Volcanic, sedimentary and tectonostratigraphic environments of the ∼3.46 Ga Warrawoona Megasequence: a review

Precambrian Research, 60 (1993) 47-67

47

Elsevier Science Publishers B.V., Amsterdam

Volcanic, sedimentary and tectonostratigraphic environments of the ~ 3.46 Ga Warrawoona Megasequence: a review M.E. Barley Key Centrefor Strategic Mineral Deposits, Department of Geology, University of WesternAustralia, Nedlands, W.A. 6009, Australia (Received April 18, 1991; accepted after revision February 23, 1992 )

ABSTRACT Barley, M.E., 1993. Volcanic, sedimentary and tectonostratigraphic environments of the ~ 3.46 Ga Warrawoona Megasequence: a review. In: T.S. Blake and A.L. Meakins (Editors), Archaean and Early Proterozoic Geology of the Pilbara Region, Western Australia. Precambrian Res., 60: 47-67. The Warrawoona Megasequence comprises a ~ 3.46 Ga assemblage of tholeiitic and calc-alkaline volcanic rocks interlayered with cherty sedimentary rocks. Deposition occurred in a range of shallow- to deeper-water environments ranging from the shoreline to distal deposits in sediment starved basins. Volcanism provided the dominant, or only, source of clastic sediment. Sedimentary rocks also contain evaporites, probable stromatolites, other evidence for the existence of microbial life and possible evidence of meteorite impact. The tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic-arc, or near-arc settings. The compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc-alkaline suites developed above subduction zones. The calc-aikaline suite has a complex petrogenesis which probably involves mantle-derived basaltic melts, intermediate to silicic melts derived from subducted mafic crust, magma mixing and fractional crystallization. Intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the Warrawoona Megasequence contains examples of early Archaean volcanic-arc and near-arc assemblages.

Introduction

The Warrawoona Megasequence in the eastern Pilbara Craton, Western Australia, is probably the best preserved early Archaean greenstone succession in the world. It comprises an assemblage of tholeiitic and calc-alkaline volcanic rocks and less abundant sedimentary rocks which contain important evidence for shallow-water sedimentary environments, early Archaean evaporites, life, meteorite impacts, and an unusual (for the Archaean) suite of metal deposits. As such it provides important Correspondence to: M.E. Barley, Key Centre for Strategic Mineral Deposits, Department of Geology, University of Western Australia, Nedlands, W.A. 6009, Australia.

evidence which is relevant to our understanding of the nature of early crust, the evolution of depositional and tectonic environments, as well as early life and the evolution of the biosphere (e.g. Schopf, 1983; Nisbet, 1987; Horgan, 1991 ). The purpose of this paper is to provide an up-to-date review and interpretation of the geology of this important succession. The Pilbara Craton comprises a composite granitoid-greenstone terrane with components which range in age from 2.85 to 3.46 Ga. Greenstone belts in the Pilbara have been interpreted in terms of a single, essentially tabular stratigraphy, the Pilbara Supergroup (Hickman, 1983). However, stratigraphic mismatches between greenstone belts and recent geochronology (Horwitz, 1986, 1990;

0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

48

Krapez and Barley, 1987; Krapez, 1989; Horwitz and Pidgeon, 1993; McNaughton et al., 1993) indicate that this stratigraphic scheme is inappropriate. Krapez ( 1993 ) summarizes these data and proposes a new sequence stratigraphy for the greenstone belts with a revised account of the tectonic evolution of the Pilbara granitoid-greenstone terrane. The Warrawoona Megasequence, as described in this paper and defined by Krapez (1993), is a ~3.46 Ga dominantly volcanic succession previously called the Warrawoona Group (Hickman and Lipple, 1978; Hickman, 1983 ). It is only developed in the eastern Pilbara (Fig. 1), and successions previously correlated with the Warrawoona Group in the western Pilbara (e.g. Hickman, 1981, 1983) are part of the Roebourne Megasequence (Krapez, 1992) which was deposited at ~3.10 Ga (Horwitz and Pidgeon, 1993). The eastern Pilbara contains greenstone belts and domes, which separate granitoid batholiths. Greenstone domes such as the North Pole and McPhee Domes (Fig. 1 ), and adjacent greenstone belts preserve extensive, very lowgrade (prehnite-pumpellyite to low-greenschist facies), low-strain domains, in which most deformation involved brittle faulting and broad open folding. These areas provide the best opportunity to study the primary structure and evolution of early Archaean volcanic and sedimentary successions. A full tectonic history of the Pilbara granitoid-greenstone terrane, including a summary of deformation, metamorphism and granitic magmatism is presented by Krapez ( 1993 ).

Stratigraphy of the Warrawoona Megasequence Published stratigraphies and estimated thicknesses for the Warrawoona Megasequence differ considerably (cf. Lipple, 1975; Hickman, 1977, 1981; Lowe, 1982; DiMarco and Lowe, 1989a, b). In this paper the nomenclature used by DiMarco and Lowe (1989a)

M.E. BARLEY

and Krapez ( 1993 ) is followed. This is based on correlation of distinctive intermediate to silicic volcanic units from greenstone belts near Marble Bar, which are well dated by both conventional and SHRIMP U-Pb in zircon methods at ~ 3460 Ma (Pidgeon, 1978; Thorpe et al., 1990; McNaughton et al., 1992). This stratigraphy (Fig. 2C) is similar to that originaly proposed by Lipple (1975), with the exception that the Duffer and Panorama Formations of DiMarco and Lowe (1989a) probably represent different facies of a single prolonged ( ~ 20 Ma) period of calc-alkaline volcanism (the Duffer Supersequence), and that the Wyman Formation is part of the Gorge Creek Megasequence. The Warrawoona Megasequence (Figs. 1 and 2) contains three supersequences (Krapez, 1993 ). The Talga Talga Supersequence (Talga Talga Subgroup of previous stratigraphic schemes) is composed mainly of gabbro, dolerite and tholeiitic basalt with minor intercalated cherty sediments. This is overlain by successions of calc-alkaline intermediate to silicic volcanic and sedimentary rocks of the Duffer and Panorama Formations, which form the Duffer Supersequence. The calc-alkaline volcanic rocks, and where they are absent, the Talga Talga Supersequence, are overlain by thick ( > 25 m) cherty sedimentary horizons, which are in turn overlain by tholeiitic and magnesian basalt interlayered with thin cherty sedimentary horizons of the Salgash Supersequence (previously the Salgash Subgroup). The distinctive thicker cherty sedimentary horizons have been given a variety of local names (Towers Formation, North Pole Chert, Marble Bar Chert, and Strelley Pool Chert). DiMarco and Lowe (1989a, b) interpret the StreUey Pool Chert and Marble Bar Chert as stratigraphically eqivalent, shallow- and deeper-water facies, respectively. These cherts overlie the Duffer Supersequence and probably represent a significant break in volcanism, prior to Salgash Supersequence basaltic volcanism.

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The tectonostratigraphic assemblage in the Warrawoona Megasequence comprising tholeiitic lavas and intrusions with calc-alkaline volcanic rocks (and granitoids), and shallowas well as deeper-water sedimentation, is similar to assemblages found in younger volcanic arc, or near-arc, settings (e.g. Busby-Spera, 1988; Hamilton, 1988; Houghton and Landis, 1989). This suggests that the Talga Talga, Duffer and Salgash Supersequences formed in tectonically related environments. The geochemistry of the volcanic rocks and distinctive

metallogeny of this association, are also compatible with this interpretation.

Hydrothermal alteration and metamorphism

It has been widely recognized that most Archaean volcanic successions experienced alteration and hydrothermal metamorphism, and the Warrawoona Megasequence is no exception. It is dominated by strongly altered and

50

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permost 100 m of the Talga Talga Supersequence and most of the Duffer and Salgash Supersequences relatively strongly altered, and is greatest in originally porous units such as pillowed flows or pyroclastic units. Basalts are typically albitized (spilitized) and/or carbonated, and locally silicified adjacent to cherty sedimentary rocks. Stable isotope data from

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Fig. 3. Wt% C O 2 in Talga Talga Supersequence basalts from the Marble Bar Belt, showing an increase in intensity of hydrothermal alteration (indicated by elevated wt% CO2) with stratigraphic height (data from Glikson and Hickman, 1981).

carbonated basalts and hyaloclastic breccias indicate 8~3C values of between - 2 and 0%0, values which are considered typical of Archaean seawater (e.g. Barley and Groves, 1989). Similar patterns of extensive carbonation, spilitization and silicification, as well as sericite-rich alteration types, are observed in the Duffer Supersequence (Barley, 1984; Barley et al., 1984; DiMarco et al., 1989). Barley (1984) compared the vertical distribution of hydrothermal alteration (e.g. Fig. 3 ) in the Warrawoona Megasequence with that in shallow-water (i.e. volcanic island) or subaerial hydrothermal systems, where boiling resuits in precipitation of silica, carbonate and feldspar which causes drastic reduction of porosity at the boundaries of hydrothermal systems. This effect and the gravitational constraint on discharge from subaerial systems

favour secondary circulation and horizontal flow, with the upper 2 km ofhydrothermal systems dominated by a zone of secondary circulation in which alteration is likely to be most intense. The distribution of metamorphic mineralogies in the Kelly Belt, McPhee Dome and North Pole Dome (Fig. 1 ) with large apparent stratigraphic thicknesses ( > 2 km) of very lowgrade (prehnite-pumpellyite facies) metamorphic mineral assemblages, indicates that hydrothermal metamorphism of the Warrawoona Megasequence is characterized by an extended facies series. This facies series is typical of that observed in many volcanic-arc and marginal-basin assemblages (Aguirre and Oftier, 1985) and contrasts with the contracted metamorphic facies series typical of the ocean floor. The style of hydrothermal alteration and

52

metamorphism observed is also similar to that in tholeiitic and calc-alkaline volcanic assemblages, representing Phanerozoic arcs and marginal basins (e.g. Hamilton, 1988 ). These environments typically have complex and prolonged histories of hydrothermal activity (cf. Aguirre and Offler, 1985; Hamilton, 1988), and it is likely that the alteration history of the Warrawoona Megasequence commenced shortly after eruption and continued during burial in an environment with a geothermal gradient similar to that in many volcanic arcs and associated basins. The alteration patterns probably reflect superimposed effects of these events, whereas very low- and low-grade metamorphic mineral assemblages probably reflect conditions during burial. Mafic volcanism

The Talga Talga and Salgash Supersequences are dominated by tholeiitic basalt, dolerite and gabbro with subordinate magnesian basalt. Lavas occur as massive flows with brecciated or pillowed flow tops or as compound pillowed flows. Individual flows range in thickness from < 5 to more than 50 m. The presence of hyaloclastic breccias at both the top and base of compound pillowed flows and the common vertical transition from massive to pillowed lava indicates deposition in a mainly subaqueous environment. Lavas are locally highly amygdaloidal indicating solidification in relatively shallow-water (Barley, 198 l; Dunlop and Buick, 1981 ). Intrusions include dolerite sills ( < 5 0 m thick) and thicker bodies of layered pyroxenite and gabbro. The rubbly outcrop which is typical of mafic sequences in most belts makes recognition of syn-volcanic dykes difficult. However, in the Kelly Belt (Fig. 1 ) a swarm of variably altered mafic dykes can be traced from the lighter coloured Duffer Supersequence into the Salgash Supersequence. These dykes have a similar compositional range to the basalts, as well as similar mineralogies, alteration and

M.E. BARLEY

metamorphism. It is difficult to estimate the percentage of dykes in the mafic succcession. However, mafic dykes large enough to be mapped on 1:40,000 scale aerial photographs occupy between 10 and 20% of the outcrop area of the Duffer Supersequence. This relationship between the two supersequences suggests that the Salgash Supersequence represents volcanism in an extensional intra-arc or back-arc setting. Mafic volcanic rocks display a continuum of textures, mineralogy and geochemistry, which suggests that they belong to a petrogenetically related suite. Successions of interlayered lava flows range in composition from magnesian basalt to iron-enriched tholeiite, but do not show consistent fractionation trends with stratigraphic height. No well-developed komatiitic successions have been recognized (Barley and Bickle, 1982 ) and aphanitic or sparsely phyric lavas with more than 12 wt% MgO are rare. In the field, lavas can be easily divided into magnesian basalts ( > 9 wt% MgO) with skeletal clinopyroxene (pyroxene spinifex) or spherulitic textures, and basalts with either prismatic clinopyroxene phenocrysts or subophitic to weakly plagioclase-phyric textures. Magnesian basalts are commonly clinopyroxene _+ olivine-phyric and the most magnesian lavas (12 to 20 wt% MgO) are generally olivine or clinopyroxene cumulates at the base of thick magnesian basalt flows. Thick magnesian basalt flows have flow tops with skeletal or prismatic clinopyroxene phenocrysts overlying cumulate zones rich in prismatic clinopyroxene with minor plagioclase. Pillow lavas, margins of flows and thin lava flows are generally aphanitic containing abundant devitrified glass and skeletal crystals. These provide the best indication of the compositional variation exhibited by parent magmas. The range of textures exhibited by aphanitic lavas in very-low grade domains parallels the textural variation observed in experimental studies relating crystal morphology to cooling rate, with the textures that reflect slowest

THE ~ 3.46 Ga WARRAWOONA

MEGASEQUENCE:

53

A REVIEW

cooling or least undercooling developed in the least magnesian lavas. Lavas with between 9 and 12 wt% MgO contain chlorite pseudomorphs after subhedral olivine microphenocrysts with skeletal overgrowths and skeletal and spherulitic relict clinopyroxene. Lavas with between 7 and 9 wt% MgO contain prismatic clinopyroxene microphenocrysts and intergrowths of spherulitic clinopyroxene and plagioclase. The least mafic lavas, with less than 7 wt% MgO, contain prismatic clinopyroxene microphenocrysts, with plagioclase as prismatic microphenocrysts with skeletal overgrowths and as spherulites. In relatively unaltered rocks, basaltic glass is replaced by chlorite, epidote, titanite and carbonate with actinolite (in greenschist-facies rocks) and prehnite and pumpellyite (in prehnite-pumpellyite facies rocks).

Geochemistry and petrogenesis Aphanitic and sparsely phyric lavas in the Kelly Belt exhibit typical tholeiitic fractionation trends towards FeO* (9.0 to 14.5 wt%) and TiO2 (0.4 to 1.3 wt%) enrichment. Contents of Zr (13 to 37 ppm), REE and most other incompatible elements also increase steadily with increasing MgO contents. Chondrite-normalized REE patterns for the Salgash Supersequence (Fig. 4a) are flat to slightly enriched in light REE and vary between approximately 5 to 8 times chondritic values for magnesian basalts to approximately 30 times chondritic values for lavas with 4.5 wt% MgO (Barley et al., 1984). Chromium (500 to 100 ppm) and Ni (200 to 70 ppm) contents also decrease as lavas become less magnesian. The chemical variation of these lavas can be explained in terms of a spectrum of low-pressure fractional crystallization trends involving the observed phenocryst phases olivine, clinopyroxene and plagioclase (Barley et al., 1984 ). Low-pressure fractionation trends are consistent with the abundance of clinopyroxenephyric basalts, the variations in phenocryst as-

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semblages and orders of crystallization observed in high-level layered sills. The more evolved lavas are related to parental magnesian basalt magmas with trace element and isotopic compositions that closely reflect their mantle source. These magnesian basalts are characterized by ratios close to chondritic for most trace elements, with variable depletion or slight enrichment of the more incompatible elements (such as K, Rb, Ba, Zr, light REE). Variations in Zr/Y and Ce/Yb in the magnesian basalts suggest that parent magmas were either derived from a heterogeneous source or were variably contaminated by crustal mate-

54

rial prior to, or during, eruption (e.g. Amdt and Jenner, 1986; Barley, 1986 ). Warrawoona Megasequence basalts generally plot in the midocean ridge (MORB), low-K tholeiite, or platemargin basalt fields (Glikson and Hickman, 1981 ) on diagrams designed to distinguish between younger basalts from different tectonic settings (e.g. Pearce and Cann, 1973). Although it can be argued that this is consistent with the interpreted tectonic setting of these lavas (i.e. arc related), interpreting the trace element signatures of even the least-altered Archaean basalts is fraught with difficulties (e.g. Arndt and Jenner, 1986; Barley, 1986), such that Archaean tectonic interpretations should not be based solely on basalt geochemistry. Calc-alkaline volcanism

Recent zircon dating of both greenstones and granitoids indicates the existence of at least two significant periods of calc-alkaline magmatism in the eastern Pilbara separated in time by more than 100 Ma. The earlier of these is represented by the ~3.46 Ga Duffer Supersequence. The Duffer Supersequence (Figs. 2 and 5) comprises successions of dominantly intermediate to silicic volcanic rocks and related sedimentary rocks; the Duffer and Panorama Formations as defined by DiMarco and Lowe (1989a, b), and un-named basalts (e.g. Barley, 1981 ). Intermediate lavas and pyroclastic rocks have been dated at 3452 _+16 Ma by conventional multigrain U - P b zircon analysis in the Coongan Belt (Fig. 1; Pidgeon, 1978), at 3464 +_2 Ma by SHRIMP U - P b zircon analysis at two localities in the Marble Bar Belt (McNaughton et al., 1993) and at 3471 + 3 Ma and 3465 + 3 at Bamboo and Yandicoogina by conventional single grain U - P b zircon analysis (Thorpe et al., 1990). As the samples analysed by SHRIMP techniques contain a small proportion of zircons which give younger ages, it is possible that the conventional multigrain analysis (Pidgeon, 1978) slightly underesti-

M.E. BARLEY

mates the age of the unit. A recent study (DiMarco and Lowe, 1989a, b) has enabled correlation of the Panorama Formation which occurs in the North Pole Dome and Pilgangoora Belt (Fig. 1 ) with distinctive volcanic and sedimentary facies at the top of the Duffer Supersequence in other greenstone belts. The Panorama Formation in the North Pole Dome has recently been dated at 3457+3 Ma by Thorpe et al. (1990). The Duffer Formation is composed dominantly of andesite and dacite (Barley et al., 1984), whereas the Panorama Formation is dominantly rhyodacite (Cullers et al., 1993). A suite ofgranitoids in the Shaw Batholith and gneisses in the Mt Edgar Batholith are similar both in composition and in age to the Duffer Supersequence, and are probably its intrusive equivalents (Bickle et al., 1983; Barley et al., 1984; Collins, 1989; Williams and Collins, 1990). Together these volcanic rocks and granitoids represent the first significant period of calc-alkaline magmatism in the eastern Pilbara. In the Marble Bar Belt, McPhee Dome, Kelly Belt and Coongan Belt (Fig. 1), the Duffer Formation is between 3 and 5 km thick and contains proximal facies which may represent the remains of volcanic centres (Barley et al., 1984; DiMarco and Lowe, 1989a). Proximal facies include lava and coarse breccia deposits. Although andesite and dacite are most abundant, there is a continuum of lava compositions from plagioclase-phyric basalt to rhyodacite. Basalts are typically compound pillowed flows, with associated hyaloclastite. Andesites and dacites occur as massive flows with columnar jointing, flow banding and autoclastic breccias. Plagioclase-phyric basalts contain tabular plagioclase phenocrysts (now composed of albite and fine granular epidote) between 2 and 5 m m in length and interlocking plagioclase laths ( < 1 m m long). The latter enclose prismatic or irregular actinolite pseudomorphs after clinopyroxene and irregular patches of chlorite, actinolite, epidote and ti-

THE ~ 3.46 Ga WARRAWOONAMEGASEQUENCE:A REVIEW

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Fig. 5. Cartoon showing the interpreted evolution of the Duffer Supersequence (modified from DiMarco and Lowe, 1989a, b).

tanite replacing basaltic glass. Partially altered iron-titanium oxides are evenly distributed as equant microphenocrysts (up to 5%). Andesites contain more abundant plagioclase phenocrysts with some relict andesine in very lowgrade domains, and quartz is a phenocryst phase in dacites and rhyodacites. Prismatic chlorite and actinolite pseudomorphs after pyroxene microphenocrysts, and iron-titanium oxide microphenocrysts are the dominant mafic phases in intermediate lavas. Pseudomorphs after igneous amphibole are also developed in some andesites and dacites. In most belts the Duffer Formation consists largely of bedded volcaniclastic breccia, of intermediate composition, interbedded with subordinate beds of turbiditic tuff (Barley et al., 1979; DiMarco and Lowe, 1989a). The breccias are poorly to very poorly sorted with a bimodal size distribution, consisting of 20 to 30%, pebble- to cobble-sized clasts of andesite and dacite suspended in a sand-sized volcani-

clastic matrix. Maximum clast sizes range up to 1 m and layers with maximum clast size less than 8 cm are rare. A few layers contain a higher proportion of pebbles and cobbles which support the sediment. Clasts are dominantly angular and equant, but many units contain 5 to 10% rounded clasts, and banded chert clasts occur in a few layers. Bedding is defined by variations in clast size and most layers are ungraded. However, normally graded and inversely graded beds sometimes occur. The texture, structure and lithologic association (with turbiditic tuff) of breccias in this facies indicates that most were deposited by subaqueous debris flows. The composition, abundance of large clasts, scarcity of lutites and presence of rounded clasts suggests rapid deposition in relatively shallow-water adjacent to emergent volcanoes (e.g. Houghton and Landis, 1989). Tuff-breccia and tuff are widely associated with, but are usually subordinate to breccias. DiMarco and Lowe (1989a) have divided

56

these into two subfacies: thin- to thick-bedded tuff-breccia interbedded with coarse volcaniclastic breccia; and thick- to very thick-bedded tuff-breccia forming successions up to several hundred metres thick. The former occurs in normally graded units, from 20 to greater than 100 cm thick, consisting, from the base upwards, of coarse-grained, structureless or normally graded tuff, flat- to cross-laminated tuff, and cross-laminated fine-grained tuff. These divisions have been interpreted as the Bouma divisions of turbidites (DiMarco and Lowe, 1989a), and in many cases only the structureless or structureless plus flat-laminated divisions are preserved. Thick units of the second subfacies are made up of cyclic units averaging 1 m thick. They consist of structureless or graded tuff-breccia capped by moderate- to well-sorted, flat-laminated tuff. Many beds exhibit clast grading, including inverse coarse-tail grading of flattened clasts which probably represent crushed pumice. The lateral continuity and sedimentary-structure sequence of both of these subfacies indicate deposition from turbidity currents. Conglomerate and sandy conglomerate facies containing thin interbeds of stratified tuff locally overlie the other facies (Barley et al., 1979; DiMarco and Lowe, 1989a). These are composed of sub-angular to rounded clasts of altered volcanic rocks in a sand-sized matrix. Disorganized matrix-supported cobble conglomerate is most abundant, and is interlayered with lenticular beds of clast-supported conglomerate. The latter are moderately wellsorted and commonly overlie the matrix-supported conglomerates on scoured contacts. Stratified tuffaceous sandstone also occupies lenses scoured into underlying conglomerate. This rock is well sorted and plane- to crosslaminated. The matrix-supported conglomerates are probably debris flow deposits with the other sediments resulting from reworking of these deposits (DiMarco and Lowe, 1989a). The range of conglomerate types, abundant evidence for scouring and lack of turbidites are

M.E. BARLEY

characteristics of subaerial conglomerate sequences, and are similar to coarse alluvial sequences deposited around Phanerozoic stratovolcanoes (e.g. Smith, 1986; Cas and Wright, 1987; Hackett and Houghton, 1989). The Panorama Formation locally overlies the Duffer Formation in the Marble Bar, Kelly and Coongan Belts, and occurs by itself in the North Pole Dome and Pilgangoora Belt (Fig. 1 ). In the Pilgangoora Belt it becomes thinner towards the west, where it is represented by a quartz-rich volcaniclastic sandstone facies (DiMarco and Lowe, 1989b). In its thicker sections this formation contains rhyodacite lava, tuff and tuff-breccia, volcaniclastic sandstone and siltstone, banded chert and silicified evaporite. These occur in four main facies associations (DiMarco and Lowe, 1989b): ( 1 ) banded chert with lesser amounts of tuff representing clastic-starved basinal environments; (2) siltstone with variable amounts of tuff representing lower shoreface and prodelta deposits; (3) cross-stratified sandstone with one or more other facies representing upper shore face, delta front and braided stream environments; and (4) turbiditic and ash-fall tuff, representing subaqueous settings under the influence of pyroclastic volcanism. Facies associations 1 to 3 are developed directly over basalt in the North Pole Dome and represent shoaling-upwards successions interpreted as sedimentation in prograding fan delta complexes linked by shoreface deposits. Facies associations 2 and 3 display evidence for wave action, such as symmetrical and chevronshaped cross-lamination, hummocky crossstratification and polymodal palaeocurrent patterns. In other belts, facies associations 3 and 4 are best developed and occur over thick sequences of the Duffer Formation (DiMarco and Lowe, 1989b). In the Pilgangoora Belt, North Pole Dome, Coongan Belt, and Kelly Belt the Panorama Formation is overlain by a

57

THE ~ 3.46 G a W A R R A W O O N A MEGASEQUENCE: A REVIEW

thick cherty sedimentary horizon (the Strelley Pool Chert) characterized by silicified shallow-water evaporites. The evolution of calc-alkaline volcanic successions, as envisaged by DiMarco and Lowe (1989a, b), is shown in Fig. 5. The Duffer Supersequence contains facies associations that are very similar to those found in and around Phanerozoic island volcanoes (e.g. Busby-Spera, 1988 ), and to the facies models for arc, or near-arc, sedimentation developed by Houghton and Landis ( 1989 ). Duffer Formation volcanism is represented by accumulations of lavas, breccias, tufts and other sedimentary rocks. These are up to several kilometres thick and were mainly deposited in subaqueous environments. Many of the early fragmental units contain only angular debris suggesting an origin as subaqueous block-andash flows or lahars. The abundance oflavas and coarse breccias indicates that the majority of deposits are the remains of broad subaqueous volcanic cones and their clastic aprons (Barley et al., 1979; DiMarco and Lowe, 1989a, b). The occurrence of rounded clasts also indicates reworking of volcanic debris in shallow-water to subaerial environments. In most belts, successions appear to shoal upwards. The youngest deposits (Fig. 5 ) represent deposition of pyroclastics and reworking of older deposits in braided-stream, shoreface and shallow-water, fan-delta environments. The relatively shallow-water environments indicated for the late stages of calc-alkaline volcanism have important implications for the abundance and nature of volcanogenic mineralization. Volcanogenic massive C u - Z n sulphide mineralization occurs in the Warrawoona Megasequence calc-alkaline volcanics at Big Stubby in the Marble Bar Belt and Yandiccoogina (Fig. 1 ). The top of the calc-alkaline volcanic succession in this belt contains a thick pile of subaqueous turbiditic and ash-fall tufts, overlain by the Marble Bar Chert, a distinctive red, white and black banded chert with little evidence for shallow-water deposition. This

appears to be the only belt in which the upper parts of the Duffer Supersequence have escaped extensive reworking in shallow-water environments. In the North Pole Dome, porphyries intruding the Panorama Formation are associated with epithermal C u - Z n - A u mineralization (Groves, 1987). This metallogenic association contrasts with mineralized deepwater Archaean calc-alkaline successions in the Murchison Province of the Yilgarn Craton (Barley, 1992) and in Canada.

Geochemistry and petrogenesis Duffer Supersequence rocks (including basalts) plot in separate fields to tholeiitic and komatiitic basalts from the Salgash Supersequence on an AFM diagram (Fig. 6a) and in plots of TiO2 and A1203 versus MgO (Barley et al., 1984). They also have higher Z r / Y ratios ( Z r / Y > 5, Fig. 6b) than the tholeiites and have fractionated REE patterns (CeN/YbN > 3 with Yb ~ 10 times chondritic concentrations Fig. 4b). The Supersequence exhibits trends towards SiO2 (52 to 70 wt%) enrichment and FeO* (9 to 2 wt%) and TiO2 (0.8 to 0.2 wt%) depletion. MgO (7to 1 wt%), Cr (250 <20 ppm) and Ni ( 150 to < 10 ppm) also decrease as SiO2 content increases. Yttrium contents increase steadily from about 15 ppm in plagioclase-phyric basalts to about 35 ppm in some andesites and dacites before decreasing to between 10 and 20 ppm in rhyodacites; Zr behaves similarly. REE patterns are fractionated with light REE contents ranging from about 30 times chondritic values in plagioclase-phyric basalts to more than 100 times chondritic values in dacites and rhyodacites, whereas heavy REE in most andesites and dacites remain relatively constant at between 5 and 10 times chondritic values (Fig. 4b). Barley and de Laeter (1984) obtained a Rb/Sr whole-rock isochron indicating an age of 3484+_ 125 Ma with a mantle-like initial 87Sr/S6Sr ratio of 0.6997+_0.0009 from a suite of least-altered andesites and dacites.

58

M.E. BARLEY

F

~Salgash (a} /~.//¢~ ~'\"/" .~//~'/'/ "~\~' 'Th0' ~,~eites andk°rnatiit~'s Supersquence

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(ppm)

Fig. 6. (a) AFM diagram comparing Duffer Supersequence calc-alkaline volcanic rocks (trend 1 ) with Whim Creek Supersequence calc-alkaline volcanic rocks (trend 2 ) and Salgash Supersequence basalts, magnesian basalts and komatiites (data from Barley et al., 1984). (b) TiO= vs Zr diagram for calc-alkaline volcanic rocks from the Duffer Supersequence (field 1 ) compared to calc-alkaline volcanic rocks from the ~ 3.0 Ga Whim Creek Supersequence (field 2), and modern St Kitts (SK), Mt Ararat (Ar) and central Chile (C) (data from Barley et al., 1984).

Although they fall within the range of modem calc-alkaline volcanics (Fig. 6b), the Duffer Supersequence rocks have less A1203 and are richer in Cr and Ni than average mode m calc-alkaline volcanics and ~ 3.0 Ga calcalkaline volcanics from the Whim Creek Supersequence in the western Pilbara. In their range of SiO2 values (including rocks with more than 60 wt% SiO2), Zr and REE contents, andesites and dacites from the Duffer Supersequence have more in common with calc-alkaline volcanics erupted through conti-

nental crust than with most volcanics from island arcs on oceanic crust (Barley et al., 1984). However, there is considerable compositional overlap between modern calc-alkaline volcanics erupted through continental and oceanic crust in the areas like the Papua New Guinea region (e.g. Johnson et al., 1978), so that by themselves, such comparisons probably have little tectonic significance. Duffer Supersequence rocks which range in composition from basalt to rhyodacite display a continuum of mineralogical and chemical variation which suggests that they developed as a petrogenetically related suite from magmas with mantle-like Sr isotopic compositions. The SiO2, MgO, Cr and Ni contents of the most magnesian plagioclase-phyric basalt precludes a crustal source for these lavas. Barley et al. (1984) considered that the geochemistry of the basalts could be best explained by partial melting of a mantle source enriched in the more incompatible elements (e.g. K, Rb, Zr, light REE), followed by fractional crystallization of olivine and clinopyroxene. The intermediate lavas could then be derived from basaltic magmas by fractional crystallization of the observed phenocryst phases (clinopyroxene + plagioclase + iron-titanium oxide _+ amphibole). Some of the Duffer Formation dacites analysed by Jahn et al. ( 1981 ) and the Panorama Formation rhyodacites analysed by Cullers et al. (1993), are more depleted in heavy REE (Yb ~ 5 times chondritic values; Fig. 4b ) than those analysed by Barley et al. (1984). This depletion in heavy REE is consistent with amphibole or garnet fractionation, and when observed in Archaean intermediate or silicic rocks is generally interpreted as the result of partial melting of amphibolite or eclogite (Jahn et al., 1981; Martin, 1986). However, simple incremental partial melting of mafic crust is not sufficient to explain the full range of compositions observed (Jahn et al., 1981; Barley et al., 1984). Consequently, petrogenetic models involving both mantle-derived basaltic magmas

THE ~ 3.46 Ga WARRAWOONA MEGASEQUENCE: A REVIEW

and intermediate to silicic melts of mafic crust, with variable amounts of magma mixing and fractional crystalization (+_ crustal contamination) are the most likely explanation for the chemical variation within this suite. The main difference between this petrogenetic scheme and the petrogenetic models commonly invoked to explain the chemical variation in modern suites of calc-alkaline volcanic rocks (e.g. Gill, 1981 ) is the generation of silicic melts from mafic crust, a process which is apparently rare in younger subduction zone environments.

Cherty sedimentary rocks The Warrawoona Megasequence contains a number of distinctive cherty sedimentary horizons (Barley et al., 1979; Hickman, 1983; Lowe, 1983; Buick and Barnes, 1984; Buick and Dunlop, 1990). These represent silicified clastic and chemical sediments deposited in a range of sedimentary environments. As such they potentially contain important evidence for the nature of early Archaean environments, the composition of the hydrosphere and atmosphere, and early life. Several of the thickest cherty sedimentary horizons are laterally continuous ( ~ 30 km in some belts) and were used as the basis for lithostratigraphic subdivisions of the Warrawoona Group. (Lipple, 1975; Hickman, 1977, 1983; DiMarco and Lowe, 1989a, b). These stratigraphies mainly differ in their interpretation of cherts in the North Pole Dome. Lipple (1975), and Hickman (1977, 1983) regard the lowermost thick chert (the North Pole Chert) as the stratigaphic equivalent of the thickest cherts in other belts. Whereas, Lowe ( 1983 ) and DiMarco and Lowe ( 1989a, b ) interpret a chert about 10 km structurally above the North Pole Chert on the eastern flank of the Dome (the Strelley Pool Chert) to be the stratigraphic equivalent to the thickest chert in other belts (Fig. 2 ), and present facies descriptions of this chert, associated silicic volcanic

59

facies below this chert and the stratigraphy of the basaltic succession above this chert, to support their case. Recent zircon dating also supports this interpretation, but further detailed stratigraphic mapping and precise geochronology are required to fully resolve the stratigraphy of the North Pole Dome. Most cherty sedimentary rocks are thin ( < 2 5 m) lenticular horizons within basaltic successions. These are dominantly black carbonaceous and pyritic chert, or banded grey, green and white chert. The black cherts comprise massive and finely laminated (0.5 to 1 cm) units with disseminated carbon and pyrite outlining a clotted or granular fabric. Banded grey and white cherts comprise a variety of lithofacies which include silicified carbonate and evaporite, and silicified volcanic ash. In basaltic successions the latter appears to be mainly derived from mafic volcanism. Distinctive horizons with abundant small spherules have been interpreted as accretionary lapilli deposits (Buick and Barnes, 1984), reworked amygdale deposits (Buick, 1987), and spherules resulting from meteorite impact (Lowe and Byerly, 1986 ). Although only a few of these cherts have been studied in detail, they contain evidence for shallow-water deposition in most greenstone belts (Barley et al., 1979; Lowe, 1983; Buick and Barnes, 1984). Where intersected below the base of weathering (typically > 150 m below the surface) in drill holes, chert horizons typically contain considerably less silica than they do in surface outcrop, implying that at least some silicification is the result of weathering. This has obvious implications for the search for microfossils in cherts as well as for stable-isotope studies which use surface samples. The North Pole Chert The lowermost chert in the North Pole Dome (the North Pole Chert) contains a substantial bedded barite deposit and is discontinuously exposed for a strike length of 25 km (Dunlop,

60 1976; Buick, 1985 ). The chert is between 5 and 40 m thick, and variations in thickness probably result from deposition on an irregular topography with low relief. The internal stratigraphy of this chert is complex and has been subdivided into more than twenty lithofacies by Buick (1985), and by Buick and Dunlop (1990). These can be grouped in four facies assemblages, which include an arenaceous assemblage consisting of volcanogenic arenites, an assemblage consisting of volcanogenic lutires, and facies comprising mainly chemical sedimentary rocks belong to either carbonate or sulphate assemblages. Silicified volcanic ash and volcaniclastic sediments comprise fine- to coarse-grained, grey-green arenites and lutites. Volcaniclastic arenites form beds that are between 0.1 and 5 m thick which are laminated, normally graded, and trough cross-bedded. Sets range in thicknes from 0.05 to 1 m, with internal laminations between 10 to 50 m m that are gently curved and inclined at angles of 10 to 20 °. The arenites are dominantly composed of pebble- to sand-sized grains, usually without a fine-grained matrix. Most clasts are derived by sedimentary reworking of basaltic debris (Dunlop and Buick, 1981 ), although embayed quartz phenocrysts derived from a silicic volcanic source occur in some units. This assemblage is interpreted as the remains of a sublittoral sandsheet formed in a high-energy environment by mobile megaripples (Dunlop and Buick, 1981 ). Lutaceous facies are composed of laminated, amorphous pale-grey microquartz (with finely dispersed chlorite and sericite) which has probably replaced a muddy sediment. Lutites are plane-laminated or ripple cross-bedded with laminations between 0.5 and 5 m m thick and ripples which are nearly symmetrical with amplitudes of up to 5 mm. Small 0.5 to 1 m m diameter siliceous pseudomorphs after diagenetic gypsum are relatively common in these facies. These units are interpreted as fine volcanic ash, or clays derived from volcanic sources, deposited on littoral and supralittoral

M.E.BARLEY mudflats (Buick, 1985). Pebbly rudite lenses containing rounded, tabular, poorly sorted, locally derived clasts occur within the lutaceous facies. These sediments are interpreted as intraclast conglomerates deposited in tidal channels (Buick, 1985). Chemical sedimentary lithofacies include plane-laminated or ripple cross-bedded lutites which are composed only of microquartz and carbonate, and which contain pseudomorphs after diagenetic gypsum. These are interpreted as carbonate mudstones and siltstones. Possibly biogenic sedimentary rocks include layers with stromatolite-like internal lamination and pebbly intraclast breccias derived from these layers. Coarse-grained sulphate evaporite facies occur in lenses up to 2 km long and 5.5 m thick (Buick and Dunlop, 1990). These are composed of coarse laminations of nearly vertically oriented barite crystals in clusters, which resemble selenite cavoli in modern and Phanerozoic gypsiferous evaporites (Dunlop et al., 1979). The interfacial angles of these crystals are those typical of gypsum rather than barite (Dunlop, 1978 ). At the tops of beds, the barite pseudomorphs after gypsum have sedimentary laminae draping over them, some of the crystals are broken and the fragments have been reworked, suggesting that the rock originally contained primary evaporitic gypsum. A detailed description of these unusual and important early Archaean sedimentary rocks is presented by Buick and Dunlop ( 1990 ). The interpreted evolution of sedimentary environments in the North Pole Chert is shown schematically in Fig. 7. The textures and structures of the clastic lithofacies and relationships between facies assemblages indicate deposition in coastal sites (Dunlop, 1976; Groves et al., 1981; Buick, 1985; Buick and Dunlop, 1990). They apparently formed in: (1) shallow sublittoral sand sheets that are structured by megaripple cross-lamination; (2) shoreline deposits that built up into barrier bars;

THE ~ 3.46 Ga WARRAWOONAMEGASEQUENCE:A REVIEW

61

Tidal channels

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(3) supralittoral mudflats (composed of volcanic ash and carbonate muds) that were cut by intraclast filled tidal channels; and (4) terrestrial debris flow fans that were partly reworked during marine incursions. Carbonate muds and diagenetic gypsum formed on supralittoral flats, and bedded sulphates formed in semi-permanent lagoons behind barrier bars. The whole unit was deposited during two shoaling cycles separated by an erosion surface (Buick, 1985 ). Soon after burial, sediments were silicified and pre-existing sulphate minerals were converted to barite when heated seawater circulated through the underlying pile ofbasalts.

The Strelley Pool Chert The Strelley Pool Chert is a 25 m thick cherty unit in the Pilgangoora Belt (Fig. 1 ), which has been described by Lowe ( 1983 ) and has been correlated with similar cherts in the North Pole Dome, Kelly Belt and Coongan Belt. In these belts, the Chert overlies silicic volcanic rocks

of the Panorama Formation (Fig. 2). DiMarco and Lowe (1989a) present evidence that the chert developed above an erosional unconformity. It contains a similar suite of lithofacies to the North Pole Chert in a predominantly regressive succession, with internal shoaling-upwards cycles (Lowe, 1983 ). The main difference between this chert and the North Pole Chert is the abundance of silicic volcanic debris, and the absence of barite (coarse-grained evaporite facies are silicifled). Stromatolite-like structures are also developed in most of the locations described by Lowe ( 1980, 1983 ). Lowe ( 1983 ) interpreted the Strelley Pool Chert as the result of sedimentation in an extensive low-energy, partially restricted hypersaline basin (or basins), in similar environments to modern shallowmarine carbonate and evaporite systems such as in the Persian Gulf.

The Marble Bar Chert The Marble Bar Chert (Fig. 2) is a distinctive 50 m thick red, white and black banded

62

chert which outcrops near Marble Bar (Fig. 1 ). Similar chert lithofacies occur locally in other belts. The colour banding is due to variations in the abundance of minute haematite inclusions in the microquartz matrix of the chert. The banding is on two scales; 1 to 5 cm thick colour bands (mesobands) which are generally planar but may pinch and swell, and 0.5 to 5 m m thick undulating laminae. This chert shows no distinctive clastic sedimentary features, but contains abundant syneresis cracks, and breccia lenses. Some of these chaotic breccias containing tabular fragments of banded chert probably result from movement of partly lithified sediment. These structures were considered typical of primary siliceous and ferruginous sediments that were originally precipitated as gelatinous colloids by Kjellgren (1976). This chert overlies intermediate turbiditic tufts and breccias and magnesian basalts and resembles iron-poor facies ofjaspilitic BIF. It probably represents a deeper-water depositional environment than the North Pole or Strelley Pool Cherts. DiMarco and Lowe (1989a) suggest that the Marble Bar Chert is correlative to the Strelley Pool Chert.

Evidence for early life Stromatolite-like structures have been described from Warrawoona Megasequence cherts (Lowe, 1980, 1983; Walter et al., 1980; Buick et al., 1981 ) and putative cellular microfossils have also been reported from these units (Dunlop et al., 1978; Awramik et al., 1983; Schopf and Packer, 1987). If these structures are biogenic and close to 3460 million years old, then they are the amongst the oldest evidence for life on earth. Lowe (1980, 1983) described relatively abundant, internally laminated, conical structures in the StreUey Pool Chert as stromatolites. These have between 2 to 6 cm relief and are similar in form to Conophyton, a group of stromatolites which is common in the Proterozoic. However, in the absence of contained

M.E. BARLEY

microfossils, these structures are not sufficiently complex to be recognized as unambiguously biogenic (Walter, 1983). Walter et al. (1980) and Buick et al. (1981) described nodular and pseudocolumnar structures from the North Pole Chert as probable stromatolites. These structures display several orders of complex, dominantly convex upwards, internal lamination and according to Buick et al. ( 1981 ) are therefore probably biogenic. However, to date no definite early Archaean microbial remains have been found within stromatolite-like structures from this locality (Buick, 1984, 1988, 1991 ). It is commonly considered that the presence of stromatolites indicates the presence of photosynthetic cyanobacteria. However, most previously described possible microfossils from the Warrawoona Megasequence are either too simple to be unambiguously recognized as biogenic structures (e.g. the microfossils described by Dunlop et al., 1978) or, if more complex, may be younger than the rocks in which they were found (e.g. the microfossils described by Awramik et al., 1983). Arguments for and against the biogenicity and ~ 3.46 Ga age of microfossil-like objects from North Pole, presented in a series of papers by Awramik et al. ( 1983, 1988 ) and Buick ( 1984, 1988 ), indicate how difficult it is to prove the existence of microfossils in Warrawoona Megasequence rocks. Reasons for this include: ( 1 ) Several features of the North Pole Chert which indicate hydrothermal activity during, or soon after deposition (Buick, 1988; Barley and Groves, 1989). (2) Metamorphism and uplift of the North Pole Dome at ~ 3.3 Ga and again at 2.77 to 2.76 Ga (during deposition of the Nullagine Supersequence; Blake, 1993). The pattern of chert dykes (possible source of contamination) which terminate at, or cut, the North Pole Chert indicates that they were emplaced during one (or both) of these events (Dunlop, 1976; Buick, 1984, 1988). Both events have disturbed the isotope (Pb-Pb and Rb-Sr;

63

THE ~ 3.46 Ga WARRAWOONA MEGASEQUENCE: A REVIEW

Blake and McNaughton, 1984) systems of Warrawoona Megasequence volcanic rocks. (3) All outcrops of the North Pole Chert (and indeed most of the Warrawoona Megasequence) have experienced Tertiary lateritic weathering and it is likely that this is the source of much of the silicification and consequently another source of possible contamination. There appears to be less doubt about the age of microfossil-like objects from cherts in the Pilgangoora and Marble Bar Belts described by Schopf and Packer ( 1987 ). These are filamentous, or comprise globular colonies of sheathenclosed spheroidal cells similar to extant cyanobacteria and Proterozoic microfossils interpreted as cyanobacteria and have been interpreted as such by Schopfand Packer ( 1987 ). However, a variety of micro-organisms have similar filamentous morphologies and interpretation of these as cyanobacteria rather than other eubacteria, although plausible, is not unequivocal (e.g. Knoll and Bauld, 1989 ). Although unambiguous recognition of Archaean biogenetic structures requires considerable care (Cloud and Morrison, 1979; Buick et al., 1981, 1991 ), some of the abundant stromatolite-like structures (stromatoloids) described from the Warrawoona Megasequence are sufficiently complex to be considered probably biogenic (Buick et al., 1981; Walter et al., 1980; Walter, 1983), and others as possibly biogenic. Also some of the microfossil-like objects described from Warrawoona Megasequence cherts are likely to be Early Archaean in age (Schopf and Packer, 1987). This evidence suggests that microbial life was probably well established on the Earth by 3.46 Ga. However, sampling beneath the weathering profile (i.e. drilling to a depth of > 150 m) is a necessity if unambiguous ca 3.46 Ga microfossils and stable isotope data (particularly oxygen) are to be obtained from Warrawoona Megasequenee cherts. Sulphur isotope studies of barite from the North Pole Chert show that fi34S values cluster around + 3%o. Lambert et al. ( 1978 ) argue that

this sulphate was derived by rapid, almost complete surficial oxidation of juvenile sulphur ( t ~ 3 4 5 ,-~ 0 ° / 0 o ) , and Buick and Dunlop (1990) suggest that the associated sulphides formed from widespread reduction of such SO42-, rather than from unoxidized juvenile sulphur. The almost complete oxidation of reduced sulphur species in a shallow-water environment is consistent with the probable existence of photosynthetic bacteria as previously suggested by Groves et al. ( 1981 ). In contrast, Ohmoto and Felder ( 1987 ) argue that sulphur isotope data on Archaean volcano-sedimentary sulphides (including those from the Warrawoona Megasequence) are in accord with widespread bacterial reduction of seawater sulphate with ~ 3 4 S of + 3%0 at water temperatures between 30 and 50°C. The presence of pseudomorphs after gypsum rather than anhydrite indicates temperatures of less than 60°C (Dunlop and Buick, 1980; Walker et al., 1983) and is a potential problem for models involving significantly warmer Archaean seawater. Evidence for meteorite impacts

Lowe and Byerly (1986, 1987) interpreted sand-sized spherules from ~ 3.5 Ga cherts in the Barberton Mountainland (in South Africa) and from the North Pole Dome as quenched, liquid silicate droplets formed during meteorite impacts. Buick (1987) questioned this interpretation for the Warrawoona Megasequence cherts, pointing out that apparently similar spherules (rounded sand-sized basalt clasts and amygdales) occurred in cherts throughout the Warrawoona Megasequence. Subsequently Lowe et al. (1989) presented evidence of Ir and spinel compositions for the South African spherules which are consistent with an origin as impact generated melt droplets. Similarities with Warrawoona Megasequence spherules suggests that some of these may also have formed during meteorite impact,

64

however, to date no geochemical evidence has been presented.

Tectonic setting Although there is no single aspect of the Warrawoona Megasequence that is by itself diagnostic of tectonic setting, the weight of evidence summarized in this paper is most consistent with the interpretation that the Megasequence comprises a tectonostratigraphic assemblage which formed in early Archaean equivalents of volcanic-arc and arc-related tectonic settings. The compositions of most tholeiitic and calc-alkaline volcanic and plutonic rocks fall within the range of m o d e m supra-subduction magmatism. Perhaps the most significant differences are the existence of silicic rocks with very low heavy REE abundances, which were probably derived by partial melting of marie crust (Jahn et al., 1981; Cullers et al., 1993 ). The abundance of heavy REE depleted silicic magmas in Archaean calcalkaline suites is thought to result from partial melting of relatively hot, young oceanic crust at a subduction zone (Martin, 1986). However, as in m o d e m supra-subduction settings, the Warrawoona Megasequence calc-alkaline suite has a complex petrogenesis involving both mantle-derived basaltic magmas and intermediate to silicic melts of marie crust, with variable amounts of magma mixing and fractional crystallization ( _+crustal contamination). The style of hydrothermal alteration in the Warrawoona Megasequence is similar to that in Phanerozoic tholeiitic and calc-alkaline arc and marginal-basin assemblages, as is the association of barite-rich Z n - P b - C u massive sulphide mineralization, and porphyry/epithermal C u - Z n - A u mineralization (Barley et al., 1992). Most importantly, associations of facies in the calc-alkaline volcanic and volcanogenie sedimentary sequences indicate deposition in a range of shore line, shallow-marine and deeper marine environments. These facies associations are similar to those in and around

M.E. BARLEY

Phanerozoic island volcanoes (e.g. BusbySpera, 1988), and to the facies model for arc and near-arc sedimentation developed by Houghton and Landis (1989). Also evaporites, carbonates and volcanogenic mud and sand were deposited in extensive sheets on basalt, in and around shallow-marine basins. These ancient shallow-water sedimentary rocks preserve some of the oldest evidence for life on Earth.

Acknowledgements Mike Bickle, Tim Blake, Clive Boulter, Roger Buick, Mike DiMarco, John Dunlop, David Groves, Rudy Horwitz and Bryan Krapez are all thanked for discussions of Pilbara geology that helped to form the synthesis presented in this paper.

References Arndt, N.T. and Jenner, G., 1986. Crustally contaminated komatiites and basalts from Kambalda, Western Australia. Chem. Geol., 56: 229-255. Aguirre, L. and Offler, R., 1985. Burial metamorphism in the Western Puruvian Trough: its relation to Andean magmatism and tectonics. In: W.S. Pitcher, M.P. Atherton, E.J. Cobbing and R.D. Beckinsale (Editors), Magmatism at a Plate Edge: the Puruvian Andes. Wiley, New York, pp. 59-71. Awramik, S.M., Schopf, J.W. and Walter, M.R., 1983. Filamentous fossil bacteria from the Archean of Western Australia. Precambrian Res., 20: 357-374. Awramik, S.M., Schopf, J.W. and Walter, M.R., 1988. Carbonaceous filaments from North Pole, Western Australia: are they fossil bacteria in Archean stromatolites? A Discussion. Precambrian Res., 39: 303-309. Barley, M.E., 1981. Relations between volcanic rocks in the Warrawoona Group: continuous or cyclic evolution? Spec. Publ. Geol. Soc. Aust., 7: 263-273. Barley, M.E., 1984. Volcanism and hydrothermal alteration, Warrawoona Group, East Pilbara. Publ. Univ. West. Aust. Geol. Dept. Ext. Serv., 9: 23-36. Barley, M.E., 1986. Incompatible element enrichment in Archean basalts: A consequence of crustal contamination rather than mantle heterogeneity. Geology, 14: 947-950. Barley, M.E., 1992. Archean volcanic-hosted massive sulfide and sulfate mineralization in Western Australia. Econ. Geol., 85: 855-872.

THE ~ 3,46 G a W A R R A W O O N A MEGASEQUENCE: A REVIEW

Barley, M.E. and Bickle, M.J., 1982. Komatiites in the Pilbara Block. In: N.T. Arndt and E.G. Nisbet (Editors), Komatiites. George Allen and Unwin, London, pp. 105-115. Barley, M.E. and De Laeter J.R., 1984. Disturbed Rb-Sr whole-rock systems of the early Archaean Duffer Formation, Eastern Pilbara Block, Western Australia. J. R. Soc. West. Aust., 66: 129-134. Barley, M.E. and Groves, D.I., 1989. Exploration significance of regional and local scale hydrothermal alteration patterns in greenstone belts. Min. Energy Res. Inst. West. Aust. Rept., 65:85 pp. Barley, M.E., Dunlop, J.S.R., Groves, D.I. and Glover, J.G., 1979. Sedimentary evidence for an Archaean shallow-water volcano-sedimentary facies, eastern Pilbara Block, Western Australia. Earth Planet. Sci. Lett., 43: 74-84. Barley, M.E., Borley G.D., Sylvester G.C., Groves D.I. and Rogers, N., 1984. Archaean calc-alkaline volcanism in the Pilbara Block, Western Australia. Precambrian Res., 24: 285-319. Barley, M.E., Groves, D.I. and Blake, T.S., 1992. Archaean metal deposits related to tectonics: evidence from Western Australia. In: J.G. Glover (Editor), Proc. Third Int. Archaean Symp., Perth, pp. 307-324. Bickle, M.J., Bettenay, L.F., Barley, M.E., Groves, D.I., Chapman, H.J., Campbell, I.H. and De Laeter, J.R., 1983. A 3500 Ma plutonic and volcanic province in the Archean east Pilbara Block. Contrib. Mineral. Petrol., 84: 25-35. Blake, T.S., 1993. Late Archaean crustal extension, sedimentary basin formation, flood basalt volcanism and continental rifting: the Nullagine and Mount Jope Supersequences, Western Australia. Precambrian Res., 60:185-242, this volume. Blake, T.S. and McNaughton, N.J., 1984. A geochronological framework for the Pilbara region. Publ. Univ. West. Aust. Geol. Dept. Ext. Serv., 9: 1-22. Buick, R., 1984. Carbonaceous filaments from North Pole, Western Australia: Are they fossil bacteria in Archaean stromatolites? Precambrian Res., 24:157-172. Buick, R., 1985. Life and conditions in the early Archaean: evidence from 3500 m.y. old shallow-water sediments in the Warrawoona Group, North Pole, Western Australia. Ph.D. thesis, Univ. West. Aust., Nedlands, unpubl. Buick, R., 1987. Early Archean silicate spherules of probable impact origin, South Africa and Western Australia: comment. Geology, 15: 180-181. Buick, R., 1988. Carbonaceous filaments from North Pole, Western Australia: are they fossil bacteria in Archaean stromatolites? A reply. Precambrian Res., 39:311-317. Buick, R., 1991. Microfossil recognition in Archean rocks: an appraisal of spheroids and filaments from a 3500 M.Y. old chert-barite unit at North Pole, Western Australia. Palaios, 5: 441-459.

65 Buick, R. and Barnes, K.R., 1984. Cherts in the Warrawoona Group: Early Archaean silicified sediments deposited in shallow-water environments. Publ. Univ. West. Aust. Geol. Dept. Ext. Serv., 9: 37-53. Buick, R. and Dunlop, J.S.R., 1990. Evaporitic sediments of early Archaean age from the Warrawoona Group, North Pole, Western Australia. Sedimentology, 37: 247-277. Buick, R., Dunlop, J.S.R. and Groves, D.I., 1981. Stromatolite recognition in ancient rocks: an appraisal of irregularly laminated structures in an early Archaean chert-barite unit from North Pole, Western Australia. Alcheringa, 5: 161-181. Busby-Spera, C.J., 1988. Speculative tectonic model for the early Mesozoic arc of the southwest Cordilleran United States. Geology, 16:1121-1125. Cas, R.A.F and Wright, J.V., 1987. Volcanic Successions: Modern and Ancient. Allen and Unwin, London, 528 PP. Cloud, P. and Morrison, G.R., 1979. On microbial contaminants, micropseudofossils and the oldest records of life. Precambrian Res., 9:81-91. Collins, W.J., 1989. Polydiapirism of the Archean Mount Edgar Batholith, Pilbara Block, Western Australia. Precambrian Res., 43:41-62. Cullers, R., DiMarco, M.J., Lowe, D.R. and Stone J., 1993. Geochemistry of a silicified felsic volcaniclastic suite from the early Archean Panorama Formation, Pilbara Block, Western Australia: an evaluation of depositional and post-depositional processes with special emphasis on the rare-earth elements. Precambrian Res., 60: 99-116, this volume. DiMarco, M.J. and Lowe, D.R., 1989a. Stratigraphy and sedimentology of an early Archean felsic volcanic sequence, eastern Pilbara Block, Western Australia, with special reference to the Duffer Formation and implications for crustal evolution. Precambrian Res., 44: 147-169. DiMarco, M.J. and Lowe, D.R., 1989b. Shallow-water volcaniclastic deposition in the early Archean Panorama Formation, Warrawoona Group, eastern Pilbara Block, Western Australia. Sed. Geol., 64: 43-63. DiMarco, M.J., Ferrell, R.E. and Lowe D.R., 1989. Polytypes of 2:1 dioctahedral micas in silicified volcaniclastic sandstones, Warrawoona Group, Pilbara Block, Western Australia. Am. J. Sci., 289: 649-660. Dunlop, J.S.R., 1976. The geology and mineralization of part of the North Pole barite deposits, Pilbara region, Western Australia. B.Sc. (Honours) thesis, Univ. West., Nedlands, unpubl. Dunlop, J.S.R., 1978. Shallow-water sedimentation at North Pole, Pilbara Block, Western Australia. Univ. W. Aust., Geol. Dept. Univ. Ext. Publ., 2: 30-38. Dunlop, J.S.R. and Buick, R., 1980. The sedimentary environment of an Archaean chert-barite deposit at North Pole, Pilbara Block, Western Australia. Extended Ab-

66 stracts, Second Int. Archaean Symp., Perth, pp. 20-21. Dunlop, J.S.R. and Buick, R., 1981. Archaean epiclastic sediments derived from mafic volcanics, North Pole, Pilbara Block, Western Australia. Spec. Publ. Geol. Soc. Aust., 7: 225-233. Dunlop, J.S.R., Muir, M.D., Milne, V.A. and Groves D.I., 1978. A new microfossil assemblage from the Archaean of Western Australia. Nature, 274: 676-678. Dunlop, J.S.R., Groves, D.I. and Buick, R., 1979. Evidence for Archaean evaporites. Open Earth, 6:15-16. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin, 390 pp. Glikson, A.Y. and Hickman, A.H., 1981. Geochemical stratigraphy and petrogenesis of Archaean basic-ultrabasic volcanic units, eastern Pilbara Block, Western Australia. Spec. Publ. Geol. Soc. Aust., 7: 287-300. Groves, D.I., Dunlop, J.S.R. and Buick, R., 1981. An early habitat of life. Sci. Am., 245: 64-73. Groves, I.M., 1987. Epithermal-porphyry style base-metal mineralization in the Miralga Creek area, eastern Pilbara Block. B.Sc. (Honours) thesis, Univ. West. Aust., Nedlands, unpubl. Hackett, W.R. and Houghton B.F., 1989. A facies model for a Quaternary andesitic composite volcano: Ruapehu, New Zealand. Bull. Volcanol., 51: 51.68. Hallberg, J.A., 1974. Whole rock geochemical orientation trip to the Pilbara. Rep. CSIRO Min. Res. Lab. Div. Mineral., FP3. Hamilton, W.B., 1988. Plate tectonics and island arcs. Geol. Soc. Am. Bull., 100: 1503-1527. Hickman, A.H., 1977. New and revised definitions of rock units in the Warrawoona Group, Pilbara Block. Ann. Rep. Geol. Surv. West. Aust., 1976, p. 53. Hickman, A.H., 1978. Nullagine, W.A.: West Aust. Geol. Surv. 1:250,000 Geol. Series Explan. Notes. Hickman, A.H., 1981. Crustal evolution of the Piibara Block, Western Australia. Spec. Publ. Geol. Soc. Aust., 7: 57-69. Hickman, A.H., 1983. The geology of the Pilbara Block and its environs. Geol. Surv. West. Aust. Bull., 127: 128 pp. Hickman, A.H. and Lipple, S.L., 1978. Marble Bar, W.A. West Aust. Geol. Surv. 1:250000 Geol. Ser. Explan. Notes: 24 pp. Horgan, J., 1991. Trends in evolution: in the begining. Sci. Am., 264: 100-109. Horwitz, R.C., 1986. Unconformities and volcanic provinces of the Pilbara Block, Western Australia. Abstracts 12th Int. Sedimentological Congr., Canberra, 1986, p. 143. Horwitz, R.C., 1990. Palaeogeographic and tectonic evolution of the Pilbara Craton, northwestern Australia. Precambrian Res., 48: 327-340. Horwitz, R.C. and Pidgeon, R.T., 1993.3.1 Ga tuff from the Sholl Belt in the West Pilbara: further evidence for diachronous volcanism in the Pilbara Craton of West-

M.E. BARLEY ern Australia. Precambrian Res., 60: 175-183, this volume. Houghton, B.F. and Landis, C.A., 1989. Sedimentation and volcanism in a Permian arc-related basin, southern New Zealand. Bull. Volcanol., 51: 433-450. Jahn, B.M., Glikson, A.Y., Peucat, J.J. and Hickman, A.H., 1981. REE geochemistry and isotope data of Archaean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution. Geochim. Cosmochim. Acta, 45: 1633-1652. Johnson, R.W., MacKenzie, D.E. and Smith, I.E.M., 1978. Volcanic rock associations from convergent plate boundaries: reappraisal of the concept using case histories from Papua New Guinea. Bull. Geol. Soc. Am., 89: 96-106. Kjellgren, G.M., 1976. The Marble Bar chert, and its geologic setting, Marble Bar, Pilbara Block, W.A.. B.Sc. (Honours) thesis, Univ. West. Aust., Nedlands, unpubl. Knoll, A.H. and Bauld, J., 1989. The evolution of ecological tolerance in prokaryotes. Trans. R. Soc. Edinburgh Earth Sci., 80: 209-223. Krapez, B., 1989. Depositional styles and geotectonic settings of Archean metasedimentary sequences: evidence from the Lalla Rookh Basin, Pilbara Block. Ph.D. thesis, Univ. West. Aust., Nedlands, unpubl. Krapez, B., 1993. Sequence statigraphy of the Archaean supracrustal-belts of the Pilbara Block, Western Australia. Precambrian Res., 60 1-45, this volume. Krapez, B. and Barley, M.E., 1987. Archaean strike-slip faulting and related ensialic basins: evidence from the Pilbara Block, Australia. Geol. Mag., 124: 555-567. Lambert, I.B., Donnelly, T.H., Dunlop, J.S.R. and Groves, D.I., 1978. Stable isotopic compositions of early Archean sulphate deposits of probable evaporitic and volcanogenic origins. Nature, 276:808-811. Lipple, S.L., 1975. Definitions of new and revised stratigraphic units of the eastern Pilbara region. West. Aust. Geol. Surv. Annual Rep., 1974, pp. 58-63. Lowe, D.R., 1980. Archean sedimentation. Ann. Rev. Earth Planet. Sci., 8: 145-167. Lowe, D.R., 1982. Comparative sedimentology of the principal volcanic sequences of Archaean greenstone belts in South Africa, Western Australia and Canada: implications for crustal evolution. Precambrian Res., 17: 1-29. Lowe, D.R., 1983. Restricted shallow-water sedimentatation of early Archean stromatolitic and evaporitic strata of the Strelley Pool chert, Pilbara Block, Western Australia. Precambrian Res., 19: 293-283. Lowe, D.R. and Byerly, G.R., 1986. Early Archean silicate spherules of probable impact origin, South Africa and Western Australia. Geology, 14: 83-86. Lowe, D.R. and Byerly, G.R., 1987. Early Archean silicate spherules of probable impact origin, South Africa

T H E ~ 3.46 G a W A R R A W O O N A M E G A S E Q U E N C E : A R E V I E W

and Western Australia: reply. Geology, 15:181-182. Lowe, D.R., Byerley, G.R., Asaro, F. and Kyte, F.J., 1989. geological and geochemical record of 3400-million year old terrestrial meteorite impacts. Science, 243: 959962. Martin, H., 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, 14: 753-756. McNaughton, N.J., Compston, W. and Barley, M.E., 1993. Constraints on the age of the Warrawoona Group, eastern Pilbara Craton, Western Australia. Precambrian Res., 60: 69-98, this volume. Nisbet, E.G., 1987. The Young Earth: an Introduction to Archaean Geology. Allen and Unwin, Boston, 402 pp. Ohmoto, H. and Felder, R.P., 1987. Bacterial activity in the warmer sulphate-bearing, Archean oceans. Nature, 328: 224-246. Pearce, J.A. and Cann J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290-300. Pidgeon, R.T., 1978. 3450 Ma. old volcanics in the Archaean layered greenstone succession of the Pilbara Block, Western Australia. Earth Planet. Sci. Lett., 37: 421-428. Schopf, J.W. (Editor), 1983. Earths Earliest Biosphere: its Origins and Evolution. Princeton Univ. Press, Princeton, 544 pp. Schopf, J.W. and Packer, B.M., 1987. Early Archean (3.3-

67

billion to 3.6-billion-year-old) microfossils from Warrawoona Group, Australia. Science, 237: 70-73. Smith, G.A., 1986. Coarse-grained nonmarine volcaniclastic sediment; terminology and depositional processes. Geol. Soc. Am. Bull., 97: 1-10. Thorpe, R.I., Hickman, A.H., Davis, D.W., Mortensen, J.K. and Trendall, A.F., 1990. Application of recent zircon U-Pb geochronology to the Marble Bar region, Pilbara Craton, to modelling Archean lead evolution. Extended Abstracts, Third Int. Archean Symp., Perth, 1990, pp. 11-13. Walker, J.C.G., Klein, C., Schidlowski, M., Schopf, J.W., Stevenson, D.J. and Walter, M.R., 1983. Environmental evolutuion of the Archean-Early Proterozoic Earth. In: J.W. Schopf (Editor), Earths Earliest Bisphere: Its Origins and Evolution. Princeton Univ, Press, Princeton, pp. 260-290. Walter, M.R., 1983. Archean stromaltolites evidence of the Earth's earliest benthos. In: J.W. Schopf (Editor), Earths Earliest Bisphere: Its Origins and Evolution. Princeton Univ. Press, Princeton, pp. 187-213. Walter, M.R., Buick, R. and Dunlop, J.S.R., 1980. Stromatolites 3400-3500 Myr old from the North Pole area, Western Australia. Nature, 284: 443-445. Williams, I.S. and Collins, W.J., 1990. Granite-greenstone terranes in the Pilbara Block, Australia, as coeval volcano-plutonic complexes: evidence from U-Pb zircon dating of the Mt Edgar Batholith. Earth Planet. Sci. Lett., 97: 41-53.