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

Abstract

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 megacrystic 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.

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

“…The mantle lithosphere below Barrington, however, clearly contains amphibole-bearing lithologies that can potentially produce corundum-normative felsic melts (Sutherland et al, 1998b). Zircon megacrysts at Barrington show magmatic growth features and although their fission track ages are reset by basalt transport events, their U-Pb formation ages cluster at 50À60 Ma and 45À46 Ma (Sutherland and Fanning, 2001). Thus, magmatic sapphire and zircon formation below the Barrington field was closely related to the Palaeogene basaltic magmatism.…”

“…The analyses used similar methods and statistical treatments to those reported for zircon megacrysts from the Barrington basaltic gemfields (see Sutherland and Fanning, 2001;Roberts et al, 2004).…”

‘Boomerang’ migratory intraplate Cenozoic volcanism, eastern Australian rift margins and the Indian-Pacific mantle boundary

“…Source signatures differ across the East Australian Plume System zone, with phlogopite-enriched sources for western leucitites, OIB-like plume components for the central volcanoes and an EMI plume and 'long term depleted' upper mantle interaction for eastern basaltic seamounts. Some basalts indicate interaction of OIB-like melts with metasomatised amphibole (+apatite)-enriched mantle lithosphere (O'Reilly & Zhang 1995;Sutherland et al 1996Sutherland et al , 2002Sutherland & Fanning 2001;Zhang et al 2001). They were called HIMU-like in their trace-element patterns, but lacked the high radiogenic Pb of true HIMU (Zhang et al 2001).…”

“…Some basalts include amphibole geochemical imprints and HIMU isotopic features but not Tasman HIMU trace-element ratios (Sutherland et al 1996). Others show Tasman HIMU-like trace-element ratios but lack amphibole (+apatite) geochemical imprints, while coexisting basalts show amphibole geochemical imprints but lack Tasman HIMU-like trace-element ratios (Sutherland & Fanning 2001;Sutherland et al 2001). The amphibole imprints largely occur outside the East Australian Plume System migratory fields, but minor diluted contributions are found within East Australian Plume System basalts (Sivell & Ashley 2000).…”

Plume-induced dynamic instabilities near cratonic blocks: Implications for P–T–t paths and metallogeny

“…Yui et al, 2003) and gem-bearing basaltic volcanism has already been suggested to derive from a mantle plume activity (Sutherland and Fanning, 2001). …”