February 2012 LIP of the Month

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A geochemical comparison of the Benagerie Volcanic Suite and the Gawler Range Volcanics: a Mesoproterozoic silicic large igneous province, South Australia

Claire E. Wade a,*, Anthony J. Reid a, Michael T.D. Wingate b, Elizabeth A. Jagodzinski a and Karin Barovich c

a Geological Survey of South Australia, Department for Manufacturing, Innovation, Trade, Resources and Energy , GPO Box 1264, Adelaide, SA 5001

b Geological Survey of Western Australia, Department of Mines and Petroleum, 100 Plain St., East Perth, WA 6004

c Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005

* Corresponding author

E-mail address: claire.wade@sa.gov.au (C.E. Wade)

Introduction

The Benagerie Volcanic Suite (BVS) comprises voluminous, A-type, rhyolite to dacite rocks extruded c. 1585 Ma in the Benagerie Ridge, north-central Curnamona Province, South Australia. New geochronological, geochemical, and whole-rock Sm-Nd isotope data for the BVS have shown geochemical and isotopic affinities with the upper Gawler Range Volcanics (GRV) of the Gawler Craton (Wade, et al., accepted). Subordinate basaltic rocks in the form of minor lava flows are also found in the Benagerie Ridge and display geochemical and isotopic signatures similar to those of the lower GRV (Wade, et al., accepted). Together, the BVS and GRV form a Mesoproterozoic, A-type silicic large igneous province (SLIP) across the Curnamona Province and the Gawler Craton, South Australia.

The occurrence of SLIPs in Proterozoic terrains is rare in the geological record. Only a few examples exist, which include the c. 2060 Ma Rooiberg Felsite, South Africa (Twist and French, 1983, Olsson, et al., 2010), the c. 1590–1580 Ma Gawler Range Volcanics (GRV) and Benagerie Volcanic Suite (BVS) (Wade, et al., accepted), South Australia, Australia, and the c. 1100 Ma North Shore Volcanic Group, Minnesota, USA (Green, 1989). Unique among these examples in terms of metallogenic significance are the extensive Mesoproterozoic volcanic rocks of South Australia: the GRV of the Gawler Craton and the BVS (Wade, 2011) of the Curnamona Province (Fig. 1). For this reason the GRV have been the focus of many research projects (e.g. Giles, 1980, Giles, 1988, Cross, et al., 1993, Allen and McPhie, 2002, Pankhurst, 2006, Allen, et al., 2008, Hand, et al., 2008, Agangi, et al., 2010); however new geochronological, geochemical and isotopic data has confirmed that the BVS is petrogenetically similar to the GRV and together they form a significant SLIP in southern Australia (Wade, et al., accepted). 


Figure 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.

Similarities between the BVS and GRV were first recognised by Giles and Teale (1979) who indentified many geochemical affinities between these two magmatic systems, which included sharing a high-temperature, lower crustal signature and enrichment in elements such as Zr, Nb, Y and Ce. Despite these initial interpretations, little is known about the age, geochemical and isotopic characteristics of the BVS. Unlike the GRV, which crop out extensively in the Gawler Craton, the BVS is overlain by widespread Phanerozoic cover in the Moorowie Sub-basin and Frome Embayment and subsequently are known only from drill holes (Fig. 1). Additionally, the two domains are separated by the Neoproterozoic Adelaide Geosyncline, obscuring any spatial connections between these volcanic domains. Importantly, 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, Skirrow, et al., 2007, Skirrow, 2009) 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).

Regional Geological Setting

Curnamona Province

The Curnamona Province is a near-circular crustal element that extends from central eastern South Australia to western New South Wales (Fig. 1; Robertson, et al., 1998, Conor and Preiss, 2008).  The oldest exposed rocks are metasedimentary and meta-igneous rocks of the late Palaeoproterozoic (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.

Two major tectonic events affected the Curnamona Province: 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). Willyama Supergroup stratigraphy is inverted and dismembered as a result of polyphase deformation and metamorphism during these orogenic events. Metamorphism reached granulite facies conditions (Clarke, et al., 1987, Powell and Downes, 1990, Page and Laing, 1992, Dutch, et al., 2005, Rutherford, et al., 2007); however  the effects of the Olarian Orogeny decrease northwards from upper amphibolite to greenschist facies (Conor and Preiss, 2008). In the Benagerie Ridge, Willyama Supergroup metasedimentary rocks are openly folded and unconformably overlain by the flat-lying and relatively undeformed BVS (Conor and Preiss, 2008). The Benagerie Ridge itself is a large area of relatively shallow, north-south-trending basement rocks obscured by younger sediments (Robertson, et al., 1998) spanning across the Mulyungarie and Mudguard Domains (Fig. 1). The southern 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 Ninnerie Supersuite

The early Mesoproterozoic Ninnerie Supersuite was generated during the waning stages of the Olarian Orogeny (c. 1620–1585 Ma) producing 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 biotite-only 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 includes felsic volcanics ranging from porphyritic rhyolite and rhyodacite, to porphyritic, amygdaloidal dacite, and basaltic to andesitic breccia, forming four units recently formally defined by Wade (2011): Finlay Dam Rhyolite, Lake Elder Rhyodacite, Benagerie 1 and Benagerie 2 respectively. These volcanic rocks are known from several drillholes in the Benagerie Ridge region (Fig. 1). 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, increasing 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, nevertheless a present-day thickness of ~1.5 km has been interpreted from seismic data (Korsch, et al., 2010).

Early geochronology, using IDTIMS multigrain zircon techniques, yielded an imprecise date of 1599 ± 40 Ma for a rhyodacite from drillhole Mudguard 1 (Fanning, et al., 1988). A SHRIMP U–Pb zircon date of 1581 ± 4 Ma for a volcanic rock in Mudguard 1 was quoted in an abstract by Fanning et al. (1998). More recently, magmatic crystallisation ages between 1587 and 1584 Ma have been obtained (Jagodzinski and Fricke, 2010, Wade, et al., accepted).

An unnamed basaltic subunit (Ninnerie 1; Wade, 2011) in the Benagerie Ridge has been identified in drillhole Bumbarlow 1 (Fig. 1). Basalts from Bumbarlow 1 comprise seven major horizons of fine-grained, amygdaloidal basalt 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.

The BVS are not considered to be direct fractionates of the basalts from Bumbarlow 1 due to paucity of outcrop, lack field relationships, lack of intermediate compositions and different geochemical and isotopic signatures.

Gawler Craton

The Gawler Craton comprises Mesoarchaean to earliest Palaeoproterozoic basement that is overlain by a series of Palaeoproterozoic basins. The basement occurs within two belts located in the north-central 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 Palaeo- to Mesoproterozoic tectonism (Daly, et al., 1998, Hand, et al., 2007).

Sedimentation in these basins was terminated by the craton-wide c.1730–1690 Ma Kimban Orogeny, which 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 relatively 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, possibly during the operation of a subduction-related magmatic arc (Swain, et al., 2008). Magmatism continued during the Mesoproterozoic in the Gawler Craton producing the voluminous Gawler Range Volcanics (1595–1590 Ma) and Hiltaba Suite (1595–1575 Ma).

Although the GRV are undeformed and many Hiltaba Suite plutons in the central Gawler Craton preserve weak or no deformation, there are abundant examples of syn-Hiltaba Suite deformation in other regions of the Gawler Craton (Hand, et al., 2007). This includes 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 GRV.

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).

The Gawler Range Volcanics and Hiltaba Suite

The Gawler Range Volcanics (GRV) are 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: Chitanilga Volcanic Complex, Kokatha (Blissett, 1975); Glyde Hill Volcanic Complex (Blissett, 1975, Giles, 1977, Ferris, 2003), Lake Everard; and Roopena Volcanics (Crawford and Forbes, 1969, Blissett, et al., 1993), Roopena (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 mafic and felsic volcanic rocks (Fricke, 2005). Volcanics from Roopena are best known from drill core, in which several basaltic flow horizons, interbedded with volcaniclastic sediments, have been identified (Blissett, et al., 1993). Basaltic and felsic volcanic rocks associated with volcaniclastic sedimentary rocks correlated with the Corunna Conglomerate also crop our near Roopena (Thomson, 1980a, Thomson, 1980b).

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 of the lower GRV yielded an upper intercept date of 1591 ± 3 Ma, interpreted as the age of crystallisation (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 crystallization 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., 1988a). 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.

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 Archaean host rocks. For example the Charleston Granite, which has εNd(1585 Ma) values of c. -14.3 (Creaser and Fanning, 1993) intrudes the Mesoarchaean domain in the north-eastern Eyre Peninsula. Mafic intrusions of hornblende-bearing quartz monzodiorite, quartz monzonite, and granodiorite are known from the north-eastern 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, Fe2O3, MgO, P2O5, 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).

Geochemistry of the BVS

The BVS are dominated by felsic (>60% SiO2) magmas and has a subordinate mafic component with silica <56% SiO2.  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) and rare earth elements (REE), and anomalous Ga/Al ratios. The overall trends for the full compositional range from basalt through to rhyolite define negative correlations for all major elements (Fig. 2), except K2O which defines a positive trend with increasing silica; Al2O3 displays an inflection at ~56% SiO2 and Na2O displays large variations (Fig. 2). Trace elements such as La, Ce, Y, Th and U all define positive correlations for the full compositional range from basalt to rhyolite (Fig. 3). 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. 3). Nb contents in the BVS are enriched relative to the basalts and display large variation across the silica range (Fig. 3).


Figure 2.  Harker variation diagrams illustrating major element characteristics of the Benagerie Volcanic Suite and subordinate basalts.


Figure 3.  Harker variation diagrams illustrating selected trace element variations in the Benagerie Volcanic Suite and subordinate basalts. Symbols as per Fig. 2.

REE signatures for the BVS are relatively enriched and show moderate to strong negative Eu anomalies (Fig. 4). The rhyolites have steep, LREE-enriched signatures with strong negative Eu anomalies (Fig. 4a). The rhyodacites and dacites have similar REE patterns (Fig. 4b & c), although the dacites have slightly flatter patterns and more moderate Eu anomalies. The rhyodacites display more variable REE patterns; one sample is depleted in LREE and enriched in HREE relative to the other rhyodacites and a second sample is LREE-enriched (Fig. 4b). REE signatures in the basalts are the least fractionated; overall REE patterns are flatter and negative Eu anomalies are less pronounced (Fig. 4d).


Figure 4.  Chondrite-normalised REE diagrams for the Benagerie Volcanic Suite; (a) rhyolite; (b) rhyodacite; (c) dacite; and (d) basalt. Normalising values for MORB are from Pearce (1983); normalising values for Chondrite from Taylor and McLennan (1995).

The Nd isotope compositions for the rhyolites and rhyodacites are indistinguishable with vales ranging from -4.3 to -2.2 (Wade, et al., accepted), indicating derivation from a common or similar crustal source. The Nd isotope compositions of the basalts are more primitive  (εNd(i) = -1.5 to 0.2;  Wade, et al., accepted) and suggest a mantle source that was not modified significantly by crustal material.

BVS and the felsic volcanic rocks of the lower GRV

When compared to felsic units of the lower GRV from Kokatha and Lake Everard, the BVS define different linear trends for most major and trace elements, in particular MgO contents are lower in the BVS and the dacites are enriched in Fe2O3T relative to the felsic volcanics from the lower GRV (Fig. 5a). The BVS are more enriched in HFSE, in particular Zr, Th, and Hf (Fig. 5a). Rare earth element signatures in the BVS are also more enriched compared to the felsic lower GRV samples (Fig. 5b).


Figure 5.  Geochemical comparison of the BVS and felsic volcanics of the lower GRV: (a) Selected major and trace element variation diagrams; (b) Chondrite-normalised REE diagram of the BVS and felsic units of the lower GRV. Data sources for the lower GRV: Giles (1980), Stewart (1994) and Fricke (2005). CVC = Chitanilga Volcanic Complex; GHVC = Glyde Hill Volcanic Complex.

Whole-rock Nd signatures in the lower GRV are extremely variable, with εNd(i) values ranging from -7 to -2.0 and -3.2 to -0.2 for the CVC and GHVC respectively (Giles, 1980, Stewart, 1994, Fricke, 2005). Although the εNd(i) values for the BVS (-4.3 to -2.2) fall within this range, the lie within the upper limit of the CVC and the lower limit of the GHVC.

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.

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 (Fig. 6a). Differences are observed in HFSE; in particular Nb, Zr, Th, and Y are more enriched in the BVS compared with the upper GRV (Fig. 6a). 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. 6b). The upper GRV display a wider array of REE abundances, which includes samples with lower REE abundances compared to the BVS. These relative enrichments may be indicative of large amounts of fractionation or an enriched source region.


Figure 6.  Geochemical comparison of the BVS and the upper GRV: (a) Selected major and trace element variation diagrams; (b) Chondrite-normalised REE diagram of the BVS and the upper GRV. Data sources for the upper GRV: Giles (1980) and Stewart (1994).

Whole-rock Nd signatures of the BVS are indistinguishable from those of the upper GRV (εNd(i) = -4.3 to -1.8 (Stewart, 1994)), which were derived from extensive melting of a crustal source. Signatures of the BVS also indicate mainly intracrustal derivation with little or no mantle input. 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).

Basaltic subunits and the lower GRV

The basaltic subunits from drill hole Bumbarlow 1 display similar major element abundances to the Roopena Volcanics (RV), although MgO contents are slightly lower (Fig. 7a). The basalts are more enriched in HFSE and LREE compared to the RV 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, particularly to the CVC basalts (Fig. 7a). LREE abundances are comparable with the CVC, although the basalts from the Benagerie Ridge are more fractionated and have more pronounced Eu anomalies (Fig. 7b).


Figure 7.  Geochemical comparison of basalts from the Benagerie Ridge and basalts from the lower GRV: (a) Selected major and trace element variation diagrams; (b) Chondrite-normalised REE diagram of the basalts from Benagerie Ridge and the lower GRV. Data sources for the lower GRV: Giles (1980), Stewart (1994) and Fricke (2005). CVC = Chitanilga Volcanic Complex; GHVC = Glyde Hill Volcanic Complex; RV = Roopena Volcanics.

The Nd isotopic signature of basalts from the lower GRV are variable between volcanic centres: CVC εNd(i) = -6.9 to -1.1; GHVC εNd(i) = 1.6; RV εNd(i) = -5.7 to 2.5 (Giles, 1980, Stewart, 1994, Fricke, 2005); however they largely indicate 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 -6.9 and -5.4 and εNd(i) value of -5.7 for the RV). Comparatively, the basalts from Bumbarlow 1 have geochemical and isotopic signatures consistent with a HFSE-enriched magma from within the mantle which has not assimilated significant amounts of crustal material.

Source of the BVS magmas

The BVS samples are enriched in HFSE, in particular Zr, Hf, Nb, and Th and REE and have elevated incompatible trace element ratios, suggestive of a dominantly crustal source. Trace element ratios in the BVS typically display a crustal signature: La/Yb >5, Nb/Y >0.3 and Zr/Nb <17. 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.

While incompatible trace element ratios of the basalts are more typical of lower crustal values rather than mantle values (Zr/Y = 3.8 – 4.9, Nb/Y = 0.2 – 0.4, Zr/Nb = 12–18), and LREE and enriched relative to HREE ((La/Yb)N = 4.5 – 5.4), juvenile εNd(i) signatures of -1.5 and 0.2 are inconsistent with crustal-derivation or contamination. A mantle-derived magma enriched in HFSE is proposed as the source material for the generation of these basalts.

 Importantly, the mafic and felsic volcanic rocks of the Benagerie Ridge likely represent two contrasting styles of magmatism produced during the same event.

A model for the generation of this Mesoproterozoic SLIP

A two-phase model, involving a developmental and a mature phase, has been proposed by previous workers such as Giles (1988), Blissett, et al., (1993) and, Stewart (1994) for the generation of the GRV-Hiltaba Suite. Here, we too invoke a two-phase model and extend it to encompass contemporaneous magmatism in the Curnamona Province. The developmental phase produced localised basaltic magmatism in the form of lava flows e.g. Roopena Volcanics and basalts in the Benagerie Ridge, and volcanic complexes with full compositional ranges from basalt through to rhyolite e.g. Chitanilga Volcanic Complex and Glyde Hill Volcanic Complex. Locally, this basaltic magmatism was accompanied by the deposition of clastic sedimentary sequences suggesting that, broadly speaking, lithospheric extension drove the developmental phase allowing asthenospheric upwelling, partial melting of the mantle and a higher heat flow which consequently primed the crust for the high temperature metamorphism and deformation associated with the Olarian and Kararan Orogenies. Additionally widespread crustal melting, potentially induced by the elevated geotherm caused by extension, ponding of mafic melt in the lower crust, or a combination of these during the developmental phase, produced rapid eruption of voluminous silicic volcanic rocks producing the BVS and upper GRV during the mature phase.

Conclusions

A-type volcanic rocks from the Benagerie Ridge comprises both a developmental mafic phase and a mature felsic phase of magmatism produced during the early Mesoproterozoic in the Curnamona Province. The mafic volcanic rocks comprise minor basaltic lava flows interlayered with coarse sandstone and siltstone and peperitic basalt. Geochemically the basaltic rocks are enriched in HFSE and have a mantle signature indicated by juvenile Nd isotopic compositions. Geochemical and isotopic signatures of the basalts from the Benagerie Ridge are similar to those of the lower GRV and slight differences indicate that each complex was derived from a separate magma chamber during the developmental phase.

The felsic volcanic rocks range from dacite to rhyolite and have A-type affinities, characterised by enriched HFSE, in particular Y, Nb, U, Zr, and Hf. Evolved Nd isotopic compositions suggest derivation from a crustal source region. 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 during the mature phase. Together, the BVS-GRV magmatic system therefore constitutes an A-type SLIP of considerable preserved extent. The association between extensive bimodal A-type magmatism, high-temperature metamorphism, and localised compressional deformation is suggestive of an intracontinental tectonic setting; although the far-field tectonic drivers for this SLIP remain uncertain.

Acknowledgements

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 on the original manuscript.

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