The petrology of high pressure xenoliths and associated Cenozoic basalts from Northeastern Tasmania
Introduction
Cenozoic volcanism is an important feature in the geological evolution of Tasmania (Sutherland, 1969). Xenoliths, cognate inclusions, xenocrysts and megacrysts of upper mantle or lower crustal origin are recorded at more than 190 localities throughout Tasmania (Wass and Irving, 1976, Varne, 1977, Everard, 2001). Most host rocks are lavas, rather than pyroclastics, in contrast to some well-studied localities in the later basaltic fields of Victoria (∼ 7–0 Ma) (Duncan and McDougall, 1989, Price et al., 2003) and in North Queensland (< 9 Ma) (Zhang et al., 2001). Because pyroclastics are relatively easily eroded, they are less commonly preserved in the older Tasmanian volcanics. Since collection and extraction is more difficult from tough lavas, Tasmanian xenoliths have been less extensively studied.
Host rocks are strongly biased towards undersaturated compositions (e.g. olivine nephelinite and basanite), with a sole occurrence in olivine tholeiite (Sutherland, 1974). Mantle xenoliths are also widespread in the more fractionated alkaline lineages of the hawaiite–mugearite and equivalent feldspathoidal lineages (e.g. Sutherland, 1974, Everard, 2001).
By far the most common inclusions are spinel lherzolites of the Cr-diopside suite. Garnet lherzolite xenoliths are known at one Tasmanian locality, Bow Hill near Oatlands (Sutherland et al., 1984). Pyroxene-rich members of the Cr-diopside suite are relatively rare in eastern Australia (O'Reilly et al., 1989) and have been located at Bow Hill and Table Cape (Sutherland et al., 2005) and Round Lagoon (Sutherland et al., 2008). However, at about 30 sites, spinel lherzolite is accompanied by Al-augite suite inclusions (mostly websterite and wehrlite). Other xenoliths, of probable lower to mid-crustal origin, include granulite, gabbro, dolerite and anorthosite. Megacryst species reported include clinopyroxene, orthopyroxene, olivine, spinels, kaersutitic amphibole, titanbiotite and titanphlogopite, apatite, alkali feldspar (anorthoclase, sanidine), plagioclase (albite to labradorite) and titanomagnetite. Zircon, corundum (sapphire), and possibly diamond occur as heavy minerals in alluvial deposits derived from Cenozoic basalts, but are very seldom found in situ (Everard, 2001). The petrology of mantle-derived and other high pressure inclusions from Australia, including Tasmania, has been reviewed by numerous authors (Frey and Green, 1974, Sutherland, 1974, Wass and Irving, 1976, Griffin et al., 1984, Wass and Shaw, 1984, Dal Negro et al., 1984, Griffin et al., 1987, Griffin et al., 1988, O'Reilly et al., 1989, Wilkinson and Stolz, 1997, Roach, 2004). More detailed information on particular Tasmanian localities is provided by Sutherland, 1974, Varne, 1977, McClenaghan et al., 1982, Everard, 1989, Sutherland et al., 1984, Sutherland et al., 1996, Sutherland et al., 2005.
Because of their small size, there are few whole rock analyses of Tasmanian xenoliths (McClenaghan et al., 1982). Varne (1977) studied nine xenoliths from eight localities in detail, and attributed the textures and chemistry of co-existing minerals to exsolution of spinel from aluminous pyroxenes with falling temperature. He noted that they were broadly similar to spinel lherzolite inclusions elsewhere in the world, and concluded that they were accidental inclusions of upper mantle, genetically unrelated to their host rocks.
This paper aims to provide new insights into the petrology and geochemistry of the lithosphere underneath Tasmania.
Section snippets
Tectonic and geological setting
Tasmania and the surrounding continental shelf comprise the southern protuberance of the Australian continent, which separated from Antarctica at 95 ± 5 Ma, although northward drift was very slow until 45 Ma (Veevers et al., 1991). Bass Strait, although mostly only 30–90 m deep (water), represents a failed rift, and locally more than 10 km of Cretaceous and Cenozoic sediments in the Bass Basin rest on thinned continental crust. Rifting east of Tasmania also commenced in the Late Cretaceous, with the
Sample localities
In Northeast Tasmania, Cenozoic basalts are widespread and display a wide range of age and composition, comparable to the rest of Tasmania, although olivine melilitites and strongly evolved alkaline types are absent. The samples of this study have been collected from localities listed in the electronic supplementary file (Appendix A) and shown in Fig. 1. The occurrences are all associated with magnetic anomalies, evident in the Northeastern Tasmanian Survey conducted by Mineral Resources
Bulk-rock major and trace element composition
Samples SDL (The Sideling) and CPJ36 (Sandy Creek) were analysed using conventional XRF methods at Mineral Resources Tasmania. Calibration was by means of international rock standards. The other samples were analysed for major and trace element concentrations, loss on ignition (LOI) and Pb, Sr, and Nd isotope abundances at the Activation Laboratories Ltd., Ancaster, Canada. Sample powders were prepared in an agate planetary ball mill. Major elements analyses were performed on ICP
Host rocks
Three of the four host rocks are porphyritic olivine nephelinites, and lack modal plagioclase. The best crystallized sample, from Sandy Creek, contains euhedral to subhedral olivine phenocrysts (≤ 2 mm) and anhedral olivine xenocrysts, grading down to a fine-grained, mostly intergranular groundmass of olivine granules, pinkish titaniferous augite prisms (≤ 200 μm), titanomagnetite granules mostly (10–50 μm) and a mesostasis of clear nepheline and finely acicular apatite. In places, poikilitic
Mineral chemistry
Xenoliths from each locality have similar mineral chemistry. Olivine falls within the range Fo88–92, with NiO contents between 0.30–0.47 wt.% and CaO ≤ 0.10 wt.% (Appendix A), typical for Type I xenoliths worldwide (Frey and Prinz, 1978). Magmatic olivine phenocrysts in the associated basanite are Mg-rich (Fo86.5) with NiO content of 0.31 wt.% , while the Mg# of the groundmass olivine ranges between (Fo77–84) with NiO content of 0.16–0.29 wt.%. The clinopyroxene in the xenoliths also has a limited
Host rocks
We present major and trace element analyses of fresh host rock samples from the Sideling, Warrentinna and Sandy Creek localities in Table 1. A sample from each locality was analysed by the XRF method, and trace elements of a further three samples each from Warrentinna and Sandy Creek were analysed by the ICP–MS method.
The host rocks are all alkaline volcanics, with low SiO2 and relatively high total alkalis. The most undersaturated host is at The Sideling, which plots in the foidite field (F)
P–T estimates
Several geothermometers have been proposed that are applicable to these xenoliths (e.g. Wells, 1977, Mercier, 1980, Sachtleben and Seck, 1981, Brey and Koehler, 1990, Witt-Eickshen and Seck, 1991, Taylor, 1998). Equilibration temperatures for the studied xenoliths have been estimated using the Wells, 1977, Brey and Koehler, 1990 two pyroxene geothermometers and the Sachtleben and Seck (1981) and the Brey and Koehler Ca-in-orthopyroxene geothermometers (Table 4). The Wells, 1977, Brey and
Oxygen fugacity
Various oxybarometers have been widely applied to spinel peridotite xenoliths as a method to estimate the redox state of the upper mantle (e.g., O'Neill and Wall, 1987, Wood, 1990, Ballhaus, 1993). We have used the oxygen barometer of O'Neill and Wall (1987) to calculate oxygen fugacities for the investigated xenoliths. These are given relative to the FMQ (fayalite–magnetite–quartz) buffer at 1.5 GPa (Table 4). Oxygen fugacity is little affected by errors in determining the ferric iron content
Mantle source characteristics
Melt modelling presented in Fig. 13 indicates that the incompatible trace element signature of the Tasmanian volcanic rocks is consistent with 2–5% partial melting of a garnet-bearing asthenospheric mantle source. However, Fig. 13d suggests that mixing of melts from both spinel and garnet lherzolite facies mantle (0.9 gt, 0.1 sp) may also to have occurred. This mixing might have occurred during diapiric mantle upwelling and/or due to remelting of a metasomatized spinel peridotite facies mantle
Conclusions
Mantle xenolith-bearing Cenozoic basanites and nephelinites are common in the northeastern parts of Tasmania. The host rocks include primitive features with OIB-like geochemical signatures. Trace element and Pb– and Sr–Nd isotope data for the rocks suggest mixing of melts derived from low degree (< 5%) of melting of both garnet- and spinel lherzolite facies mantle source with HIMU and EMII characteristics.
The mantle xenoliths are mainly anhydrous spinel lherzolite and spinel pyroxenite. Mantle
Acknowledgements
Constructive reviews of this paper by Lin Sutherland and an anonymous reviewer and valuable comments from Robert Stern are gratefully acknowledged. We thank Andrew Kerr for his excellent editorial work. We also thank Thomas Theye for his help with microprobe analyses and David Seymour (MRT) for help with the geophysical modelling.
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