The age and tectonic significance of the Warraweena Volcanics and related rocks, southern Thomson Orogen

Abstract

Magmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417 ± 3.5 and 414 ± 4.0 Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNd_t values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411 ± 5.5 Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNd_t values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty.

The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian.

Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100 km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system.

KEYWORDS:

• Warraweena Volcanics
• Oxley volcanics
• Booda Formation
• zircon
• U-Pb
• Sm-Nd
• geochronology
• geochemistry
• provenance
• southern Thomson Orogen

Acknowledgements

Rosemary Hegarty and Phil Gilmore, Geological Survey of New South Wales, for generous assistance with sampling and advice throughout the project. Roland Maas for Sm–Nd analytical data. Bill Collins for sharing his knowledge and reviewing an earlier version of the manuscript. Pete Betts and Geoff Fraser for constructive and critical comments on an earlier version of the manuscript that inspired ACH, RCD and HH to get more data. David Purdy and Charlotte Allen are thanked for helpful reviews, which improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Figure 12. NMORB-normalised multielement plots (modified after Sun & McDonough, 1989). (a) CAB WV show similar and parallel patterns to the HSB. Slopes between Th/Nb (subduction influence), Nb/Ti (enrichment of source) and Ti/Yb (depth of source) are broadly similar and suggest these two volcanic suites may have been produced under similar conditions in a similar setting (continental rift to incipient back-arc). (b) Select Phase 4 Macquarie arc samples show in comparison to the CAB in (a). The multielement pattern of the Macquarie arc are quite similar to the WV samples but there are some subtle differences. A shallower Th/Nb, steeper Nb/Ti and Ti/Yb may reflect the IAB nature of the Macquarie arc compared with the WV. Isotopic values for Phase 4 Macquarie arc do become more evolved over time likely reflecting the incorporation of SSZ material or an enriched older SCLM component and might explain why rift-related rocks of the southern Thomson Orogen/Lachlan Orogen appear so similar to IAB of the terminal phase of the Macquarie arc. (c) EMORB WV sample from ACDWE010 displays a flatter pattern that lacks the element spiking observed in subduction influenced rocks typified by the almost flat slope between Th/Nb. (d) EMORB to OIB samples from Louth L1 and L3 show similar but generally more enriched patterns to ACDWE010. Fossil evidence (Zhen, Hegarty, Percival, & Pickett, 2017) from the closely associated Louth L2 suggests these volcanic rocks might be Silurian–Devonian in age and may represent rift-related volcanism older than the Permo-Triassic alkaline Louth Volcanics.

Appendix 1. Sample preparation and analytical methods

Table A1. LA-ICP-MS analytical conditions.

Table A2 U–Th–Pb LA-ICPMS data (excel spreadsheet).

Geochemical comparison of magmatic rocks from the STO and NLO.

Additional information

Funding

This research was funded by the Australian Research Council grant LP140100874.