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 of kimberlitic pipes (the Webb Province) in the Gibson Desert of central Australia. The Webb pipes comprise sparse macrocrystic olivine set in a groundmass of olivine, phlogopite, perovskite, spinel, clinopyroxene, titanian‐andradite and carbonate. The pipes resemble ultramafic lamprophyres (notably aillikites) in their mineralogy, major and minor oxide chemistry, and initial 87Sr/86Sr and εNd‐εHf isotopic compositions. Ion probe U‐Pb geochronology on perovskite (806 ± 22 Ma) indicates the eruption of the pipes was co‐eval with plume‐related magmatism within central Australia (Willouran‐Gairdner Volcanic Event) associated with the opening of the Centralian Superbasin and Rodinia supercontinent break‐up. The equilibration pressure and temperature of mantle‐derived garnet and chromian (Cr) diopside xenocrysts range between 17 and 40 kbar and 750–1320°C and define a paleo‐lithospheric thickness of 140 ± 10 km. Chemical variations of xenocrysts define litho‐chemical horizons within the shallow, middle, and deep sub‐continental lithospheric mantle (SCLM). The shallow SCLM (50–70 km), which includes garnet‐spinel and spinel lherzolite, contains Cr diopside with weakly refertilized rare earth element compositions and unenriched compositions. The mid‐lithosphere (70–85 km) has lower modal abundances of Cr diopside. This layer corresponds to a seismic mid‐lithosphere discontinuity interpreted as pargasite‐bearing lherzolite. The deep SCLM (>90 km) comprises refertilized garnet lherzolite that was metasomatized by a silicate‐carbonatite melt.

Ancient Craton‐Wide Mid‐Lithosphere Discontinuity Controlled by Pargasite Channels

“…(2022) (Gawler), Sudholz and Reddicliffe et al. (2023) (Webb), and Sudholz and Jaques et al. (2023) (W. Kimberley).…”

“…The MLD is considered to play an essential role in controlling long‐term stability and evolution of the cratonic lithospheric mantle (e.g., Slave Craton: Chen et al., 2009; North America: Hopper & Fischer, 2015) as well as unusual cases of its destruction and modification (e.g., North China Craton: Chen et al., 2014). Several mechanisms have been proposed for the cause of MLDs, including (a) elastically accommodated grain boundary sliding (Karato et al., 2015), (b) layers of partial melt, saline brines and melt‐modified peridotite (Bettac et al., 2023; Liu et al., 2022; Thybo & Perchuć, 1997), (c) compositional layering of variably depleted peridotite, or changes in the anisotropic fabric of the lithospheric mantle (Boyce et al., 2024; Ford et al., 2016; Wirth & Long, 2014; Yuan & Romanowicz, 2010), and (d) hydrous minerals (Aulbach et al., 2017; Kovács et al., 2017, 2021; Liu et al., 2023; Rader et al., 2015; Saha et al., 2021; Selway et al., 2015; Sudholz et al., 2022; Sudholz and Reddicliffe et al., 2023). Despite growing interest, previous investigations have mainly focused on independent evidence from seismic observations (e.g., Birkey et al., 2021; Ford et al., 2010; Krueger et al., 2021) and geochemical data (e.g., Aulbach et al., 2017; Liu et al., 2022; Saha et al., 2021; Sudholz and Jaques et al., 2023).…”

“…To study the lithology and geochemistry of the MLD we have collected ∼3,000 clinopyroxene mantle xenocrysts from six volcanic provinces across Australia (Figure 1) (see Sudholz, 2024 for online repository of xenocryst compositions). We use recently published xenocryst data from the Gawler Craton and Adelaide Fold Belt (Sudholz et al., 2022), Kimberley Craton (Sudholz and Jaques et al., 2023) and Arunta Inlier (Sudholz and Reddicliffe et al., 2023) (Figure 1). Our data set includes new xenocryst data for the Yilgarn Craton and Delamarian Orogen (Figure 1).…”

“…The MLD is interpreted to have formed from bottom‐up percolation of H 2 O‐bearing melts from the deep lithospheric mantle or shallow asthenosphere (Figure 4a). H 2 O may have been transported by proto‐ or failed kimberlites, plumes, slab‐triggered upwellings or other mechanisms (Freitas & Manthilake, 2019; Green, 2015; Kuritani et al., 2019; Sudholz et al., 2022; Sudholz and Reddicliffe et al., 2023; Yang et al., 2018). Given the temporal and spatial extent of the MLD in the global lithosphere, the supply of H 2 O to the mid‐lithosphere is a universal, albeit poorly understood process that has persisted throughout geological time.…”