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A precise Ar-Ar age for the Kalkarinji low-Ti Continental Flood Basalt Province of northern Australia

L.M. Glass and D. Phillips1 1 School of Earth Sciences, The University of Melbourne, Victoria

The Antrim Plateau Volcanics and their stratigraphic equivalents (Helen Springs, Nutwood Downs, Peaker Piker and Colless Volcanics) comprise Australia's largest Phanerozoic age flood basalt province. However, the eruption age of the province has not been precisely determined to date. Stratigraphic constraints suggest the province is Early Cambrian in age (545 - 508 Ma) consistent with a recently determined SHRIMP U-Pb zircon age of 513 ± 12 Ma and older K-Ar measurements giving ages of ca 510 Ma. In order to obtain more precise eruptive ages for basalts over the geographical province, petrographically least altered rocks from the Limbunya (western NT), Helen Springs, Daly River and Katherine regions were selected for plagioclase separation. The freshest feldspar grains were carefully handpicked from the mineral separates. Of the four samples analysed by stepwise degassing, two samples yielded well-defined plateau ages (Figure 1). The others (Daly River and Katherine) had discordant saddle-shaped spectra, suggestive of excess argon and/or alteration.

Table 1: Isochron data for the Antrim Plateau and Helen Springs basalts

40Ar-39Ar plateau ages
Region
Sample No.
Age
%39Ar included
MSWD
Helen Springs
HS002(1)
508±2 (1 sigma)
99.2
0.89
Helen Spring
HS002(2)
508±2 (1 sigma)
52.8
1.30
Limbunya
LB011(1)
504±2 (1 sigma)
74.9
1.80

 

Figure 1: 40Ar-39Ar step heating age spectrum for Antrim sample HS002 (1) plagioclase separate.

The new high-precision Ar-Ar age is younger than previously obtained and indicates the eruptive event occurred at the Lower Cambrian - Middle Cambrian boundary. In view of the new ages, we now have geochronological evidence that definitively links the Antrim Plateau Volcanics and their stratigraphic equivalent, the Helens Springs Volcanics. Because the Antrim Plateau Volcanics and their stratigraphic equivalents are geochemically, isotopically and geochronologically indistinguishable we introduce a new name for the province: the Kalkarinji Continental Flood Basalt Province. The name has been chosen firstly to minimise confusion with the more well known northern hemisphere Antrim basalts in northern Ireland and also to provide an all encompassing name to embrace the north Australian Cambrian flood basalt event. It is possible that coeval basalts in the Officer (Table Hill Volcanics) and Savory Basins (M Wingate, pers. comm. 2001) may also be part of the eruptive event and warrants further investigation.