Garnet websterites and associated ultramafic inclusions from a nepheline mugearite in the Walcha area, New South Wales, Australia

Abstract

ABSTRACT. A nepheline mugearite dyke in the Walcha area, New South Wales, contains xenoliths of Cr-diopside lherzolite, garnet-and spinel websterites, and twopyroxene-garnet granulites. The Cr-diopside lherzolite xenoliths are regarded as refractory upper mantle which finally equilibrated at c. 950-980 ~ The garnet websterites are considered to represent spinel websterite segregations from an Mg-rich olivine tholeiite magma at 14 16 kbar which on isobaric cooling to 1000 1050 ~ passed into the garnet pyroxenite stability field. Spinel-amphibole websterites which probably last equilibrated at c. 950 ~ and pressures of 9 12 kbar have dose mineralogical affinites with the Cr-diopside lherzolite xenoliths. The mafic granulites may represent metamorphosed crystal segregates resulting from relatively large degrees of crystallization of tholeiitic basaltic magma which finally equilibrated at c. 14 kbar and 1000 ~ Subcalcic dinopyroxene and spinel megacrysts crystallized from the host nepheline mugearite at c. 13 20 kbar.

Diamonds from nepheline mugearite? A discussion of ‘Garnet websterites and associated ultramafic inclusions from a nepheline mugearite in the Walcha area, New South Wales, Australia’

“…The Brickclay Creek xenoliths record mantle pressures and temperatures around 14-16 kbar and 1000-1050 ~ (Ellis and Green, and Harley and Green methods; Stolz, 1984). Thus, this mantle sample records a relatively 'hot' palaeogeotherm, considered typical of the eastern Australian continental margin since the Palaeozoic (Griffin et al, 1984).…”

“…Kaersutite is recorded at Brickclay Creek with TiO 2 4.8 Y/o (Stolz, 1984). This suggests its possible crystallization around 14-17 kbar based on the geobarometer of Oba et al (1982) as used by Hollis et al (1983).…”

“…Its role in fractionation was considered unlikely by Stolz (1984), as it is not a near-liquidus phase in most experimental runs on natural basaltic compositions above 10 kbar. However, Arculus et al (1977), found it up to 15 kbar in the presence of CO2 in the basaltic basanitic system.…”

“…A discussion of 'Garnet websterites and associated ultramafic inclusions from a nepheline mugearite in the Walcha area, New South Wales, Australia' STOLZ (1984) gives a thorough description of xenoliths in this dyke and discusses their likely origins and pressure-temperature (PT) equilibration regimes. He also comments on the megacrysts and host rock.…”

“…Stolz (1984) divided Brickclay Creek xenoliths into Group I and II types representing Cr and Ti-A1 enriched series in the classification of Wilshire and Shervais (1975). The spinel-amphibole websterites were included with Group II, but are transitional between the two extremes and indicate hydrous crystallization in the mantle assemblage.…”