The You Yangs batholith in Southeastern Australia, the sources of its magmas and inferences for local crustal architecture

Abstract

The 365-Ma You Yangs batholith is a mainly I-type monzogranitic body, containing rocks with both clinopyroxene and hornblende, but with a 2–2.5 km-wide rim of S-type rocks. In places, the margins of the intrusion wedge out laterally. A laccolithic shape may explain there being only low-grade contact metamorphism of the Ordovician metasedimentary wall rocks. The chemical and isotopic characteristics of the granitic rocks suggest that the magmas formed by partial melting of a source that contained some meta-igneous rocks but was dominated by chemically immature metasedimentary types, to impart an evolved Sr isotope signature (⁸⁷Sr/⁸⁶Sr_t = 0.70877–0.71066 for the main monzogranitic rocks), combined with relatively non-radiogenic εNd_t (–2.4 to –1.9). Crystal fractionation played little role in shaping the compositions of the granitic magmas, with the main variations interpreted to be source-inherited. Igneous-textured microgranular enclaves (IMEs) are prominent in the monzogranitic rocks. The IMEs probably had an ultimate enriched-mantle source, and their magmas did not mix significantly with the crustally derived granitic host magmas. The characteristics of the monzogranitic rocks hosting the enclaves suggest the possibility that an unrecognised metasediment-dominated terrane of ancient arc crust may be present beneath the Bendigo Zone.

KEYWORDS:

• You Yangs
• batholith
• granite
• igneous microgranular enclaves
• petrogenesis
• basement geology

Acknowledgements

Dr Bill Birch (Museums Victoria) provided a copy of George Baker’s 1935 paper, arranged access to and assisted with collecting samples within the Mount Rothwell Conservation and Research Centre (north of the You Yangs Regional Park) in November 2014, accompanied the author on fieldwork in the regional park (in December 2016) and took the photographs used in Figures 4a and 6a. He also discovered a duplicate set of thin-sections made by George Baker in 1935, arranged for the loan of these, and read and commented on an earlier draft of the manuscript. I also acknowledge Parks Victoria and the Victorian Department of Environment, Land, Water and Planning for providing a permit to carry out the geological research, and according access to the park. Accommodation in 2017 was kindly provided by Prof. Neil Phillips and Profs. Pat and Tom Rich. Neil also read and commented on a draft of the manuscript. The oxygen isotope analysis of sample YY1 was kindly supplied by Prof. Chris Harris at the University of Cape Town. The paper was reviewed by Ron Vernon and Todd Waight, who provided many useful comments and suggestions.

Disclosure statement

No potential conflict of interest was reported by the author.

Funding

Some travel, accommodation, fieldwork and analytical costs associated with this paper were funded through the former South African NRF scheme of Incentive Funds for Rated Researchers and a Stellenbosch University DRD Travel Grant to undertake a research visit abroad. In connexion with the DRD grant I also thank the University of Stellenbosch for a 2-month leave of absence to take up the research opportunity, and Monash University (Australia) and Prof. A. R. (Sandy) Cruden (then Head of the School of Earth, Atmosphere and Environment) for hosting the visit in 2017. I also thank Dr Julian Vearncombe, of SJS Resource Management and Geosymposia.com.au, for covering my air fare from Cape Town to Melbourne in 2017, in connexion with the Granites2017@Benalla field symposium organised under the aegis of the Australian Institute of Geoscientists and the Geological Society of Australia.

Supplementary papers

Appendix A1. Atlas of textures in the rocks of the You Yangs pluton.

Appendix A2. Analytical details.

Appendix A3. Whole-rock major- and trace-element data (excel file).