New 40 Ar/ 39 Ar dating of the Antrim Plateau Volcanics, Australia: clarifying an age for the eruptive phase of the Kalkarindji continental flood basalt province

Abstract

The Kalkarindji flood basalt province of northern Australia erupted in the mid-Cambrian. Today, the province consists of scattered volcanic and intrusive suites, the largest being the Antrim Plateau Volcanics (APV) in Northern Territory. Accurate dating of the Kalkarindji flood basalt province has proved challenging, with previous studies focused on minor volcanic rocks and intrusive dykes in Northern Territory and Western Australia. These previously published data, corrected to the same decay constants, range from 512.8 to 509.6 ± 2.5 Ma [2σ], placing the Kalkarindji flood basalt province in apparent synchronicity with the Cambrian Stage 4–5 biotic crisis at 510 ± 1 Ma. This study utilizes 40 Ar/ 39 Ar dating of basalts from the APV to accurately date the major volcanic eruptions in this province. The results yield an age of 508.0–498.3 ± 5.5 Ma [2σ], indicating that the APV is younger than the intrusive rocks. These dates allude to a relative timing discrepancy, where intrusive activity in the North Australian Craton preceded the eruption of the APV as the last magmatic activity in the region. The determination of these largest eruptions to be later than 510 Ma effectively disassociates the Kalkarindji lavas from being a major cause of the 510 Ma biotic crisis, but cannot definitively discount any deleterious effects on the fragile Cambrian ecosystem. Supplementary material : 40 Ar/ 39 Ar analysis data tables, XRF geochemical data, alteration study and photomicrographs of all five samples are available at: https://doi.org/10.6084/m9.figshare.c.4176674