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

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 ac tivity on western and southern flanks after 45 Ma. Eruptions brought up meg acrystic gemstones (ruby, sapphire and zircon) throughout the volcanism, pa rticularly during quieter eruptive periods. Zircon fission track dating (th ermal 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 c rystallisation between 60 and 50 Ma and 46-45 Ma. Zircons show growth and s ector twinning typical of magmatic crystallisation and include low-U, moder ate-U and high-U types. The 46 Ma high-U zircons exhibit trace and rare-ear th element patterns that approach those of zircon inclusions in sapphires a nd may mark a sapphire formation time at Barrington. Two Barrington basalti c episodes include primary lavas with trace-element signatures suggesting a mphibole/apatite-enriched lithospheric mantle sources. Other basalts less-e nriched in Th, Sr, P and light rare-earth elements have trace-element ratio s that overlap those of HIMU-related South Tasman basalts. Zircon and sapph ire formation is attributed to crystallisation from minor felsic melts deri ved by incipient melting of amphibole-enriched mantle during lesser thermal activity. Ruby from Barrington volcano is a metamorphic type, and a metamo rphic/metasomatic origin associated with basement ultramafic bodies is favo ured. Migratory plate/plume paths constructed through Barrington basaltic e pisodes intersect approximately 80% of dated Palaeogene basaltic activity ( 65-30 Ma) along the Tasman margin (27-37 degreesS) supporting a migratory p lume-linked origin. Neogene Barrington activity dwindled to sporadic gem-be aring eruptions, the last possibly marking a minor plume trace. The present subdued thermal profile in northeastern New South Wales mantle suggests fu ture Barrington activity will be minimal.