Abstract
Intraplate alkaline provinces, particularly those forming fields of small volcanoes, often display a range in basaltic composition spanning the alkaline to subalkaline spectrum. Traditional geochemical models justify this range by various degrees of partial melting of garnet to spinel lherzolite as well as involvement of nonperidotitic source lithologies. Alternative scenarios involve stepwise interaction and metasomatism of the lithospheric mantle by asthenosphere-derived silicate fluids and subsequent remelting and percolative reaction to form alkaline to subalkaline primary magmas. The Auckland Volcanic Field (AVF) is a unique case study to unravel the "primary magma conundrum" because of the detailed knowledge of products from its approximately 50 eruptions and occurrence of mantle xenoliths. Here we present the first microchemical investigation of mantle peridotite xenoliths from the Auckland Volcanic Field with the aim of understanding metasomatic modification of the lithospheric mantle. Xenoliths consist of lherzolite, harzburgite and dunite with occurrence of phlogopite and Cr- and Na-rich diopside. These phases show sieve/breakdown textures and are surrounded by secondary olivine+ or -clinopyroxene and vesiculated glass. Glass associated with green clinopyroxene is characterized by HFSE depletion, Th, Pb and Sr enrichment and concave up MREE to HREE enriched patterns. 87Sr/86Sr ratios of metasomatic glass and clinopyroxene are <0.7027 and overlap with the most alkaline samples of the AVF. Glass surrounding phlogopite has Rb, Nb, Ta, K, Pb, Zr and Hf enrichment and variable Th, U, La and Ce depletion. Mineral and glass record involvement of different metasomatic fluids, partial melting and crystallization. Metasomatic green clinopyroxene likely formed from percolative reaction of ancient subduction derived fluids and may be a late stage percolative fractionate after amphibole vein formation, whereas phlogopite formed in response to percolation of asthenospheric OIB-like fluids. The lithospheric mantle beneath Auckland has experienced several episodes of metasomatic modification that impacted the alkaline-subalkaline spectrum of erupted lavas