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Abstract
Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.
Acknowledgements
We thank other members of the Yarrol Project Team (Simon Crouch, Jan Domagala, Barry Fordham, Mark Hayward, Allan Robertson, and Glenn Simpson) for discussions on the tectonic evolution of the Yarrol Province, and for provision of samples. John Jell coped with extremely wet and difficult conditions to assist with the collection of basalt samples for this study. Phil Blevin, Bob Stern, and Julian Pearce generously provided unpublished geochemical data on the Mt Morgan Trondhjemite and Mariana Trough basalts. We are grateful to Queensland Health Scientific Services, in particular Henry Olszowy and John Hegarty, for XRF analyses on major and trace elements. Together with trace-element analyses by laser ablation and ICP-MS by Greg Yaxley of PRISE at the Research School of Earth Sciences, Australian National University, the geochemical data comprise a comprehensive and consistent dataset. The figures were drawn by Lesley Blight of the Graphic Services Unit, Natural Resource Sciences. Paul Messenger, Vince Morand, and Alex Taube readily responded to requests for information about specific issues. Reviews by Paul Messenger and Peter Flood improved the manuscript by focusing our arguments on crucial issues. Finally, we wish to thank Scott Bryan for breathing new life into the ongoing debate about the early history of the Yarrol Province: although we disagree with his model, his new ideas provided the impetus for us to re-examine and refine ours.
Notes
*Appendix 1 [indicated by an asterisk (*) in the text and listed at the end of the paper] is a Supplementary Paper; copies may be obtained from the Geological Society of Australia's Web site (www.gsa.org.au) or from the National Library of Australia's Pandora archive (http://nla.gov.au/nla.arc-25194).