August 2018 LIP of the Month

Basaltic magmatism in the 1800-1640 Ma Leichhardt and Calvert superbasins of northern Australia: a cautionary note on the use of large igneous provinces (LIPs) as temporal markers of continental breakup

Gibson1, G.M., Champion2, D.C., Withnall3, I.W., Neumann2, N.L. and Hutton3, L.J.

1Research School of Earth Sciences, Australian National University, Canberra, ACT, 2601, Australia; 2Geoscience Australia, Symonston, Canberra, ACT 2609; 3Geological Survey of Queensland, Brisbane, Queensland 4001;

The following paper is a summary and modification of a paper recently published in Precambrian Research Volume 213: 148-169 (2018).


Basaltic magmatism in northern Australia from 1790-1650 Ma occurred in six discrete episodes separated by ca. 20 Myr periods when there was little or no magmatism. The oldest and most voluminous of these magmatic events dominates basin fill in the 1780-1740 Ma Leichhardt Superbasin and comprises mainly pillow lavas and hyaloclastites of the 1780 Ma Eastern Creek Volcanics. They share many of the same geochemical features as the Karoo basalts of southern Africa and likely formed in a similar intracontinental rift environment. Together with basaltic rocks of the same age in the McArthur Basin (Seigal Volcanics) and 1780 Ma felsic volcanic rocks in Mount Isa’s Eastern Fold Belt (Argylla Formation), they make up a large igneous province ≥ 0.1 Mkm3 in volume. Subsequent magmatic events were less voluminous and occurred in a progressively deepening marine sedimentary environment driven by ongoing crustal extension and lithospheric thinning. An upper plate back-arc extensional basin linked to eastward retreat of a west-dipping subduction is thought to have been the most likely driver for these magmatic events. Basaltic magmatism ceased at around 1655 Ma following passive margin formation and break off of the contemporary magmatic arc by which time the depositional environment was dominated by deep water turbidites and basaltic rocks had become increasingly MORB-like in character. This phase of basaltic magmatism occurred some 140 Myr after eruption of the Eastern Creek Volcanics, pointing to the need for caution when using large igneous provinces (LIPs) as a either a piercing point or timing constraint in reconstructions of the Nuna and Rodinia supercontinents.


Introduction and geological context for north Australian basaltic magmatism


Continental flood basalts and other large igneous provinces (LIPs) associated with continental rifting typically extend over many 100’s if not 1000s of square kilometres and take less than 5-10 Myr to be emplaced (Coffin and Eldholm, 2005; Duncan et al., 1997; Ernst et al., 2005; Jerram and Widdowson, 2005; Sheth, 2007). Although still being created today along the East African Rift (Corti, 2009; Courtillot et al., 1999; Ebinger, 2005), many more are of Mesozoic or Cenozoic age (Coffin and Eldholm, 2005; Ernst et al., 2005) and date back to breakup of the Pangea and Gondwana supercontinents (Sensarma et al., 2018). Many of these basalts still retain some vestige of their syn-rift origin, either infilling half-grabens (Catuneanu et al., 2005) or thickening seaward towards their respective continental margins (Planke and Eldholm, 1994; Skogseid, 2001). In contrast, few such clues to origin are preserved in LIPs of Neoproterozoic or older age. With a few notable exceptions like the 830 Ma Gairdner Dyke Swarm in South Australia (Wingate et al., 1998), these LIPs either occur distal to any presently recognised plate margin or have had their temporally-related volcanic sequences completely removed by erosion so that all that remains is a regionally extensive dyke swarm and occasional intrusive centre through which comparably large volumes of basaltic magma are thought to have passed on their way to the surface (Ernst et al., 2005; Ernst et al., 2008). Basaltic rocks making up a significant component of the 1790-1740 Ma Leichhardt and 1730-1640 Ma Calvert superbasins in northern Australia (Fig. 1) present no such problems and constitute one of the least eroded and best preserved examples of an older Paleoproterozoic magmatic province anywhere in the world. However, unlike the Karoo and many other LIPs linked to breakup of the Pangea and Gondwana supercontinents with which they share many geochemical similarities (Gibson et al., 2018; Scott et al., 2000; Wilson, 1987), basaltic magmatism in this much older province occurred in discrete pulses, the oldest and most voluminous of which (1780 Ma Eastern Creek Volcanics) predated continental breakup and passive margin formation by as much as 140 Myr. Magmatism also became progressively less voluminous over time so that basaltic rocks erupted during younger episodes and closer to the time of continental rupture do not always qualify for the term LIP (for a recent definition see Ernst & Youbi, 2017). Only the older Eastern Creek Volcanics and their 1780 Ma correlatives in the McArthur Basin are sufficiently voluminous to warrant use of this term. Yet successive stages of basaltic magmatism developed under very similar tectonic circumstances and had a common trigger, illustrating the need for caution when using any one basaltic magmatic event in supercontinent reconstructions and/or determining the time of continental breakup. These north Australian basaltic rocks are all thought to have formed in an upper-plate backarc extensional basin or marginal sea (Gibson et al., 2018; Gibson et al., 2017) and in this respect preclude any tectonic model that has Australia juxtaposed against western Laurentia from 1800-1650 Ma (etts et al., 2016; Holland et al., 2018). This back-arc basin subsequently collapsed during arc-continent collision between 1650 and 1640 Ma, and by 1620 Ma was already involved in a second major orogenic event during the course of which Australia collided with Laurentia and the arc rocks (Bonnetia) were obducted onto the west Laurentian margin (Furlanetto et al., 2016; Gibson et al., 2018; Thorkelson and Laughton, 2016). Although variably deformed and metamorphosed during both these collisional events, most basaltic rocks and their sedimentary host basins in northern Australia still retain much of their primary structure and character thereby ensuring that the depositional and tectonic environment at the time of magmatism is both independently determined and reasonably well understood.

Figure 1. Maps of exposed (a & b) and interpreted solid geology (c) for the Mount Isa region showing distribution of principal basaltic and felsic magmatic events discussed in the text. Note that many magmatic units are interpreted to extend well beyond the present limits of outcrop (heavy red line in a & c). The particularly voluminous 1785-1780 Ma Eastern Creek Volcanics are best exposed in the Leichhardt River Fault Trough (after Bain et al., 1992; Blake, 1987; Derrick, 1982) but are interpreted from geophysical and seismic reflection data (Gibson et al., 2016) to extend eastwards and northwards at depth (c) into their age correlatives in the McArthur Basin (Seigal Volcanics) and Eastern Fold Belt (Argylla Formation). Note limited extent of 1655 Ma Toole Creek Volcanics in (a) compared to (c).

Table 1: Aerial extent of volcanic units described in text (from mapped and inferred solid geology)

Magmatic Suite/Event

Mapped exposed extent (km2)

Inferred total extent (km2)

Average Thickness (km)

Estimated volume (km3)

Age and comments

Eastern Creek Volcanics (ECV)


14 160



1780 Ma; wholly basaltic in composition

Argylla Formation





1780 Ma; felsic part of same bimodal event as ECV

Bulonga/Marraba Volcanics


10 750


ca. 26800

1760 Ma; bimodal as per Argylla Fm and ECV

Toole Creek Volcanics


16 670


ca. 25 000

1655 Ma; mainly buried beneath younger cover

Aerial extents estimated from ArcMap and do not include temporal equivalents further afield in the McArthur Basin, Curnamona or Gawler cratons (Raveggi et al., 2007; Rutherford et al., 2006; Szpunar et al., 2011). The ECV and Argylla Formation belong to the same 1780 Ma magmatic event which when combined with the Seigal Volcanics and other volcanic rocks of this age in the McArthur Basin exceeds 100 000 km3 in volume and thus falls within recent definitions of what constitutes a large igneous province (Ernst et al., 2005; Ernst and Youbi, 2017).

Depositional environment and age of basaltic magmatism in northern Australia

The Leichhardt and Calvert superbasins (Fig. 1) encompass six major magmatic events of relatively short duration (Gibson et al., 2018), most of which are not wholly basaltic in composition but include subordinate amounts of rhyolite and/or ignimbrite. Inclusion within any one magmatic event (Fig. 1a) is based on similarities in age and the observation that magmatism appears to have occurred in discrete pulses, separated by intervals of time up to 20 Myr long when there was little or no obvious igneous activity (Fig. 2). The Eastern Creek Volcanics are by far the most voluminous of these events and have time equivalents in both the Eastern Fold Belt (Argylla Formation) and McArthur Basin (Seigal Volcanics), making for a magmatic province of considerable aerial extent and volume (Table 1). Based on a combination of mapped (Fig. 1a) and interpreted solid geology (Fig. 1c), magma volumes erupted during this event exceeded 100 000 km3. Volumes of magma erupted during younger events in the Calvert Superbasin are likely to be similarly greater than their surface exposures suggest (Fig. 1c) and some once extended eastwards as far as Georgetown (Fig. 2) and southward into the formerly contiguous Curnamona and Gawler cratons where basaltic rocks of comparable 1700-1690 Ma age are widely exposed (Conor and Preiss, 2008; Gibson et al., 2018; Raveggi et al., 2007; Rutherford et al., 2006). Volumes overall are nevertheless much less than the Eastern Creek Volcanics and its correlatives. They include the predominantly intrusive 1690-1670 Ma Sybella and 1740 Ma Wonga events, neither of which is listed in Table 1 or qualifies for the term large igneous province based on their known aerial extents. They have few if any volcanic equivalents and mainly comprise granites and gabbro intruded syn-extensionally along shear zones during the close of rifting in both basins when extension was presumably at a maximum. Age constraints on the duration of intrusion and, other magmatic events, have been obtained either directly through U-Pb dating of magmatic zircon or the use of zircon held in intercalated tuffaceous material; other age constraints are based on U-Pb dating of detrital zircon from sedimentary rocks that either host volcanic units or are cut by dykes and sills of dolerite and gabbro.

Figure 2. Simplified stratigraphic columns for north Australian tectonostratigraphic elements and their interpreted along-strike temporal equivalents in south-central Australia and Antarctica. Stratigraphy and time range of individual geological units within each element are from multiple sources (Baker et al., 2010; Black et al., 1998; Black et al., 2005; Carson et al., 2009; Carson et al., 2011; Derrick, 1980; Fanning et al., 2007; Neumann and Fraser, 2007; Neumann et al., 2009; Neumann et al., 2006; Page et al., 2005; Page et al., 2000; Page and Sun, 1998; Page and Sweet, 1998; Peucat et al., 1999; Withnall, 1985; Withnall and Hutton, 2013).

Similarly missing from Table 1 are magmatic rocks from the 1640-1575 Ma Isa Superbasin (Fig. 1b). This basin overlies both the Leichhardt and Calvert superbasins and, except for a few rhyolites and tuffaceous units, is almost totally amagmatic in character. Unlike the two older basins, its few magmatic rocks are unrelated to continental breakup. Coeval basaltic rocks are absent.

1790-1780 Ma Eastern Creek Volcanic event

The oldest episode of rift-related volcanic activity commenced around 1790 Ma ± 9 Ma (Fig. 2) in the NNW-trending Leichhardt River Fault Trough (Fig. 1) with rhyolites and minor basalt of the Bottletree Formation (Page, 1983) and concluded no later than 1779 ± 5 Ma with extrusion of the Eastern Creek Volcanics (Neumann et al., 2006). The Eastern Creek Volcanics are wholly

basaltic in composition and were emplaced in two discrete pulses (Cromwell and Pickwick metabasalt members; Fig. 2), separated by the intervening 750 m-thick Lena Quartzite Member (Derrick, 1982; Glikson et al., 1976). They form a volcanic pile 5-8 km thick but overall retain a wedge-like geometry consistent with extrusion into half-graben bounded by normal faults that formed during west-east extension and remained active for the duration of basaltic magmatism (Bain et al., 1992; Eriksson and Simpson, 2009; Gibson et al., 2012; Gibson et al., 2008).

Basaltic flows making up the older Cromwell Metabasalt Member (Fig. 2) typically have rubbly to vesicular (amygdaloidal) tops and were erupted under subaerial conditions into a continental environment dominated by braided river systems (Eriksson and Simpson, 2009). These flows reach thicknesses of 1-140 metres and are almost everywhere interstratified with subordinate amounts of cross-bedded sandstone or quartzite (Bain et al., 1992; Gregory et al., 2008b). In contrast, the Pickwick Metabasalt Member (Fig. 2) contains a higher proportion of peperite and hyaloclastites consistent with emplacement of hot basaltic magma into wet, poorly consolidated sediment or a paleo-landscape in which lakes or bodies of standing water had become common (Gibson et al., 2016). Fluviatile-lacustrine conditions persisted during deposition of the overlying Alsace Quartzite which, apart from a few intrusions of dolerite near its base, largely post-dates basaltic magmatism and has a maximum depositional age of 1779 ± 5 Ma (Neumann et al., 2006). This is no different to the 1779 ± 4 Ma age obtained for the Lena Quartzite (Neumann et al., 2006) and is within error of the 1790 ± 9 Ma age determined for the Bottletree Formation (Page, 1983), indicating that the Eastern Creek Volcanics were not only emplaced over a remarkably short period of time but that this occurred within less than 10-15 Myr of the onset of rifting.

Emplacement of the Eastern Creek Volcanics was accompanied elsewhere in the Mount Isa region by the intrusion of dolerite dykes and eruption of the 1780 Ma Argylla Formation. The Argylla Formation comprises mainly rhyolite and ignimbrite intercalated with subordinate amounts of cross-bedded quartzite and sandstone deposited under fluviatile to shallow marine conditions. Unlike the more widely distributed Eastern Creek Volcanics, the Argylla Formation is only known to occur in the Eastern Fold Belt and lies east of the Kalkadoon-Leichhardt basement block.

1760-1755 Ma Bulonga event

Following a 20 Myr period of little or no volcanic activity, bimodal magmatism resumed in the east with emplacement (Figs. 1 & 2) of the 1760 Ma Bulonga and Marraba Volcanics (Neumann et al., 2009). The former are compositionally and mineralogically indistinguishable from the underlying older Argylla Formation and consist mainly of felsic volcanic rocks whereas the overlying 700m-thick Marraba Volcanics share more similarities with younger parts of the Eastern Creek Volcanics and formed through the eruption of basaltic magma into a subaqueous, highly saline shallow-marine environment (Derrick, 1980). Pillow lavas occur widely throughout the Marraba Volcanics and locally form thin 1-3 m thick layers interstratified with fine-grained sandstone or dolostone containing stromatolites and silica pseudomorphs after gypsum (“cauliflower chert”). No equivalent to the Bulonga and Marraba volcanics has ever been identified farther west in the Leichhardt River Fault Trough or among the rocks farther north in the McArthur Basin. This is in keeping with the idea that by 1760 Ma the locus of crustal thinning and bimodal magmatism had shifted eastwards (Gibson et al., 2012; Glikson et al., 1976).

Wonga Extensional Event (1740-1735 Ma)

Subsequent to 1755 Ma, the depositional environment continued to evolve with fluviatile to lacustrine conditions persisting in the west until after 1750 Ma when the first stromatolite-bearing dolostones began to appear in the Lochness Formation at the very top of the Leichhardt Superbasin (Gibson et al., 2012; Gibson et al., 2008). Coincidently, shallow marine conditions became ever more widespread in the east, leading to deposition of a further 2000m of coarse to fine sandstones (upper Mitakoodi Quartzite), carbonate rocks and black carbonaceous shale (Overhang Jaspillite and lowermost Corella Formation; Fig. 2) in one or more rift-related basins (Neumann et al., 2009; Withnall and Hutton, 2013). Other than the occasional thin layer of basalt or rhyolite, volcanic rocks are largely absent or not preserved, and magmatic activity mainly took place at mid-crustal levels in the Wonga Extensional Belt to the east of the Kalkadoon-Leichhardt basement block (Fig. 1). Variably deformed or mylonitised plutonic rocks of 1740-1735 Ma age make up a significant portion of this belt and include both granites and gabbro.

Fiery Creek event (1725-1710 Ma)

Lithospheric extension resumed after 1730 Ma, leading to a resurgence in rifting and basaltic magmatism whose most obvious expression is eruption of the 1725-1710 Ma Fiery Creek and Peters Creek Volcanics (Fig. 2) into fault-angle depressions and half-graben (Jackson et al., 2000; Page and Sweet, 1998). These rocks mark the onset of crustal thinning associated with formation of the Calvert Superbasin (Fig. 1b) and, like the Eastern Creek Volcanics, may have been erupted in two pulses rather than one continuous event: (i) an older 1725-1720 Ma pulse (Peters Creek Volcanics) that occurs towards the very base of the superbasin and is best developed in the southern McArthur Basin; and (ii) a younger phase erupted mainly around 1710-1705 Ma (Fiery Creek Volcanics) and best exposed on the Lawn Hill Platform from whence it extends southward into the Leichhardt River Fault Trough (Betts et al., 1999; Jackson et al., 2000). Basaltic flows in these two units are commonly vesicular to amygdaloidal and were erupted sub-aerially into a non-marine environment dominated by red-beds, fanglomerates and quartzite. Except for intrusion of the 1711 ± 3 Ma Weberra Granite on the Lawn Hill Platform (Neumann et al., 2006), plutonic rocks of felsic composition are for the most part absent. 

1695-1670 Late Calvert (Sybella) Event

Like the Wonga extensional event before it, magmatism in the Calvert Superbasin from 1695-1670 Ma was bimodal in character and dominated by intrusive rocks emplaced at shallow to mid-crustal depths. They include thin sheets of felsite (“pinkite”) on the Lawn Hill Platform dated at 1694 Ma (Page et al., 2000) and an even greater volume of granite making up the 1675-1670 Ma Sybella Batholith (Neumann et al., 2006) west of the Leichhardt River Fault Trough (Fig. 1). Basaltic rocks make up only a small fraction of the Sybella Batholith and are much more pervasively developed farther east where intrusion occurred synchronously with turbidite deposition (Kuridala and Soldiers Cap groups; Fig. 2) in a deep marine basin developed off the continental shelf (Gibson et al., 2012; Southgate et al., 2013; Withnall and Hutton, 2013).

Figure 3. (a) Classification of north Australian mafic rocks based on plot of alkali metals K2O+Na2O against SiO2 (TAS). Note greater amount of hydrothermal alteration in the Fiery Creek and Peters Creek suite so that samples from these two units show greater spread and often plot well within the alkaline field.

Figure 4. Selected major elements for Leichhart and Calvert basin basalts plotted against wt% MgO. Fiery Creek and Peters Creek samples again show considerable spread towards abnormally high K2O (a) but less mobile elements are thought here to have retained their original distribution and values. Despite some compositional overlap, note how the bulk of samples fall naturally into high- and low-Ti suites and that this subdivision is evident at both the regional and local level as exemplified by the Eastern Creek Volcanics and its two constituent volcanic units (Cromwell and Pickwick metabasalt members). Symbols as for figure 3.

Turbidites of this same age have also been identified in the Georgetown region to the east (Fig. 2) where they have been intruded by 1675 Ma dolerite (now amphibolite) and two deformed granites for which magmatic ages of 1695.8 ± 1.5 Ma and 1684.2 ± 2.1 Ma have been reported (Black et al., 1998).

1660-1655 Ma Toole Creek event

Significant volumes of basalt and dolerite hosted by sediments of deep marine origin, including turbidites, dolomitic siltstones and carbonaceous shale, occur towards the top of the Soldiers Cap Group where they make up the 1658-1654 Ma Toole Creek Volcanics (Figs. 1 & 2). They and their sedimentary host rocks have long been identified with the transition from rift to drift and in this respect have more in common with basaltic rocks in the Georgetown region (Baker et al., 2010; Gibson et al., 2012; Gibson et al., 2017; Gibson et al., 2008; Glikson et al., 1976; Withnall and Hutton, 2013). The latter comprise the 1000 m-thick Dead Horse Metabasalt and co-magmatic Cobbold Metadolerite and appear to have been emplaced around the same time (Withnall, 1985). Moreover, despite having been metamorphosed up to the amphibolite facies,the former still retains many of its original volcanic features and consists predominantly of non-vesicular aphanitic pillow lava and hyaloclastite (Withnall, 1985). The Dead Horse Metabasalthas also provided a 1663 ± 14 Ma age that not only serves to constrain the age of basaltic eruption but many of the dolerite intrusions thought to have supplied magma to this basaltic unit (Baker et al., 2010; Withnall and Hutton, 2013). Other dolerites in the Cobbold Metadolerite were intruded later and cut both the Dead Horse Metabasalt and all overlying sedimentary units up to but not including the immediately overlying ≤1650 Ma Townley and Helliman formations (Neumann and Kositcin, 2011).

Geochemistry of basaltic magmatism in northern Australian

Geochemical data for all but two of the north Australian magmatic events are plotted in figures 3-7, and compared to the composition of 180-170 Ma basalts from the younger Karoo Basin in figure 5. The Wonga and Sybella magmatic events (Fig. 1) produced mainly intrusions of granitic composition and are omitted from the plots. The 1780 Ma Eastern Creek Volcanics are particularly well represented in the dataset but, as one of the most voluminous and aerially extensive of all the basaltic units, compositions are far from uniform. For this reason, the Cromwell and Pickwick Metabasalt members are plotted separately and no attempt has been made to merge analyses from these two units with other basalts from the Eastern Creek Volcanics for which the stratigraphic affinities are unknown. A full list of samples and their compositions can be obtained from the authors.

Figure 5. (a) NMORB- and (b) chondrite-normalised trace and REE plots for north Australian basalts. Enrichment in Th and LREE relative to HREE is common to all basaltic suites (a) and shows clear trend towards progressively lesser amounts of enrichment with decreasing age as in the two younger Georgetown units (Dead Horse Metabasalt and Cobbold Metadolerite). These two units also have flatter REE patterns (b) and more subdued negative anomalies in incompatible elements like Nb, Pb, and to a lesser extent Ti which make for more jagged compositional profiles (a). Compositional average for Karoo basalts is added for comparison. Colours as for figures 3 & 4.

Major and minor elemental plots

As previously recognised (Glikson et al., 1976; Gregory et al., 2008a; Scott et al., 2000; Wilson, 1987; Wyborn et al., 1987), the Eastern Creek Volcanics and most other basaltic rock units in the Mount Isa and Georgetown regions share many similarities with the Karoo and exhibit mainly sub-alkaline tholeiitic compositions. Major exceptions include the Peters Creek and Fiery Creek

Volcanics for which alkaline compositions which have been reported (e.g., Wyborn et al., 1987). The biggest difference between the Mount Isa and Karoo basaltic magmatism is in the degree of scatter within the Mount Isa data (Fig. 3), largely as a result of alteration. This alteration has led to both reduced and increased SiO2, and elevated K2O values, particularly at higher stratigraphic levels in the alkaline Fiery Creek and Peters Creek Volcanics where K2O contents can exceed 8-10 wt% (Figs. 3 & 4). The elevated K2O is accompanied by greatly reduced Na2O (<0.5%) and CaO (<1%). Samples of basalt from these two units consequently often plot well within the alkaline field and are compositionally trachybasalts or basaltic trachyandesites (Fig. 3). This contrasts with the less strongly altered Eastern Creek Volcanics, Toole Creek Volcanics and Georgetown rocks (Dead Horse Basalt and Cobbold Metadolerite), which range from basalt to basaltic andesite and are mainly low-K tholeiites (Fig. 3). Compared with the Karoo, altered rocks in the Fiery Creek and Peters Creek volcanics extend to abnormally low MgO (≤ 3 wt%). MgO contents for basalts from the Mount Isa and Georgetown regions, on the other hand, more typically range from 4-8 wt% and only occasionally extend upward of 10% (Fig. 4).

In addition, for any single value of MgO, there is a consistent separation of basaltic compositions into two groupings based on other commonly used fractionation indices (e.g. total Fe’; Fig. 4b) or incompatible elements: one with significantly greater amounts of TiO2 (≥ 2.0 wt%) whereas the other has lower TiO2 (1-2 wt%) and reduced concentrations of the other incompatible elements (Gibson et al., 2018; Fig. 4). This grouping into high- and low-Ti basalts is particularly evident in the Eastern Creek Volcanics and broadly corresponds to the existing two-fold division into Cromwell and Pickwick Metabasalt Members (Fig. 4a).

Figure 6. Compositional data for north Australian basalts for which REE data (ICPMS) are available plotted on standard discriminatory diagrams for tectonic setting after (a) Meschede (1986) and (b) Wood (1980). Strong linear trends in composition away from within plate and continental arc basalts towards MORB are evident in both plots. WPT = within-plate type basalt; WPAB = within-plate arc basalt; VAB = volcanic arc basalt; EMORB and NMORB = enriched and normal mid-ocean ridge basalt. Symbols as for figure 3.

Trace and REE elemental plots

NMORB-normalised multi-element plots for all basaltic units in the Leichhardt and Calvert superbasins for which the full complement of trace and REE data is available are presented in Figure 5. No attempt was made to discriminate between hydrothermally altered and unaltered rocks and compositional data from both groups are included. Previously observed compositional similarities with the Karoo (Scott et al., 2000) are no less striking (Fig. 5a) especially among the LILE, HFSE and LREE, but now extend to all basaltic units in the Leichhardt and Calvert superbasins, including those in the Georgetown region to the east (Fig. 5a). Moreover, even though the degree of enrichment is spread over a considerable range, the least enriched basalts are unmistakably concentrated towards the younger end of the age spectrum and include the Toole Creek Volcanics as well as the Georgetown rocks (e.g. Th/YN values mostly in the range 1-6 compared to 5-20 for other units). Conversely, levels of enrichment and depletion are greatest for the Fiery and Peters Creek volcanics whose Pb, Sr, Rb, K and Ba concentrations (Fig. 5a) are most likely to have been affected by secondary alteration as previously noted. Removing the more mobile elements (Fig. 5b), shows broadly similar average patterns in  HFSE and REE for all units, including the overall general enrichment and equally conspicuous negative anomalies in Nb, Ta, P and Ti (Fig. 5b). This same elemental pattern is shared by all basaltic units irrespective of whether they have been hydrothermally altered or not (e.g. Fiery Creek vs Toole Creek Volcanics). It therefore seems likely that most, if not all, of this enrichment in Th and LREE and corresponding depletion in HFSE is a primary feature and not the result of later metasomatism or metamorphism at 1640 Ma.

Magma source and tectonic setting for north Australian basaltic rocks

In Figures 6a and 6b, standard discriminatory diagrams (Meschede, 1986; Wood, 1980) have been employed to classify basalts from the Leichhardt and Calvert superbasin according to their possible tectonic settings. Unsurprisingly, for rocks that have been likened to the Karoo and other flood basalts emplaced during continental rifting, the majority of analyses plot together and have the compositional characteristics of volcanic arc or within-plate basalts (Fig. 6). A few samples from the 1780 Ma Seigal Volcanics (Fig. 1) have compositions more akin to within-plate alkaline basalts but otherwise the other most notable feature in the data is the linear nature of the plot, and concomitant spread towards more MORB-like compositions in the younger units, and more particularly within the Toole Creek Volcanics and Georgetown rocks (Figs. 6a & 6b). This is the same age-related trend observed in the HFSE and REE patterns (Fig. 5) and largely corresponds to decreasing levels of LILE, LREE and HFSE enrichment in these rocks relative to their older counterparts in the Eastern Creek and Fiery Creek/Peters Creek volcanics. The tholeiitic nature of the rocks, coupled with the Th (and LILE and LREE) enrichment and relative depletion in other incompatible elements like Nb, Ta and Ti (Fig. 6a), negates any possibility that these basaltic rocks are directly arc-related but is permissive of alternative interpretations (e.g. Hergt et al., 1991) in which the parental magmas were generated in a backarc environment through melting and/or incorporation of subduction-modified mantle lithosphere, along with the assimilation of continental crust and more particularly sedimentary rocks (Plank and Langmuir, 1998). In the case of the southern Karoo and other continental flood basalts (Ferrar, North Atlantic), derivation from an enriched or metasomatically refertilised lithospheric mantle source is thought to have been the norm rather than the exception (Cox, 1992; Elliot, 1992; Fitton et al., 1998; Heinonen et al., 2014; Hergt et al., 1991; Luttinen and Furnes, 2000; Storey et al., 1992).

Scott et al (2000) drew the same conclusion for the basalts of northern Australia and alluded to the possibility of a calc-alkaline or subduction-related signature in these rocks, citing HFSE and REE patterns, including Nb-Ta anomalies (cf Figs. 5a), that bore a strong resemblance to those reported from basalts formed in continental or island arc settings. This signature is no less evident in the expanded dataset used here and is made even more explicit in a plot of Th/Yb against Nb/Yb (Fig. 7). This plot serves as a proxy for the amount of crustal input which is higher for basaltic rocks erupted at continental margins or in subduction zones compared to those in oceanic environments (Pearce, 2008). Basalts from the Leichhardt and Calvert superbasins plot almost exclusively above the MORB-OIB array in the magmatic arc field and map out a steep and broadly linear trend consistent with mixing between a crustal (or sedimentary) and enriched mantle source (Fig. 7). Importantly, the crustal contribution is highest for samples of 1780 Ma or older age. With decreasing age, basaltic compositions are displaced towards the MORB-OIB array and in a few instances overlap or lie within the array. This trend towards reduced crustal input is particularly evident in basalts of younger 1666-1655 Ma age from the Toole Creek Volcanics and Georgetown units (Dead Horse Metabasalt and Cobbold Metadolerite) whose oceanic affinities have long been recognised and used as evidence for continental breakup around this time (Baker et al., 2010; Gibson et al., 2012; Gibson et al., 2008). As is already clear from some of the other compositional plots (Figs. 8a & 8b), basalts from these three units are compositionally the most MORB-like and would appear to have been sourced directly from the asthenosphere rather than the subcontinental lithospheric mantle (or via interaction with continental crust). An enriched mantle source region is still evidently required but by the time these younger basalts were being erupted, the lithospheric mantle had become vanishingly thin or been removed altogether.

Figure 7. Th-Nb plot as proxy (Pearce, 2008) for the determination of crustal input in compositional data from north Australian basaltic rocks Modern day basalts from oceanic environments all plot within the MORB-OIB array whereas basalts containing a large component of recycled crustal material are displaced above the array and lie within the volcanic arc array (the so-called calc-alkaline or subduction signal). The Georgetown rocks have the least amount of crustal contamination and plot within or close to the MORB-OIB array than the older basaltic units consistent with eruption through thinner lithosphere at or close to the point of continental breakup. Symbols and colours as for figure 3.


A subduction-related back-arc setting for north Australian basaltic magmatism

Changes in basalt geochemistry contingent on lithospheric thinning and upwelling of the underlying asthenosphere have been reported from many of the world’s rifted continental margins and would appear to be no different in the example described here from Paleoproterozoic northern Australia. As elsewhere, magmatism was initially dominated by the eruption of continental flood basalts but, after some tens of millions of years, basaltic melts became more MORB-like and were increasingly sourced from the asthenosphere. In the case of northern Australia, this compositional change occurred between 1675 Ma and 1655 Ma by which time the majority of basaltic rocks were being erupted into a deep water environment dominated by turbidites whose distribution extended as far east as the Georgetown area. Total accumulated sedimentary thickness across the region then exceeded 10-12 km and was followed by deposition of a further 1-2 km of black shales and dolomitic siltstones subsequent to the conclusion of basaltic magmatism. The magmatic and sedimentary records would thus appear to be in complete accord and provide complementary evidence for a tectonic environment that had evolved beyond intracontinental rifting and reached the point of continental breakup and passive margin formation by 1655 Ma. Moreover, as previously recognised by Scott et al (2000) and reaffirmed in this paper, most of the older basaltic units, including the more voluminous Eastern Creek Volcanics, share a great many more similarities with the Karoo, not least of which is a high probability that the parental melts were derived from a mantle that had been enriched in crustal components during a subduction-related event. Given their similar REE and trace element patterns (Figs. 5-7), it may be further surmised that the north Australian basalts were all derived from the same subduction-modified lithospheric source albeit one whose contribution either diminished over time or was subjected to lesser amount of crustal contamination, or both. Moreover, this trend towards reduced amounts of crustal input is in the same west to east direction as the lithosphere must have thinned to accommodate an ever increasing thickness of deep water sediments and eventual passive margin formation. Only the younger Georgetown rocks and their temporal equivalents in the Toole Creek Volcanics have compositions consistent with melting of a depleted upper mantle source (MORB).

Figure 8. Basin evolution in northern Australia from 1800-1650 Ma. (a) Onset of extensional faulting and rift-related magmatism (Bottletree Formation) at 1790 Ma in a backarc extensional setting. Subduction commenced ≥ 1840 Ma following separation of Laurentia from Australia; (b) Subduction rollback and lithospheric thinning are well advanced by the end of Leichhardt time (1740 Ma); flat-slab subduction leads to reduced amounts of basaltic magmatism but rifting continues accompanied by extensional unroofing of earlier formed 1780 Ma granites and mid-crustal rocks. Higher heat flow promotes crustal thinning, doming and low pressure-high temperature metamorphism; (c) Following rupture of the lithosphere at 1655 Ma, basaltic magmatism is driven by decompressional melting of the asthenosphere. Passive margin conditions established in the Mount Isa and Georgetown regions while subduction and arc magmatism continue to east; (d) Arc-continent collision commencing at ca. 1650 Ma with arc and its underlying continental substrate (Bonnetia) obducted onto the Laurentian margin; backarc basin begins to collapse as Laurentian crust continues to enter the subduction channel.

Both the Fiery and Eastern Creek volcanics are similarly enriched in LILE, LREE and HFSE but otherwise share depletions in Nb, Ta and Ti relative to the LILE and LREE (e.g. La/Nb). These geochemical features are more typical of basalts erupted in subduction-related (Kelemen et al., 1993; Sun and McDonough, 1989) or back-arc basin settings (Arculus, 1987; Hirahara et al., 2015). Furthermore, even though the peaks and troughs in elements like Sr and Pb may have been exacerbated in the Fiery and Eastern Creek Volcanics by later alteration or direct assimilation of upper crustal material (Gregory et al., 2008a), this is unlikely to have affected the overall trace element patterns. Comparably large negative anomalies in these same elements have been documented for the Karoo (Fig. 5a) and its correlatives in the Ferrar LIP and widely attributed to large-scale reworking of subduction-modified subcontinental lithospheric mantle

(Cox, 1980; Duncan, 1987; Elliot, 1992; Heinonen et al., 2014; Hergt et al., 1991; Luttinen and Furnes, 2000; Storey et al., 1992). It is therefore not unreasonable to suggest that some similar metasomatic or subduction-related process may have led to enrichment of the subcontinental lithospheric mantle beneath the Mount Isa-Georgetown region. However, unlike the Karoo LIP which was emplaced over a short time frame and some tens of millions of years before continental breakup took place, this was evidently not the case in Proterozoic northern Australia where magmatism was far more protracted and occurred in several pulses over a 140 Myr period (Gibson et al., 2018). Not one but several LIPs may be present in this region and only the youngest of these was emplaced at the point of continental rupture, providing a cautionary tale in the use of LIPs as a marker of continental breakup.

Gibson et al (2018) attributed compositional trends in these basalt rocks to back-arc extension and the splitting off a magmatic arc that had been developing along the eastern margin of Australia since ca. 1800 Ma (Fig. 8). These events culminated in formation of a marginal sea or western Pacific-style back-arc extensional basin that separated the arc from the continental margin and its newly formed passive margin sequences. Accordingly, Australia and Laurentia cannot have been juxtaposed during this period because at least one ocean basin or back-arc basin and its volcanic arc associated intervened. Following on-going subduction, this backarc basin closed and the arc was re-accreted to the Australian margin (Fig. 8). This was followed by the Isan-Racklan Orogeny and collision of Australia with Laurentia, during the course of which the arc (Bonnetia) was obducted onto the opposing Laurentian margin (Fig. 8) where remnants now reside in the ≤ 1640 Ma Wernecke Breccia (Furlanetto et al., 2013; Furlanetto et al., 2016; Thorkelson and Laughton, 2016). A similar history of late Paleoproterozoic orogenesis and terrane accretion subsequent to backarc extension and arc magmatism has been reported from the 1780-1650 Ma Mojave, Yavapai and Mazatzal provinces in southern Laurentia (Hill and Bickford, 2001; Whitmeyer and Karlstrom, 2007), inviting speculation (Gibson et al., 2018) that one or more of these three provinces, like Bonnetia, originated elsewhere and once formed part of the opposing Australian-Antarctic continental margin before being accreted to Laurentia. As in northern Australia, magmatism in these three Laurentian provinces terminated before 1650 Ma by which time all three were engaged in a collisional/accretionary event that brought backarc extension and basin formation in southern Laurentia to a close (Mazatzal Orogeny; Whitmeyer and Karlstrom, 2007). Reconstructions of the late Paleoproterozoic Nuna supercontinent that juxtapose eastern Australia against western Laurentia commonly make use of such striking similar geological histories but importantly this proposed match is not based on any single geological event let alone any single LIP or magmatic event. Northern Australia encompasses several such events but, as already stated, all but the youngest 1655 Ma event predate continental breakup and provide no real constraint on either the age or timing of breakup relative to basin formation. Significantly, compared to the other magmatic events, and the Eastern Creek Volcanics in particular, this 1655 Ma event is neither well exposed nor especially voluminous to even warrant the term large igneous province (Table 1), pointing to the need for caution when drawing conclusions about the tectonic importance of any one magmatic event based on size and volume alone. In northern Australia, the smallest and volumetrically least conspicuous magmatic event best constrains the locus and timing of continental breakup but then only in a back-arc setting, further suggesting that LIPs be used judiciously in reconstructions of the Nuna and Rodinia supercontinents.


GMG and IWW thank Geoscience Australia and the Geological Survey of Queensland for their support of this research. DCC and NLN both publish with permission of the CEO Geoscience Australia; LJH publishes with permission of the Director, Geological Survey of Queensland.

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