Subcalcic clinopyroxenites and associated ultramafic xenoliths in alkali basalt near Glen Innes, northeastern New South Wales, Australia

Abstract

An alkali basalt near Glen Innes, northeastern New South Wales, contains a suite of Cr-diopside group ultrama®c xenoliths which includes some spinel peridotites but which is dominated by a diverse spinel pyroxenite assemblage. Pyroxenite xenoliths range from subcalcic clinopyroxenites (composed largely of unmixed prismatic subcalcic clinopyroxene megacrystals and lesser orthopyroxene megacrystals) to equant mosaic textured websterites (orthopyroxene and Ca-rich clinopyroxene AEspinel. Rare orthopyroxenite xenoliths also occur. The pyroxenite xenoliths are characterised by high 100MgaMg Fe 2 ratios (M$ 90 and low concentrations of Ti, K, P, La, Ce and Zr. The websterites are mineralogically and chemically similar to many spinel pyroxenites occurring as layers or dykes in peridotite massifs such as those at Ronda in southern Spain and at ArieÁ ge (French Pyre ne es). a estimates indicate crystallization temperatures of 1250±1350°C for subcalcic clinopyroxene-orthopyroxene megacrystal pairs and 900± 1000°C for the equilibrated mosaic textured websterites and associated peridotites at pressures of 9±13 kbar. Subcalcic clinopyroxene megacrystals, websterites and orthopyroxenites have LREE-depleted chondrite-normalised REE abundances with La/Yb CN`1 and their convex-upwards REE patterns are typical of subcalcic clinopyroxene-dominated cumulates. The pyroxenites are not residua from partially melted pyroxenite layers or dykes in mantle peridotites nor are they completely crystallized protobasaltic or protopicritic magmas. They are interpreted as high-pressure crystal segregations from basaltic magmas (probably mildly alkaline or transitional)¯owing within narrow mantle conduits (the¯ow crystallization model of Irving, 1980). The parental magma(s) was Ti-poor (0.6±0.7% TiO 2 ) and relatively Mg-rich (M $ 74À70. Pyroxenite genesis was a two-stage process involving crystallization of tschermakitic subcalcic clinopyroxenes and orthopyroxenes AEspinel as liquidus or near-liquidus phases at 1250±1350°C and 9± 13 kbar to yield``primary'' subcalcic clinopyroxenites which then re-equilibrated at 900±1000°C and similar pressures to produce the mosaic textured``secondary'' websterites. The pyroxenites show a wide range of 143 Nda 144 Nd and 87 Sra 86 Sr values (0.513298±0.512473 and 0.702689±0.704659, respectively). Their isotopic ratios appear to have been variably modi®ed by exchange with adjacent mantle peridotites or migrating basaltic melts.Editorial responsibility: R.A. Binns Petrography and mineralogy of pyroxenite xenolithsPetrography Four basic mineralogical and textural types of pyroxenite can be distinguished.

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

“…Such high temperatures, beyond the stability of Ti-rich amphibole (Niida and Green 1999), may explain the lack of amphibole in all Group VI pyroxenites. This P-T evolution-cooling from crystallisation temperatures to *950-1,100°C at pressures between 0.9 and 1.7 GPa-is similar to that of other pyroxenite xenoliths entrained by intraplate alkaline magma (Wilkinson and Stolz 1997).…”

Geochemistry of pyroxenitic and hornblenditic xenoliths in alkaline lamprophyres from the Spanish Central System

“…Most pyrox- enites are interpreted as fragments of high pressure crystal-segregations from melts flowing through conduits in the mantle or crystallised in magma chambers (Wilshire and Shervais , 1975;Frey, 1980;Irving, 1980;Bodinier et al, 1987a,b;Suen and Frey, 1987;Wilkinson and Stolz, 1997;Ho et al, 2000), providing infolTIlation about fractionation of basaltic magmas in the mantle. Nonetheless, other origins have been advocated, such as solid-state recycling of subducted lithosphere (Allegre and Tur cott e, 1986) and derivation by metasomatic fluids or by melt-rock reactions (Garrido and Bodinier, 1999).…”

Petrogenesis of anhydrous clinopyroxenite xenoliths and clinopyroxene megacrysts in alkali basalts from the Ganseong area of South Korea

“…Meanwhile, the Al-Ti-augite group is dominated by clinopyroxenerich pyroxenites, such as clinopyroxenites and wehrlites, with generally Mg# < 85. Websterites, orthopyroxenites, and dunites are relatively rare in the both groups (Frey & Prinz 1978;O'Reilly & Griffin 1987;Wilshire et al 1988;Wilkinson & Stolz 1997). Composite xenoliths containing pyroxenite veins in peridotites are found in many localities worldwide.…”