1600–1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions

doi: 10.1111/j.1365-3121.2007.00778.x

1600–1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions Peter G. Betts,1 David Giles,2 Bruce F. Schaefer1 and Geordie Mark1 1

School of Geosciences, Monash University, Clayton, Vic. 3800, Australia; 2Department of Geology and Geophysics, University of Adelaide, Adelaide, SA, Australia

ABSTRACT Mesoproterozoic A-type magmatic rocks in the Gawler Craton, Curnamona Province and eastern Mount Isa Inlier, form a palaeocurvilinear belt for reconstructed plate orientations. The oldest igneous rocks in the Gawler Craton are the Hiltaba Granite Suite: c. 1600–1575 Ma. The youngest in the Mount Isa Inlier are the Williams-Naraku Batholiths: c. 1545–1500 Ma. The belt is interpreted as a segment of a hotspot track that evolved between c. 1600 and 1500 Ma. This hotspot track may define a quasilinear part of Australia!s motion between 1636 and 1500 Ma, and

Introduction There are 48 recognized oceanic and continental hotspots on the modern Earth (Seidler et al., 1999). Hotspots, and the mantle plumes that are thought to cause them, are considered to be an intrinsic feature of the Earth and have persisted throughout its history. Hotspot tracks in the ancient geologic record provide important constraints on geodynamic models and palaeogeographical reconstructions because they should have a spatial, temporal and geochemical pattern with unambiguous pinning points between continents. Unlike plume heads which have a relatively characteristic geologic signal, there have been relatively few hotspot tracks recognized in the Precambrian record. This dearth is attributed to reworking ancient lithosphere, reorganization of Precambrian plates through geologic time, and erosion of the surface expression of hotspots. We present evidence for a segment of a hotspot track defined by the distribution of A-type magmatism between c. 1600 and 1500 Ma in the eastern Proterozoic provinces of Australia. This period defines a major Correspondence: Peter G. Betts, School of Geosciences, Monash University, Clayton, Vic. 3800, Australia. Tel.: +613 99054150; fax: +613 99054903; e-mail: peter.betts@ sci.monash.edu.au 496

suggests that Australia drifted to high latitudes. An implication of this interpretation is that Australia and Laurentia may not have been fellow travellers leading to the formation of Rodinia. A hotspot model for A-type magmatism in Australia differs from geodynamic models for this style of magmatism on other continents. This suggests that multiple geologic processes may be responsible for the genesis of Proterozoic A-type magmas.

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decoupling in the geologic record of Australia and Laurentia in the supercontinent Columbia (Zhao et al., 2002). Leading up to this time, palaeomagnetic data allow for an Australia–Laurentia connection in a modified-SWEAT position at c. 1750 Ma (Betts et al., in press). Published palaeomagnetic data from the Albany-Fraser metamorphics and the Fraser Dykes preclude a connection between Australia and Laurention at c. 1200 Ma (Pisarevsky et al., 2003), although Wingate et al., 2002 speculated that Australia and Laurentia may have been connected in the AUSMEX configuration. The dearth of primary palaeomagnetic poles in Australia precludes assessment of Australia!s drift between c. 1590 and 1200 Ma. However, if our proposed hotspot track is feasible, Australia appears to have migrated to higher latitudes between c. 1600 and 1500 Ma.

Palaeogeographic context Two palaeogeographic reconstructions have been proposed for Australia between c. 1600 and 1500 Ma (Betts and Giles, 2006; Wade et al., 2006). The reconstruction that is consistent with both geologic and palaeomagnetic data is that of Giles et al. (2004) (Wingate and Evans, 2003). This reconstruction requires the Gawler Craton and the Curnamona Province (Fig. 1b) to be rotated counterclockwise !52! such that the

northern Curnamona Province is juxtaposed against the Mount Isa Inlier (Fig. 1b). We apply this reconstruction and address a spatial and temporal pattern of magmatism with the hallmarks of a hotspot trail that arises from it. Although the exact mechanism of accretion may have differed, Australia and Laurentia both appear to have undergone protracted crustal growth along their southern margin between c. 1800 and 1600 Ma (Karlstrom et al., 2001; Betts and Giles, 2006; Duebendorfer et al., 2006). In the continental interior of Australia and Laurentia back-arc basins evolved (e.g. Giles et al., 2002; Rainbird et al., 2003). Accretion was interrupted in Australia by an era of widespread A-type magmatism and orogenesis throughout eastern Proterozoic Australia (Fig. 2).

Spatial–temporal relationships of A-type granites in eastern Australia Rocks of three spatially and temporally distinct Mesoproterozoic (1600– 1500 Ma) A-type magmatic provinces are exposed in the Gawler Craton, Curnamona Province and the Eastern Fold Belt in the Mount Isa Inlier (Fig. 1a). A-type magmatism initiated in the Gawler Craton at c. 1600 Ma after a period of arc-related magmatism on the south-western edge of the craton, and widespread orogenesis in the Curnamona Province (Betts and " 2007 Blackwell Publishing Ltd

P.G. Betts et al. • Australian Mesoproterozoic A-type hotspot track

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............................................................................................................................................................. (a)

(b)

15°

MAIC (~1520 Ma)

Naraku Granite (~1500 Ma)

MountI sa Inlier

20°

Eastern Fold Belt Petermorra Volcanics (~1565 Ma)

25°

Benagerie Volcanics (~1580 Ma)

Mount Neill Granite (~1575 Ma)

Mount Painter Inlier Curnamona Province

30°

Gawler Range Volcanics (~1595 Ma)

Hiltaba Suite Granites (~1590–1575 Ma)

35°

Gawler Craton 135°

MAIC:Mt Angeley Igneous Complex

Pluton rock Volcanic rock

140°

500 km

Mount Painter Inlier

Curnamona

Williams/ Naraku

GRV: Gawler Range Volcanics BV: Benagerie Volcanics PV: Petermorra Volcanics

Mount Isa Inlier Isan Orogeny II

(?)

Hil

Olarian Orogeny High-T metamorphic event Bimodal A-type granite pluton

1600

Isan Orogeny I

GRV

1580

1560

1540

1520

Gawler Craton

1500

Fig. 1 (a) Distribution of c. 1600–1500 Ma A-type magmas and geological provinces superimposed on total magnetic intensity image of eastern Precambrian Australia. (b) Distribution of c. 1600–1500 Ma A-type magmas in the reconstruction space of Giles et al. (2004).

Time (Ma)

Sedimentation event Felsic Volcanic Bimodal Volcanic Orogenic event

Fig. 2 Time–space plot of geologic events for eastern Australia between c. 1600 and 1500 Ma.

Giles, 2006). In the Gawler Craton the Hiltaba Suite granites and their eruptive equivalents, the Gawler " 2007 Blackwell Publishing Ltd

Range Volcanics, were emplaced over an area of !320 000 km2 between c. 1595 and 1575 Ma (Fig. 1a). The

Gawler Range Volcanics record the eruption of more than 200 000 km3 of dominantly felsic lavas during a c. 2 Myr interval at c. 1592 Ma (Fanning et al., 1988; Creaser and White, 1991). Correspondingly, A-type magmatism is restricted to the central and northern parts of the Curnamona Province following the Olarian Orogeny (c. 1600–1592 Ma: Page et al., 2005). The c. 1580 Ma Benagerie Volcanics (Robertson et al., 1998) were erupted in the central and northern Curnamona Province and form an extensive volcanic province (!5200 km2) of unknown thickness. Volumetrically smaller A-type granites and felsic volcanics were emplaced throughout the Mount Painter Inlier at c. 1575 Ma and at c. 1555 Ma (Elburg et al., 2001) (Fig. 1a). In the Mount Isa Inlier, numerous A-type granite batholiths were emplaced after the Isan Orogeny (Mark, 1999). These A-type batholiths form a north–south trending belt (!80 km wide) in the Eastern Fold Belt (Fig. 1a). Individual batholiths are up to 30 km wide and 60 km long and were emplaced into the middle crust in discrete events at c. 1540–1520 Ma, c. 1520–1510 Ma and c. 1510–1500 Ma (Page and Sun, 1998; Wyborn, 1998) (Fig. 2). The overall age distribution of A-type magmatism of eastern Australia shows a general decreasing age to the north (Fig. 1b). The Gawler Craton and the Curnamona Province preserve an upper crustal and surficial record of magmatism; the eastern Mount Isa Inlier preserves a midcrustal expression. The width of the magmatic province decreases from !500 km in the Gawler Craton to 80 km in the Eastern Fold Belt (Fig. 1). Elsewhere in Australia at this time, felsic magmatism was restricted to I-type granites with magmatic arc affinities, consistent with a broadly east–west trending convergent margin (Wade et al., 2006), and volumetrically minor S-type granites in the Curnamona Province.

A-type magma geochemistry The Australian Proterozoic A-type magmatic province is typified not only by enriched REE and Ga contents, which are common among A-type clan, but also by enrichment in 497

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............................................................................................................................................................. heat-producing elements (U, Th and K). Additionally, each magmatic system considered here is bimodal in composition, with small but volumetrically important mafic co-magmatic rocks. Isotopically both the felsic and mafic parts of this system in the Gawler Craton trend towards juvenile isotopic compositions with uncontaminated end-members typically preserving eNd(i) values of +2 to +3 (Stewart and Foden, 2001) strongly indicative of a mantle component, whereas mafic parts in the Mount Isa Inlier have eNd(i) values between )0.1 and )2.5 (Mark, 1999). Magmatic temperatures are also generally high; pigeonite inversion textures are preserved in rhyolites of the Upper Gawler Range Volcanics corresponding to unusually high eruption temperatures (950–1150 !C: Creaser and White, 1991) compared with typical eruptive temperatures in the range of !700–900 !C (Cas and Wright, 1988). Elevated heat fluxes are also associated with the Mount Painter A-type granites as indicated by average zircon saturation temperatures that range between 800 and 900 !C (Stewart and Foden, 2001). Apatite and zircon saturation models using P2O5 and Zr concentrations within the Eastern Fold Belt suggest the initial temperature of felsic and mafic melts were >900 and >960 !C, respectively (Mark, 1999). The combination of high magmatic temperatures for A-type magmas and evidence for a mantle component in the least contaminated parts of the magmatic systems are strongly suggestive of an active mantle influence for magmatism. In each of the systems this may vary from simply providing heat to drive melting of lower crust or depleted sub-crustal lithospheric mantle, through to actively supplying a modest amount of mantle-derived material into the melts. A mantle plume is capable of providing both heat and material.

Hotspot model of A-type magmatism Today, hotspot magmatism is characterized by early, voluminous volcanism that subsequently contracts into relatively focused narrow belts (Campbell and Griffiths, 1990), explained by a transition from domi498

nantly plume head to plume tail magmatism. Modern analogues display a preponderance of mafic magmatism, although the Yellowstone and the Parana-Etendeka hotspot tracks have significant parts of felsic A-type magmas (Kirstein et al., 2000, 2001; Perkins and Nash, 2002; Trumbull et al., 2004). In the context of Late Palaeoproterzoic and Mesoproterozoic A-type magmatism, many models for their formation have been proposed, including plume generated or extensional settings (Frost and Frost, 1997), synorogenic settings (Nyman et al., 1994) and late-orogenic settings, in which Atype magmatism formed temporally discrete belts parallel with the plate margin (Aha¨ll et al., 2000). A-type magmatism along the eastern Australian Proterozoic provinces involved elevated temperatures of emplacement and anhydrous melting, magma genesis involved a mantle component and emplacement occurred after a major period of crustal thickening and orogenesis, similar to the Laurentia and Baltica examples of this type of magmatism. However, unlike many Laurentia and Baltica examples, A-type magmatism was not superimposed on juvenile accretion terranes, rather upon continental crust with Archaean to early Palaeoproterozoic Nd model ages (Wyborn, 1998). The orientation of the eastern Australia belt is approximately orthogonal to inferred plate margin at c. 1600 Ma (Betts and Giles, 2006), and the age of magmatism decreases away from the plate margin, whereas A-type magmatic belts in Laurentia and Baltica are characterized by discretely aged belts that evolved parallel with a plate margin (Aha¨ll et al., 2000). We propose that A-type magmas along the eastern margin of the Australian continent were generated in response to the interaction of a mantle plume and continental lithosphere and that distribution records a hotspot track generated between c. 1600 and 1500 Ma. This interpretation is supported by the geometry of the belt and the northward age progression of magmatism. Geochemically, the plume model is supported by the elevated heat flux throughout the magmatic belt with a corresponding mantle component (Johnson and McCulloch, 1995; Mark, 1999).

The plume head is interpreted to have interacted with the Gawler Craton continental lithosphere, where the hotspot track is thickest, whereas the narrower magmatic belt in the Mount Isa Inlier is more consistent with a plume tail interacting with continental lithosphere. It is possible that the hotspot initiated in Terra Adelie Land (East Antarctica), where c. 1600 Ma granite clasts, with similar chemistry to granites in the Gawler Craton, have been collected in moraine deposits sourced from Antarctica (Peucat et al., 2002). The preserved hotspot track in Australia is !1500 km in length and evolved over 100 Myr, which suggests Australia was travelling at a modest 1.5 cm year)1. The absence of geologic features typically expressed in modern hotspots such as radial dykes and large mafic contributions has been used to reject hotspot models for the generation of Mesoproterozoic A-type magmas. In Australia, A-type magmatism occurred after major episodes of orogenesis and crustal thickening. We argue that the thickened crust may have restricted (or inhibited) adiabatic melting of the plume and that magma was generated dominantly via melting of a thickened lower crust due to heat conduction, with only a relatively minor mantle component.

Implications for palaeogeographic reconstructions Australia and Laurentia have been interpreted to have been travelled together for c. 950 Myr between c. 1800 and 850 Ma (Hoffman, 1991). Many reconstructions placing Australia adjacent to Laurentia have been proposed (Moores, 1991; Karlstrom et al., 2001; Wingate et al., 2002). Critical verification of these reconstructions suffers from the lack of palaeomagnetic data for Laurentia between c. 1595 and 1500 Ma and lack of primary poles from Australia during the same interval (Tanaka and Idnurm, 1994). The position of Australia in the preceding 40 Myr is defined by palaeomagnetic data from sedimentary units from the McArthur Basin in the North Australian Craton (Idnurm, 2000). The poles used to define this trajectory of Australia!s Apparent Polar Wander Path yield one polarity " 2007 Blackwell Publishing Ltd

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............................................................................................................................................................. of magnetization, and have not been subjected to field tests (e.g. conglomerate tests, reversals test) to assess for primary magnetization and thus may not be reliable primary palaeomagnetic poles (J. Meert, personal communication). However, if we consider these poles to define Australia!s Apparent Polar Wander Path between c. 1636 and 1595 Ma such as published by Idnurm (2000) then Australia!s trajectory is defined by a quasilinear track approximated as a small circle with an Euler pole of 5.2N, 92.8E, with a rotation angle of 23.7! (Fig. 3a).

(a)

When the spatial distributions of A-type magmatism is superimposed on the Giles et al. (2004) reconstruction of Australia at c. 1595 Ma, these rocks also occur on the same small circle that defines the c. 1636– 1595 Ma palaeomagnetic track (Fig. 3b). This represents a remarkable coincidence if the McArthur Basin poles are not primary. One implication of this result is that Australia continued to drift along approximately the same trajectory defined by the McArthur Basin poles between c. 1595 and 1500 Ma, above a

hotspot that generated A-type magmas. During this interval Australia drifted from mid to high latitudes along a unidirectional path that had begun c. 40 Myr prior to plume magmatism. A continuous quasilinear unidirectional continental drift for c. 140 Myr is not recorded in the Phanerozoic (McElhinny and McFadden, 2000), although there appears to be a minor deviation in the hotspot track between the Curnamona Province and the Mount Isa Inlier (Fig. 1b), which suggests there was a minor restructuring of the Australian

(b)

Lynott Formation pole (~1636 Ma)

Euler pole 5.2N, 92.8E

?

Upper Balbirini Dolomite pole ~1589 Ma

rotation 23.7°

Upper Balbirini Dolomite pole ~1589 Ma Lynott Fm. pole ~1636 Ma

Hotspot ca 1595 Ma (c)

Australia ~1500 Ma

Australia ca 1595 Ma

Hotspot over Mount Isa ~1500 Ma

Australia ~1636 Ma

Australia ~1595 Ma

(d) Australia ~1500 Ma

Australia ca 1636 Ma Laurentia ~1475 Ma

Fig. 3 (a) Trajectory of Australia between c. 1636 Ma and c. 1589 Ma based on palaeomagnetic poles from the McArthur Basin, North Australia Craton (Idnurm, 2000) leading up to the onset of the arrival of the mantle plume beneath the Gawler Craton. These data suggest a 23.7! rotation about an Euler pole located at 5.2!N, 92.8!E. (b) Quadrant of an equal area stereographic projection showing the motion of Australia between c. 1636 Ma and c. 1589 Ma (dashed small circle). The projection is centred on the Euler pole of rotation for Australia leading up to the arrival of the mantle plume. The distribution of A-type magmas also occurs on the small circle. (c) Motion of Australia between c. 1636 Ma and c. 1500 Ma using a combination of palaeomagnetic data and the location of inferred hotspot track. (d) Location of Australia at c. 1500 Ma based on our interpretation and the position of Laurentia at c. 1475 Ma based on Meert and Stuckey (2002). " 2007 Blackwell Publishing Ltd

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............................................................................................................................................................. plate, possibly related to the switch in the shortening direction associated with the Isan Orogeny (Betts et al., 2006). The migration of Australia into high latitudes between c. 1640 and 1500 Ma has clear and important palaeogeographical implications. Although, the position of Laurentia is unconstrained by palaeomagnetic data during this period there is a well-defined pole from the c. 1475 Ma rhyolite dominated St Francois terrane, which suggest Laurentia was at low latitudes (Meert and Stuckey, 2002) (Fig. 3c,d). If Australia and Laurentia remained fellow travellers during the Mesoproterozoic it would require a rapid migration of Australia and Laurentia from moderate to high latitudes to low latitudes in the interval between c. 1500 Ma and c. 1475 Ma. Our favoured interpretation is that Australia and Laurentia separated and began different journeys, possibly as early as the beginning of the Mesoproterozoic, which is supported in the geological record. An initial separation of Australia and Laurentia has been proposed at c. 1650 Ma based on a digression in the interior basin evolutions, although they may have re-amalgamated at c. 1540 Ma, possibly in a different configuration (Betts et al., 2002; Betts and Giles, 2006). The final separation of the two continents is likely to have occurred after the c. 1540–1500 Ma orogenesis in the Mount Isa Inlier, when Australia underwent widespread continental extension and erosion to supply detritus into the c. 1475– 1450 Ma Belt Supergroup (Ross et al., 1992). Australia!s trajectory between c. 1600 and 1500 Ma may define part of its path during the transition from a modified-SWEAT reconstruction at c. 1750 Ma (Betts et al., in press), to a palaeomagnetic-based reconstruction where Australia and Laurentia were separated by c. 1200 Ma (Pisarevsky et al., 2003). If this is the case, then Australia!s position in Rodinia will need to be reconsidered.

Acknowledgements We would like to acknowledge the support from ARC Linkage grant no. LP0454301 in collaboration with Primary Industry and Resources South Australia. Reviews by S. Pisarevsky, J. Meert, E. Duebendorfer, and

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an anonymous review greatly improved the manuscript.

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