Origin of metasedimentary and igneous rocks from the Entia Dome, eastern Arunta region, central Australia: a U – Pb LA-ICPMS, SHRIMP and Sm – Nd isotope study

Abstract

Analysis of detrital zircon from previously geochronologically unconstrained metasedimentary rocks in the eastern Arunta region, using U – Pb LA-ICPMS and SHRIMP, provides ages reconcilable with Australian sources. Maximum depositional ages of the analysed samples fall into two groups, three samples having maximum depositional ages of ca 1780–1760 Ma and two at ca 2510–2490 Ma. Metamorphic monazite from two pelitic rocks provides bimodal age populations of 1773 ± 9 Ma and 325 ± 8 Ma from one sample, and 1719 ± 9 Ma and 341 ± 5 Ma from the second. The age of 1773 ± 9 Ma is the first direct evidence of monazite growth in the interval ca 1780–1770 Ma, and is attributed to high-grade contact metamorphism associated with the proximal emplacement of granodioritic bodies previously thought to be subduction-related. The metamorphic age of 1719 ± 9 Ma is attributed to the Strangways Orogeny (1730–1710 Ma). The Carboniferous ages of 325 ± 8 Ma and 341 ± 5 Ma in both samples are attributed to partial resetting and new growth of monazite during the Alice Springs Orogeny (ca 400–300 Ma). Sm – Nd isotope systematics of the metasedimentary rocks display a large variation ranging from ε _Nd(1760) =  – 1.4 to  – 8.8 and, when coupled with the detrital-zircon age populations, allow for a locally derived (North Australian Craton) source for the metasedimentary rocks. Analysis of igneous zircon from a felsic layer of the Entia Gneiss Complex intimately interlayered with the metasedimentary rocks yielded a ²⁰⁷Pb/²⁰⁶Pb weighted average age of 1771 ± 10 Ma, interpreted as the crystallisation age of the igneous precursor. This age is indistinguishable within error from that of the granodioritic intrusions (Inkamulla Granodiorite 1773 ± 4 Ma), here thought to be responsible for the high-grade metamorphism in the metasedimentary rocks. Given that the granodioritic bodies have previously been characterised as subduction-related intrusions, the approximately coincident ages of metamorphism, deposition of at least some of the metasedimentary rocks, and the intrusion of voluminous felsic and mafic magmas, suggests that the deposition and metamorphism of the precursor to the metasedimentary rocks may be related to active subduction along the southern margin of the Arunta region.

Key Words:

• Arunta region
• LA-ICPMS
• orogenesis
• samarium – neodymium dating
• SHRIMP
• uranium – lead dating
• zircon

Acknowledgements

Thanks must go to Justin Payne and David Kelsey for reading through preliminary copies of the manuscript, and also to Christine Edgoose from the Northern Territory Geological Survey for discussions on the geology of the eastern Arunta. We also appreciate helpful and constructive comments given by Russell Shaw and Alastair Stewart which helped to considerably improve the manuscript.

Notes

^∗Tables 3–5 [indicated by an asterisk (∗) in the text and listed at the end of the paper] are Supplementary Papers; copies may be obtained from the Geological Society of Australia's website (<http://www.gsa.org.au>) or from the National Library of Australia's Pandora archive (<http://nla.gov.au/nla.arc-25194>).

∗OZCHRON is a database of Australian geochronological data managed and maintained by Geoscience Australia (<http://www.ga.gov.au/oracle/ozchron/index.jsp>).