Journal of Volcanology and Geothermal Research 99 (2000) 55–78 www.elsevier.nl/locate/jvolgeores The Whitsunday Volcanic Province, Central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province S.E. Bryana,*, A. Ewart1,a, C.J. Stephens2,b, J. Parianos3,c, P.J. Downes4,d aDepartment of Earth Sciences, University of Queensland, St Lucia, Queensland 4072, Australia bCentral Norseman Gold Corporation Limited, P.O. Box 56, Norseman, Western Australia 6443, Australia cQNI Limited, PO Box 7879 Waterfront Place, Brisbane, Queensland 4001, Australia dDepartment of Earth and Planetary Sciences, Western Australian Museum of Natural Science, Francis St, Perth, Western Australia 6000, Australia Received 16 June 1999; accepted 21 December 1999 Abstract Contrary to general belief, not all large igneous provinces (LIPs) are characterised by rocks of basaltic composition. Silicic- dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly recognised in the rock record. These silicic LIPs are consistent in being: (1) volumetrically dominated by ignimbrite; (2) active over prolonged periods (40–50 m.y.), based on available age data; and (3) spatially and temporally associated with plate break-up. This silicic- dominated LIP, related to the break-up of eastern continental Gondwana, is also significant for being the source of Ͼ 1:4 × 106 km3 of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia. The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal sections. Pyroclastic surge and fallout deposits, subordinate basaltic to rhyolitic lavas, phreatomagmatic deposits, and locally significant thicknesses of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignim- brites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (up to 50 m in width), sills and comagmatic granite. Dyke orientations are primarily from NW to NNE. The volcanic sequences are characterised by the interstratification of proximal/near-vent lithofacies such as rhyolite domes and lavas, and basaltic agglomerate, with medial to distal facies of ignimbrite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicates a low-relief depositional environment. Furthermore, the volcanic succession records a temporal change in: (1) eruptive styles; (2) the nature of source vents; and (3) erupted compositions. An early explosive dacitic pyroclastic phase was succeeded by a later mixed pyroclastic- effusive phase producing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From the nature and distribution of * Corresponding author. Tel.: ϩ 61-7-3365-3057; fax: ϩ 61-7-3365-1277. E-mail addresses: [email protected] (S.E. Bryan), [email protected] (A. Ewart), [email protected] (C.J. Stephens), [email protected] (J. Parianos), [email protected] (P.J. Downes). 1 Fax: ϩ 61-7-3365-1277. 2 Fax: ϩ 61-8-9039-9801. 3 Fax: ϩ 61-7-3229-2398. 4 Fax: ϩ 61-8-9427-2882. 0377-0273/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved. PII: S0377-0273(00)00157-8 56 S.E. Bryan et al. / Journal of Volcanology and Geothermal Research 99 (2000) 55–78 volcanic lithofacies, the volcanic sequences are interpreted to record the evolution of a multiple vent, low-relief volcanic region, dominated by several large caldera centres. ᭧ 2000 Elsevier Science B.V. All rights reserved. Keywords: Early Cretaceous; large igneous province; Eastern Australia; ignimbrite; volcanic evolution 1. Introduction nature of the crustal source and absence(s) of precursory magmatic episodes, which will remove Large igneous provinces (LIPs) are classically the more fusible components from the lithosphere considered to be massive crustal emplacements of (e.g. Gibson et al., 1992), are thought to facilitate predominantly extrusive and intrusive mafic magma crustal partial melting. Considerable debate has that are manifest as continental and ocean basin flood centred on the petrogenesis of large volumes of rhyo- basalts, oceanic plateaux, volcanic passive margins, lite magma, and the relative roles of fractional crystal- submarine ridges, and seamount groups (Coffin and lisation, assimilation/fractional crystallisation, and Eldholm, 1994). They show a temporal relationship to magma mixing (see discussion in Pankhurst et al., continental plate break-up, and particularly character- 1998; Ewart et al., 1998a,b). ise the Jurassic–Cretaceous break-up history of Gond- Little is known in detail of these silicic-dominated wana (e.g. Karoo, Parana–Etendeka continental flood LIPs. Only overviews of the lithological variation basalts, Ferrar Group and oceanic plateaux: Ontong- have been provided, while the stratigraphy, volcanic Java, Kerguelen, Manihiki; Coffin and Eldholm, 1994 architecture and evolutionary trends remain uncon- and references therein; Elliot, 1992; Cox, 1992). strained. Caldera complexes, thought to be source Emplacement of continental flood basalt provinces vents for the silicic volcanism, have proved difficult typically occurs over time scales of 105–106 years to identify due to the scale of volcanism (e.g. up to (Coffin and Eldholm, 1994). 2000 km strike length), burial, faulting, and later Some LIPs are known to have a significant silicic deformation. component, such as the Lebombo part of the This paper describes in detail, the lithology and Karoo (e.g. Cleverly et al., 1984), and the Parana– stratigraphy of Early Cretaceous volcanic rocks of Etendeka (e.g. Milner et al.,1992) continental flood the Whitsunday Volcanic Province (Fig. 1), thereby basalt provinces of southern Africa. However, it allowing constraints to be placed on the volcanic is increasingly recognised that LIPs can be architecture and evolution of this silicic-dominated silicic-dominated, such as the Early Cretaceous volca- LIP. This is achieved by focussing on the well nic passive margin of eastern Australia (Bryan et al., exposed and tilted volcanic sequences in the Molle 1997) and the almost entirely rhyolitic Lower–Middle Group of islands (Bryan, 1991), which are located Jurassic Chon-Aike Province of South America centrally within the main part of the province (Fig. (Pankhurst et al., 1998; Riley and Leat, 1999). 2). Detailed accounts of the mainland, Lindeman and These silicic-dominated LIPs are: (1) volumetrically southeast Whitsunday Island sequences (Fig. 2) by dominated by ignimbrite; (2) active over prolonged Parianos (1993); Downes (1991); Finnis (1999), periods (40–50 m.y.), based on available age data; respectively, are used to provide regional constraints and (3) spatially and temporally associated with on the stratigraphy and volcanic architecture. This LIP plate break-up. in eastern Australia is unusual in that a large propor- The generation of large volumes of rhyolite, ulti- tion of its products are preserved as huge volumes mately related to hot mantle upwelling and intrusion Ͼ1:4 × 106 km3 of coeval volcanogenic sediment of basaltic magma into the crust, is thought to be due in adjacent sedimentary basins (Fig. 1; see Bryan et to the fertile nature of the crust. For both eastern al., 1997). Such substantial volumes of coeval volca- Australia and the Chon-Aike Province, the basement nogenic sediment are not characteristic of other LIPs. comprises Palaeozoic–Mesozoic volcanic and sedi- Determining the character of this volcanic source mentary rocks accreted and/or deposited along conti- terrane is important to shed light on why so much nental margins (Ewart et al., 1992; Stephens et al., fine-grained volcanogenic sediment was generated at 1995; Pankhurst et al., 1998). The relatively hydrous this time. S.E. Bryan et al. / Journal of Volcanology and Geothermal Research 99 (2000) 55–78 57 o 148 E Townsville • 129 o 103 ? 150 o 120,106 20 S 116 119 Whitsunday Volcanic 142,134 131 Province (132-95Ma) 126 Zone of Early Cretaceous 132 120 silicic pyroclastic volcanism 136-142 125-131• of Clarke et al. (1971) Mackay 120 o 152 E o 22 122-130 Shoalwater Bay 136 ? 134o E 152o E Rockhampton • Maryborough 12o S Laura Basin Co ral Se 12o S Basin a B as o in 24 18o Marion Plateau 141 rough 0200142 o Km 24 Great Cato T Artesian 99 101 Basin system Grahams Creek 30o Formation 140 138,140 AUSTRALIA o 101 Age date (ages in bold 26 S o Tasman are SHRIMP dates, 36 Gippsland 136 Basin Basin all others are 36o S 140,144-146 Otway K/Ar dates) Basin 0 500 109,145 Km ? o 42 S • Town/city 137 o o o o o 137 134 E 140 146 152 158 E Brisbane• Fig. 1. Distribution of Early Cretaceous magmatism in eastern Queensland (Australia). The light shaded region represents the Whitsunday Volcanic Province, whereas the darker shade represents other Early Cretaceous igneous units of eastern Queensland. Age data sources: Allen et al. (1998); Ewart et al., (1992); Groen (1993); Parianos (1993) and references therein; Everndern and Richards (1962); Webb and McDougall (1968); Ellis (1968). K/Ar age dates have been corrected with reference to Steiger and Ja¨ger (1977). Inset map shows the location of Early Cretaceous magmatism in eastern Queensland