Mafic intrusions in southwestern Australia related to supercontinent assembly or breakup?

Abstract

Variably oriented dolerite intrusions outcrop in the Albany–Fraser Orogen along the south coast of Western Australia with previously unknown ages but where previous studies interpreted Mesoproterozoic to Cretaceous emplacement. Here, we place temporal constraints on seven mafic intrusions across ∼150 km of coast using zircon U–Pb, apatite U–Pb, and plagioclase ⁴⁰Ar/³⁹Ar geochronology, coupled with whole-rock major and trace-element geochemistry, that reveal late Mesoproterozoic to potentially Early Cretaceous crystallisation ages. Three intrusions metamorphosed to greenschist facies are likely associated with either the emplacement of the ca 1210 Ma Marnda Moorn large igneous province or Stage II Albany–Fraser Orogeny, both of which were associated with the assembly of Rodinia. Three unmetamorphosed dykes have (probable) Neoproterozoic to lower Cambrian emplacement ages, likely associated with the ca 550–500 Ma Kuunga Orogeny during Gondwana assembly. The final sill, also unmetamorphosed, strikes perpendicular to the other six intrusions, shows unusual Pb anomalies and contains inherited zircon that has been reset by a Permian or younger event, pointing towards magmatism in southwestern Australia during the breakup of Gondwana. The new results provide hitherto unrecognised mafic intrusive evidence for modification of Proterozoic crust, potentially associated with Rodinia assembly, Gondwana assembly and Gondwana breakup in southwestern Australia.

KEY POINTS

1. Variably oriented mafic dykes in southwest Australia are dated by zircon U–Pb, apatite U–Pb and plagioclase ⁴⁰Ar/³⁹Ar methods.

2. The dykes are related to Rodinia assembly (ca 1200 Ma), Gondwana assembly (ca 550 Ma) and, probably, Gondwana breakup (ca 135 Ma).

3. These new ages provide evidence for mafic activity clearly linked to the supercontinent cycle.

Keywords:

• dyke
• ⁴⁰Ar/³⁹Ar
• Albany–Fraser orogen
• U–Pb
• Rodinia

Acknowledgements

The authors thank the following for permission to sample rocks: the Department of Parks and Wildlife for permission to sample from national parks (Knapp Head, Elephant Rocks, Point Irwin and Long Point; permit no. CE005761 and CE005770), the City of Albany (Lowlands Beach, permit no. EF18303832), the Shire of Denmark (Parry Beach, permit no. A5585) and the Department of Planning, Lands and Heritage (Whaling Cove). R. E. Ernst, L. Harris, E. Scibiorski and three anonymous reviewers are thanked for critical insight into earlier versions of this manuscript. The Wood family is thanked for sampling assistance.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Additional information

Funding

This work was partially funded by Australian Antarctic Science Project #4446 to H.K.H.O. and F.J. and internal grants to C.E. Analysis in the JdLC GeoHistory Facility is enabled by AuScope (auscope.org.au) and the Australian Government via the National Collaborative Research Infrastructure Strategy (NCRIS). The Tescan Mira3 FEG-SEM, Tescan TIMA FEG-SEM with four PulsTor SDD X-ray detectors, and the SelFrag at the JdLC were acquired through Australian Research Council LIEF program (LEI130100053, LE140100150 and LE130100219, respectively).