Geochemistry and Petrogenesis of a Continuous Stratigraphic Succession of the Mid-Cambrian Kalkarindji Continental Flood Basalt Province, Northern Australia

The Kalkarindgi CFBP (c. 505 - 510 Ma) is an extremely rare ancient CFBP for which significant thicknesses of the lava succession still remain preserved. Accordingly it provides a unique insight regarding Late Proterozoic - early Phanerozoic mantle dynamics and associated volcanism. Due to erosion, its remnants now consist of scattered basaltic suites occurring across northern and central Australia, and which together suggest a minimum eruptive volume of c. 1.5 x10⁵ km³, though its original volume is estimated to have significantly exceeded 5 x 10⁵ km³ [1]. Extensive, flat lying lava flows are present in the Waterloo region (c. 16° 15' - 30'S 129° 00' - 30'E) of the Northern Territory as low mesas rising c. 160m from the basement topography; the flanks of these allow access to a detailed volcanostratigraphy. The lava flows are predominantly simple sheet flows of up to 60m thick of mostly aphanitic massive basalt with vesicular or brecciated/rubbly flow tops and, less commonly, plagioclase-phyric porphyritic lavas. Subordinate compound-type flows also occur. Interbedded with this volcanic succession are clean, arenitic sandstones (up to 10m thick) derived from a mature continental setting. These indicate that, during eruption, this part of the basalt province was topographically low. Geochemically, the lavas are predominantly evolved low Ti-tholeiitic basaltic andesites and basaltic trachy-andesites with relatively high K, Na and Si and low Ca and Ti. The lavas are LREE enriched (La/Yb 7.3) with slight negative Eu anomalies (Eu/Eu* 0.8) and flat HREE. The lavas also demonstrate extreme crustal signatures with Th/Nb >1, enrichment in Pb and depletion in Sr. Importantly, this distinctive Low-Ti and incompatible element enriched geochemical signature is far removed from typical LIP tholeiitic basalts, and implies significant crustal involvement during the genesis of the Waterloo, and wider Kalkarindji, basalt stratigraphy. Further, the high Si content and low Ca and Ti content is considered indicative of a high volatile content during crystallization. The atypical geochemistry of the Kalkarindji basalts is likely due to either derivation from a hydrous metasomatised mantle source or due to the assimilation of hydrous crustal material during passage through the continental crust. [1] Glass & Philips (2006) Geology 34: 461-464