October 2006 LIP of the Month

Corresponds to event #49 in LIP record database (see also October 2005 LIP of the Month).

The Kalkarindji Continental Flood Basalt Province: A new Large Igneous Province in Australia

Linda Glass
Northern Territory Geological Survey, Department of Primary Industry, Fisheries and Mines Centrepoint Building  Smith Street Mall GPO Box 3000  DARWIN  NT  0801 AUSTRALIA
linda.glass@nt.gov.au

The Kalkarindji continental flood basalt province is the suggested new stratigraphic name  (Glass & Phillips, 2006) for a number of scattered basalt suites across northern and central Australia, including the sub-aerial Antrim Plateau Volcanics (minimum volume of 0.15 x 10⁶ km³) and intrusive Milliwindi dolerite dyke in the north, and the stratigraphically correlated Nutwood Downs, Helen Springs, Peaker Piker and Colless Volcanics in the east (Dunn, 1963; Bultitude, 1976; Hanley and Wingate, 2000). The name was chosen to avoid confusion with the well-known Antrim basalts of the British Tertiary Igneous Province and to provide a definitive link between correlative igneous suites. This vast Cambrian igneous event is Australia’s oldest and largest Phanerozoic large igneous province. Recent aerial magnetic imagery across northern Australia, reveals the presence of extensive basalt sub-crops under Palaeozoic and Mesozoic cover, with flows up to 200 km long, filling palaeo-valley and stratigraphic lows. The province thus extends across a vast area (>10⁶ km²) (Fig. 1), with maximum thickness attained in the north-western part of the province within the Antrim Plateau Volcanics (~1500m; Mory and Beere, 1988). The original pre-erosion volume of the province is difficult to estimate, although it is likely to have exceeded 5x10⁵ km³ (based on an areal extent of >1 x 10⁶ km² and an average thickness of ~0.5 km).

The Antrim Plateau basalts typically comprise ~20-60m (up to 200 m) thick lava flows of mostly aphanitic massive basalt with vesicular or brecciated flow tops, and less common plagioclase-phyric porphyritic basalt. Glomeroporphyritic, often columnar-jointed and hydrothermally altered basalts occur locally in the west. Individual basalt flows may be intercalated with thin beds of aeolian sandstone, conglomerate, siltstone and stromatolitic chert (Randal and Brown, 1967); however, age-diagnostic fossils have yet to be found (Bultitude, 1976).

Stratigraphic constraints suggest the Kalkarindji volcanics are at least Middle Cambrian in age (>500 Ma; Kruse et al., 1994; Bowring and Erwin, 1998) consistent with the oldest previous K-Ar ages for Antrim Plateau and Helen Springs basalt samples of ca. 510 Ma (Bultitude, 1976) and with a recent SHRIMP U-Pb zircon age of 513 ± 12 Ma for the related Milliwindi dolerite (Hanley and Wingate, 2000).

In western-central Australia, the Boondawari dolerite, located within the large Neoproterozoic to Palaeozoic Officer Basin (Figure 1) has been dated at 508 ± 10 Ma (2σ), using the U-Pb zircon method (Macdonald et al., 2005). Age constraints for the Table Hill Volcanics in the south (Fig. 1), include a Rb-Sr result of 575 ± 40 Ma (Compston, 1974) and K-Ar whole rock ages of 500 ± 14 Ma (2σ; Veevers, 2000) and 484 ± 8 Ma (2σ; Stevens and Apak, 1999), the latter being in the Ordovician.

Recent high precision ⁴⁰Ar/³⁹Ar analyses of plagioclase feldspar separates from basalt flows yield ages of 508 ± 2 Ma and 505 ± 2 Ma (2σ), indistinguishable from previous U-Pb zircon ages for related dolerites (Glass & Phillips, 2006).

The basalts are low-Ti tholeiites and are geochemically distinctive having unusual elevated incompatible element signatures (high Th/U, La/Sm, Rb/Ba) Figure 2. Results from 80 Antrim Plateau samples, spanning an Mg# range of 34-72 are compared with data for representative samples from the northern and western-central Australian stratigraphic equivalents. The stratigraphic correlatives are geochemically indistinguishable from the Antrim Plateau Volcanics, with near constant trace element ratios (e.g. Ba/Th, Nb/La and P/Nd), indicating a common parentage. However, for the Table Hill Volcanics, abundances of the highly incompatible elements Rb and Cs, are likely affected by alteration.

Relative to mid-ocean ridge basalts, the Kalkarindji province is characterised by low FeO_total and TiO₂ contents at fixed Mg# (e.g. at Mg#₆₅, FeO_total ~8.4 wt% and TiO₂ ~0.8 wt%). Compared to most other continental flood basalts worldwide, the Kalkarindji basalts are distinguished by low HFSE abundances (Ti, P, Nb) relative to the incompatible elements and extreme enrichment in the most incompatible elements, such as Th, U and LREE (Glass, 2002). The distinctive Low-Ti and incompatible element enriched geochemical signature is far removed from typical tholeiitic basalts and implies crustal involvement in the genesis of the Kalkarindji basalts. A detailed geochemical and isotopic study involving petrogenetic modelling of the basalts will be the focus of a manuscript to be published in the near future.

REFERENCES CITED