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THE ARCHAEAN and Earllest PROTEROZOIC EVOLUTION and METALLOGENY of Australla

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THE ARCHAEAN and Earllest PROTEROZOIC EVOLUTION and METALLOGENY of Australla Revista Brasileira de Geociências 12(1-3): 135-148, Mar.-Sel.. 1982 - Silo Paulo THE ARCHAEAN AND EARLlEST PROTEROZOIC EVOLUTION AND METALLOGENY OF AUSTRALlA DA VID I. OROVES' ABSTRACT Proterozoic fold belts in Austrália developed by lhe reworking of Archaean base­ mcnt. The nature of this basement and the record of Archaean-earliest Proterozoic evolution and metallogeny is best prescrved in the Western Australian Shield. ln the Yilgarn Craton. a poorly-mineralized high-grade gneiss terrain rccords a complex,ca. 1.0 b.y. history back to ca. 3.6b.y. This terrain is probably basement to lhe ca. 2.9~2.7 b.y. granitoid­ -greenstone terrains to lhe east-Cratonization was essentially complete by ca, 2.6 b.y. Evolution of the granitoid-greenstone terrains ofthe Pilbara Craton occurred between ca. 3.5b.y. ano 2.8 b.y. The Iectonic seuing of ali granitoid-greenstone terrains rcmains equivocaI. Despitc coincidcnt cale­ -alkalinc volcanism and granitoid emplacemcnt , and broad polarity analogous to modem are and marginal basin systcrns. thcre is no direct evidencc for plate tectonic processes. Important diffcrences in regional continuity of volcanic scqucnccs, lithofacies. regional tectonic pauerns and meta1Jogeny of lhe terrains may relate to the amount of crusta! extension during basin formation. At onc extreme, basins possibly reprcsenting low total cxrensíon (e.g. east Pilbara l are poorly mi­ ncralizcd with some porphyry-stylc Mo-Cu and small sulphute-rich volcanogenic 01' evaporitic deposits reflecting the resultam subaerial to shaJlow-water environment. ln contrast, basins inter­ prctcd to have formcd during greater crusta! cxrcnsion (e.g. Norseman-Wiluna Belt) are riehly mineralized witb Ni-Cu and Au dcposits. duc lo widesprcad eruption of komatiitcs and rapid subsidencc of Ihe volcanic pile. Some" greenstone basins show intermediate character. Deposition of the eartiest Protcrozoic volcanics and shell' scdiments of the Hamersley Basin predated stabilization 01' lhe Y ilgarn Craton. Thc changc in Iectonic seuing heraldcda major ehange in mctallogencsis from dominantfy volcanic-related Arehaean deposits lo major sedimcntary ore associations. lhe earliesl of whieh were lhe iron ores 01' lhe Hamersley Group. INTRODUCTION The Precambrian shield of Austrália ln Western Austrália. the term Arehaean has generally encompasses severaI major metallagenic provinces, Its tec­ becn applied to rocks older than 2.4 b.y., whereas 2.5 b.y. tonic evolution has been discussed recently by several au­ is more widely adopted (James, 1978). ln contrast, the Bu­ thors (e.g. Rutland 1976'; 1981; Gee, 1979; Plumb, 1979). reau of Mineral Resourees (Rutland, 1981) have used a It is not intended here to discuss in detail the entire shield time-rock systern, defining the base of the platforrn Fortes­ (e.g. Rutland, 1981), but to concentrare on the evolution cue Group in the Pilbara region of Western Australia as lhe and metallogeny of its Archaean and earliest Proterozoie lower Proterozoic boundary. With more geochronological segments. Emphasis is placed on the Western Australian data, Ihis age has increased from ca. 2.3 b.y. to the present Shield which ineludes the most extensive, preserved Archae­ estimare of > 2.7 b.y. Thus, platforrn cover sequenees were an bascment. and its oldest, gently folded and very low mela­ deposited on the Pilbara Craton before stabilization of the morphic grade cover sequenees (Gee, 1979); both eontain Yílgarn Craton, The Archaean-Proterozoic boundary, as major metallogenic provinces. This review firstly covers well defined in a tectonic sense and adapted here, is therefore established aspeets of lhe crustaI evolution of lhe Shield and diachronous. examines tectonic models in terms of constraints imposed by the nature and quality of available data. Metallogeny is then discussed in terms of tectonic and crustal evolution TECTONIC FRAMEWORK OF AUSTRALlA The and is shown to be an integral part of this evolution. gross structurc 01' Australia is dominated by Archaean A slightly modified terminology of Oee (1979) is used to cratons ln the.extreme wcst, thelate Precambrian-Phanero­ describe tectonic units. B/ock is used as a general term for zoic Tasman Fold Belt System in the east and the intervening a coherent outcropping part of the Precambrian ba..,ement. Early to Late Proterozoic provinces and basins ofcentral and Where ane 01' more of these blocks, 01' segments within northern Australia (Fig. I). thern, have tectonic characteristics that distinguish them The Pilbara and Yilgarn Bloeks represent the exposed from adjacent blocks ar segments they are termcd prol'inces. parts of Arehaean era tons. They represent segments of Ar­ The scale of provinces defined by various authors' (e.g. chaean crusi (> 3,6 to <'a. 2.7 b.y.) lhat were essentially Rutland, 1976, 1981; Oee 1979) is varied; the terminology stabilized by 2.8 to 2.6 b.y. and subsequently inlruded by in eurrent use by the Oeological Survey of Western Aus­ mafie dykes at ca. 2.5-2-4 b.y.; these bloeks largely eseaped o'alia is adopted. The larger scale provinces of Rutland Prolerozoic reworking that affected other Arehaean crus!. (1981) are, however, discllssed (Fig. I). The (erms craton, They represent major components of the Archaean super­ mobile bel, and basín are used in the normal tcctonic sense. province of Rutland (1981). *Geology Depal'trnenl, University 01' Western Australia, Nedlands. Weslern Australia, Australia 6009 136 Revista Brasileira de Geocíências, Volume 12 (l·3), 1982 ......, Maior D,o;>vinclI bounda,y Approx;mau, boundary 01 .......... P'ohHozoic provinces delmed by Rulland (1981) .......... Subp'ovlnca bound.'y ....,_ ..... WeSlG,n Australi"" bordol Q Lere ProtelOlo;c·PMnarow,c lold bells 13 P,oterolo,ç blocks (orogcnic doma,nsl ~ P,ote<Olo;o blocks (orogemc domal"o) ,,' !22Zi:'l wh,cM Archaean basemenl ,s ,eco,do<:1 [II] AreMo"n 9raniiord-gronnslo"e to""i",, D Alchaean h'ijh·g,ade gne,ss I""al". Figure J -- The major tectonic units (~r Australia (odaptrd fram Rutland, /981) Thc Proterozoic supcrprovince of Rutland (19811 eom­ from reactivation of Early Proterozoíc, and possibly Ar­ prises a nurnber of blocks overlain by plaiform cover se­ chaean, erust and partIy reflcct teetonie trends of the base­ quenees. Rutland (1981) suggests that unity of the super­ ment. provinee is demonstrated by a ubiquitons phase of pluto­ The marked eontrast in geophysieally derived strueture nism, commonly related to acid volcanism and associated of lhe extensively reworked Precambrian crust and that of with deformation and metamorphism, in the range 1.9 to the largely Phanerozoie Tasman Fold Belt System (e.g. 1.7 b.y., which pre-dated an early platform eover stage Finlayson, 1982) represents one ofthe fundamental teetonic (Central Australian Platform Cover). Mafic dykes at ca. features of Australia (Fig. I). It probably relates to the 1.1 to 1.0 b.y, mark stabilization of the superprovinee. longevity of global heat-flow deeay (e.g. Rutland, in prep.). Rutland (! 981) defined three main Proterozoie crustaI seg­ The Tasman Fold Belt System (Seheibner, 1978), whieh ments (his Northern Australian, Gawler and Gascoyne shows progressive stabilization frorn west to east and incor­ Provinees - Fig. I) separated by zones of crustaI reworking porates blocks of oceanic crust, was not cratonized until (Albany-Fraser Provinee and Paterson-Musgrave belt of the Triassic. There is current debate on whether this formed 'Fig. 3). Ali segments eontain relies of Arehaean basement or by lateral accretion or by vertical accretionary growth as inferred Archaean basement (sec summaries by Plumb et at., for the Proterozoie superprovince (e.g. Rutland, 1981). 1981; RutIand et al., 1981). The existenee of Arehaean erust beneath the Kimberley Basin has been suggested, but not THE WESTERN AUSTRALlAN SHIELD Theevolu­ substantiated. The North Australian and Gawler Provinces tion and metallogeny ofthe eariiest Precambrian ofAustralia of RutIand (] 98.1) have eontrasting styles of sedimentation is rccorded in the Western Australian Shicld. Its ovcrall and metamorphism/deforrnation. The former comprises tectonic history is briefly reviewed to placc in perspective trough successions, generally of low metarnorphic grade those aspects 01' its earlicst evolution discussed below. with loealized zones of higher grade, whereas the latter The most aneient teetonie units, the Arehaean Pilbara eomprises strongly deformed and high metamorphie grade and Yilgarn Blocks, are partly covered by Proterozoic basins thinner platform sequences. and partly surrounded by Proterozoie mobile belts (Fig. 2). The style of sedimentation, magmatism and tectonics of Gravity data define the margins of the Pilbara and Yilgarn the Proterozoie bloeks suggests that the lower Proterozoie Cratons and indicare a further craton in the south-east fold belts formed in an ensia1ie setting and developed lar­ (Fig. 3; Gee, 1979). The Pilbara and Yilgarn Cratons differ gely by reworking of Arehaean erust (e.g. Rutland, 1981). in i) erustal thiekness and structure, ii) relative heteroge­ Teetonie trends do not appear to mimie trends within adja­ neity, iii) exposure of high-grade gneiss belts, ir) age and, eent granitoid-greenstone terrains, but may be partly eon­ at least in part, stratigraphic continuity and gross tectonic trolled by strueturally elevated segments of high-grade pattern of their granitoid-greenstone terraíns, and v) metal­ gneiss terrains in the basement (e.g. Gee, 1979). A further logeny. They are interpreted as produets of independent stage of orogenie reworking is refleeted by linear belts of diachronous evolution. They appear to have -been in the high thermal and teetonie activity whieh appear to result same relative position since 2.4 b.y. (Embleton, 1978). RevistaBrasileira de Geocilncias. Volume 12 (1-3), 1982 137 PHANEROZOIC The Archaean granitoid-greenstone terrains behaved es­ o Sedimenta'y bas;n~ sentially as cratonic blocks during the Proterozoic, and are PROTEROZOIC SEOIMENTARY BASINS ~12b;Y~_:n: ly:u::_e~nd largely covered by gently folded Proterozoiccoversequences, hd,";,,:.:\i:;' õ youngar The earliest of these, the Fortescue and Hamersley Groups LU ca 2.6 b,y, and younger of the Hamersley Basin (Fig.
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