Geochronology and Geochemistry of Complex Ancient Volcanic Sequences in Western Tasmania

Abstract:

Recent detailed (1:25,000 scale) regional geological mapping near Waratah in Western Tasmania has revealed numerous probable thrust-emplaced volcanic and non-volcanic sequences. Segments of fault-bound tholeiitic volcanic units of the Luina Group have been structurally emplaced with picritic and boninitic sequences of supra-subduction zone affinities.

The geochemically distinct rift-related volcanic units of the Luina Group include multiple tabular and pillowed lava flows and re-sedimented shard-rich volcaniclastic facies. These units are Early Cambrian, an age constrained by laser ablation inductively coupled mass spectrometry (LA-ICPMS) analysis of detrital zircons. The youngest rocks were emplaced after ~536 Ma, and supports the pre-Tyennan Orogeny (>510 Ma) age inferred from field relations.

Newly mapped units of picritic lavas are fault bound and comprise well preserved lavas with euhedral, skeletal and needle shaped clinopyroxene and olivine phenocrysts set in a glassy groundmass with microlites of finely spinifex-textured to dendritic olivine. These ‘fresh’ lavas transition into highly altered, monomictic breccia facies. The boninitic units are extremely weathered, and contain remnant orthopyroxene phenocrysts. They are texturally diverse with remnant pillow lobes, autoclastite and resedimented volcaniclastic facies. New geochemical analyses of Cr-spinel grains included in olivine phenocrysts (from the picrites) support a tectonic setting consistent with the formation of a supra-subduction zone in the fore-arc region of an intra-oceanic island arc (Berry and Crawford, 1988; Crawford and Berry, 1992).

The area preserves two texturally diverse and geochemically disparate volcanic sequences. Both of which were erupted in the Early Cambrian and tectonically emplaced in Early to Middle Cambrian. The area records a rapid and structurally complex transition from a passive margin setting to an intra-oceanic island arc.

References

BERRY R. F. & CRAWFORD A. J. 1988. The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35, 523533.

CRAWFORD A. J. & BERRY R. F. 1992. Tectonic implications of Late Proterozoic- Early Palaeozoic igneous rock associations in western Tasmania. Tectonophysics 214, 37-56.