Geochemistry and provenance of the Turquoise Bluff Slate, northeastern Tasmania: tectonic significance

Abstract

The Ordovician Turquoise Bluff Slate in northeastern Tasmania is a 2 km-thick sequence of deep-marine siliceous black slates. It is dominated by meta-siltstones with bimodal grainsize distributions typical of turbidite T_E-1 and T_E-2 facies. The slates have high SiO₂ indicating they are hemipelagites. The high Ba and V indicate they were deposited in an anoxic environment associated with high oceanic productivity. All these features are common in muddy turbidites. U–Th–Pb dating of detrital monazite and authigenic xenotime in the slates supports previous evidence that the dominant cleavage, in this unit, formed during the Benambran Orogeny. The whole-rock composition of the slates is similar to black slates in the Adaminaby Group, NSW. A review of Paleozoic whole-rock compositions from the Lachlan Orogen confirms they all have trace element contents similar to average Australian shale. However, there are subtle differences in composition. The Turquoise Bluff Slate and other Mathinna Supergroup rocks from the Eastern Tasmania Terrane have higher average Cr content than similar age turbidites from Victoria and NSW. This probably reflects a small contribution from Tasmania Cambrian ultramafic rocks in the provenance. If this were correct, northeastern Tasmania was closer to western Tasmania in the Paleozoic than other provinces of the Lachlan Orogen, southeastern Australia. Other subtle features of the whole-rock composition of Paleozoic sedimentary rocks from the Lachlan Orogen indicate it may be possible to recognise provincial variations in composition that will provide new constraints on tectonic models of southeastern Australia.

KEYWORDS:

• Mathinna Supergroup
• Benambran Orogeny
• xenotime
• whole-rock geochemistry
• hemipelagic
• muddy turbidite

Acknowledgements

We thank Terry Mernagh and David Champion for help with background information on the analyses sourced from the OZchem database, and Michael Bruce and Robin Offler for their constructive reviews, which improved the paper. This paper is published with the permission of the Director of Mines, Tasmania.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary papers

Table S1. Whole-rock XRF results, methods and sample locations. Table S2. LA-ICP-MS methods and results. Table S3. EPMA xenotime analyses. Table S4. Detrital mineral abundance.

Additional information

Funding

The paper was partly supported by ARC Linkage LP160100483 Ore deposits and tectonic evolution of the Lachlan Orogen, SE Australia, and partly by the ARC Research Hub for Transforming the Mining Value Chain (project number IH130200004).