Petrology, Age, and Rift Origin of Ultramafic Lamprophyres (Aillikites) at Mount Webb, a New Alkaline Province in Central Australia

Abstract

Diamond exploration over the past decade has led to the discovery of a new province ofkimberlitic pipes (the Webb Province) in the Gibson Desert of central Australia. The Webb pipes comprisesparse macrocrystic olivine set in a groundmass of olivine, phlogopite, perovskite, spinel, clinopyroxene,titanian-andradite and carbonate. The pipes resemble ultramafic lamprophyres (notably aillikites) in theirmineralogy, major and minor oxide chemistry, and initial 87Sr/ 86Sr and εNd-εHf isotopic compositions. Ionprobe U-Pb geochronology on perovskite (806 ± 22 Ma) indicates the eruption of the pipes was co-eval withplume-related magmatism within central Australia (Willouran-Gairdner Volcanic Event) associated withthe opening of the Centralian Superbasin and Rodinia supercontinent break-up. The equilibration pressureand temperature of mantle-derived garnet and chromian (Cr) diopside xenocrysts range between 17 and40 kbar and 750–1320°C and define a paleo-lithospheric thickness of 140 ± 10 km. Chemical variations ofxenocrysts define litho-chemical horizons within the shallow, middle, and deep sub-continental lithosphericmantle (SCLM). The shallow SCLM (50–70 km), which includes garnet-spinel and spinel lherzolite, containsCr diopside with weakly refertilized rare earth element compositions and unenriched compositions. Themid-lithosphere (70–85 km) has lower modal abundances of Cr diopside. This layer corresponds to a seismicmid-lithosphere discontinuity interpreted as pargasite-bearing lherzolite. The deep SCLM (>90 km) comprisesrefertilized garnet lherzolite that was metasomatized by a silicate-carbonatite melt. Show more