Australia: The Land Where Time Began
Australian Tectonic and Metallogenic Evolution - A Summery Pt.1
According to the authors Huston et al. Australia's geological evolution is linked closely to supercontinent cycles that have characterised the Earth's tectonic evolution, with most geological and metallogenic events being related to the assembly and breakup of supercontinents/supercratons. The Australian continent mostly grew from west to east, The Yilgarn Craton and the Pilbara Craton, 2 major cratons from the Archaean, formed the oldest part of the continent as the West Australian Element. Mostly Palaeoproterozoic-Mesoproterozoic North Australian Element and the South Australian Element formed the central part of the continent, and the eastern part of the continent is dominated by the Tasman Element from the Neoproterozoic-Mesozoic. The initial assembly involving the West, North and South Australian Elements took place during the amalgamation of Nuna in the Palaeoproterozoic, with the Tasman Element forming mostly as an accretionary margin in the Palaeozoic during the assembly of Gondwana-Pangaea. The breakup of Gondwana resulted in the present position of Australia as a continent that is relatively stable. The current movement of Australia is towards Southeast Asia, which the authors1 say is probably a reflection of the earliest stages of assembly of the next supercontinent, Amasia.
The mineral and energy resources of Australia are linked to its tectonic evolution and the supercontinent cycle. The most important gold province was produced by the assembly of Kenorland, and its major zinc-lead-silver deposits, as well as it iron oxide-copper-gold deposits formed as Nuna was breaking up. The assembly of Pangaea-Gondwana produced the diverse metallogeny of the Tasman Element, and the breakup of this supercontinent produced most of the hydrocarbon resources of the Australian continent.
As the Australian continent moved away from Antarctica the last connections were finally broken about 34 Ma (Veevers et al., 1991), a process which largely isolated the Australian continent from active plate margins. The complex geological history of Australia, that is contentious, as recorded in the rocks of the Australian landmass, can be traced back to the Eoarchean. In this article the authors1 present 1 interpretation of this history, in which they explore some of the related controversies, as well as linking the geological heritage of Australia to its resources, geography, flora and fauna to this tectonic narrative. In Australia much of the geology has resulted from the amalgamation and breakup of supercontinents and supercratons over geological time, including such supercontinents of the past as Vaalbara, Kenorland, Nuna (Columbia), Rodinia and Pangaea that includes Gondwana.
The distribution of resources, such as petroleum, gas and minerals has been controlled by supercontinent history, as well as influences on the processes that have, and will continue to have, a role in the shaping of Australia of the present. Many floral and faunal affinities are shared between Australia and South America and Africa, though few affinities have been found between Australia and the continents of the northern hemisphere (e.g., Couper, 1960;Fooden, 1972), the breakup separated Gondwana from Pangaea isolating it from Eurasia and North America. The separation of Australia from Antarctica also opened a seaway that allowed the polar circulation of ocean currents that eventually became the Southern Ocean, and ultimately the Antarctic polar ice cap (Livermore et al., 2005), which made Australia, as the rest of the world, drier and cooler (Fujioka & Chappell, 2010).
The Australian continent grew mostly from west to east, the west being comprised of rocks from the Archaean, in the centre there are rocks from the Proterozoic, and in the east the rocks are of Phanerozoic age. The present state of the continent is that the western 2/3 is comprised of 3 elements that are mostly Precambrian, The West Australian, North Australian and South Australian Elements, while the Tasman Element makes up the eastern 1/3. According to Huston et al. these spatial and temporal growth patterns are consistent with the growth of supercontinents and supercratons, especially Kenorland, Nuna and Pangaea-Gondwana. This broad pattern also governs the distribution of Australia's mineral and energy resources, with distinctive deposit assemblages characterising each of the 4 major cratonic elements, both in space and composition.
There were 4 broad time periods during which the Australian continent evolved, 3,800-2,200 Ma, 2,200-1,300 Ma, 1,300-700 Ma and 700-0 Ma. In the first period the growth of nuclei took place, the cratonic elements growing around these nuclei, during the latter 3 periods amalgamation and dispersal took place of Nuna, Rodinia and Pangaea-Gondwana, respectively. Huston et al. have prepared sections showing a history of the growth of the Australian continent of the present by use of this framework, though they note that there is significant uncertainty in many cases, and disagreement about specific details and abut whether some of these processes even occurred. Context for events that have shaped and changed Australia, as well as the Earth, is provided by this geohistory, such as the evolution of life and changes in atmospheric composition and the composition of the hydrosphere. The evolution of the mineral and petroleum systems in Australia provides a framework for the evolution of the continent as a whole, as they have been increasingly linked to geodynamic processes.
On the Australian continent the oldest rocks with ages between 3731-3655 Ma are anorthosite and orthogneiss in the Narryer Terrane of the Yilgarn Craton (Kinny et l., 1988; Wilde et al., 2001), and in the Pilbara Craton, as a xenoliths in plutons of a younger age (Van Kranendonk et al., 2002). These 2 cratons comprising the West Australian Element are the most extensive exposures of old rocks in Australia, though recently rocks from the Archaean-Palaeoproterozoic have been increasingly identified within the North and South Australian Elements where they form the nuclei onto which these elements were accreted (Fraser et al., 2010; Hollis et al., 2011).
The Meeberrie Gneiss, dated to 3731 ± 4 Ma, is the oldest known rock in continental Australia (Kinny et al., 1988) is contained in the Narryer Terrane, that also contains the oldest known mineral on Earth, a detrital zircon that has been dated to about 4404 Ma that is found in the sedimentary rocks of Jack Hills that is of Palaeoproterozoic age (Wilde et al., 2001). Together with other zircons that are 150-350 My younger than the age of the Earth have implications for the earliest part of the history of the Earth. Their presence at that time indicates that continental crust had formed very early, and from their heavy oxygen isotope characteristics indicate that there were already oceans present in the Hadean (Wilde et al., 2001).
The Yilgarn Craton declines in age from west to east. Rocks comprising the Narryer, Youanmi and Southwest Gneiss Terranes formed between 3730-2900 Ma in the west and those of the Kalgoorlie, Kurnalpi, Burtville and Yamarna Terranes, which comprise the Eastern Goldfields Superterrane, were formed between 2940-2660 Ma in the east. The eastern Goldfields Superterrane is envisaged in most tectonic reconstructions as arc-related accretion (e.g., Barley et al., 1989; Krapez et al., 2008), though there is some variation in the number of arcs and their polarity. Assembly along a series of east- and west-dipping subduction zones that closed between 2780-2655 Ma are inferred (e.g., Korsch et al., 2011a). An alternative suggestion (Czarnota et al., 2010) infers that Eastern Goldfields Superterrane growth relates to a west-dipping subduction zone, that was long-lived, to the east of the Burtville Terrane. The authors1 suggest it is likely the accretionary processes have some broad similarities to subduction processes of the present, with backarc basins and major orogenic events forming as fragments collide (Barley et al., 1989). Crust-penetrating shear zones, that were laterally continuous, which accessed a mantle that was fertilised by subduction, were produced by these processes. For gold mineralisation these shear zones were important conduits (Blewett et al., 2010), which are suggested to possibly be one of the keys to the gold deposits of the Eastern Goldfields Superterrane.
The Eastern Goldfields Superterrane is part of the Yilgarn Craton, and had has a global resource (production and reserves) exceeding 8,500 tonnes of gold and is one of the 2 largest global Archaean gold provinces. Most of the deposits in the Yilgarn Craton, particularly in the Eastern Goldfields Superterrane are considered to be lode gold deposits, though in the Saddleback island arc, dated to 2714-2696 Ma (Qiu et al., 1997; Korsch et al., 2011a), situated in the southwestern part of the craton, the Boddington gold-copper deposit is considered to be polygenetic (McCuaig et al., 2001). At about 2707 Ma, the earliest phase of mineralisation, is interpreted as a porphyry-style, while the second mineralisation phase, at about 2629 Ma, is of a similar age to lode gold mineralisation in the Eastern Goldfields Superterrane (Stein et al., 2001). One of the earliest examples of arc-related mineralisation known is the first porphyry-related stage. The Yilgarn Craton is also a major nickel province, with individual nickel deposits being hosted by komatiites, high-temperature ultramafic volcanic and shallow intrusive rocks that are believed to be a product of the Archaean Earth that was hotter than at present (Nisbet et al., 1993).
According to the authors1 the Yilgarn Craton appears to have been part of the Kenorland supercontinent, believed also to have included Abitibi Superprovince in Canada, both provinces forming over the same time period, and they are also the 2 most richly mineralised provinces from the Archaean that are known. The amalgamation of Kenorland had begun by about 2660 Ma and by about 2480 Ma had begun to break up (Barley et al., 2005).
The Pilbara Craton, 3530-2930 Ma, is overlain by the Fortescue Basin and the Hamersley Basin, 2870-2450 Ma. The oldest part of the Pilbara Craton, that formed before 3200 Ma, and the proposed mechanisms by which this occurred are controversial, with suggestions ranging from crustal overturn (Van Kranendonk al., 2002) to the formation of an oceanic plateau consequent on mantle plume activity (Smithies et al., 2005a), and to tectonic process that are analogous to those of modern plate tectonics (Bickle et al., 1983; Barley et al., 1894; Zegers et al., 1996; Blewett, 2002). Plate-tectonic-like processes must have been active by about 3120 Ma, as the Whundo Greenschist Belt is the earliest known oceanic arc system in Australia and one of the earliest known in the world (Smithies et al., 2005b). The Pilbara Craton contains the earliest evidence of many geological processes active at the present, as it is one of the best preserved blocks of old crust known of on earth. An example is the Strelley Pool Formation, dated to about 3430 Ma, that is the oldest known unconformity that has been preserved (Buick et al., 1995). The earliest known weathering (regolith) profile is overlain by this highly angular unconformity. The earliest known unequivocal evidence of life on Earth is contained in the Pilbara Craton, the Dresser Formation hosted stromatolites by about 3490 Ma (Walter et al., 1980). Also hosted by the Dresser Formation is the oldest known ore deposit, the North Pole barite deposit, which produced 129,000 tonnes of barite that was used as mud in the Northwest Shelf petroleum province. It is common to find stromatolites closely associated with hydrothermal barite (Van Kranendonk, 2008), and this supports the idea that the initial evolution of life might have occurred in a hydrothermal environment (Baross & Hoffman, 1985).
Also contained in the Pilbara Craton are the earliest known examples of mineral deposits, though most are small and of little economic interest, that includes volcanic-hosted massive sulphide, lode gold, epithermal precious metal and porphyry copper deposits (Huston et al., 2007; Hickman & Van Kranendonk, 2012). As these deposits share many features with examples that are geologically young it indicates that many processes of mineralising have persisted through geological time. Hydrocarbons that are the oldest known, found within ore-related fluid inclusions, are associated with massive sulphide deposits that are volcanic-hosted, dating to about 3240 Ma, in the Panorama district (Rasmussen & Buick, 2000).
The authors1 suggest the Pilbara Craton was probably a part of Vaalbara, the oldest supercontinent. Comprised of the Pilbara Craton and the Kaapvaal Craton in South Africa, this supercontinent began forming about 3600 Ma, then the breakup began a bit after 2800 Ma (Zegers et al., 1998; Barley et al., 2005). Supercratons, that were probably smaller than most modern continents, seem to have been longer lived than supercontinents. The breakup of Vaalbara at about 2800 Ma (Barley et al., 2005) resulted in the formation of the Fortescue and Hamersley Basins, as well as their equivalents in South Africa. The earliest preserved passive margin successions on Earth are constituted by the rocks of these basins (Bradley, 2008). Thick successions of mafic and felsic volcanic rocks and sedimentary rocks dominate the fill of these basins, the most important being the banded-iron formations (Nelson et al., 1999). These banded-iron formations, dated to 2590-2450 Ma, formed when bottom waters that were rich in reduced Fe2+- were oxidised as they welled up onto the wide passive margin where they deposited the iron (e.g., Cloud, 1973). 2600-1800 Ma was the time period over which most of the banded iron formations of Australia and the world were deposited, a time during which the hydrosphere had mostly low oxygen levels (Bekker et al., 2010).
There are a number of important differences between the Yilgarn Craton and the Pilbara Craton. Both cratons have extended geological histories, though a characteristic of the Yilgarn Craton is short period, even catastrophic events that formed the crust. Between 2720-2655 Ma the final assembly of the Yilgarn Craton corresponds with a short, sharp peak in igneous rock ages. In the period between 3500-2850 Ma there were several small indistinct peaks, which has been suggested to reflect a slower rate of overall crust production, in the Pilbara Craton particularly. The same pattern is seen in the global data, with the largest peak in the juvenile crust occurring between 2700-2600 Ma, with a peak that is more diffuse at 3000-2800 Ma (Condie, 2005). It has been noted (Hawkesworth et al., 2010) that these peaks correspond to 'a particular stage in the cooling of the Earth' and possibly to a change in the mantle mode of convection (Korenaga, 2006).
It has been indicated by recent geochronology that the Archaean nuclei of the North Australian Element and South Australian Element are more widespread and older than previously believed. An example is the identification recently of granites from about 3150 Ma in the South Australian Element extending its geological history by about 600 Ma (Fraser et al., 2010). The extent of the known Archaean rocks in the North Australian Element has been greatly increased by recent dating (Hollis et al., 2011). When these new data are taken together they indicate that the history in both the North and South Australian Elements in the Archaean is much more significant and prolonged.
2200-1300 Ma - Nuna, amalgamation and break up
Another section of the evolution of the Australian continent that is controversial is the period from the Palaeoproterozoic-Mesoproterozoic, for which there are 2 groups of models, the 'fixist' group and the 'mobilist' group. According to the fixist models (e.g., Etheridge et al., 1987) it is suggested that little lateral movement occurs between crustal blocks, while according to the mobilist models (e.g., Giles et al., 2004; Betts & Giles 2006; Cawood & Korsch, 2008) large lateral movements occurred between blocks. In this review the authors1 adopted a mobilist model that infers the assembly of the 3 major elements of Precambrian age in Australia took place mostly in the Palaeoproterozoic as part of the Nuna supercontinent, though significant differences of opinion remains regarding the details of this assembly (Myers et al., 1996; Betts & Giles, 2006; Cawood & Korsch, 2008). They followed the concept of the North Australian element's southern margin being convergent through the late Palaeoproterozoic-early Mesoproterozoic (Scott et al., 2000; Giles et al., 2002). Although this is consistent with the broad model proposed by Betts & Giles (2006), some have proposed alternative models (Gibson et al., 2008; Payne et al., 2009). The boundaries between provinces have been determined upon contrasts in seismic and/or magnetotelluric data, at least partly, in many cases.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|