K-Ar geochronology of basalt petrogenesis, Newer Volcanic Province, Victoria

Abstract

The largest component of the basaltic Newer Volcanic Province of Victoria, Australia, the Western Plains subprovince, comprises extensive tholeiitic to alkalic lava flows and pyroclastics. The subprovince is subdivided into eastern and western sectors with distinctive Sr isotopic signatures; the isotopic discontinuity corresponds to an underlying Paleozoic tectonic boundary. Fifty-one new K-Ar ages plus previously published results encompass the geographic, lithological and isotopic variants of the subprovince, and are used to date extensive flow units, provide time calibration of regolith mapping units, and to compile eruptive and petrogenetic histories. The onset of volcanism at 4.6 Ma, the peak of activity at 3.0-1.8 Ma, and the progression from tholeiitic to alkalic compositions towards the present day are subprovince-wide, indicating that the basis of activity is independent of the lithospheric substrate. Local activity appears to have little pattern: some subareas have a conservative aspect, erupting similar tholeiites over much of the time span of the field; others show the complete range of lithological and isotopic composition. Peak activity (3.0-1.8 Ma) produced the maximum range of rock and isotopic compositions erupted subprovince-wide: the western sector has alkalic basalt to tholeiite (87Sr/86Sr 0.7041-0.7044), and the eastern sector transitional basalt to basaltic icelandite with much larger isotopic variation (87Sr/86Sr 0.7042-0.7055). In contrast to this apparently erratic behaviour, plots of 87Sr/ 86Sr vs time reveal a coherent system with the distinct eastern and western sectors expressing parallel isotopic change, a progressive decline in 87Sr/86Sr as magma compositions change from tholeiitic to more alkalic. Potential explanations, mantle chemical heterogeneity and crustal contamination, are examined in the light of chronological constraints. The tholeiitic to alkalic progression is analogous to that of oceanic islands and considered characteristic of intraplate volcanism; it implies that the most recent phase of Cenozoic volcanic activity in Victoria is coming to a close.