Geochronology of early to middle Proterozoic fold belts in northern Australia: a review

PrecambrianResearch, 40/41 (1988) 1-19

1

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

GEOCHRONOLOGY OF EARLY TO MIDDLE PROTEROZOIC FOLD BELTS IN NORTHERN AUSTRALIA: A REVIEW R.W. PAGE Bureau of Mineral Resources, Canberra, A. C. T. (Australia) (Received March 30, 1987; revision accepted December 21, 1987)

Abstract Page, R.W., 1988. Geochronology of early to middle Proterozoic fold belts in northern Australia: a review. Precambrian Res., 40/41: 1-19. This geochronological review integrates geologically relevant isotopic data from the several early to middle Proterozoic fold belts of northern Australia. A number of firm chronological benchmarks, largely established by means of U-Pb zircon studies, now provide a time framework within which to better quantify the tectonostratigraphic history of many of these terranes. The apparent, world-wide dearth of crustal events in the earliest Proterozoic, between 2500 and 2000 Ma, was broken in northern Australia with the advent of a major crust-forming episode between 1880 and 1850 Ma ago. These rocks may have evolved from an initial mantle or lower crustal melting and fractionation process about 400-300 Ma earlier, that is evidenced from Sm-Nd model ages. The extent, involvement, and/or degree of assimilation of any pre-existing Archaean crust is small, but whether or not it is negligible or zero is not satisfactorily known. The 1880-1850 Ma event, the Barramundi Orogeny, is well represented in northern Australia, both lithologically and analytically, and the rocks that pre- and post-date it have a similar tectonic setting over very wide areas of the continent. Two consistent and widespread elements of this tectonism are regional metamorphism and deformation of the first cycle of supracrustals (Barramundi Orogeny), and crystallization of I-type, K-rich felsic magmas closely following and, in some cases, overlapping with deformational processes. In the Halls Creek Inlier, the ages of this rapid deep crustal to supracrustal tectonic transition cannot be distinguished within the range 1860-1850 Ma. This major yet short-lived tectonism is mirrored in other fold belts such as Pine Creek, Tennant Creek, and Mount Isa, in which U-Pb zircon ages give slightly older constraints of 1885-1870 Ma, 1920-1870 Ma and 1885 ± 10 Ma, respectively. The ensuing felsic magmatism in these latter terranes continued for a few tens of millions of years, with the crystallization and emplacement of comagmatic volcanic/plutonic pairs throughout, in the interval 1870-1860 ]Via. In the Georgetown and Arunta Inliers, direct evidence of this otherwise ubiquitous event, if present, is masked by younger high-grade metamorphism. In the latter, however, metamorphic crustal components having approximate protolith ages of this order can be inferred from their evolved isotopic signatures. The geochronological and geochemical coherency of the extensive early Proterozoic, ~ 1870 Ma magmatism contrasts with that of post-1800 Ma magmatism, which is less extensive, generally anorogenic, commonly bimodal in character, and varies in age from one terrane to another. Several major sedimentary basins were being filled at this time. Major igneous events at 1790-1740 Ma, 1670 Ma and ~ 1500 Ma are recognized. Elongate, sublinear volcanic and plutonic belts, 1790-1740 Ma old, are present at Mount Isa. In high-grade granitic gneisses of the Arunta Inlier, a few U-Pb zircon ages and extrapolated Rb-Sr protolith ages are coincident at about 1760-1750 Ma, indicating that this event may be important in the development of the 'Division 2' sequences of that fold belt. The platformal Hart Dolerite and Oenpelli Dolerite, two of the largest mafic intrusions in the world, were emplaced at 1762 + 25 Ma and 1688 _+13 Ma. Younger felsic plutonism and volcanism gave rise to rapakivi-type granites in the Mount Isa, Tennant Creek and The Granites-Tanami Inliers at ~ 1670 Ma, a time close to that of the Aileron deformational/metamorphic event in central Australia. Further, at ~ 1650 Ma, there is growing evidence of an unexpectedly coherent hydrothermal episode

that reset or disturbed many Rb-Sr ages, gave rise to pegmatites, and was possibly associated with mineralization in the Davenport and eastern Arunta Inliers. Discrete middle Proterozoic deformationalevents, between 1610 and 1470 Ma, punctuate the tectonostratigraphic history in the Arunta, Mount Isa and Georgetownfold belts where they are best documented.

Introduction

The early to middle Proterozoic (2500-1600 Ma to 1600-900 Ma, following Plumb and James (1986)) was an important period of crustal development, particularly in northern Australia where several inliers of this age, separated from each other by Phanerozoic basins, dominate much of the continental area and are host to some of Australia's largest mineral deposits. In order to define and trace the principal tectonic and petrogenetic trends within and between these early to middle Proterozoic fold belts (e.g., Rutland, 1973; Plumb, 1979; Plumb et al., 1981 ), and to quantify model (s) for such orogenies, we need to be confident of the numerical database that reveals the growth and changes of such terranes through time. In the last decade, more detailed characterization of the variety and chronology of these crustal processes has become possible with the wider application of isotopic techniques that, together with field and geochemical criteria, can now be more assuredly employed to date specific events, be they igneous crystallization, metamorphism, thermal history, or even pre-crustal processes in the mantle. In Proterozoic or older terranes, the geological utility and precision of the U - P b clock in zircons, whether by conventional or ion microprobe methods, is widely recognized, and zircon geochronology has now become a most important corner-stone for furthering the understanding of tectonostratigraphic evolution. This paper endeavours to integrate and assess the most reliable, relevant geochronological data in the early to middle Proterozoic domains of northern Australia, from the Halls Creek-King Leopold Inliers in the west, to the Mount Isa and Georgetown-Coen Inliers in the

east (Fig. 1). The isotopic data can also shed light on the extent of crustal recycling, that is, whether geologically younger parts of the continental mass represent addition of new material, or are in fact products of metamorphism of older, pre-existing provinces. These considerations are central to any age framework that may arise from such an isotopic synthesis. The data are treated within the stratigraphic and tectonic framework recently presented by Etheridge et al. (1987). T h a t synthesis of northern Australian early to middle Proterozoic orogenesis describes two major periods of basin formation, involving an earlier pre-orogenic sedimentary-volcanic cycle (cycle 1, or ~ Lower Proterozoic of D u n n et al. (1966)), tectonically separated from a younger depositional package (cycle 2, or ~ C a r p e n t a r i a n of D u n n et al. (1966)) by a major deformation/ metamorphic event of continental proportions: the Barramundi Orogeny. The geochemistry of associated igneous rocks and structural style of this orogenesis appear to be consistent over very large areas so, by critically reviewing the geochronology of important benchmarks in this development, we would hope to improve understanding of the relationships of the various lithologic units to one another and more accurately reconstruct the orogenic history, or at least recognize the problems or gaps in doing so. This review of early to middle Proterozoic geochronological work in northern Australia follows previous appraisals of Australian Precambrian geochronology by Compston and Arriens (1968) and Page et al. (1984). However, this paper is not intended to be an exhaustive review of all isotopic studies, but rather a summary of the well-dated events and unambiguous studies that have a major bearing on more

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accurately quantifying early to middle Proterozoic geological evolution. Since the review of four years ago, further U - P b zircon data have emerged on a number of Proterozoic terranes, in particular on the older cycle 1 sequences in the Mount Isa, Davenport, and Halls Creek Inliers and their time(s) of metamorphism/deformation. Some of these data have been obtained by ion microprobe analyses of single zircons or zones therein, and reveal zircon inheritance information relevant to the age of possible earliest Proterozoic and Archaean source materials. Additionally, new U - P b zircon studies in the Halls Creek, Arunta, and Davenport Inliers now provide further constraints on the age of cycle 2 sequences and associated magmatism.

The integration of these geochronological data for the north Australian early to middle Proterozoic can become a useful stepping-stone towards comparing these orogenic episodes and processes with events in southern Australian Proterozoic terranes, and with well-dated Proterozoic orogenies in north America and Scandinavia. All ages referred to in this review are based on (or recalculated with) decay constants recommended by the I.U.G.S. Subcommission on Geochronology (Steiger and J~iger, i977). Errors quoted are at the 95% or 2a confidence level. This summary incorporates only published data, with the exception of a few unpublished results (those not specifically referenced in the text) determined by the author.

L a t e A r c h a e a n e l e m e n t s in n o r t h e r n Australia

Prior to discussing the geochronology of the earliest recognized Proterozoic sequences in northern Australia, it is useful to summarize the isotopic ages for the small inliers of crust which became basement to these sequences. In the ensialic tectonic model of Etheridge et al. (1987), crust of this age has an important role, as the 'anchor' on to which newly underplated early Proterozoic material is accreted from the mantle, and which later becomes attenuated. The apparent paradox that stratigraphically and geochronologically well-documented late Archaean crust is only known in a few places, is best addressed by continuing not only the direct search for that crust, but also, within the early Proterozoic terranes, the search for old isotopic signatures and zircon relics inherited from such a source. Granitic rocks, gneisses, and metasediments of the Rum Jungle Complex, identified as structural basement to early Proterozoic successions in the Pine Creek Inlier, were known to be at least 2400 Ma old two decades ago (Richards et al., 1966, 1977). Further RbSr total-rock data on the nearby Waterhouse Complex show similarly old granitic gneisses or, in some cases, -~ 1800 Ma ages with high initial STSr/S6Sr, also indicative of a late Archaean protolith. McCulloch (1987) documents older S m - N d ages based on a depleted mantle model, in the range 2890-2710 Ma, but one of the Rum Jungle gneisses gives a very much older result (3300 Ma ) suggesting the presence of even earlier Archaean components. To the east, the age of gneissic granitoid in a drill core of the otherwise concealed Woolner Dome has been determined by zircon U - P b ion microprobe measurements at 2675 + 15 Ma (McAndrew et al., 1985). Rb-Sr total-rock and U - P b zircon data (Page et al., 1980) from a further eastward extension of this basement i n the Nanambu Complex give an age of 2470+26 Ma (initial STSr/S6Sr 0.704_+1), although the S m - N d

model age is again somewhat older, 2620 Ma. These late Archaean basement ages, the local reworking of whole-rock ages, and the pattern of younger mineral ages reflect a history similar to that in the Archaean-Proterozoic Gawler Craton of South Australia (Webb et al., 1986). Previous suggestions that the Halls Creek Group (Bofinger, 1967; Compston and Arriens, 1968; Gellatly, 1971 ) and part of the Arunta Inlier (Rossiter and Ferguson, 1980) are Archaean, are no longer considered valid (Page, 1976) and are unsubstantiated by recent work (McCulloch, 1987; Page and Hancock, 1988). However, there are suggestions of late Archaean crustal involvement from limited and somewhat ambiguous S m - N d data on Proterozoic rocks in the Tennant Creek and Georgetown Inliers {Black and McCulloch, 1984). The above represents the full, yet rather patchy, isotopic evidence on the age and nature of the preProterozoic basement in northern Australia, but there is new, yet subtle evidence obtained from zircon relics in the early Proterozoic terranes (summarized below ) that indicates that the late Archaean sialic crust may have been considerably more extensive.

Early Proterozoic supracrustal sequences - cycle 1

A period of upper crustal quiescence in the earliest Proterozoic (2500-2000 Ma) of northern Australia was broken with the advent of major crust-forming events between 2000 and 1800 Ma ago (Fig. 2). Initially, this involved a number of basin-forming episodes that are considered to have resulted from local extension of the pre-existing Archaean continental crust (Etheridge et al., 1987). The tripartite sedimentary-volcanic sequences so formed consist of (i) an older rift sequence commonly of fluviatile clastic sediments and bimodal volcanics, (ii) a middle finer-grained clastic and volcaniclastic sequence-thermal subsidence phase, and

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Fig. 2. Summary of U-Pb zircon ages on volcanic, plutonic and metamorphic rocks in northern Australian Proterozoic terranes. Also shown are several Rb-Sr results that define medium- and high-grade tectonothermal events. The stippled vertical areas represent the available age definition of the Barramundi Orogeny in different inliers. Cycle 1 sequences predate this 1890-1850 Ma event, and cycle 2 sequences, that dominate the lithostratigraphic and geochronological record, are post-orogenic, i.e., post-1870 Ma. The four open boxes show the limits of the principal igneous events: from left to right, the 1880-1850 Ma Barramundi orogenic suite, and the anorogenic suites at 1790-1740 Ma, ~ 1670 Ma and ~ 1500 Ma.

(iii) a younger flysch sequence, marking the beginning of orogenesis. Until recently very little isotopic work had been done on these cycle 1 successions or their metamorphic products, owing to the paucity of igneous rocks in the sequences and their dominantly mafic character. Nevertheless, some general age constraints were available from RbSr total-rock data on high-grade equivalents. For example, schists and paragneisses in the Ranger 'Footwall Sequence' (Namoona Group, Pine Creek Inlier) imply a maximum depositional age between 2200 and 2000 Ma (Page et al., 1980). In the Halls Creek Inlier, Bofinger's (1967) Rb-Sr data on Halls Creek Group volcanics suggest protolith ages of 2050 Ma or younger (Page, 1976). His pooled regional isochron for the Tickalara Metamorphics and syntectonic Mabel Downs Granodiorite is 1920_+27 Ma

(0.7025+ 10), and although perhaps marginally too old owing to probable Rb-Sr mixingline effects, this result had long been accepted as dating an important orogenic event affecting cycle 1 sequences in this Inlier. Page and Hancock (1988) separately regress these Mabel Downs granitic gneiss (1894 + 53 Ma, 0.702 _+1 ) and Tickalara Metamorphics data (1891_+ 78 Ma, 0.703 + 2 ), and suggest that these are more realistic ages for the time of regional metamorphism. A similar Rb-Sr total-rock age of 1920 ___60 Ma was interpreted by Black ( 1977 ) as dating amphibolite-grade tectonism in basement amphibolites of the T e n n a n t Creek Inlier. Although somewhat imprecise, this 1920+60 Ma deformation age is in accord with what is known so far of the ages of felsic volcanics in the deformed Warramunga Group in the Davenport Inlier (1880-1870 Ma, Blake and Page (1988)) and the unconformably overlying

Hatches Creek Group (1870-1815 Ma). Recently acquired U - P b zircon data on other cycle 1 sequences, summarized below, confirm their relatively short duration, within the time range 1880-1860 Ma. Page and Hancock (1988) report U - P b zircon data on the Halls Creek Group succession (cycle 1, Halls Creek Inlier). High-level alkali 'rhyolite' and quartz latite sills that are thought to be comagmatic with tuffs in the Biscay Formation indirectly determine the stratigraphic age at 1856 + 5 Ma. This is younger than dated cycle 1 sequences in other terranes (see below), although, in strict terms, it is only a minimum age for the Biscay Formation. More recent, preliminary evidence from U - P b zircon study of a Biscay Formation tuff suggests that the stratigraphic age might be closer to ~ 1880 Ma (Page, unpublished data). A felsic anatectic segregation in granulite facies pelitic and mafic rocks of the Tickalara Metamorphics provides a zircon U - P b age, 1854 + 6 Ma, indistinguishable from the 1856_+5 Ma 'rhyolite' sill age, and thereby a direct determination of the age of the high-grade Barramundi event in this province. The rapidity of tectonism implied from these results (underthrusting and metamorphism of the supracrustal terrane at lower crustal depths, possibly within a few million years) is further emphasized by the proximity in time of cycle 2 deposition, the unconformably overlying, lateto post-Barramundi Whitewater Volcanics ( 1850 _+5 Ma zircon age ). The presence of older Archaean crust and lithosphere beneath the Halls Creek Inlier is postulated from Nd isotopic data on the syndepositional Woodward Dolerite, clastic sediments from elsewhere in the Halls Creek Group, and ultramafic rocks from the Lamboo Complex (Sun et al., 1986). Pb and Nd isotopic studies on younger Proterozoic lamproites also provide indirect evidence of very early Proterozoic ( > 2.1 Ga) mantle evolution (Nelson et al., 1986), although it must be emphasized that no late Archaean rocks have yet been identified in the Halls Creek Inlier in any

direct geochronological, stratigraphic, or tectonic sense. Early Proterozoic cycle 1 sequences are also well represented in the Pine Creek succession and in the Arnhem Inlier. Recent U - P b zircon work on volcaniclastic rocks in the South Alligator Group yields near concordant data and a crystallization/deposition age of 1885 +_2 Ma (Needham et al., 1988). This is a maximum local age for the ensuing Barramundi Orogeny that deformed the sequence. When considered with the zircon ages on unconformably younger, post-orogenic volcaniclastic rocks in the E1 Sherana and Edith River Groups (Page and Williams, 1988), the intervening Barramundi tectonism in the Pine Creek Inlier is bracketed to the time frame 1885-1870 Ma. Independent corroborative evidence as to the regional significance and integrity of this event within the Pine Creek province is present from zircon studies of Nimbuwah granulites (1886 + 5 Ma, Page et al. (1980)), and from Rb-Sr total-rock data on the syntectonic Bradshaw complex in the Arnhem Inlier to the east (1902_+ 75 Ma; A.W. Webb, in Plumb and Derrick (1975)). The Mount Isa Inlier is largely composed of the younger cycle 2 depositional package, but there are older, isolated regions of pre-Barramundi (cycle 1 ) rocks that have been deformed in that orogeny. Zircons from paragneisses and an anatectic 'sweat' pegmatite in the Yaringa Metamorphics have been used with U - P b ion microprobe techniques to date the high-grade event at 1885+10 Ma (Page and Williams, 1988). Conventional U - P b work on multi-grain zircon samples of the gneiss gives a geologically meaningless age (2090+40 Ma) due to the presence of inherited grains (from 2.55 to 2.2 Ga sources) amongst the dominant population that crystallized in situ during 1885 Ma highgrade metamorphism. The Nd depleted mantle model age of one such paragneiss (2230 Ma, McCulloch (1987)) can also be interpreted as a mixed age of no direct geological consequence. A comparably old deformation/metamorphism age to that of the Yaringa Metamorphics is es-

tablished for another basement complex within the Kalkadoon-Leichhardt belt, where zircon U - P b data on migmatitic gneiss and an intrusive granitic dyke together constrain the migmatite formation age to 1870-1860 Ma (Blake and Page, 1988). This is in accord with other minimum age limits imposed by the 1865-1850 Ma ages for post-migmatite felsic volcanism in the Leichhardt Metamorphics (see below). There is thus mounting isotopic evidence that the Barramundi Orogeny, so widespread and uniform in its tectonostratigraphic and geochemical characteristics, is also reasonably coherent in time (to within 20-30 Ma) from one early Proterozoic terrane to another. The best determination for the age of this orogenic event in the Mount Isa Inlier is 1885 _+10 Ma, in the Pine Creek Inlier 1885-1870 Ma, in the Halls Creek Inlier 1854 + 6 Ma, and in the Tennant Creek Inlier 1920-1870 Ma. We can predict a similar age in The Granites-Tanami, Murphy, and Arnhem Inliers. Previous interpretations (Plumb, 1979; Rutland, 1981) that advanced older and younger 'domains' or 'subprovinces' within these early Proterozoic terranes can no longer be supported, particularly as one of the older tectonic 'domains', Halls Creek Inlier, appears to be marginally younger than the others.

Early Proterozoic orogenic felsic magmatism (cycle 2) A major period of felsic volcanism and granite emplacement closely followed and in some cases overlapped the Barramundi metamorphic and deformational processes that occurred between 1885 and 1860 Ma ago. The petrology and geochemistry of these magmatic episodes is reviewed by Wyborn (1988). The rocks are I-type, K-rich felsic magmas and are compositionally uniform over several terranes with a known outcrop area of at least 37 000 km 2. In a reconnaissance study of the S m - N d isotopic systematics, McCulloch (1987) found a relatively narrow range of depleted mantle model ages, from 2290 to 2130 Ma. These ages have no stra-

tigraphic application or significance. McCulloch (1987) interprets them as signifying times of primary crustal formation or mantle extraction processes, thus implying a ~- 400 Ma protolith pre-history since, as will be seen, the crystallization and stratigraphic age of the resultant supracrustals and granites is about 1850-1880 Ma. However, the geological ambiguity of such 2.3-2.1 Ga Nd model ages is raised by Arndt and Goldstein (1987) who prefer to consider them, not as crust formation ages, but as average ages of mixed Archaean and Proterozoic sources. Early geochronological work on granitic rocks in the Mount Isa and other Proterozoic inliers of northern Australia was based largely on K Ar and Rb-Sr mineral ages (Richards et al., 1963; McDougall et al., 1965; Richards, 1966). These and many of the subsequent Rb-Sr total-rock studies on granitic and volcanic rocks provided useful information on alteration and/ or cooling ages, but primary ages were rarely obtained. Likewise, initial 87Sr/86Sr ratios derived from such studies are mostly too high, and are artifacts of post-crystallization alteration events. Zircon U-Pb work appears to provide the most reliable and precise ages for the early to middle Proterozoic magmatic events, and for the most part this has been achieved by conventional multi-grain zircon techniques developed by Krogh (1973), and more recently by zircon ion microprobe analysis (Compston et al., 1986). Felsic volcanic rocks in the Leichhardt Metamorphics and associated intrusives in the Kalkadoon and Ewen Granites are the principal magmatic components that overlapped and followed the Barramundi Orogeny at the initiation of renewed basin formation (cycle 2) in the Mount Isa Inlier. This volcanic/plutonic pair forms a sublinear belt > 300 km in length. U Pb zircon ages in one part of the belt confirm the synchroneity of the pair, with igneous crystallization ages of 1865_+ 3 Ma and 1862 +_24 Ma, respectively (Page, 1978). Zircon data from other parts of this belt give statistically indis-

tinguishable ages of 1856 _+10 Ma for granites in the south, and less precise volcanic crystallization ages of 1886 _+33 Ma and 1852 _+27 Ma. The Ewen Volcanics in the far northern area of the belt may be marginally younger, having a U - P b age of 1852 _+7 Ma (Page, 1983a; Wyborn and Page, 1983 ). As indicated earlier, these ages, as well as the 1860 + 32 Ma result for post-migmatite granitic dyke zircons (Blake and Page, 1988), provide minimum age constraints for cessation of the Barramundi Orogeny in the Mount Isa Inlier. Mica ages, both K-Ar and RbSr, invariably reflect final cratonization and cooling of the terrane, 1500-1400 Ma ago. This geochemically distinctive igneous suite is found in each of the other early to middle Proterozoic inliers in northern Australia, and its chronology, as so far delineated from U - P b zircon studies, is strikingly uniform. In Pine Creek, the E1 Sherana and Edith River Groups that post-date Barramundi tectonism both contain subaerial volcanic sequences deposited in an active rift setting between 1885 and 1860 Ma ago. Subvolcanic intrusive equivalents (Grace Creek Granite ) also have this U-Pb age (1863+5 Ma), but as is commonly the case, relatively precise Rb-Sr total-rock isochron ages are significantly younger at 1778_+ 9 Ma (Page, unpublished data) .Of the several zircon suites analysed by conventional techniques from these volcanic units, there are some where the presence of inherited zircon components precludes the determination of meaningful stratigraphic ages by conventional zircon techniques (Needham et al., 1988; Page, unpublished data). Compositionally similar, I-type granitic bodies that are l a t e - t o post-tectonic (Cullen, Burnside, Bundy, Prices Springs, Margaret, and Shoobridge Granites), intrude deformed Pine Creek sequences and commonly contain highU zircon populations that have rather discordant U-Pb systems. These indicate crystallization over an extended time period, 1830-1750 Ma, some 100-40 Ma younger than the 1870 Ma regime of the Barramundi suite. The ages for

these Pine Creek granites are more similar to the 1800 Ma Yeldham and Big Toby Granites in the Mount Isa Inlier. In the Western Pine Creek Inlier, the Litchfield province granites give U-Pb ages for zircon and xenotime that do not quite agree at 1840 _+5 Ma and 1850 _+2 Ma, respectively. Most of these granitic rocks are deformed, and as a result have updated Rb-Sr total-rock systems (1768___16 Ma), but a 1852+33 Ma (0.7039 + 24) grouping of granodiorites appears to have retained primary age features (Page et al., 1985). The available granite ages in the Litchfield province thus closely mimic the 1855-1850 Ma ages of deformation and transitional felsic magmatism in the Halls Creek Inlier to the immediate southwest, as earlier inferred by Hancock and Rutland (1984). There are also close geochronological analogies between the granitoid isotopic data from the East Alligator River region (eastern Pine Creek Inlier) and the Arnhem Inlier (Fig. 1), although so far there are few data from the latter. The largely syntectonic Nimbuwah Complex has U-Pb zircon ages of 1886_+ 5 Ma for granulitic rocks, and 1866 _+8 Ma for granitic/ tonalitic gneisses (Page et al., 1980). The latter gneisses lie on a regional Rb-Sr total-rock isochron (1800+24 Ma, 0.7064_+8), which was interpreted as dating the main amphibolitefacies regional metamorphism, but is now considered to reflect a younger --~1800-1770 Ma late-tectonic overprint. Rb-Sr and K-At mica ages are also of this order. The syntectonic Bradshaw complex has similar (minimum) KAr and Rb-Sr mineral ages of 1800-1750 Ma (McDougall et al., 1965), and an older totalrock isochron age (1902 + 75 Ma) that probably dates the same high-grade event (Barramundi Orogeny) as seen in the Nimbuwah Complex. The post-tectonic Giddy and Caledon Granites have 1800-1750 Ma ages (McDougall et al., 1965), akin to mineral ages for granites in the main part of the Pine Creek Inlier (Riley, 1980). In the Murphy Inlier (Fig.

1 ), further minimum ages of this magnitude are recorded in Rb-Sr total-rock systems of the Cliffdale Volcanics (1733 ___20 Ma), and in 1800 Ma K-Ar mineral ages for intrusive granites (McDougall et al., 1965; A.W. Webb, in Plumb and Derrick, 1975). In the Halls Creek Inlier, the intensely deformed early Proterozoic cycle 1 metasediments and metavolcanics of the Halls Creek Group and Tickalara Metamorphics (1854 + 6 Ma) are unconformably overlain by the latetectonic Whitewater Volcanics (1850 + 5 Ma) and intruded by cogenetic, post-tectonic batholiths such as the Bow River Granite (Page and Hancock, 1988). These also form part of the north Australian Barramundi igneous suite documented by Wyborn (1988). Regional RbSr studies by Bofinger (1967) gave isochrons then interpreted as dating the Whitewater felsic volcanism at 1784+17 Ma (0.714+3). Page's (1976) reassessment of these and the Bennett and Gellatly (1970) data on similar West Kimberley felsic volcanics indicated that the generally disturbed Rb-Sr systems could only be interpreted within rather wide limits of 1912_+ 107 Ma (0.707+6). Similarly, Rb-Sr data give pooled 'ages' for granitic magmatism at 1840 + 50 Ma ( ~ 0.709) in the Lennard River suite of the King Leopold zone, and 1815 _+14 Ma (0.703 + 2) for the post-tectonic Bow River and Sophie Downs Granites. These granite ages are likely to be minima, but can be no older than the 1850 + 5 Ma zircon crystallization age of the Whitewater Volcanics. A depleted mantle SmNd model age of the Sophie Downs Granite reflects earlier mantle fractionation processes ( ~ 2.29 Ga; McCulloch, 1987) that are evident in the other 1870-1840 Ma terranes. The above constraints from the Halls Creek Inlier render previous Rb-Sr results (Page et al., 1976) on correlated magmatic/stratigraphic units in The Granites-Tanami Inlier open to reinterpretation. For example, the Mount Winnecke Formation felsic volcanic suite and associated subvolcanic granitic intrusions (Winnecke Granophyre) overlie and in-

trude basement metamorphics in a situation analogous to the Whitewater/Bow River suite at Halls Creek. Although no U-Pb zircon work has been done on the Winnecke extrusive/intrusive suite, their present Rb-Sr total-rock ages of 1770 ___15 Ma ( 0.705 + 4 ) and 1764 + 15 Ma (0.707 _+4) and slightly variable biotite ages ( 1784-1746 Ma) will almost certainly be found to be some 5% younger than the magmatic crystallization age. Metasedimentary and metavolcanic rocks of the Warramunga Group are some of the oldest rocks in the Tennant Creek Inlier, although their relationship to the previously mentioned 1920 + 60 Ma amphibolite-grade metamorphics is not known. Deformed Warramunga Group rocks, about 100 km southeast of Tennant Creek in the Davenport Inlier, have apparent U-Pb zircon ages of ~ 1935 Ma, anomalously old because of the presence of inherited components (Blake and Page, 1988). Another age of ~ 1870 Ma for the Warramunga Group near Tennant Creek derives from U - P b zircon data reported by Black (1981) for felsic volcanics in the Bernborough Formation. Black (1984) reinterpreted the primary stratigraphic age for the group as 1870___15 Ma. These discordant data still show signs of secondary recent Pb loss and/ or inheritance in the most discordant zircon fractions. However, taking this into account, reassessment of the five remaining data indicate a similar crystallization age, 1875 + 33 Ma (lower intercept 296 + 142 Ma). The intrusive Tennant Creek Granite, dated at 1870 + 20 Ma, also has evidence of inheritance in two coarser zircon fractions and their deletion results in a slightly younger, but more precise age of 1864 + 7 Ma (lower intercept 235 + 25 Ma). The Cabbage Gum Granite has a significantly younger U-Pb zircon age of 1846 + 8 Ma but, as the unit is gneissic, it is not clear whether or not this age relates to igneous crystallization. Other recent U - P b zircon data on felsic volcanic suites in the Davenport Inlier are plagued by the same problems as seen in the Tennant Creek Inlier, namely, recent secondary Pb loss

10 superimposed on zircon systems with variable inheritance. For example, Epenarra Formation dacitic rocks at the base of the Hatches Creek Group and unconformable on the Warramunga Group, yield an apparent discordia age that is clearly too old, 1894 _+28 Ma (Blake and Page, 1988). The two coarsest fractions control the discordia trajectory and thus the unusually old age. If deleted on the grounds of their anomalous U content, the apparent age becomes 1862 _+45 Ma. Neither of these results is particularly convincing but if, as seems likely, there is inheritance, the younger result is more meaningful. Felsic porphyry in the overlying Treasure Volcanics gives U - P b zircon data again clearly indicating inheritance in some fractions, but it is ambiguous as to whether its interpreted crystallization age of 1815+5 Ma reflects an unexpectedly young intrusive event or, less likely, a depositional age. More detailed work clearly is needed to resolve this chronology, and the present data provide warning that zircon inheritance in early Proterozoic terranes can significantly mask the true stratigraphic age. Nearly all volcanic rocks in the vicinity of Hatches Creek mining field (Davenport Inlier) are altered and metasomatized. Their Rb-Sr total-rock systematics are fairly coherent, indicating an age of 1645+44 Ma (0.715+5). This result is close to G. Riley's 1660 Ma RbSr total-rock and mineral age for the nearby E1kedra Granite (referred to by Compston and Arriens, 1968), and may be approximately dating the cessation of this metasomatic/mineralization event in the Hatches Creek Group. It is interesting to note that similar Rb-Sr ages (1648 _+57 Ma, 1642 _+26 Ma, 1625 _+50 Ma) are found by Black (1980) in pegmatites and schists associated with W and base-metal mineralization in the Jervois Range (eastern Arunta block), but at Tennant Creek the major ~ 1810 Ma alteration and deformation associated with Au-Cu mineralization (Black, 1977) is clearly earlier. The above synthesis demonstrates the tem-

poral coherence of a major felsic magmatic event at 1870-1850 Ma. This event is well represented in northern Australia, both analytically and lithologically, and its uniformity in age extends to a uniformity in the chemical composition of the magmas (Wyborn, 1988), and to a similar tectonic setting over very wide areas of the continent. The S m - N d isotopic evidence (McCulloch, 1987) and limited initial 87Sr/86Sr data from undisturbed systems also point to a uniform crustal-formation process, occurring no earlier than 2290-2130 Ma ago, although there is a tentative suggestion of late Archaean prehistory in both the Tennant Creek and Georgetown Inliers (Black and McCulloch, 1984). These Sm-Nd constraints apply to all the early Proterozoic terranes discussed above, and also to the Arunta and Georgetown Inliers, even though the available geochronological data from these last-mentioned blocks do not readily fit the general stratigraphic-geochronological framework that has emerged from lower metamorphic grade terranes. The Arunta and Georgetown data are largely influenced by highgrade mid-Proterozoic (and Palaeozoic) metamorphic events. Crustal reworking has all but masked the early Proterozoic history. However, based on the geological constraints implicit in the 1860-1820 Ma age of the younger Davenport Inlier sequence, as well as the ~ 1900-1850 Ma isotopic pre-history evident from much of the published Rb-Sr data on Arunta Division 1 granulites and gneissic granitoids (Black et al., 1983), and the Sm-Nd ages (1980 + 190 Ma, Black and McCulloch (1984); 2070_+ 125 Ma, Windrim and McCulloch (1986)) ascribed to initial protolith formation, it is now probable that at least Division I rocks in the Arunta Inlier contain elements of the 1870-1850 Ma felsic igneous event, the extent of which must be further evaluated by future U-Pb zircon studies.

Anorogenic magmatism and middle Proterozoic regional metamorphism A depositional break of 50-70 Ma occurred in most of northern Australia at the wane of the

11 1870-1850 Ma Barramundi igneous activity. Further rifting, followed by laterally extensive thermal subsidence, subsequently led to the development of several large sedimentary basins, which included major episodes of bimodal igneous activity, in the period from 1800 to 1600 Ma ago. In their consideration of the overall Australian Proterozoic record, Wyborn et al. (1987) conclude that principal igneous suites are largely developed within three discernible age groups, 1800-1780 Ma, 1760-1740 Ma and 1670-1640 Ma. This time framework is thus much less coherent than for the 1870-1850 Ma Barramundi magmatic event, and it differs significantly in age from province to province. These intrusives and extrusives are in anorogenic settings, have dominantly A-type affinities, and can be chemically distinguished from the older Barramundi suite on the basis of their much higher incompatible element contents, especially Zr, Nb, Y and Ti, and relatively high K, U and Th. The earliest evidence of this younger magmatism in the Mount Isa Inlier is in bimodal volcanic sequences that unconformably overlie the 1870 Ma Kalkadoon-Leichhardt suites. The age of this rhyodacitic volcanism in the Argylla Formation, and of higher grade recrystallized, garnet-bearing metadacites in the Bottletree Formation, is well controlled by consistent U Pb zircon results of 1783 + 5 Ma and 1790___9 Ma, respectively (Page, 1978; 1983a). The Argylla Formation has a S m - N d depleted mantle model age of 2180 Ma, suggesting the same source age as for Leichhardt-Kalkadoon magmatism (McCulloch, 1987). The Magna Lynn Metabasalt underlies these formations, but is not independently dated. Eastern Creek Volcanics (Haslingden Group ), a 3-6-km-thick sequence of basaltic flows, tuffs and sediments, conformably overlies the Argylla felsic lavas, and gives a regional total-rock P b - P b isochron of 1710+50 Ma (Gulson et al., 1983). This is somewhat younger than, but within error of, an independent m i n i m u m age for the Haslingden Group (and Mary Kathleen Group) of 1740-

1730 Ma, derived from the U - P b zircon age of the intrusive Burstall Granite (Page, 1983b). Recent conventional and ion microprobe U - P b work (Page, unpublished data) on xenocrystic zircons in an amygdaloidal basalt of the Eastern Creek Volcanics reveals the presence of much older inherited zircon components, of variable ages in two groupings: 2100-1800 Ma and 2700-2500 Ma. These are interpreted as relics of the magmatic source (s) a n d / o r from passage through older underlying crust. This is important as the first direct evidence for involvement of Archaean crust in Proterozoic magmatism in northern Australia, and provides new insights into the age and nature of the deep crust on to which the early to middle Proterozoic fold belts have been accreted. The regional extent of this involvement now needs to be established by further work. The 1740 Ma Burstall Granite, mentioned above, is part of a 100-km-long belt of bimodal granitic/gabbroic complexes in the central Mount Isa Inlier. Within the belt, the granite is intruded by compositionally similar rhyolite dykes and by the Lunch Creek Gabbro, both having close U - P b zircon ages of 1737 + 15 Ma and 1740___24 Ma, respectively. The nearby Wonga Granite is geochronologically complex: a phase dated at 1671+8 Ma (Page, 1978) is possibly the youngest, as recent unpublished zircon results on other phases are indicating apparent ages between 1760 and 1740 Ma. The presence of inherited zircon has so far precluded the determination of final, more precise ages for these Wonga Belt granites and associated structural events. However, it is clear from the present field and geochronological evidence that this belt underwent a major deformation event prior to emplacement of the 1670 Ma granite, and thus some time in the interval 1740-1670 Ma (R.W. Page, unpublished data). Stratigraphically higher sequences contain rhyolites with zircon U - P b ages of 1720 _+7 Ma and 1678 + 3 Ma. Zircons from concordant tuff horizons in the stratiform Mount Isa A g - P b Zn orebody (above the 1678 _+3 Ma sequence)

12 give the depositional age of the base-metal deposit at 1670 -+ 19 Ma. This is indistinguishable from the 1690 _+27 Ma age for a tuff in a mineralized member of the McArthur Group, and confirms the geological synchroneity of these economically important successions (Page, 1981 ). Throughout northern Australia there are a number of granitic bodies with similar I-type chemical features (Wyborn et al., 1987) emplaced at this time. At Mount Isa these include various phases of the Sybella Granite ( 1671 _+8 Ma, 1668_+24 Ma, 1610+10 Ma) and the Weberra Granite (1698___25 Ma, Wyborn et al. (1988)). A series of deformational and metamorphic episodes between 1620 and 1500 Ma overprints all rocks of the inlier, and mostly give rise to spurious Rb-Sr total-rock ages, and Rb-Sr and K-Ar cooling ages for micas of around 1490 Ma and 1450-1400 Ma (Richards et al., 1963; Page, 1978). In addition to the earlier deformational events at 1885 _+ i0 Ma and 1670-1740 Ma, three later separate deformations, evidenced from Rb-Sr total-rock isochrons within structurally discrete domains, occurred at 1610_+13 Ma, 1544 _+12 Ma and 1510 _+13 Ma (Page and Bell, 1986). U and REE ore formation in the Mary Kathleen deposit post-dates the emplacement of the nearby 1730-1740 Ma Burstall Granite by a few hundred million years and took place during one of the deformation events, as indicated by the U - P b uraninite age ( 1550 _+15 Ma; Page, 1983b), and the S m - N d mineral/totalrock isochron age (1472_+ 40 Ma, Maas et al. (1987)). Post-tectonic emplacement of granites in the Yellow Waterhole (Williams), Wimberu and Naraku batholiths in the eastern part of the Mount Isa Inlier occurred between 1560 and 1480 Ma ago (Wyborn et al., 1988). These bodies and dolerite dykes at ~ 1100 Ma are the youngest magmatic occurrences evident in the Mount Isa Proterozoic record. In other Proterozoic terranes in northern Australia, the chronology of younger, post-Barramundi magmatism, the enveloping sedimentary sequences, and their metamorphism has

mostly been studied through Rb-Sr total-rock work. The Hart Dolerite in the Kimberleys, one of the largest gabbro bodies in the world, has a model 1 total-rock and mineral isochron (controlled by granophyre) of 1762+25 Ma (0.7041 _+6, Bofinger (1967)). This is a minim u m age for the Kimberley Basin platform cover sequences that are intruded by the Hart Dolerite. This basin's maximum age is 1850 _+5 Ma, the age of the underlying Whitewater Volcanics. The Kimberley Basin is very likely to be of similar age to the Haslingden Group at Mount Isa. The McArthur Basin platform cover sediments (in the western region of the Arnhem Shelf) are considerably younger, as given by the 1688 + 13 Ma age (0.7044 + 2) for the underlying Oenpelli Dolerite (Page et al., 1980). The best estimate for the age of the Kombolgie Formation is the Rb-Sr isochron age (minimum) of interbedded basaltic flows, 1648 + 29 Ma. This lower McArthur Basin sequence unconformably overlies Pine Creek Inlier metamorphic rocks (1885-1800 Ma) that are host to several U deposits. Isotopic work on pitchblende, associated galena and other sulphides indicates extensive U mobility at 900-800 Ma and 600-400 Ma (Hills and Richards, 1976), but primary mineralization, at least in the Ranger and Jabiluka orebodies, occurred much earlier, as defined by U - P b total-rock ore studies giving ages of 1737 + 20 Ma and 1437 + 40 Ma, respectively (Ludwig et al., 1987). A number of granitic bodies in the T e n n a n t Creek, The Granites-Tanami and Arunta Inliers have Rb-Sr ages (minima for igneous crystallization) between 1750 and 1650 Ma, but there are few zircon ages. Many that have been worked on geochemically show trends interpreted as indicating a common source/petrogenetic history similar to the I-type, 1670 Ma Sybella Granite complex at Mount Isa (Wyborn et al., 1987), but at this stage the Rb-Sr results are ambiguous other than for implied minimum age constraints. For example, muscovite Rb-Sr ages of 1692 Ma for the Warrego Granite (Tennant Creek) are older than the to-

13 tal-rock isochron interpretation (1662 + 20 Ma, 0.702 + 8, Black ( 1977 ) ). Both values are probably minima for igneous crystallization ages, especially as the 'total-rock' sampling was via relatively small drill-core in which any totalrock/mineral-age discordance can be a problem. Two phases of the nearby Red Bluff Granite have similar total-rock ages (1648 + 50 Ma, 0.716_+5 and 1640_+36 Ma, 0.7077+_6) which, because of the high initial STSr/S6Sr ratios, can be interpreted possibly in terms of low-temperature hydrothermal disturbance of granitoids that were emplaced between 1900 and 1740 Ma ago. If this alternative interpretation is supported by future zircon work, the Rb-Sr results will remain as evidence, further to that previously alluded to in the Davenport and Arunta Inliers, for a major hydrothermal/metasomatic overprint in central Australia at ~ 1650 Ma ago. Similar reservations must apply to the RbSr total-rock isochron ages of 1740-1685 Ma for various granitic bodies (Lewis, Slatey Creek, The Granites) in The Granites-Tanami Inlier (Page et al., 1976). However, these ages may not be grossly updated, as the apparent initial STSr/S6Sr ratios (0.707-0.709) are only moderate, despite very high Rb/Sr. In higher-grade, polymetamorphosed granitic-gneiss terranes of the Arunta Inlier, six well-controlled Rb-Sr total-rock isochrons (Black et al., 1983) and a semi-regional Rb-Sr isochron (1690 + 25 Ma, Windrim and McCulloch (1986) ) define the age of the Aileron deformational event between 1700 and 1650 Ma. This was a major, regional retrograde event that was superimposed on the earlier ~ 1800 Ma Strangways granulite event (Shaw et al., 1984). The age for this Aileron event closely corresponds with mineral and 'disturbed' total-rock ages in the adjacent Tennant Creek and Davenport Inliers. It also correlates in time with the early deformation recognized in the Wonga Belt east of Mount Isa, and the 1670 Ma I-type, felsic magmatic episode. It is thus reflected over a very wide path in the north Australian Proterozoic and in a variety of different tectonic settings. Wyborn et

al. (1988) suggest that the magmatic rocks emplaced at this time in the Mount Isa Inlier may be the deep crustal expression of an important extensional event associated with a generally high geothermal gradient over a large part of northern Australia. Much of the reset Rb-Sr data reported by Black et al. (1983) in the high-grade Arunta granitic gneisses can be interpreted now as Aileron metamorphic overprints on protoliths whose primary ages, as inferred from their reset high initial STSr/S6Sr, were in the range 17901720 Ma. As the post-tectonic Jinka and Jervois Granites in the eastern Arunta also have this Rb-Sr age (1775-1750 Ma) and no evident crustal pre-history (Black, 1980 ), it seems likely that these bodies can now be linked to a major episode of felsic magmatism, temporally akin to The Granites Granite (protolith age ~1780 Ma) and Slatey Creek Granite (protolith age ~ 1750 Ma) in The Granites-Tanami area, or Burstall Granite equivalents ( ~ 1740 Ma) in the Mount Isa area. Corroborative evidence supporting the significance of this magmatic event within Division 2 rocks in the Harts Range, eastern Arunta Inlier, is given by recent U - P b zircon emplacement ages of 1762 _+2 Ma for a gneissic granitoid intruding supracrustal paragneisses whose age is a 'few tens of million of years older' (Cooper et al., 1986). The nearby Bruna granitic gneiss and an associated aplitic gneiss also intrude basement granulitic gneiss and supracrustal cover, and have similar U - P b zircon ages, 1748_+ 5 Ma and 1752 + 14 Ma, respectively, considered by Mortimer et al. ( 1985 ) to be the time of igneous emplacement, synchronous with a thrusting event that post-dates gneiss formation in the basement. It is not clear whether a slightly younger zircon age, 1730 + 1 Ma, for a migmatitic amphibolite relates to igneous crystallization or a younger event. These initial zircon results, together with extrapolation of the Black et al. (1983) Sr isotopic data for several of the updated granitic gneisses, lead to the tentative conclusion that ~1750 Ma magmatism, long evident in Mount Isa and

14

other adjacent north Australian terranes, may be well represented in Division 2 rocks of the Arunta Inlier. Other Arunta granitic gneisses affected in the younger Anmatjira ( ~ 1400 Ma) or Ormiston (900-1050 Ma) events, have isotopic signatures that testify to similar crustal pre-histories in the range 1840-1710 Ma, although these latter extrapolations from the original Black et al. (1983) data are not as well defined. These midProterozoic geochronological analogues between the Arunta, Mount Isa and other inliers are further extended by examples of largely post-tectonic, ~1500 Ma emplacement of granitic bodies such as the Wangala Granite (Black et al., 1983), Mount Webb Granite (Page et al., 1976) and Yellow WaterholeWimberu suite (Wyborn et al., 1988). The Georgetown and Coen Inliers are large Proterozoic terranes in northeastern Australia for which, like the Arunta Inlier, there are few primary ages. This is because of the generally high metamorphic grade and the fact that little zircon geochronology has yet been undertaken. Multiply-deformed Einasleigh Metamorphics paragneisses have a depleted mantle S m - N d model age of ~2250 Ma (McCulloch, 1987). Black and McCulloch (1984) considered an isochron grouping of S m - N d data that indicated a much older pre-history (2490+_70 Ma), but the validity of pooling such a variety of rock types is debatable. Nevertheless, there is evidence, from 2°TPb-2°6pb ages up to 2350 Ma from detrital zircons in schist of the Robertson River Formation, that early Proterozoic crust may once have been exposed in this block. Crustal xenoliths in late Tertiary alkali basalts from this region contain zircon components that provide further evidence of early to middle Proterozoic lower crust (Rudnick et al., 1986). One group of apparently igneous zircons in a metasedimentary xenolith has an age of 1579 -+ 6 Ma, and in another 1670-+170 Ma. Older xenocrysts at 2020 and 2200 Ma are of similar ages to zircon xenocrysts found in 1870 Ma volcanic rocks and paragneisses in the Pine Creek and

Mount Isa Inliers (Page and Williams, 1988). At this stage there are no geochronological data in the Georgetown Inlier that can be related directly to any early Proterozoic supracrustal sequence or primary igneous event. The multiple deformations and high-grade metamorphism of the Einasleigh Metamorphics and Robertson River Formation schists and gneisses have been dated by Rb-Sr total-rock studies at 1570 + 20 Ma, 1470+20 Ma and 976_+28 Ma (Black et al., 1979). Mid-Proterozoic (and Palaeozoic) granitic and volcanic rocks are considered to post-date the 1470 Ma deformation. The comagmatic Croydon Volcanics and Esmeralda Granite are the oldest such magmatic suites and have minimum Rb-Sr total-rock and muscovite ages of 1400 and 1445 Ma (Oversby et al., 1975). The Coen Inlier probably contains stratigraphic equivalents to the Georgetown rocks, but Rb-Sr and K-Ar dating show strong isotopic overprints in the Siluro-Devonian (Cooper et al., 1975 ). The only Proterozoic age on the metamorphic sequence is a Rb-Sr totalrock age for the Holroyd Metamorphics (1246_+ 72 Ma), almost certainly a minimum. The Dargalong and Coen Metamorphics largely have Siluro-Devonian metamorphic isochrons (360-480 Ma), all with high initial STSr/S6Sr ( ~ 0.74). Modelling of these data, using an averaged S~Rb/86Sr in each suite as measured by Cooper et al. (1975), allows approximate extrapolations of the protolith ages, which, for the Dargalong and Coen Metamorphics are 25002400 Ma. The convergence towards a late Archaean to early Proterozoic protolith age, and the report of early Proterozoic S m - N d depleted mantle model ages (2130, 1940 and 2110 Ma, McCulloch ( 1987 ) ) on these rocks suggest that the assumptions involved in these estimates are reasonably valid. This is the only isotopic evidence of the early Proterozoic character of the Coen Inlier, other than K-Ar total-rock and plagioclase ages in excess of 1800 Ma for dolerite from the Yambo Inlier to the south.

15 Conclusions Growth of continental crust during early to middle Proterozoic orogenic activity can be traced and quantified more confidently as a result of the integrated geological, geochemical and geochronological studies being pursued in northern Australia. The chronological input into this framework (Fig. 2 ) that has been the subject of this review has been substantially modified over the past several years, and will continue to be refined, particularly as further precise U - P b zircon studies are undertaken. Many of the problems and gaps in the present geochronological record need to be addressed by further ion microprobe study of zircons, not only in felsic but also in mafic lithologies. Although in its geological infancy, ion microprobe research has already provided new insights into crustal growth and age patterns in sequences that have been difficult or impossible to date accurately by conventional zircon techniques. Once the ages of the rocks are known, concurrent study of S m - N d model ages will enable clearer interpretation of the mantle fractionation and crustal formation processes that they trace. At present, reconnaissance S m - N d ages in the 1900-1700 Ma early Proterozoic terranes of northern Australia indicate that lower crustal progenitors of this material had ~ 400 Ma prehistory, that is, that initial mantle fractionation took place ~ 2300-2100 Ma ago. This is supported by the presence of minor relic zircon components of these ages (and older), recently found in some early Proterozoic rocks. Largescale assimilation of evolved Archaean crust can be ruled out, using both the S m - N d and the UPb zircon systematics. However, the S m - N d data cannot rule out a small degree of Archaean contamination, and evidence is emerging from ion microprobe zircon studies that minor late Archaean relics are also present in some early Proterozoic magmatic products. This geochronological synthesis broadly defines a continent-wide coincidence of much of

the depositional and orogenic activity in the early to middle Proterozoic. The earliest recognized supracrustal sequences, cycle 1 of Etheridge et al. (1987), remain the least well defined, partly because of paucity of suitable felsic volcanic horizons and generally high metamorphic grade. The best evidence indicates that cycle 1 deposition is 1890-1880 Ma old. In the Halls Creek Inlier there is preliminary evidence for that same age (1890-1880 Ma), but the age of sills (minimum) comagmatic with cycle 1 tuffs suggest this sequence may be some 30 million years younger than elsewhere in northern Australia. The Barramundi Orogeny, marked by increasing proportions of turbidite in the cycle 1 sequences and deformation and metamorphism of these supracrustals, occurred in the Mount Isa and Pine Creek areas between 1885 and 1870 Ma, and apparently some 15-20 million years later (1854+6 Ma) in the Halls Creek Inlier. Accompanying orogenic magmatism produced I-type, K-rich felsic igneous complexes whose ages (and geochemistry) are remarkably coherent across the early Proterozoic terranes, between 1880 and 1850 Ma. In the high metamorphic grade Arunta and Georgetown Inliers, rocks of this antiquity are not directly recognized, although amongst Arunta Division 1 granulites, geological, and Sr and Nd isotopic constraints strongly suggest igneous protoliths of this approximate age. The north Australian geochronological data thus firmly substantiate the contemporaneity of a world-wide early Proterozoic orogeny, recently documented in the Penokean, trans-Hudson and Wopmay orogens of the north American continent (van Schmus, 1980; Hoffman and Bowring, 1984; van Schmus et al., 1987) and in early Proterozoic terranes of Scandinavia (Park, 1985; Huhma, 1986; Welin, 1987). In contrast with the early Proterozoic 18801850 Ma magmatism, post-1800 Ma anorogenic magmatism in northern Australia is much less coherent in age and geochemistry. The majority of these younger, bimodal intrusive and ex-

16

trusive suites fall into four age ranges: 18001780 Ma, 1760-1740Ma, 1670-1640 Ma and ~ 1500 Ma. Granitic rocks having primary zircon crystallization ages of ~ 1750 Ma have been recognized recently amongst high-grade Arunta Inlier gneisses, and are also evident from updated Rb-Sr systematics in some of the gneisses, suggesting that this phase of anorogenic magmatism may be well represented in this terrane. Additionally, felsic volcanic magmatism has now been documented between 1790 and 1740 Ma in the Gawler Craton, South Australia (Fanning et al., 1988), pointing yet further to the very wide geographic distribution of this magmatic episode. The 1670-1640 Ma magmatic activity is present in the Mount Isa and central Australian terranes. This activity corresponds in time with the well-documented Aileron deformational event in the Arunta Inlier, and with a number of hydrothermal and pegmatitic episodes evidenced in mineral and 'disturbed' total-rock Rb-Sr ages in all of these inliers. This suggests a high geothermal gradient at ~ 1670 Ma over very large areas of northern Australia. The available geochronology of early to middle Proterozoic mineral deposits in these fold belts reveals that pre-1800 Ma and post-1800 Ma terranes contain an unbalanced share of the known economic mineralization. The former, largely cycle 1 sequences, are essentially devoid of mineral deposits. In contrast, post-1800 Ma depositional or igneous packages, namely timeequivalents of the anorogenic igneous suites whose ages lie in the interval 1790-1500 Ma, contain a disproportionately large share of rich and varied mineralization episodes.

Acknowledgements There are a number of past and present colleagues, both field and isotope geologists, with whom I have had fruitful exchanges on the geochronology, geochemistry and tectonics of Proterozoic terranes. Most of these are frequently acknowledged in the text above and are listed

in the bibliography below. Geochronological research on the Proterozoic terranes of northern Australia has been very much enhanced through our longstanding joint collaboration with the Research School of Earth Sciences, A.N.U. Additionally, I would like to thank Tom Krogh, who for a long time has freely shared his analytical skills with many people, and whose indirect contribution to the advancement of Australian Proterozoic geochronology and, for that matter, to Precambrian geology in general, has been invaluable. The paper benefits from critical reviews by Ian Williams and Ken Plumb, and is published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics.

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