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Lena Z. Evins, School of Earth and Geographical Sciences (Australia)
David Phillips, School of Earth Sciences (Australia)
Fred Jourdan, WAAIF, Department of Applied Geology (Australia)
Tom Blenkinsop, School of Earth and Environmental Sciences (Australia)
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The Early Cambrian Kalkarindji LIP is still relatively unknown in spite of its impressive size and temporal link to the Early-Middle Cambrian mass extinction. Remnants of this LIP are found over an area of at least 2.1 million km2, from Queensland to the Kimberleys, Western Australia (WA), to the Officer basin in south eastern WA and South Australia (SA). New geochemistry and preliminary 40Ar/39Ar geochronology from the Table Hill Volcanics in the Officer basin indicate that these, and correlated intrusives, are part of the province. Possible Kalkarindji rocks are found in the Harts Range, Northern Territory (NT), the Kanmantoo sedimentary rocks (SA), and in southern WA. Remnants may also be found on other continents; the Tarim block of western China was rifted from Australia at the time of the Kalkarindji LIP formation, and the dikes of SW Australia have counterparts in Antarctica.
The thickest part of this LIP is found in the East Kimberley, near the WA-NT border. Here it is locally up to c. 1500 m thick, and four members have been defined, from youngest to oldest: 1) the c. 70 m thick Blackfella Rockhole member (mainly volcanic breccia), 2) the c. 40 m thick glomeroporphyritic Bingy Bingy basalt member, 3) the 5 m thick Mt Close Chert member and 4) the 5 m Malley Spring member (sandstone). Outcrops of the Bingy Bingy basalt member are found over an area of c. 10 500 km2 and, assuming a thickness of 40 m, represent c. 400 km3 of erupted lava. The extent of the Blackfella Rockhole member is similar and there appears to be a close spatial correlation between these two members. Major and trace element geochemistry from basalt samples collected in the region around the WA-NT border show that the early eruptive rocks have 7-8 wt% MgO, while the Bingy Bingy basalt member and other samples from the later stages have 4-5 wt% MgO.
Previous, contradicting interpretations of the formation of the Blackfella Rockhole member suggest highly explosive activity or autobrecciation. The morphology of clasts have been analysed using the approach of Maria and Carey, in which variations in fractal dimensions of the clast outlines are obtained from the dilation method as a function of increasing scale. The general increase of fractal dimension with scale observed in this study is similar to that reported previously for volcanic clasts, and has been interpreted as the influence of vesicles of different sizes on the clast outlines. The relatively high maximum values of the fractal dimensions (2.2 - 2.4) from the Blackfella Rockhole member, combined with the irregular increase in fractal dimension with scale, suggest that phreatomagmatic or magmatic tephra fall processes were responsible for fragmentation, by comparison with previous measurements. A highly explosive origin for this extensive member strengthens the hypothesis that these eruptions injected gases and aerosols into the stratosphere, causing environmental change.
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