The Nature of the Upper-Most Mantle Beneath Victoria, Australia as Deduced from UL Tramafic Xenoliths

Abstract

Utramafic inclusions from two neighbouring localities in Victoria, Australia include lherzolites with and without hydrous phases, wehrlites, pyroxenites, and hornblendites. Mineral chemistry provides evidence for equilibrium crystallization for individual nodules at a depth near 45 km but over a range of temperatures. Temperature estimates yield systematic differences with anhydrous assemblages > phlogopite-bearing assemblages > amphibole-bearing assemblages. Bulk rock variation of MgO, CaO, Al 2 O 3 and compatible element contents in lherzolite and harzburgite has been modelled as an early partial melting event, giving rise to various degrees of depletion. The extracted liquid was of picritic composition. Amphiboles in lherzolites are developed independently and postdate the partial melting event, as a response to near-isochemical metamorphic reaction, consequent on addition of water. The hydration events predate but are not precursor conditions for production of basanite. Hydration-metasomatism occurs in the uppermost mantle above the LVZ, the source region for basanites lies at deeper levels (?LVZ). The emplacement and passage of alkaline magmas through the lithosphere/upper mantle may be the cause of local metasomatism and of hydration. Wehrlites, pyroxenites, hornblendites and some lherzolites are precipitates from magmas fractionating and/or crystallizing at mantle depths. Observable wall-rock reaction is extremely restricted (about 1 cm) as evidenced by composite xenoliths.