Abstract

The temporal relationship between tectonic and volcanic activity on passive continental margins immediately before and after the initiation of mid-ocean ridge spreading is poorly understood because of the scarcity of volcanic samples on which to perform isotope geochronology. We present the first accurate geochronological constraints from a suite of volcanic and volcaniclastic rocks dredged from the 70,000 km2 submerged Wallaby Plateau situated on the Western Australian passive margin. Plagioclase 40Ar/39Ar and zircon U-Pb sensitive high-resolution ion microprobe ages indicate that a portion of the plateau formed at ca. 124 Ma. These ages are at least 6 m.y. younger than the oldest oceanic crust in adjacent abyssal plains (minimum = 130 Ma). Geochemical data indicate that the Wallaby Plateau volcanic samples are enriched tholeiitic basalt, similar to continental flood basalts, including the spatially and temporally proximal Bunbury Basalt in southwestern Australia. Thus, the Wallaby Plateau volcanism could be regarded as a (small) flood basalt event on the order of 104–105 km3. We suggest that magma could not erupt prior to 124 Ma because of the lack of space adjacent to the plateau. Eruption was made possible at 124 Ma via the opening of the Indian Ocean during the breakup of Greater India and Australia along the Wallaby-Zenith Fracture Zone. The scale of volcanism and the temporal proximity to breakup challenges the prevailing theory that the Western Australian margin formed as a volcanic passive margin. Given that the volume of volcanism is too small for typical flood basalts associated with volcanic passive margins, we suggest that the two end members, magma-poor and volcanic passive margins, should rather be treated as a continuum.

INTRODUCTION

Passive margins are the locus of complex tectonic and magmatic processes leading from continental rifting to oceanic spreading. Understanding these processes is critical to deciphering how the continental lithosphere eventually breaks up and how the first oceanic lithosphere is generated by the mid-ocean ridge. Specifically, the tectonic and chronological mechanisms that drive the switch from rifting to spreading and how associated magmatism is generated are still not well constrained. Relatively little has been published on many of the known passive margins, in part due to the prohibitive cost associated with offshore sample recovery expeditions, including on the Western Australian passive margin that formed during the breakup of Greater India and Australia. The relict northeastern junction between Greater India and Australia is enigmatic. Evidence of seaward-dipping reflectors and high-velocity lower crust led researchers to consider this margin as a volcanic passive margin (Coffin and Eldholm, 1994; Goncharov and Nelson, 2012; Symonds et al., 1998). Estimations of the volume of volcanic rocks are not well constrained, but such volume is probably on the order of 104–105 km3 (Goncharov and Nelson, 2012), which is not as voluminous as “true” volcanic passive margins (Courtillot et al., 1999; Franke, 2013). However, no high-quality geochronological data exist for offshore volcanic rocks along the Western Australian margin, and so the timing between breakup and magmatism remains poorly understood (Ludden, 1992; von Stackelberg et al., 1980). In this contribution, we present radioisotopic data on a set of volcanic and volcano-detrital rocks dredged from the Wallaby Plateau, a continental fragment that was once situated along the relict northeastern junction between Greater India and Australia prior to the breakup of eastern Gondwana. The data provide the first constraints on the timing of volcanism and new constraints on the geochemical composition of the Wallaby Plateau volcanic rocks associated with the continental breakup on the northeasternmost conjugate paleo–Greater Indian and paleo-Australian margins.

GEOLOGICAL SETTING

The Wallaby Plateau is a large bathymetric high with an areal extent of ∼70,000 km2, ∼500 km off the northwest coast of Australia (Fig. 1) (Daniell et al., 2009). The current understanding of the nature and evolution of the Wallaby Plateau is based largely on interpretations of seafloor-spreading magnetic anomalies (e.g., Gibbons et al., 2012; Mihut and Müller, 1998; Robb et al., 2005), seismic reflection profiles (e.g., Goncharov and Nelson, 2012; Rey et al., 2008), and dredge sampling data (Colwell et al., 1994; Daniell et al., 2009; von Stackelberg et al., 1980). These studies have indicated that the Wallaby Plateau is probably composed of continental crust thinned during lithospheric rifting, which is buried beneath a volcanic sequence, which is in turn overlain by a sedimentary carapace (Daniell et al., 2009). Geochemical and paleontological studies of dredged rocks indicate a continental origin for the Wallaby Plateau, and the plateau is therefore interpreted as a continental fragment (Colwell et al., 1994; Stilwell et al., 2012; von Stackelberg et al., 1980).

The thick volcanic sequence overlying the continental basement is thought to have formed during a magmatic event located along the adjoining northwest-southeast–trending Wallaby-Zenith Fracture Zone (WZFZ) (Mihut and Müller, 1998). Seaward-dipping reflectors interpreted on two-dimensional seismic reflection images indicate 320,000 km3 of volcanic flows interspersed with sedimentary strata (Goncharov and Nelson, 2012; Symonds et al., 1998). Volcanic rocks are estimated to comprise ∼10%–90% of the total volume, which approximates to 104–105 km3. Interpretations from seafloor-spreading anomalies imply that this volcanism could either be rift related and coeval with breakup (Robb et al., 2005) or be pseudo-intraplate and postdate breakup by ∼20–30 m.y. (Mihut and Müller, 1998). As there are no accurate ages for these volcanic rocks, it remains unknown whether the Wallaby Plateau volcanism occurred relatively quickly during breakup or during a prolonged and/or episodic period of volcanism after breakup.

RESULTS

Samples dredged from the seafloor on the Wallaby Plateau during the marine reconnaissance survey GA2746 in 2009 (Daniell et al., 2009) provide an opportunity to improve temporal constraints on magmatic activity along the rifted margin of northwest Australia. Zircons from three volcaniclastic samples were dated by the sensitive high-resolution ion microprobe (SHRIMP) U-Pb method, and plagioclase separates from three basaltic samples were dated by the 40Ar/39Ar method. Four samples were analyzed for major and trace element geochemistry.

SHRIMP U-Pb Geochronology of Detrital Zircons from the WZFZ

Three volcaniclastic samples from two dredge sites (sites 53 and 61; see Fig. 1; see Table DR1 in the GSA Data Repository1 for full sample names) along the WZFZ <1 km apart were dated using zircon U-Pb SHRIMP. Zircons were separated using conventional techniques (see the Data Repository for analytical techniques).

The three samples display age clusters at ca. 124 Ma and scattered ages ranging from Paleozoic to Archean (Fig. 2). Zircon grains from volcaniclastic samples 53-1 and 53-2 from dredge site 53 yielded weighted average 206Pb/238U ages of 123.9 ± 1.0 Ma (mean square of weighted deviates [MSWD] = 1.0; P = 0.42) and 123.9 ± 1.3 Ma (MSWD = 1.8; P = 0.10), respectively (Fig. 2). The sample from dredge site 61 produced a similar apparent age range, with individual spot 206Pb/238U ages ranging from 118 to 134 Ma, but the MSWD and P-value of 4.9 and <0.001 indicate that the data are scattered beyond statistical expectation for a single population and thus a reliable weighted mean age could not be calculated.

In the three samples, the 14 analyses yielding Paleozoic, Proterozoic, and Archean ages indicate that the grains were derived from older continental sources, either directly available to sedimentary transport at the time of deposition or via recycling from preexisting sedimentary units.

40Ar/39Ar Plagioclase Ages from the WZFZ, Sonne Ridge, and Sonne Seamount

Three plagioclase separates from basaltic samples of the Wallaby Plateau yielded statistically reliable 40Ar/39Ar plateau ages (Fig. 3; Table DR2; see the Data Repository). The first sample was collected along the WZFZ (46B from dredge site 52; Fig. 1). Two aliquots of plagioclase from this sample yielded plateau ages of 125.12 ± 0.90 Ma (MSWD = 0.99; P = 0.45) and 123.8 ± 1.0 Ma (MSWD = 1.80 P = 0.09) with at least 97% of the total 39Ar released included in the plateau calculation (Fig. 3). A weighted mean age between the two aliquots yields an age of 124.53 ± 0.54 Ma for this sample. The 37Ar/39Ar ratio, a proxy for Ca/K, was abnormally low (∼0.1; Fig. 3), which is incompatible with pure plagioclase and illustrates the quasi-complete sericitization of the crystals, a process commonly observed for altered basaltic samples (Verati and Jourdan, 2014). Therefore, this age is interpreted as the age of the alteration process (Verati and Jourdan, 2014) and thus provides a minimum age for the eruption of the basalt.

The second sample was collected on the Sonne Seamount (49B from dredge site 56; Fig. 1). Two aliquots of plagioclase from this sample yielded plateau and mini-plateau ages of 59.39 ± 0.64 Ma (MSWD = 1.08; P = 0.38) and 63.49 ± 0.79 Ma (MSWD = 1.02; P = 0.39), respectively, which include 94% and 69% of 39Ar, respectively (Fig. 3). Similar to the previous sample, the 37Ar/39Ar ratio was abnormally low (∼0.1; Fig. 3), which is interpreted to result from sericitic replacement of plagioclase, and thus these ages represent the age of the alteration event(s) and provide minimum ages for the timing of the eruption of the host basalt.

The third sample was recovered from the Sonne Ridge (51A from dredge site 57; Fig. 1). Two aliquots of plagioclase yielded imprecise mini-plateau apparent ages of 120 ± 14 Ma and 123 ± 11 Ma (Fig. 3) indicating an Early Cretaceous age for the Sonne Ridge samples, but the relatively poor resolution of the age spectra prevent detailed age comparison (Fig. 3).

Major and Trace Element Geochemistry from the WZFZ, Sonne Ridge, and Sonne Seamount

Two samples from the WZFZ (52 and 61), one sample from the Sonne Ridge (57), and one sample from the Sonne Seamount (56) were analyzed for major and trace elements (Table DR2; see the Data Repository). All samples are basaltic or slightly differentiated as shown by their low Mg content. Three of the samples show negative Nb and positive Pb anomalies, which are characteristic of many continental flood basalts. Incompatible and rare earth element (REE) patterns are similar to those of enriched tholeiitic basalts such as enriched mid-oceanic ridge basalts (Fig. 4; Dadd et al., 2015). They are remarkably similar to that of the ca. 132 Ma Bunbury Basalt in southwestern Australia (Fig. 4) (Coffin et al., 2002; Frey et al., 1996). The sample from the Sonne Seamount presents a stronger light REE (LREE) enrichment relative to heavy REE (HREE) compared to the other samples, and shows positive Nb and negative Pb anomalies.

DISCUSSION

The chronology of volcanism on the Wallaby Plateau as indicated by zircon and sericite/plagioclase isotopic ages provides new constraints on the duration of magmatism as Greater India drifted away from Australia. The concordant U-Pb ages from volcaniclastic detrital zircon indicate that an episode of volcanism occurred along the WZFZ at ca. 124 Ma on the edge of the Wallaby Plateau. The sericite minimum 40Ar/39Ar age of ≥124 Ma further west along the WZFZ confirms the importance of this volcanic phase along the WZFZ. Despite the sericitization age being very similar to the age recorded by the zircon and thus indicating strong hydrothermal activity at the time, we note that the true age of the basaltic eruption could range from being synchronous with the alteration process (cf. Jourdan et al., 2009) to up to a few million years older (Verati and Jourdan, 2014). Therefore, we can say with certainty that some of the Wallaby Plateau volcanic rocks formed at ca. 124 Ma, but the total duration of volcanism remains uncertain. The sericite 40Ar/39Ar age from the Sonne Seamount has indicated a much younger age of alteration of ≥60 Ma. As a result, the true age of the eruption remains elusive. The Sonne Seamount sample could be related to the rest of the Wallaby Plateau volcanism at ca. 124 Ma and have been severely altered at ca. 60 Ma. However, the geochemistry of the Sonne Seamount is more typical of an ocean island basalt (Fig. 4). Alternatively, we note that the closest magmatic occurrences of ca. 60 Ma are located in the Christmas Island Seamount Province (CHRISP; Hoernle et al., 2011), which was, at the time, only a few hundred kilometers away from the Sonne Seamount, suggesting a possible connection between the two. In any case, the difference in age and chemistry points to a distinct origin for the Sonne Seamount relative to the rest of the Wallaby Plateau.

The age of the bordering oceanic crust in adjacent abyssal plains is significantly older than that of the 124 Ma Wallaby Plateau volcanic rocks, taking into account the uncertainties associated with the ages of the magnetic anomalies. Although most authors agree that the oldest magnetic anomalies adjacent to the Wallaby Plateau are M11 (Perth Abyssal Plain) and M10 (Cuvier Abyssal Plain), in the absence of isotopic ages, there is current debate about the absolute ages of these chrons, ranging from 136 to 132 Ma (M11) and 134 to 130 Ma (M10) (cf. Heine et al., 2013). In any case, some portion of the volcanic rocks on the Wallaby Plateau erupted at least 6 m.y. after the onset of oceanic spreading, although the initiation of the volcanism on the plateau could have happened a few million years earlier.

Furthermore, we showed that the Wallaby Plateau has rocks with enriched compositions similar to that of continental tholeiitic basalt (Fig. 4). Such geochemical signatures clearly point to the involvement of continental material during the melt generation for the Wallaby Plateau volcanic rocks, either by contamination of the basaltic melts ascending through the continental lithosphere or by derivation from a fertile subcontinental lithospheric mantle. Temporally and spatially, the closest known basalt with similar geochemical signatures is the ca. 132 Ma Bunbury Basalt in southwestern Australia (Fig. 4) (Coffin et al., 2002), although it only has a volume of ∼102 km3 (Olierook et al., 2015). The Bunbury Basalt and volcanism on the Wallaby Plateau have a similar composition despite an 8 m.y. gap, suggesting that both eruptive processes share some affinities. Nevertheless, the volumetric approximations for the Wallaby Plateau volcanism are two to three orders of magnitude greater than for the Bunbury Basalt flood basalt (Goncharov and Nelson, 2012). Plate reconstruction models indicate that the position of the dredge samples along the WZFZ coincide with the opening of the Indian Ocean along this major lithospheric discontinuity (Gibbons et al., 2012; Hall et al., 2013) (Fig. 5). This would imply that while the Bunbury Basalt volcanism was restricted to southwest Australia at ca. 132 Ma, the Wallaby Plateau could have initiated only when space opened sufficiently at ca. 124 Ma that could allow basaltic lavas to flow freely along the WZFZ and over the Wallaby Plateau. The sericite age of 124 Ma could be explained by intense hydrothermal activity associated with the magmatism along the WZFZ.

Incubation of a mantle plume head underneath the lithosphere could provide an origin for the Wallaby Plateau volcanism that is consistent with the volcanic rock geochemistry (e.g., Xu et al., 2014). Upwelling plume material is stored at the base of the thick continental lithosphere below its solidus before continental breakup. This plume can only rise after breakup, crossing its solidus and melting by decompression. This provides an alternative mechanism to generate plume-type magmatism after continental breakup and is probably the direct consequence of mantle melting caused by plate breakup.

Despite 104–105 km3 of basalt on the Wallaby Plateau, these contiguous margins are still like neither magma-poor (e.g., Iberia-Newfoundland, <103 km3) nor volcanic passive margins (e.g., East Greenland, 106–107 km3) (Courtillot et al., 1999; Eldholm and Grue, 1994). We suggest that magma-poor and volcanic passive margins are only “end members” of a continuous spectrum for continental breakup. The age, geochemistry, and volume of the Wallaby Plateau volcanic rocks indicate that intermediate levels of volcanism exist within what we suggest should be a continuum between the two end members.

CONCLUSIONS

New zircon U-Pb and 40Ar/39Ar ages of volcaniclastic and volcanic rocks at 124 Ma along the WZFZ indicate a significant magmatic event shortly after the onset of seafloor spreading. Geochemical and volume constraints indicate that the Wallaby Plateau volcanic rocks are part of a small continental flood basalt province with a volume of 104–105 km3. Known plate reconstruction constraints imply that this volcanism occurred when space was generated as the northeastern Indian Ocean opened adjacent to the Wallaby Plateau, potentially by means of an incubating plume. These intermediary volumes of continental flood basalts related to continental breakup challenge the prevailing theory that passive margins are either magma poor or volcanic. We suggest that a continuum between these end members is far more suitable.

We would like to thank the shipboard crew from Geoscience Australia marine reconnaissance survey GA2476 aboard the RV Sonne that dredged the rocks used in this study, and the German Ministry of Education and Research for funding the RV Sonne. This is contribution 633 of the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1014 of the GEMOC Key Centre (www.gemoc.mq.edu.au). The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners, Macquarie University, and Curtin University. We also thank S. Planke, J. O’Conner, and three anonymous reviewers for significantly improving the manuscript.

1GSA Data Repository item 2015328, tabulated U-Pb SHRIMP, 40Ar/39Ar, and geochemistry results, zircon photomicrographs, and detailed analytical techniques, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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Current address: Department of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK; E-mail: h.olierook@liverpool.ac.uk.