Australian Journal of Earth Sciences (2001) 48, 221–237 INTRODUCTION Geological setting of Barrington volcano Barrington volcano is an eroded basaltic shield forming an isolated residual plateau east of the Great Escarpment (Pain 1983), 100 km north-northeast of Newcastle, New South Wales. It forms part of the larger Barrington volcanic province, mostly constructed in the Early Palaeogene (Wellman & McDougall 1974a). The basic stratigraphy, erosional features and petrology of this remnant volcano are described in Wellman et al. (1969), Mason (1982, 1989), Pain (1983), and O’Reilly and Zhang (1995). The volcano formed during widespread Cenozoic volcanism in east Australia (Wellman & McDougall 1974b; Johnson 1989; Sutherland 1991) and its relationship to this activity is shown in Figure 1. A feature of Barrington volcano is its association with gem minerals, including corundum (ruby, sapphire) and zircon (Sutherland 1996; Webb 1997). Detailed studies of the corundums indicate that both metamorphic and magmatic underlying source rocks were tapped by the basaltic eruptions (Sutherland & Coenraads 1996; Sutherland et al. 1998b). Preliminary fission track and U–Pb isotope dating of zircons has suggested several ages of zircon formation and eruptive discharge during Early Palaeogene to Late Neogene time (Sutherland 1993). This paper presents further age determinations and geochem- istry on basalts and zircons from Barrington volcano, particularly from the eastern side, which have received little attention in previous studies. Grid locations cited in the paper are taken from 1:100 000 topographic sheets [e.g. Upper Manning (sheet no. 9234)]. The present basalt distribution and drainage in relation to published dating sites and sampling sites in this study are shown in Figure 2. The new data provide a basis to discuss the long-term evolution of this gem-bearing volcano. Gem-bearing basaltic volcanism, Barrington, New South Wales: Cenozoic evolution, based on basalt K–Ar ages and zircon fission track and U–Pb isotope dating* F. L. SUTHERLAND 1 † AND C. M. FANNING 2 1 Mineralogy/Petrology Section, Australian Museum, 6 College Street, Sydney, NSW 2010, Australia. 2 Research School of Earth Sciences, Australian National University, ACT 0200, Australia. Barrington shield volcano was active for 55 million years, based on basalt K–Ar and zircon fission track dating. Activity in the northeast, at 59 Ma, preceded more substantial activity between 55 and 51 Ma and more limited activity on western and southern flanks after 45 Ma. Eruptions brought up mega- crystic gemstones (ruby, sapphire and zircon) throughout the volcanism, particularly during quieter eruptive periods. Zircon fission track dating (thermal reset ages) indicates gem-bearing eruptions at 57, 43, 38, 28 and 4–5 Ma, while U–Pb isotope SHRIMP dating suggests two main periods of zircon crystallisation between 60 and 50 Ma and 46–45 Ma. Zircons show growth and sector twinning typical of magmatic crystallisation and include low-U, moderate-U and high-U types. The 46 Ma high-U zircons exhibit trace and rare-earth element patterns that approach those of zircon inclusions in sapphires and may mark a sapphire formation time at Barrington. Two Barrington basaltic episodes include primary lavas with trace-element signatures suggesting amphibole/apatite-enriched lithospheric mantle sources. Other basalts less-enriched in Th, Sr, P and light rare-earth elements have trace-element ratios that overlap those of HIMU-related South Tasman basalts. Zircon and sapphire formation is attributed to crystallisation from minor felsic melts derived by incipient melting of amphibole-enriched mantle during lesser thermal activity. Ruby from Barrington volcano is a metamorphic type, and a metamorphic/metasomatic origin associated with basement ultramafic bodies is favoured. Migratory plate/plume paths constructed through Barrington basaltic episodes intersect approximately 80% of dated Palaeogene basaltic activity (65–30 Ma) along the Tasman margin (27–37°S) supporting a migratory plume-linked origin. Neogene Barrington activity dwindled to sporadic gem-bearing eruptions, the last possibly marking a minor plume trace. The present subdued thermal profile in northeastern New South Wales mantle suggests future Barrington activity will be minimal. KEY WORDS: basalt, Cenozoic, fission track dating, potassium–argon dating, ruby, sapphire, uranium–lead dating, zircon. *Tables 7–9 [indicated by an asterisk (*) in the text and listed at the end of the paper] are Supplementary Papers lodged with the National Library of Australia (Manuscript Section); copies may be obtained from the Business Manager, Geological Society of Australia. † Corresponding author: lins@austmus.gov.au