is intruded by more than 2,000, white chert-barite dykes that are up to 10 m thick and >1 km long (downwards) and black chert dykes that increase in width and abundance to the capping of barite-bearing chert beds (Nijman et al., 1998). The silica dykes have been interpreted as the fossil pathways of hydrothermal fluids (Kitajima et al., 2001). There is an increase downwards through the units in the metamorphic grade of the greenstone (prehnite-pumpellyite, transitional to greenschist) which is comparable to that seen in modern ocean floor metamorphism (Terabayashi et al., 2003). The 5 units are demonstrated to decrease in age progressively downwards by zircon ages from the tops of Units III and IV and the underlying monozodiorite. It was argued (Kitajima et al., 2001) that the overall downwards younging stratigraphy combined with the right-way-up stratigraphy of all the units, each of which is separated by thrust complexes, is so similar to that of circum-Pacific complexes of the present that the Lower Warrawoona Group is a subduction-accretionary complex with prominent OPS that formed by ridge-trench shortening and imbricated during accretion by layer-parallel thrusting.

Cherts and overlying clastics in the 3.463-3.454 Ga OPS within the Warrawoona Group, at Marble Bar 50 km to the East of North Pole, were studied geochemically in detail (Kato & Nakamura, 2003). Pillowed and massive basalts that are 1 km thick are Fe-rich, Low-K tholeiites with a MORB-type geochemical signature, with the exception of high CO₂ that is caused by seafloor hydrothermal carbonisation (Nakamura & Kato, 2004). An anastomosing network of black silica-barite dykes that are 5-30 m thick, up to the base of the overlying bedded cherts, traverse the upper 500 m of the basalts, though they do not penetrate upward through the cherts; they are interpreted as hydrothermal feeders onto the seafloor where they deposited silica for the bedded cherts. Some massive and pillowed komatiitic basalts have undergone upwards silicification up to the overlying chert caps. It was concluded (Van Kranendonk, 2006) that bedded cherts, that are associated with fossilised stromatolites, were silicified by the hydrothermal fluids that had circulated through the underlying basalts by way of the silica-barite dykes. The main bedded cherts, which are multi-coloured and are more than 45 m thick, are overlain by chlorite-bearing volcaniclastic cherts, which are likely to have been derived from low-T, Si-precipitating hydrothermal solutions from a hot spot over which the inferred oceanic plate has passed. It is suggested by the uppermost volcaniclastic cherts which are enriched in Zr, Nb, Hf and Th, and have high Th/Sc and (La/Yb)_N, that the depositional site was approaching a continental source or margin. According to Bolhar et al. (2005) silica-jasper couplets in the uppermost cherts contain incompatible trace elements, which indicated input into shallow water that is saturated in silica from a terrigenous source via volcanic ash. Siliceous mudstones, sandstone/mudstone alternations, fine- to coarse sandstones, and topmost conglomerates overlay the cherts. Kato & Nakamura (2003) interpreted these clastic sediments as turbidites that were deposited near or from a continental margin.  Trace elements are contained in cherts, which indicate an increasing flux of debris that was continent-derived, though that geochemical signature cannot be observed in the field is an important observation. Kusky et al. suggest that the remarkable similarity of these stratigraphic and geochemical features of the marble Bar OPS with those of Permian-Triassic OPS in Japan (Matsuda & Isozaki, 1991; Kato et al., 2000) has provided robust support for the conclusion that in the Early Archaean the depositional environment changed from mid-ocean spreading centre with a strong hydrothermal flux, via a hotspot, towards a trench that had been infilled with clastic sediments. They suggest that if this conclusion is correct, then the ridge-trench transition formed a plate opening, seafloor spreading, and subduction implies that horizontal plate tectonics, that were not appreciably different from that of the Earth in the Phanerozoic, was operating in the East Pilbara by 3.46 Ga.

General model of OPS in accretionary orogens

Oceanic plate stratigraphy (OPS) is defined in this paper as the sequence of basement igneous rocks that were deposited on the seafloor as the oceanic basement beneath it inevitably moved from the mid oceanic ridge where it was formed towards its deep sea trench at a subduction zone where it slipped beneath an adjacent plate. Among the many possible variations of OPS there are types of oceanic basement in which the secondary carapace it is deposited upon, to types of early sediments (such as carbonates, if the initial ridge is above the CCCD), to variations that were acquired in the younger sediments as the oceanic basement approaches different types of convergent margins. In this paper, Kusky et al. first discussed a “standard” model of OPS, and then discuss some of the possible variations.

Standard model of OPS

Oceanic crust and lithosphere of normal character (i.e., corresponding to the typical Penrose type ophiolite, grading down from pelagic sediments, to pillow basalts, a sheeted dike complex, gabbros, ultramafic cumulates, then tectonised harzburgite) is overlain by pelagic sediments that were initially deposited at the ridge. If the ridge is elevated above the CCCD, these can include pelagic carbonates, which are then overlain by deep sea cherts and pelagic shales.

Summary

In the Pilbara Craton there are basalt-chert-clastic successions that are remarkably well preserved, which can be interpreted convincingly in terms of OPS and ridge-trench tectonics. Though this environment is incompatible with the idea that is favoured by e.g. Von Kranendonk et al. (2007 and earlier papers) which is currently popular (well-advertised) according to which in the Early Archaean the East Pilbara evolved by several massive outpourings of mantle plumes which formed a continuous lava pile that was 12 km thick, or indeed an eruption onto a continental basement (Green et al., 2000), or a volcanic arc setting (Barley, 1993), or an oceanic plateau setting (Condie, 1997). Blewett (2002) supported the case for horizontal shortening (plate interaction), based on structural studies around Marble Bar and extended across the whole of the Pilbara Craton, and earlier by Bickle et al. (1980). Kusky et al. emphasise that it is the OPS in Pilbara, which provides the most definitive and diagnostic evidence for plate tectonics in the Early Earth, a feature and conclusion that is noticeably ignored, or not taken account of, in most models of Early Archaean evolution.

Sources & Further reading

Kusky, T. M., et al. (2013). "Recognition of ocean plate stratigraphy in accretionary orogens through Earth history: A record of 3.8 billion years of sea floor spreading, subduction, and accretion." Gondwana Research 24(2): 501-547.