Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province

Precambrian Research 206–207 (2012) 17–35

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Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province Claire E. Wade a,∗ , Anthony J. Reid a , Michael T.D. Wingate b , Elizabeth A. Jagodzinski a , Karin Barovich c a

Geological Survey of South Australia, Department for Manufacturing, Innovation, Trade, Resources and Energy, GPO Box 1264, Adelaide, SA 5001, Australia Geological Survey of Western Australia, Department of Mines and Petroleum, 100 Plain St., East Perth, WA 6004, Australia c Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia b

a r t i c l e

i n f o

Article history: Received 1 June 2011 Received in revised form 20 February 2012 Accepted 29 February 2012 Available online 8 March 2012 Keywords: Silicic large igneous province Benagerie Volcanic Suite Gawler Range Volcanics Mesoproterozoic Geochemistry Geochronology

a b s t r a c t New ion microprobe (SHRIMP) U–Pb zircon data indicate that voluminous A-type volcanic rocks were extruded c. 1585 Ma in the Benagerie Ridge region of the north-central Curnamona Province in southern Australia. Incompatible trace element ratios and whole-rock Sm–Nd isotope compositions (εNd (1585) = −4.3 to −2.2) suggest a dominant crustal source region for the felsic rocks. Incompatible trace element ratios in the basalts, enriched light REE/heavy REE ratios (La/Yb = 6–7.5), and εNd (1585) values between −1.5 and 0.2 suggest that the basalts were derived from a HFSE-enriched magma from within the mantle. These new data show that the Benagerie Volcanic Suite share geochronological, geochemical, and isotopic affinities with the upper Gawler Range Volcanics of the Gawler Craton indicating these two volcanics suites can be considered part of a formerly contiguous Mesoproterozoic silicic large igneous province. We invoke aspects of the two-phase model for the generation of this A-type silicic large igneous province presented by previous workers and attempt to place the initial ‘developmental’ and subsequent ‘mature’ phases of this event into a tectonic framework for the early Mesoproterozoic of the Curnamona Province and the adjacent Gawler Craton. Initiation of the developmental phase likely resulted from lithospheric extension and was accompanied by localised basaltic magmatism. The mature phase, which involved eruption of voluminous felsic volcanic rocks, was a result of widespread crustal melting, potentially induced by the elevated geotherm caused by extension, by ponding of mafic melt in the lower crust, or a combination of these factors. The high geothermal gradient that resulted from extension and magmatism likely primed the crust for the early Mesoproterozoic regional deformation and metamorphism that occurred across the Curnamona Province and Gawler Craton. The far-field tectonic drivers for this silicic large igneous province remain uncertain, although the association between extensive bimodal A-type magmatism, high-temperature metamorphism, and localised compressional deformation, is suggestive of an intracontinental setting, possibly mechanically connected to a far-field subduction zone. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Silicic large igneous provinces (SLIPs) are rare phenomena in the geological record (Bryan, 2007; Bryan and Ernst, 2008; Pankhurst et al., 2011). Most large igneous provinces (LIPs) consist of mainly mafic rocks, and typical examples include the latest Cretaceous Deccan Traps of India (Bondre et al., 2004) and the Mesoproterozoic Warakurna LIP of west-central Australia (Wingate et al., 2004). Of the known SLIPs, only few Proterozoic examples exist, which include the c. 2060 Ma Rooiberg Felsite, South Africa (Twist and French, 1983; Olsson et al., 2010), and the c. 1100 Ma North

∗ Corresponding author. Fax: +61 8 8226 3200. E-mail address: [email protected] (C.E. Wade). 0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2012.02.020

Shore Volcanic Group, Minnesota, USA (Green, 1989); Mesozoic to Cenozoic provinces include the Paraná-Etendeka traps, Brazil and Namibia (Kirstein et al., 2001) and the Snake River Plain province, USA (Rogers et al., 1990). Documented SLIPs are typically dominated by high-temperature, A-type magmas capable of extensive flows when erupted (Smith et al., 1996; Riley et al., 2001; Allen and McPhie, 2002; Briand et al., 2002; Pankhurst et al., 2011). In general, A-type magmatism forms as a result of either fractional crystallisation, or assimilation and fractional crystallisation (AFC), of an extensive basaltic magma source (Turner et al., 1992), partial melting of crustal material (Collins et al., 1982; Whalen et al., 1987), or some combination of crustal and mantle sources (Mingram et al., 2000; Mushkin et al., 2003; Trumball et al., 2004). Such broad magmatic processes may operate in a variety of tectonic settings, including continental margins (Hawkesworth et al.,

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2000; Kirstein et al., 2001), rifts (Sears et al., 2005), or back-arcs (Rivers and Corrigan, 2000), and within intracontinental settings associated with mantle plumes and mafic underplating (Stein and Hofmann, 1994). In some instances, widespread A-type magmatism has been linked to large-scale mantle upwelling beneath a supercontinent (Giles, 1988; Hoffman, 1989), a process that has commonly, but not exclusively, been followed by extension and fragmentation of the supercontinent (Anderson, 1982; Storey and Kyle, 1997; White, 1997; Briand et al., 2002) e.g. Karoo and Ferrar continental flood basalts and Chon Aile felsic LIPs associated with the break-up of Pangea (Pankhurst et al., 1998; Dalziel et al., 2000). Such diverse modes of formation and tectonic settings for A-type magmatism mean that understanding the petrogenesis of any given A-type magmatic suite requires the integration of geochemical and isotopic datasets, together with an understanding of the tectonic setting in which the magmatic rocks were generated. Only a few examples of SLIPs exist within Australia. Phanerozoic provinces include the 132–95 Ma Whitsunday Volcanic Province of eastern Australia and the 320–280 Ma Kennedy–Conners–Auburn Volcanic Province of northeastern Australia (Bryan et al., 1997, 2000). Unique among the Australian examples in terms of size and metallogenic significance are the extensive Mesoproterozoic volcanic rocks of South Australia: the Gawler Range Volcanics (GRV) of the Gawler Craton and the Benagerie Volcanic Suite (BVS) (Wade, 2011) of the Curnamona Province (Fig. 1). In common with other early Mesoproterozoic (c. 1600–1500 Ma) magmatic rocks in Australia (Wyborn et al., 1987), the GRV and BVS are A-type in character and dominated by felsic (>60% SiO2 ) magmas and has a subordinate mafic component. Based on outcrop extent, together with drillhole intersections and geophysical data, a conservative estimate for the total preserved volume of the combined GRV and BVS is in the order of 1.11 × 105 km3 , indicating this was a major episode of magmatic differentiation and crustal reorganisation within the southern Australian region. Although the total eruptive volume is less than other SLIPs (e.g. the c. 2060 Ma Rooiberg Felsite, South Africa (Olsson et al., 2010), has a total volume of 3 × 105 km3 of rhyolitic to dacitic volcanic material (Twist and French, 1983) and the Phanerozoic Whitsunday Volcanic Province includes 2.2 × 106 km3 (Bryan et al., 2000)), the GRV and BVS is unique in that the >1.11 × 105 km3 was erupted over a few million years, with at least three individual felsic units each representing 1000–3000 km3 (Allen et al., 2008). Undeformed and weakly metamorphosed felsic volcanic rocks were first identified in the Curnamona Province from exploration drilling in the late 1970s and early 1980s. Initial geochemical investigations showed these felsic volcanic rocks to have a hightemperature, lower-crustal signature with elevated Zr, Nb, Y, and Ce contents that had many similarities to the Gawler Range Volcanics of the Gawler Craton (Giles and Teale, 1979). Further mineral exploration drilling, together with geophysical data sets acquired throughout the 1980s and 1990s, helped to reveal the significant extent of these volcanic rocks (Flint et al., 1993), an extent that has increased due to intersections of felsic volcanic rocks in recent drilling to the west of the main Benagerie Ridge region (Fig. 1). These felsic volcanic rocks have recently been formally defined as the BVS as part of a regional synthesis of magmatism in the Curnamona Province (Wade, 2011). The BVS is overlain by widespread cover of the Phanerozoic Moorowie Sub-Basin and Frome Embayment (Fig. 1), and is not exposed. Consequently, compared to the GRV in the Gawler Craton which crop out extensively, little is known about the age, geochemical, or isotopic signature of the BVS. Furthermore, because the north-south-trending Neoproterozoic to Cambrian failed rift of the Adelaide Geosyncline separates the Curnamona Province from the Gawler Craton (Fig. 1), there is no spatial connection between the two volcanic domains that might help to clarify their

relationship. Consequently, although it has long been considered that the GRV and BVS form part of a related magmatic system (Giles and Teale, 1979), there have been few attempts to assess the geochronological and geochemical similarities that potentially unite them. Given the relationship between the GRV and the extensive iron oxide–copper–gold (IOCG) deposits of the Olympic IOCG Province in the eastern Gawler Craton (Fig. 1; Skirrow et al., 2002, 2007; Skirrow, 2009) the similarities already identified between the BVS and the GRV (Giles and Teale, 1979; Flint et al., 1993), understanding the relationship between these two magmatic systems will have important implications for the potential metal endowment of the Curnamona Province (Conor and Preiss, 2008). In this contribution, we explore the nature of the BVS by presenting new SHRIMP U–Pb zircon ages for three felsic volcanic units together with new geochemical and whole-rock Sm–Nd isotope data for felsic and subordinate associated mafic units. Our new results confirm that the BVS is petrogenetically similar to the Gawler Range Volcanics and that together they form a significant A-type SLIP within the southern Curnamona Province and eastern Gawler Craton. We also summarise recent work on the nature of the tectonic setting of this SLIP and develop a model for its formation.

2. Regional geological setting 2.1. Curnamona Province The Curnamona Province is a roughly circular crustal element located in eastern South Australia and western New South Wales (Fig. 1), and is mostly unconformably overlain by Neoproterozoic to Holocene sediments. The oldest exposed rocks are metasedimentary and meta-igneous rocks of the late Paleoproterozoic (1720–1640 Ma) Willyama Supergroup (Conor and Preiss, 2008) in the southern Curnamona Province (Fig. 1). The Willyama Supergroup is intruded by granites and overlain by volcanic rocks of the Ninnerie Supersuite, both of which are of early Mesoproterozoic age (Cook et al., 1994; Fanning et al., 1998; Page et al., 2000). The boundaries of the Curnamona Province are clearly visible on geophysical images and are a result of Neoproterozoic and Paleozoic tectonic events (Conor and Preiss, 2008). The southern Curnamona Province was affected by the c. 1620–1585 Ma Olarian Orogeny (Page et al., 2000; Forbes et al., 2005; Rutherford et al., 2007) and the c. 500 Ma Delamerian Orogeny (Harrison and McDougall, 1981; Wingate et al., 1998; Paul et al., 2000; Dutch et al., 2005). Polyphase deformation and metamorphism during these orogenic events inverted and dismembered Willyama Supergroup stratigraphy and reached granulite facies conditions (Clarke et al., 1987; Powell and Downes, 1990; Page and Laing, 1992; Dutch et al., 2005; Rutherford et al., 2007). The effects of the Olarian Orogeny decrease northwards from upper amphibolite to greenschist facies (Conor and Preiss, 2008). Willyama Supergroup metasedimentary rocks in the Mulyungarie Domain are openly folded and unconformably overlain by the flat-lying and relatively undeformed BVS (Conor and Preiss, 2008). The Benagerie Ridge is a large area of relatively shallow, northsouth-trending basement rocks obscured by younger sediments (Robertson et al., 1998), located in the Mudguard Domain of the central Curnamona Province (Fig. 1). The southern portion of the Benagerie Ridge consists of primarily low-grade metasedimentary rocks that host Cu–Au–Mo mineralisation (e.g. Portia, North Portia, and Kalkaroo Prospects) (Teale, 2000). The northern Benagerie Ridge comprises Willyama Supergroup metasedimentary rocks and granites that are overlain by the flat-lying BVS (Robertson et al., 1998).

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Fig. 1. Location diagram and regional geology of the Curnamona Province and Gawler Craton, illustrating the distribution of the Benagerie Volcanic Suite in the Curnamona Province and the Gawler Range Volcanics in the Gawler Craton. The location of drillholes are shown on the regional geology of the Curnamona Province. Felsic tuffs belonging to the Benagerie Volcanic Suite have recently been reported at >3950 m depth within the geothermal well Paralana 2 (Reid et al., 2011).

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2.1.1. Early Mesoproterozoic magmatism in the Curnamona Province: the Ninnerie Supersuite The early Mesoproterozoic Ninnerie Supersuite was generated during the waning stages of the Olarian Orogeny and includes vast quantities of S- and I-type granites and A-type volcanic rocks (Fricke, 2006; Wade, 2011). The supersuite comprises four suites, three of which are exposed at the surface in the Curnamona Province (Fig. 1). These include muscovite-biotite granites of the Bimbowrie Suite, sodic and biotite-muscovite and biotiteonly granites of the Crocker Well Suite, and granites and volcanic rocks of the Coulthard Suite in the Mount Painter Inlier (Fig. 1; Wade, 2011). The BVS comprises the fourth remaining suite and is found only at depth around the Benagerie Ridge region (Fig. 1; Wade, 2011). The majority of Ninnerie Supersuite granites appear to have been largely derived from partial melting of the Willyama Supergroup and lower crust, although the presence of more mafic I-type magmas indicates that there was some contribution from a mantle source (Barovich and Foden, 2002). The BVS is the extrusive part of the Ninnerie Supersuite and by definition includes the felsic volcanic units, the Finlay Dam Rhyolite and the Lake Elder Rhyodacite (Wade, 2011), which unconformably overlie Willyama Supergroup metasedimentary rocks (Fricke, 2006; Conor and Preiss, 2008; Wade, 2011). The BVS is a flat-lying sheet of A-type felsic volcanic rocks found in the Mudguard Domain. More recently, deep drilling in the Moorowie Sub-basin has discovered felsic volcanic rocks at depths >1600 m in two drillholes: Paralana 2 (Reid et al., 2011) and Frome 13 (Fig. 1). These represent the most northwesterly and southeasterly extents of the BVS, respectively, and increase the distribution of the BVS further to the west, beneath the Cambrian Moorowie Sub-basin and Erudina Domain (Fig. 1). The contact between the base of the BVS and Willyama Supergroup metasedimentary rocks has not been observed in drill core, although a present-day thickness of the BVS of ∼1.5 km has been interpreted from seismic data (Korsch et al., 2010). The Mudguard Domain is mostly formed by the buried Benagerie Ridge but is also inferred to extend further to the west below the Moorowie Sub-basin (Fig. 1; Burtt and Betts, 2003; Williams and Ailleres, 2009). Felsic volcanic rocks have been intersected in seven drillholes; samples, drill holes, locations and lithologies are listed in Appendix A in Supplementary Material. The BVS is generally porphyritic, and range in composition from rhyolite to dacite (Fig. 2). Pervasive hydrothermal alteration (hematite, sericite, carbonate, K-feldspar, and albite) is common (Fig. 2). Early geochronology, using IDTIMS multigrain zircon techniques, yielded an imprecise date of 1599 ± 40 Ma for a rhyodacite from drillhole Mudguard 1 (Table 1; Fanning et al., 1988). More recently, a SHRIMP U–Pb zircon date of 1581 ± 4 Ma for a volcanic rock in Mudguard 1 (Table 1) was quoted in an abstract by Fanning et al. (1998). Aside from these two results, no other isotopic ages are available for the BVS. An unnamed basaltic subunit in the Benagerie Ridge has been identified in drillhole Bumbarlow 1 (Fig. 1). Basalts intersected in Bumbarlow 1 comprise seven major horizons of fine-grained, amygdaloidal basalt (Fig. 2) interlayered with coarse sandstone and siltstone and peperitic basalt and most likely represent several lava flows, some with vesicular and brecciated flow tops (Teale and Flint, 1993). In Bumbarlow 1, an older limit for extrusion of the basalts is provided by a maximum depositional age of 1591 ± 6 Ma for an underlying white sandstone (Fraser and Neumann, 2010). An overlying red sandstone produced a maximum depositional age of 1550 ± 6 Ma (Fraser and Neumann, 2010). These results suggest that the basaltic magmatism was broadly contemporaneous, if not coeval, with the BVS.

2.2. Gawler Craton The Gawler Craton comprises Mesoarchean to earliest Paleoproterozoic basement that is overlain by a series of Paleoproterozoic basins. The basement occurs within two belts located in the northcentral and southern portions of the craton, which contain similar lithologies of similar ages and which are inferred to represent a formerly contiguous rock system now disrupted by Paleo- to Mesoproterozoic tectonism (Daly et al., 1998; Hand et al., 2007). The Paleoproterozoic basins include, in the eastern portion of the craton, the c. 1860 Ma Darke Peake Group (formerly part of the Hutchison Group; Szpunar et al., 2011), the c. 1790 Ma Myola Volcanics (Fanning et al., 1988), the c. 1780 Ma Cleve Group (Szpunar et al., 2011), and the c. 1760 Ma Wallaroo Group (Cowley et al., 2003). Sedimentation between c. 1860 and 1730 Ma also occurred at this time across the western and northern Gawler Craton (Payne et al., 2006), indicating that this interval was a time of intense rift activity, widespread sedimentation, and associated bimodal magmatism (Wyborn et al., 1987). Sedimentation in these basins was terminated by the cratonwide c. 1730–1690 Ma Kimban Orogeny, which resulted in high-strain deformation and metamorphism up to granulite facies that was largely partitioned into regional-scale transpressional belts (Parker, 1993; Vassallo and Wilson, 2002; Dutch et al., 2010). Following the Kimban Orogeny, only localised sedimentation is preserved in the central Gawler Craton as the Tarcoola Formation, which is associated with volumetrically minor mafic magmatism at c. 1657 Ma (Daly et al., 1998). The c. 1630–1608 Ma interval saw the formation of a significant volume of moderately juvenile bimodal magmatism in the southwestern Gawler Craton. This magmatic event comprises the c. 1630 Ma Nuyts Volcanics (Rankin et al., 1990) together with the c. 1620–1608 Ma St Peter Suite (Flint et al., 1990) the latter of which shows juvenile εNd signatures and calc-alkaline affinities and has been interpreted to have formed as a result of fractional crystallisation of an enriched mantle source with only minor crustal contamination (Swain et al., 2008). 2.2.1. Early Mesoproterozoic magmatism in the Gawler Craton: the Gawler Range Volcanics and Hiltaba Suite The Gawler Range Volcanics (GRV) is divided into lower and upper sequences (Blissett et al., 1993). The lower GRV is a developmental phase comprising bimodal mafic and felsic volcanic rocks, ranging in composition through basalt, andesite, dacite, rhyodacite, and rhyolite. The lower GRV forms at least three discrete volcanic centres: Kokatha (Chitanilga Volcanic Complex), Lake Everard (Glyde Hill Volcanic Complex), and Roopena (Roopena Volcanics) (Fig. 1). The volcanic complexes at Kokatha and Lake Everard include the complete volcanic stratigraphy from basalt to rhyolite. Fractionation trends indicate that felsic units in the Chitanilga Volcanic Complex (CVC) and the Glyde Hill Volcanic Complex (GHVC) are direct fractionates of the mafic units (Stewart, 1994; Fricke, 2005). Assimilation and fractional crystallisation (AFC) and crustal contamination processes also modified the geochemical signature of the felsic volcanic rocks (Fricke, 2005). Volcanics from Roopena are best known from drill core, in which several basaltic flow horizons have been identified. The upper GRV consists of extensive, flat-lying, relatively undeformed felsic (rhyolite–rhyodacite–dacite) volcanic rocks up to ∼1.5 km thick (Blissett et al., 1993; Allen et al., 2003). The Yardea Dacite is the most extensive unit, and is exposed over 12 000 km2 and represents a total erupted volume of 3000 km3 (Blissett et al., 1993). The upper GRV is high in silica (>65% SiO2 ) enriched in HFSE and REE, and has a predominantly crustal signature with εNd values ranging between −5.8 and −2.2 (Stewart, 1994). Isotope dilution thermal ionization mass spectrometric (IDTIMS) dating of the Waganny Dacite, one of the lowermost units

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Fig. 2. Rock types of the Benagerie Volcanic Suite: (a) porphyritic rhyolite from Culberta 1, (b) plane-polarised light photomicrograph of rhyolite from Culberta 1 illustrating abundant quartz (qtz) phenocrysts, hematite-stained orthoclase (hem-st feld), and sericitised (ser) plagioclase in a groundmass of microcrystalline quartz and hematitestained K-feldspar, (c) rhyodacite from BRD 003, (d) photomicrograph, under crossed polarizers, of rhyodacite from BRD 003, with fayalite (fay) and clinopyroxene (cpx) phenocrysts in a groundmass of microgranular quartz and k-feldspar with minor plagioclase, clay minerals, clinopyroxene, and opaque oxide, (e) dacite from WK1, (f) photomicrograph of dacite from WK1 displaying smectite (sm) – sericite (ser) – leucoxene (leuc) alteration in a groundmass of microgranular quartz clouded by smectite, sericite, and clay minerals, (g) basalt from Bumbarlow 1, (h) photomicrograph of basalt from Bumbarlow 1 displaying randomly oriented plagioclase (plag) laths in a fine-grained matrix.

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Table 1 Summary table of geochronological data for the Benagerie Volcanic Suite (BVS) and Gawler Range Volcanics (GRV). Location

Suite

Lithology

Frome 13

BVS

Rhyolite

Age (Ma)

Mudguard 1 Mudguard 1 Culberta 1 DDH SAE-11, Menninnie Dam prospect, DDH MD0030 Menninnie Dam prospect, DDH MD0030 “Cultana” Roopeena DDH6 Gawler Ranges Gawler Ranges Lake Everard Tarcoola-Kingoonya Tarcoola-Kingoonya Acropolis, WMC ACD 5

BVS BVS BVS GRV GRV GRV GRV Yardea Dacite, GRV Waganny Dacite, GRV GRV Childera Dacite, GRV Ealbara Rhyolite, GRV Labyrinth Formation GRV

Porphyritic rhyolite Rhyolite Rhyolite Porphyry Altered rhyolite dyke Altered rhyolite dyke Granophyre Tuff Dacite Dacite Dacite Rhyolite

Acropolis, WMC ACD 5

GRV

Dacite

1578

Olympic Dam

GRV

Lapilli tuff

Olympic Dam

GRV

Peperite dyke

1587 ± 6

Dacite

of the lower GRV yielded an upper intercept date of 1591 ± 3 Ma, interpreted as the age of crystallisation (Table 1; Fanning et al., 1988). Similarly, IDTIMS dating of the Yardea Dacite, the uppermost unit of the upper GRV in the Lake Everard region, yielded a crystallisation age of 1592 ± 3 Ma (Fanning et al., 1988), suggesting that the entire GRV erupted within only a few million years. A less precise IDTIMS age of 1576 +22/−17 Ma was reported from a felsic volcanic rock within drill core from the northern Olympic IOCG Province (Mortimer et al., 1988b). Several SHRIMP ages between 1583 ± 12 Ma and 1604 ± 11 Ma have been reported for porphyritic units interpreted to be equivalent to the Gawler Range Volcanics (Johnson, 1993; Jagodzinski et al., 2006; Fanning et al., 2007). The IDTIMS data suggest that the exposed volcanic units were erupted over a short time interval, although the SHRIMP dating suggests the possibility that high-level intrusions associated with the GRV were emplaced some time around c. 1600 Ma and that magmatism continued until at least c. 1583 Ma (Table 1). The Hiltaba Suite is a bimodal intrusive suite, although granites predominate (Flint et al., 1993). The Hiltaba Suite displays considerable geochemical variation, even within the granitic units. Granites of the Hiltaba Suite are widespread across the central and southern Gawler Craton and are mostly fractionated, enriched in HFSE, U, Th, and K, with silica contents generally >70% (Flint et al., 1993). Nd isotopic data from across the craton indicate that the more evolved Hiltaba Suite granites are associated with regions of Archean host rocks. For example the Charleston Granite, which has εNd 1585 Ma values of c. −14.3 (Creaser and Fanning, 1993) intrudes the Mesoarchean domain in the north-eastern Eyre Peninsula. Mafic intrusions of hornblende-bearing quartz monzodiorite, quartz monzonite, and granodiorite are known from the northeastern Gawler Craton (Flint et al., 1993), and include the Curramulka Gabbronorite on Yorke Peninsula (Zang et al., 2007). The mafic units have SiO2 contents <65%, and are characterised by elevated TiO2 , Fe2 O3 , MgO, P2 O5 , CaO, Ba, Sr, and Zr (Flint et al., 1993). Also associated with the Hiltaba Suite is a subordinate suite of S-type intrusive rocks, the Munjeela Suite, known chiefly from the western Gawler Craton (Payne et al., 2010). The Munjeela Suite intruded at c. 1585 Ma and formed by partial melting of a metasedimentary protolith (Payne et al., 2010). The Munjeela Suite is therefore the magmatic end-product of high-geothermal-gradient metamorphism that affected metasedimentary rocks in the

1581 ± 4 1584 ± 5 1584 ± 6 1583 ± 12 1604 ± 11 1594 ± 11 1584 ± 3 1587 ± 15 1592 ± 3 1591 ± 3 1592 ± 17 1589 ± 16 ∼1640–1600 1591 ± 10

Method

Reference

SHRIMP II

Jagodzinski and Fricke (2010) and this article Fanning et al. (1998) This article This article Fanning et al. (2007) Fanning et al. (2007) Fanning et al. (2007) Fanning (1990) Johnson (1993) Fanning et al. (1988) Fanning et al. (1988) Fanning (1987a) Fanning (1987a) Fanning (1987b) Creaser and Cooper (1993)

SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP TIMS SHRIMP TIMS TIMS TIMS TIMS TIMS SHRIMP +13 −12

TIMS

Mortimer et al. (1988a)

1586 ± 7

SHRIMP

1593 ± 7

SHRIMP

Johnson and Cross (1991) Johnson and Cross (1991)

western Gawler Craton during the early Mesoproterozoic across much of the Gawler Craton. Although many Hiltaba Suite plutons in the central Gawler Craton adjacent to the undeformed, or weakly deformed, Gawler Range Volcanics show no deformation fabrics at all, there are abundant examples of syn-Hiltaba deformation across the Gawler Craton (Hand et al., 2007), including the formation and/or reactivation of shear zones in the central part of the craton (McLean and Betts, 2003; Swain et al., 2005; Fraser and Lyons, 2006), high-temperature metamorphism and thrusting in the Mount Woods inlier (Forbes et al., 2010) and adjacent Coober Pedy Ridge (Fanning et al., 2007; Cutts et al., 2011), along with deformation of the c. 1760 Ma Wallaroo Group in the Moonta region in the southeastern part of the craton that was synchronous with emplacement of the 1577 ± 7 Ma Tickera Granite (Conor, 1995; Fanning et al., 2007). Thus, it is clear that compressional deformation and high-temperature metamorphism accompanied this magmatism in a phase of orogenesis known as the Kararan Orogeny (Daly et al., 1998; Hand et al., 2007). Kararan deformation appears to have been partitioned at the craton-scale into belts of reworking, such as the Coober Pedy Ridge and the Yorke Peninsula, that bound the central ‘core’ of the craton defined by the weakly deformed Gawler Range Volcanics. Compressional or transpressional deformation continued across the northern Gawler Craton following the GRV-Hiltaba Suite magmatic event, with reactivation along major shear zones occurring between c. 1550 and 1450 Ma in the western (Fraser and Lyons, 2006) and northern Gawler Craton (Howard et al., 2011). Major tectonism in the Gawler Craton terminated following these events. The eastern Gawler Craton underwent localised Neoproterozoic extension and c. 820 Ma mafic dyke emplacement recording initial rifting associated with formation of the Adelaide Geosyncline (Wingate et al., 1998), which resulted in the separation of formerly contiguous Curnamona Province and the Gawler Craton (Szpunar et al., 2007). 3. Ion microprobe (SHRIMP) U–Pb geochronology 3.1. Analytical details Three samples for SHRIMP U–Pb zircon dating were collected from three drillholes: Culberta 1, Mudguard 1, and Frome 13 (Fig. 1). Geochronological data are reported in Appendix B in

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Supplementary Material. Two samples (R668303 and R668422) were analysed using the SHRIMP II ion microprobes at Curtin University, Perth, and a third sample (R1709059) was analysed using the SHRIMP IIe instrument at Geoscience Australia, Canberra. Over 100 zircons were separated from each of samples R668303, R668422, and R1709059, which are from drillholes Culberta 1, Mudguard 1, and Frome 13, respectively. The zircons were cast, together with zircon reference standards, in epoxy discs, which were then polished to expose the interiors of the crystals. The zircons were examined and documented in transmitted and reflected light, and with a cathodoluminescence (CL) detector on a JEOL XL30 SEM at Curtin University of Technology (samples R668303 and R668422). Absolute U and Th concentrations were estimated by comparison with the CZ3 zircon standard (551 ppm 238 U) and 206 Pb/238 U ratios were determined relative to the Temora zircon standard (206 Pb/238 U = 0.0668 [417 Ma] (Black et al., 2003)), analyses of which were interspersed with those of unknown zircon grains. For sample R1709059, the OG1 standard (3465.4 ± 0.6 Ma; Stern et al., 2009) was used to monitor 207 Pb*/206 Pb* reproducibility and accuracy (Pb* = radiogenic Pb). Data were reduced and analysed using Squid and Isoplot (Ludwig, 2001, 2003), using decay constants recommended by Steiger and Jäger (1977). Correction for initial or common Pb was made using measured 204 Pb/206 Pb and contemporaneous common-Pb isotopic compositions determined according to the model of Stacey and Kramers (1975). Mean ages are reported with 95% confidence intervals. 3.2. Zircon descriptions and U–Pb results Zircons isolated from R668303 are sub- to euhedral, equant to elongate, and up to 300 ␮m long, with aspect ratios up to 5:1 (Fig. 3a). Zircons from R668422 are irregular to subhedral, equant to elongate, and up to 300 ␮m long, with aspect ratios up to 4:1 (Fig. 3b). Many grains in these two samples are broken fragments of larger crystals. Zircons from R1709059 have a uniform morphology, consisting of magmatically zoned euhedral, elongate grains, about 50–120 ␮m in length, with simple prismatic faces and blunt pyramidal terminations, which are clear in transmitted light, with few cracks and inclusions (Fig. 3c). In CL images, the zircons exhibit euhedral growth zoning and some show sector zoning (Fig. 3). No indication of inherited cores was apparent. 3.3. Culberta 1 rhyolite (sample R668303) Thirty analyses were obtained of 28 zircons from sample R668303 during two sessions. Uranium concentrations are low to moderate, ranging from 61 to 277 ppm, with a mean of c. 120 ppm. Ratios of Th/U fall within a narrow range, from 0.30 to 0.51, with a mean of 0.45. The data are concordant to slightly discordant (Fig. 4a), and indicate dates of 1642–1550 Ma. Twenty-nine of 30 analyses yield a weighted mean 207 Pb*/206 Pb* date of 1584 ± 6 Ma (MSWD = 1.3). The single excluded analysis (12.1) is the most discordant (6%), and indicates a significantly older date of 1642 Ma, although a second analysis (12.2) of the same zircon indicates a younger date of 1627 Ma. The mean date of 1584 ± 6 Ma is interpreted as the crystallisation age of the rhyolite in Culberta 1. 3.4. Mudguard 1 rhyolite (sample R668422) Twenty analyses were obtained of 20 zircons from sample R668422. Uranium concentrations are slightly higher than in R668303, and range from 65 to 430 ppm, with a mean of c. 210 ppm. Ratios of Th/U are very similar to those in R668303 zircons, however, at 0.29–0.55, with a mean of 0.41. Most data are concordant to slightly discordant (Fig. 4b). Four analyses between 6 and 11%

Fig. 3. Cathodoluminescence (CL) images of zircons from rhyolite samples from drillholes: (a) Culberta 1 (R668303), (b) Mudguard 1 (R668422), and (c) Frome 1 (R1709059).

discordant appear to have undergone mainly recent loss of radiogenic Pb. All 20 analyses indicate 207 Pb*/206 Pb* dates of 1599–1571 Ma, and yield a weighted mean date of 1584 ± 5 Ma (MSWD = 0.45). Excluding the four most discordant results does not change the mean significantly, and there is no reason to exclude them. The mean date of 1584 ± 5 Ma is interpreted as the age of crystallisation of the rhyolite in Mudguard 1. 3.5. Frome 13 rhyolite (sample R1709059) Twenty-two zircons were analysed from R1709059. Uranium concentrations are moderate, ranging from 193 to 353 ppm, and averaging c. 272 ppm. Th/U ratios are closely grouped, with a

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4.1. Major and trace elements

Fig. 4. U–Pb analytical data for zircons from rhyolite samples from drillholes: (a) Culberta 1 (R668303), (b) Mudguard 1 (R668422), and (c) Frome 1 (R1709059). Pb*, radiogenic Pb. Error bars are 1 sigma; mean ages are quoted with 95% uncertainties.

similar range to those in R668303 and R668422 zircons, from 0.33 to 0.63, averaging 0.45. These values are typical for zircons from felsic igneous rocks (0.1–1.0) (Heaman et al., 1990; Kröner et al., 1994; Klötzli-Chowanetz et al., 1997). The data are concordant to slightly discordant (Fig. 4c). All 22 analyses yield a 207 Pb*/206 Pb* date of 1587 ± 6 Ma (MSWD = 0.79), interpreted as the age of crystallisation of the rhyolite in Frome 13. 4. Geochemistry Sixty-nine samples were collected from eight diamond drillholes. Eight basalt samples were collected from one drillhole and 61 felsic volcanic samples were collected from seven drillholes. Three rhyolites, six rhyodacites, and two basalts were analysed for whole-rock Sm–Nd isotope compositions.

Major and trace element analyses were performed at a commercial geochemical laboratory in Adelaide (Amdel Laboratories: http://www.amdel.com/wps/wcm/connect/bv comau/Local). Major elements were analysed by Inductively Coupled Plasma with Optical Emission Spectroscopy (ICP-OES) and trace-elements were analysed by Inductively Coupled Plasma Mass Spectrometry (ICPMS). Whole-rock geochemical results are reported in Appendix C in Supplementary Material. The overall trends for the full compositional range from basalt through to rhyolite define negative correlations for all major elements (Fig. 5), except K2 O which defines a positive trend with increasing silica, and Al2 O3 displays an inflection at ∼56% SiO2 (Fig. 5). Trace elements Y, Th and U all define positive correlations for the full compositional range from basalt to rhyolite while Sc and Sr define negative trends (Fig. 6). Zr displays an inflection at ∼70% SiO2 ; defined by the basalts, dacites and rhyolites, while the rhyodacites define a separate trend with extreme enrichment in Zr values (Fig. 6). Rhyolites have a narrow silica range from 70.3 to 74.3% SiO2 . Alkali contents are generally high, and negative correlations with increasing silica are defined for Al2 O3 , Fe2 O3 Total and TiO2 (Fig. 5). Clusters are formed for CaO and MgO (Fig. 5). Rhyolites display tight clusters in trace element abundances; although Nb, Y, U, and Rb (not shown) are variable (Fig. 6). Rhyolites define negative correlations with silica for Zr, Pb, Sr, Ba, Ti, Co (not shown), and Ni, indicating fractionation of olivine, clinopyroxene, and zircon or baddeleyite. The rhyolites also show increased U, Y, Yb, Rb, Ce (not shown), and Th with increasing silica. The SiO2 content in the rhyodacites is also narrow, ranging from 66 to 68.4% Alkali contents are also generally high, and negative correlations with increasing silica are defined for all major elements shown on Fig. 5. The rhyodacites contain higher Al2 O3 , CaO, Na2 O (not shown), P2 O5 , TiO2 , and Fe2 O3 Total contents relative to rhyolites. Rhyodacites also display a wide variation in the large ion lithophile elements (LILE) and the high field strength elements (HFSE) Zr, Y, Nb, Th, and Sc, with extreme enrichment in Zr, across the very narrow silica range (Fig. 6). Silica content within the dacite samples is broad, ranging between 59.7 and 67.7% and large variation is observed in major element abundances across this range (Fig. 5). High aluminium saturation index (ASI) values, alkali, P2 O5 , and Al2 O3 contents may be due to alteration and or weathering in these volcanic rocks. The dacites display steeper trends for P2 O5 , and K2 O and Al2 O3 contents increase with increasing silica relative to the other volcanic rocks. TiO2 , CaO, and MgO contents in the dacites do not display significant variation with increasing silica, defining shallower trends (Fig. 6). Trends defined for trace elements in the dacites differ from those in the rhyolites and rhyodacites and define increasing Zr, U, and Rb (not shown), decreasing Nb, Pb (not shown), and Sr, homogeneous Th, Ti, V (not shown), and Ni, and dispersed Y, Yb, Ce, Co, and Cr (not shown) contents with increasing silica (Fig. 6). Patterns for the BVS normalised to mid-ocean ridge basalt (MORB) display common features such as strong negative Sr, P, and Ti anomalies, positive Th and Ce anomalies, elevated Zr, Hf, Y, and Yb anomalies, and an absence of significant negative Nb anomalies (Fig. 7). Rhyolites display the strongest negative P and Ti anomalies that become weaker in the rhyodacites and are the weakest in the dacites (Fig. 7), which is indicative of fractionation. Rhyolites also have the strongest positive Th anomalies, which decrease with decreasing silica. The rhyodacites have the highest abundances of Zr, Hf, La, Ce, Y, Yb, and Ba (Fig. 7). Strong negative Sr, P, and Ti anomalies in the felsic volcanic rocks indicate plagioclase, apatite, and Ti-bearing minerals (rutile,

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Fig. 5. Harker variation diagrams illustrating major element characteristics of the Benagerie Volcanic Suite.

ilmenite, or titanite) as main fractionating phases, respectively. In the felsic volcanic rocks, fractionation increases from the dacites to the rhyolites, indicated by increases in anomalies. The moderate negative Nb anomalies observed in the BVS is indicative of contamination by crustal material. The felsic volcanic rocks also have geochemical signatures characteristic of A-type magmas, with

mild to strong enrichments in Zr, Nb, Y (Zr + Ce + Y + Nb = 635–1360) (Fig. 8a), and rare earth elements (REE), and anomalous Ga/Al ratios (Fig. 8b). The rhyolites and rhyodacites have high Na2 O/K2 O (>8.5) and K2 O/MgO (>16) ratios. Basalts from Bumbarlow 1 are metaluminous and have a wide range of SiO2 contents, between 45.9 and 55.6% (Fig. 5). The basalts

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Fig. 6. Harker variation diagrams illustrating selected trace element variations in the Benagerie Volcanic Suite.

define negative correlations for TiO2 , and Fe2 O3 Total . MgO, Na2 O, and Al2 O3 contents increase with increasing SiO2 (Fig. 5), although the high Al2 O3 contents may be due to alteration. The basalts generally have homogeneous trace element concentrations. However, the basalts are generally enriched in trace elements such as Zr, Nb and Y (Fig. 6) in addition to Ce, La and Yb (not shown), and are depleted in Th, Ni, Cr and Co (not shown). The basalts are the least enriched in trace elements and display a relatively strong negative Sr anomaly, indicating some plagioclase fractionation. Apatite is not considered to be a major fractionating phase, because P anomalies are slightly negative to flat (Fig. 7), whereas slight to moderate negative Ti

and Nb anomalies indicate fractionation of ilmenite, rutile, or titanite. 4.2. Rare-earth elements (REE) REE signatures for the BVS are relatively enriched and show moderate to strong negative Eu anomalies (Fig. 7). The Eu troughs increase with increasing fractionation of plagioclase. The rhyolites have the strongest negative anomalies (Eu/Eu* = 0.18–0.53), whereas the dacites and rhyodacites have moderate negative Eu anomalies (Eu/Eu* = 0.53–0.64 and 0.40–0.84, respectively). The rhyolites also have steep,

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Fig. 7. MORB-normalised multi-element diagrams and chondrite-normalised REE diagrams for the Benagerie Volcanic Suite. Normalising values for MORB are from Pearce (1983); normalising values for chondrite from Taylor and McLennan (1995).

LREE-enriched signatures, reflected by their (La/Yb)N values of 7.50–12.66, and indicate increasing fractionation. The rhyodacites and dacites have similar REE patterns, although the dacites have slightly flatter patterns which is reflected by their

(La/Yb)N values (4.09–9.84) and more moderate Eu anomalies (Fig. 7). One rhyodacite (sample R1709062) is HREE-enriched, as reflected by its (La/Yb)N value of 2.1. REE signatures in the basalts are the least fractionated, with flatter REE patterns

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Fig. 8. Classification diagrams illustrating the A-type character of the Benagerie Volcanic Suite.

((La/Yb)N = 4.51–5.34) and moderate negative Eu anomalies (Eu/Eu* = 0.59–0.79) (Fig. 7).

4.3. Sm–Nd isotopes Sm–Nd isotope analyses were performed at the University of Adelaide following the method of Wade et al. (2006). Nd analyses were conducted using a Finnigan MAT262 thermal ionization mass spectrometer, in static and quadruple-cup dynamic measurement modes, normalized to 146/144 Nd = 0.721903 and Nd concentrations corrected for 200 pg blank. Sm analyses were carried out on a Finnigan MAT261 thermal ionisation mass spectrometer, in single-cup dynamic mode, and Sm concentrations were corrected for 150 pg blank. Nd isotope compositions for three rhyolite, six rhyodacite, and two basalt samples are recorded in Table 2. Initial εNd(i) values for the BVS and basalts were calculated for 1585 Ma. The 147 Sm/144 Nd ratios for the rhyolites range between 0.1180 and 0.1191 and initial εNd(i) values for the rhyolites are also well-grouped, between −4.0 and −2.2 (Table 2). 147 Sm/144 Nd ratios for the rhyodacites range between 0.1164 and 0.1331 and εNd(i) values are −4.3 to −3.6 (Table 2). The 147 Sm/144 Nd ratios for the basalts are 0.2217 and 0.1267, with εNd(i) values of −1.5 and 0.2, respectively (Table 2). The Nd isotope compositions for the rhyolites and rhyodacites are indistinguishable, indicating derivation from a common or similar crustal source (Fig. 9). The Nd isotope compositions of the basalts suggest a mantle source that was not modified significantly by crustal material (Fig. 9).

5. Discussion 5.1. Age of the Benagerie Volcanic Suite Three rhyolite samples collected from three drillholes: Culberta 1, Mudguard 1, and Frome 13, were dated using SHRIMP U–Pb zircon dating methods. The three samples yielded crystallisation ages between 1587 and 1584 Ma (Appendix B). These new results are consistent with the earlier SHRIMP U–Pb zircon date of 1581 ± 4 Ma for a volcanic rock from drill hole Mudguard 1 by Fanning et al. (1998).

The new SHRIMP geochronology from the BVS has yielded weighted mean ages of 1584 ± 6 Ma, 1584 ± 5 Ma and 1587 ± 6 Ma, which are within analytical uncertainty of one another, and importantly are within analytical uncertainty of the range of ages derived from the GRV or porphyritic equivalents thereof (Table 1). Based on the available data, it appears that the BVS and the GRV are broadly contemporaneous and, hence, were likely generated as part of the same tectonothermal event. 5.2. Source of the BVS magmas As indicated earlier, A-type magmas can be formed by fractionation from a mantle source, by intracrustal melting of granulitic or felsic crustal material, or by mixing of crustal and mantle material. Each source region has a distinct geochemical and isotopic signature which will be reflected by rocks from those regions. For example, a mantle source will have a distinctly juvenile Sm/Nd signature with high Sm/Nd and 143 Nd/144 Nd, positive εNd , Zr/Y < 3, Nb/Y < 0.05, Zr/Nb = 34–49, and depleted LREE/HREE ratios (La/Yb < 1) (Salters and Stracke, 2004; Workman and Hart, 2005). Magmas from a dominantly granulitic or felsic crustal material will, conversely, possess an evolved Sm/Nd signature with low Sm/Nd, low 143 Nd/144 Nd, negative εNd , highly evolved REE patterns (enriched LREE/HREE ratios: La/Yb > 5), enriched LILE, Zr/Y > 5, Nb/Y > 0.3, and Zr/Nb = 11–14 (Taylor and McLennan, 1995; Rudnick and Gao, 2003). A mixed source will necessarily have an intermediate signature, dependant on the proportion of the two end-member source regions. The BVS samples are enriched in HFSE, in particular Zr, Hf, Nb, and Th and REE (Appendix C; Fig. 7) and incompatible trace element ratios, which suggests they were derived from a dominantly crustal source (Fig. 9). Trace element ratios in the rhyolites typically display a crustal signature: (La/Yb) La/Yb > 7, Nb/Y > 0.3, and Zr/Nb < 17, with the exception of Zr/Y < 6.3. Similarly, the rhyodacite and dacite samples display trace element ratios typical of crustal values (La/Yb > 5, Nb/Y > 0.4, Zr/Nb < 27, and Zr/Y > 3.7). Negative whole-rock Sm–Nd isotope compositions (εNd(i) = −4.3 to −2.2) also indicate a dominant intracrustal derivation with no or little mantle input. Incompatible trace element ratios in the basalt are closer to a lower crustal source rather than a purely mantle source

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Table 2 Sm–Nd isotopic data for the Benagerie Volcanic Suite and associated subordinate basalts. Sample

Lithology

Age (Ga)

Nd (ppm)

Sm (ppm)

RS687236 RS687237 R1709061 RS687225 RS687227 RS687235 RS687238 RS687244 R1709062 RS668268 RS668269

Rhyolite Rhyolite Rhyolite Rhyodacite Rhyodacite Rhyodacite Rhyodacite Rhyodacite Rhyodacite Basalt Basalt

1.585 1.585 1.587 1.585 1.585 1.585 1.585 1.585 1.585 1.585 1.585

99.5 104.1 85 80.4 78.7 78.1 98.6 112.3 35.2 37 49.5

19.6 20.3 17.5 16.4 16.1 16 19.6 21.6 7.8 8.2 10.2

147

Sm/144 Nd

0.1189 0.1180 0.1191 0.1232 0.1237 0.1238 0.1199 0.1164 0.1349 0.1341 0.1267

143

Nd/144 Nd

0.511662 0.511617 0.511716 0.511673 0.511696 0.511662 0.511652 0.51161 0.511787 0.511911 0.511921

2sa

eNd(0)

eNd(i)

8 9 8 9 9 10 10 7 10 8 8.2

−19 −19.9 −18 −18.8 −18.4 −19 −19.2 −20.1 −16.6 −14.2 −14

−3.3 −4 −2.2 −3.9 −3.6 −4.3 −3.7 −3.8 −3.7 −1.5 0.2

TDM 2.381 2.43 2.301 2.475 2.451 2.51 2.423 2.401 2.564 2.338 2.145

143

Nd/144 NdCHUR(0) = 0.512638, 147 Sm/144 NdCHUR = 0.1967. Nd/144 NdDM(0) = 0.51315, 147 Sm/144 NdDM = 0.2145. CHUR and DM values from Goldstein et al. (1984). a Isotope error measurements are 2 standard errors. 143

with Zr/Y = 3.8–4.9, Nb/Y = 0.3–0.4, Zr/Nb = 13–18, and enriched LREE/HREE ratios (La/Yb = 6.0–7.5). However, εNd(i) signatures of −1.5 and 0.2 (Fig. 9) are inconsistent with crustal contamination, suggesting that the basalts were derived from a HFSE-enriched magma from within the mantle. Importantly, simple fractionation trends are not observed between the mafic and felsic volcanic rocks and the absence of intermediate compositions suggests that the mafic and felsic rocks are not genetically related, but instead represent two contrasting styles of magmatism during the same event.

The enriched mantle source of the BVS-related basalts is an interesting feature. Paleoproterozoic c. 1685 Ma tholeiites of the southern Curnamona Province yield positive εNd values and geochemically enriched signatures that Rutherford et al. (2006) interpreted to be derived from a subductionenriched subcontinental lithospheric mantle source. Conceivably, early Mesoproterozoic mafic magmatism in the Curnamona Province could have sampled the same pre-existing fertile mantle material.

Fig. 9. Incompatible-element ratio plots versus εNd values for the BVS.

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Fig. 10. Geochemical comparison of felsic volcanic rocks from Benagerie Ridge and the lower and upper GRV. (a) Mid-ocean ridge basalt (MORB)-normalised multi-element diagram of the BVS and the lower GRV; (b) chondrite-normalised rare earth element (REE) diagram of the BVS and the lower GRV; (c) MORB-normalised multi-element diagram of the BVS and the upper GRV; (d) chondrite-normalised REE diagram of the BVS and the upper GRV. Data sources for the lower GRV: Giles (1980a), Stewart (1994) and Fricke (2005). CVC: Chitanilga Volcanic Complex and GHVC: Glyde Hill Volcanic Complex.

5.3. Geochemical comparisons with the Gawler Range Volcanics 5.3.1. BVS and felsic volcanic rocks of the lower GRV The BVS define different linear trends to the lower GRV for most major and trace elements, and are more enriched in HFSE, in particular Zr, Th, and Hf, relative to the felsic lower GRV samples from Kokatha and Lake Everard (Fig. 10a). The BVS also have enriched REE signatures, particularly HREE, compared to the felsic lower GRV samples (Fig. 10b). Whole-rock Nd signatures in the lower GRV are extremely variable, with εNd(i) values ranging from −7 to −0.2 (Fig. 11). Although the εNd(i) values for the BVS fall within this range, their values are much less dispersed and lie within the upper limit of the CVC and the lower limit of the GHVC (Fig. 11). These enriched geochemical signatures, in addition to the absence of clear fractionation trends with the basalts from the Benagerie Ridge, indicate a different, crustal-dominated source for the BVS and do not appear to represent an equivalent to the lower GRV. 5.3.2. BVS and the upper GRV The geochemical signatures of the BVS are more indicative of the upper GRV, with similar linear trends for all major elements and most trace elements. The BVS are more enriched in Nb, U, and Y compared with the upper GRV and the rhyodacites from the BVS display elevated Zr and Hf contents (Fig. 10c). The dacites and rhyodacites have very similar REE signatures to the upper GRV, although the rhyolites are more enriched in REE and have more pronounced negative Eu anomalies (Fig. 10d). These relative enrichments may

be indicative of large amounts of fractionation or an enriched source region. Whole-rock Nd signatures of the BVS indicate mainly intracrustal derivation with little or no mantle input (Fig. 11). This differs to the developmental phase rhyolites to dacites of the lower GRV which are direct fractionates of the basalts and have assimilated varying amounts of crustal material (Fricke, 2005). The Nd signatures of the BVS are indistinguishable from those of the upper GRV (εNd(i) = −4.3 to −1.8; Fig. 11), which were derived from extensive melting of a crustal source. 5.3.3. Basaltic subunits and the lower GRV The unnamed basalts from the Bumbarlow drillhole display similar major element abundances as the mafic Roopena Volcanics, although MgO contents are slightly lower. The basalts are more enriched in HFSE and LREE (Fig. 12) compared to the Roopena Volcanics suggesting that their source region may have been elevated in these elements. Similarities between the basalts and the CVC and GHVC are also observed, although the basalts from the Benagerie Ridge are generally more mafic but lie on similar linear trends (Fig. 12). The basalts are particularly enriched in U and Nb compared to all basalts from the lower GRV. LREE abundances are comparable with the CVC, although the basalts from the Benagerie Ridge are more fractionated and have more pronounced Eu anomalies (Fig. 12). The Nd isotopic signature of the lower GRV (εNd(i) = −6.9 to 2.5; Fig. 11) indicates a mantle source which, at some volcanic centres, has subsequently been modified by varying amounts of assimilated crustal material (e.g. basalts from the CVC with εNd(i) values of

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Fig. 11. Nd-evolution plot comparing the BVS and associated subordinate basalts with the lower and upper GRV. Data sources for the GRV: Giles (1980a,b), Stewart (1994) and Fricke (2005). CVC: Chitanilga Volcanic Complex and GHVC: Glyde Hill Volcanic Complex.

−6.9 and −5.4 and εNd(i) value of −5.7 for the Roopena Volcanics). Comparatively, the unnamed basalts from Bumbarlow 1 have geochemical and isotopic signatures consistent with a HFSE-enriched magma from within the mantle. It is possible that basalts from the Benagerie Ridge represent a discrete volcanic centre also formed during the developmental phase and thus form part of the lower GRV, and while no fractionates of this basalt have been identified to date, a large area of the Benagerie Ridge is still unknown and it possible that the fractionated felsic volcanic rocks exist. 5.4. Petrogenesis of early Mesoproterozoic magmatism in the Curnamona Province and adjacent Gawler Craton and relationship to regionally distributed, broadly synchronous orogenesis Given the abundance of weakly deformed or undeformed granites and volcanic rocks within the Curnamona Province and Gawler

31

Craton, the early Mesoproterozoic magmatism in southern Proterozoic Australia was initially considered to have formed in a mid-cratonic, anorogenic, or weakly extensional setting (Flint et al., 1993; Creaser, 1995). The extensive felsic magmatism was interpreted to have formed via underplating of mafic material from the mantle into the lower crust via a mantle plume resulting in high heat flow, widespread partial melting and hence voluminous felsic magmatism (Giles, 1980b, 1988; Blissett et al., 1993; Flint et al., 1993; Betts et al., 2007). One problem with this model is the comparatively small volume of mafic rocks within the upper crust, more of which might be expected if a mantle plume is invoked to explain the mafic underplating because plume upwelling is commonly associated with continental flood basalts (Richards et al., 1989; White and McKenzie, 1989; Duncan and Richards, 1991). A second problem for a plume-related model is to explain how the hypothesised plume-related intracrustal melting is related to the broadly synchronous Olarian and Kararan Orogenies. The BVS–GRV magmatic system occurred at around the same time as major regional metamorphism and associated shortening across both the Curnamona Province and Gawler Craton. In the Curnamona Province, the major deformation event at this time is the c. 1620–1585 Ma Olarian Orogeny (Page et al., 2000). Early Mesoproterozoic high-temperature metamorphism and associated deformation is also recorded within the northern Gawler Craton (e.g. Coober Pedy Ridge (McLean and Betts, 2003; Fanning et al., 2007)); Mt Woods Domain (Jagodzinski et al., 2007; Forbes et al., 2010) and from regions to the south and east of the Gawler Range Volcanics (e.g. Barossa Complex (Szpunar et al., 2007), northern Yorke Peninsula (Conor, 1995; Fanning et al., 2007)). In fact, parts of the lower GRV preserve steep dips whereas the upper GRV is essentially undeformed (Hand et al., 2008), suggesting that even as deposition of the volcanic rocks was ongoing, some portion of the lower material was being deformed. Therefore, although A-type magmas may easily be generated during lithospheric extension in an active rift environment (Sears et al., 2005), it seems likely that rather than a strictly anorogenic setting for the BVS–GRV magmatic system, this time period in the southern Australian region actually includes major, regional-scale high-temperature metamorphism and compressional deformation. Consequently, any model for generation of the BVS and GRV must account for the presence of: (1) early but volumetrically minor mafic volcanic rocks during the developmental phase, (2) an overall A-type geochemical signature, (3) extensive crustally derived felsic magmas during the mature phase, and (4) the generation of these melts in concert with

Fig. 12. Comparison of mafic volcanic rocks from Benagerie Ridge and the lower GRV. (a) MORB-normalised multi-element diagram of the basalts and the lower GRV; (b) chondrite-normalised REE diagram of the basalts and the lower GRV. Data sources for the GRV: Giles (1980a), Stewart (1994), and Fricke (2005). CVC: Chitanilga Volcanic Complex, GHVC: Glyde Hill Volcanic Complex and RV: Roopena Volcanics.

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regionally distributed high-temperature metamorphism and attendant deformation. Volumetrically minor mafic magmatism occurred during the developmental phase of the BVS and GRV, with isolated volcanic centres located in the Benagerie Ridge and at Kokatha, Lake Everard, and Roopena (Fig. 1). There is considerable geochemical variation between the individual volcanic centres during this development phase of the volcanic event (Blissett, 1975; Blissett et al., 1993; Stewart, 1994; Fricke, 2005). Locally, deposition of clastic sedimentary sequences accompanied basaltic magmatism as a result of space being created to accommodate immature sediments, such as the locally sourced clastics within the Bumbarlow 1 drillhole (Fraser and Neumann, 2010) and the Corunna Conglomerate in the Gawler Craton (Daly et al., 1998). The presence of basaltic volcanism together with sedimentation suggests that, at the broad scale, this early phase of magmatism was driven by lithospheric extension, resulting in asthenospheric mantle ‘upwelling’, mantle partial melting, and associated higher mantle heat flow. The fact that the developmental phase appears localised to discrete volcanic centres may indicate that mantle melting was restricted to separate, distinct locations. However, mafic intrusions are known across the Gawler Craton, including the 1589 ± 5 Ma Curramulka Gabbronorite on Yorke Peninsula (Zang et al., 2007) and gabbros of the Coober Pedy Ridge in the north-central Gawler Craton (Rusak, 2011), and across the Curnamona Province, including the Lake Charles Diorite (Wade, 2011) of the Benagerie Ridge (Fig. 1), clearly indicating that not all mafic melt was erupted. Thus it is likely that the restricted basaltic flows indicate that more extensive mafic magmas ponded in the lower crust or upper mantle rather than rising to the surface (Flint et al., 1993). A regionally extensive mafic magma component in the lower crust or upper mantle likely provided the heat required for the extensive intracrustal melting that produced the voluminous ‘mature’ phase of this magmatic event, during which the BVS and upper GRV, together with their intrusive equivalents, the Ninnerie Supersuite, Hiltaba Suite, and Munjeela Suite were formed. Across the Curnamona Province and Gawler Craton, the c. 1620–1586 Ma Olarian Orogeny and the c. 1590–1555 Ma Kararan Orogeny were broadly coeval with formation of the BVS, GRV, Ninnerie Supersuite, Hiltaba Suite and Munjeela Suite. A feature of both of these orogenic events is high-geothermal-gradient metamorphism (e.g. Clarke et al., 1987; Powell and Downes, 1990; Forbes et al., 2010; Cutts et al., 2011). It is possible that the high geothermal gradient that both caused the extensive A-type BVS–GRV magmatic system and the I- and S-type intrusive system (i.e. Ninnerie Supersuite, Hiltaba Suite and Munjeela Suite) and as a result of its development, likely primed the crust across the Curnamona Province and Gawler Craton for these deformation and metamorphic events. Interestingly, however, although the deep crust underneath the BVS–GRV magmatic system underwent extensive partial melting, the deformation associated with the Olarian–Kararan orogenic systems was not localised in this region – a region which could reasonably be expected to deform due to the thermal weakening effects of the high heat flow and partial melt. Instead, deformation was partitioned around the edges of the volcanic provinces to such an extent that these upper-crustal volcanic rocks remained in the upper crust during the orogenesis, and during the subsequent c. 1580 m.y. of geological evolution of the southern Australian region. This would appear to suggest that the BVS–GRV magmatic system developed above buoyant, refractory, and hence comparatively rigid lithosphere which resisted deformation even during a period of intracrustal melting. In the Curnamona Province, the strongest deformation and highest metamorphic grade corresponds to the locus of deepest sedimentation (Conor and Preiss, 2008), where the rifted deep crust in the province is likely to have been thinnest

in the lead-up to the Olarian Orogeny. In the Gawler Craton, zones of significant deformation during this time interval include reactivated Paleoproterozoic structures, such as the Kalinjala Shear Zone in the south (Hand et al., 2007) and the Karari Shear Zone in the north (Fanning et al., 2007; Cutts et al., 2011). Therefore, it appears that the pre-existing crustal architecture played a significant role in both the localisation of extensive magmatism and in the nearsynchronous partitioning of deformation about a relatively rigid central zone within the two provinces. Are there any clues as to possible far-field tectonic drivers that could help to indicate the broad tectonic setting for this SLIP? Recent work in the Musgrave Province to the north of the Gawler Craton has identified a c. 1590–1550 Ma juvenile magmatic suite in the Musgrave Province (Fig. 1; Wade et al., 2006; Smithies et al., 2011). The felsic units within this suite show features such as negative Nb, Ti, and Y anomalies, steep LREE patterns, and comparatively juvenile Nd isotopic compositions (εNd(1550) = −1.2 to 0.9), which Wade et al. (2006) suggested compare favourably with melts expected within island arc magmatic systems. If an island arc magmatic system was in operation to the north of the Curnamona Province and Gawler Craton, with subduction being south-directed (in present coordinates) as interpreted by Wade et al. (2006), this would imply that the latter two provinces lay within a far-field continental back-arc setting at this time;. Such a broad-scale tectonic setting provides a plausible thermomechanical environment within which both lithospheric thinning and compressional tectonism could operate. Changes in subduction zone dynamics, such as the speed or angle of subduction result in extension or compression in the over-riding plate with switching between these two states occurring over relatively short periods of time, c. 5–10 Ma, e.g. Collins (2002). Conceivably, therefore, the A-type BVS–GRV magmatic system could have resulted from back-arc extension in an intracontinental setting while also being associated with regional high-grade metamorphism and crustal reworking during the early Mesoproterozoic. Importantly however, we recognise there is considerable ambiguity in the identification of subductionrelated geochemical signatures in the Australian Proterozoic in general (Fraser et al., 2007; Hayward and Skirrow, 2010) and the suggestion of a far-field back-arc setting for the BVS-Gawler Range Volcanics SLIP, while conceptually appealing, relies heavily on such an interpretation from the Musgrave Province. Further studies are therefore required into the nature of the Mesoproterozoic magmatism of the Musgrave Province and indeed of the structures and chemistry along the boundary between the Gawler Craton/Curnamona Province and the Musgrave Province to clarify the nature of this boundary and the Mesoproterozoic tectonic relationships between these major crustal elements.

6. Conclusions A-type volcanic rocks from the Benagerie Ridge comprise an older or developmental mafic phase and a younger or mature felsic phase of magmatism during the early Mesoproterozoic in the Curnamona Province. The mafic volcanic rocks comprise minor basaltic lava flows which are enriched in HFSE and have a mantle signature. The felsic volcanic rocks range from dacite to rhyolite and do not represent direct fractionates of the basalts. The felsic volcanic rocks are also enriched in HFSE, in particular Y, Nb, U, Zr, and Hf, suggesting inheritance from an enriched, crustal source region. New geochronological results indicate that the three rhyolite samples of the BVS analysed yielded magmatic crystallisation ages between 1587 and 1584 Ma. These results indicate that the BVS was contemporaneous with intrusive magmatism in the southern Curnamona Province and the Gawler Craton, and the extensive GRV of the Gawler Craton. Geochemical and isotopic signatures of

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the basalts from the Benagerie Ridge are similar to those of the lower GRV, although slight differences indicate that each complex was derived from a separate magma chamber during the developmental phase. Geochemical and isotopic compositions of the BVS are indistinguishable from the upper GRV, suggesting that similar source regions and magmatic conditions were driving the early Mesoproterozoic magmatism in the mature phase. Together, the BVS–GRV magmatic system therefore constitutes an A-type SLIP of considerable preserved extent, with an estimated volume of c. 1.11 × 105 km3 . Mafic magmatism in the developmental phase could have been generated by transient lithospheric extension, a scenario readily explained if the BVS–GRV magmatic system occurred in a continental back-arc setting relative to juvenile magmatism recently recognised in the Musgrave Province (Wade et al., 2006; Smithies et al., 2011). Shortly following the developmental phase, widespread felsic magmatism, reflecting large-scale crustal melting, occurred during the mature phase. This melting was likely assisted by high heat flow as a result of the underplating of asthenospheric mantle and likely primed the crust for high-temperature metamorphism and partitioned deformation that occurred essentially synchronously with the eruption of this Mesoproterozoic silicic large igneous province. Acknowledgements U–Pb measurements for two samples were conducted using the SHRIMP ion microprobes at the John de Laeter Centre of Mass Spectrometry at Curtin University in Perth. The Frome 13 sample was analysed at Geoscience Australia geochronology laboratory, as part of a PACE-funded collaborative geochronology project between the PIRSA and Geothermal Resources under the auspices of a National Geoscience Agreement between Geoscience Australia and PIRSA. Geoff Stolz of Geothermal Resources in particular is thanked for his support of this program. David Bruce, University of Adelaide, is thanked for assistance with Sm–Nd isotope analysis. Martin Hand is acknowledged for many fruitful discussions. CE Wade, AJ Reid, and EA Jagodzinski publish with permission of the director, Geological Survey of South Australia. MTD Wingate publishes with permission of the Executive Director of the Geological Survey of Western Australia. We thank Richard Ernst for his valued comments and useful suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2012.02.020. References Allen, S.R., McPhie, J., 2002. The Eucarro Rhyolite, Gawler Range Volcanics, South Australia; a >675 km3 compositionally zoned lava of Mesoproterozoic age. Geological Society of America Bulletin 114 (12), 1592–1609. Allen, S.R., McPhie, J., Ferris, G., Cadd, A.G., 2008. Evolution and architecture of a large felsic igneous province in western Laurentia: the 1.6 Ga Gawler Range Volcanics, South Australia. Journal of Volcanology and Geothermal Research 172, 132–147. Allen, S.R., Simpson, C.J., McPhie, J., Daly, S.J., 2003. Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia. Australian Journal of Earth Sciences 50 (1), 97–112. Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature 297 (5865), 391–393. Barovich, K., Foden, J., 2002. Nd isotope constraints on the origin of 1580 Ma Curnamona Province granitoid magmatism. In: Preiss, V.P. (Ed.), Geoscience 2002; Expanding Horizons; Abstracts of the 16th Australian Geological Convention, 67. Sydney, N.S.W., Australia. Geological Society of Australia, p. 156. Betts, P.G., Giles, D., Schaefer, B.F., Mark, G., 2007. 1600–1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions. Terra Nova 19, 496–501.

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