ORIGIN OF FE-RICH LHERZOLITES AND WEHRLITES FROM TOK, SE SIBERIA BY REACTIVE MELT PERCOLATION IN REFRACTORY MANTLE PERIDOTITES

Abstract

Lherzolite–wehrlite (LW) series xenoliths from the quaternary Tok volcanic field in the southeastern Siberian craton are distinguished from the more common lherzolite–harzburgite (LH) series by (a) low Mg numbers (0.84–0.89) at high modal olivine (66–84%) and (b) widespread replacement of orthopyroxene (0–12%) and spinel by clinopyroxene (7–22%). The LW series peridotites are typically enriched in Ca, Fe, Mn and Ti, and depleted in Si, Ni and Cr relative to refractory LH series rocks (Mg number ≥0.89), which are metasomatised partial melting residues. Numerical modelling of Fe–Mg solid/liquid exchange during melt percolation demonstrates that LW series rocks can form by reaction of host refractory peridotites with evolved (Mg numbers 0.6–0.7), silica-undersaturated silicate melts at high melt/rock ratios, which replace orthopyroxene with clinopyroxene and decrease Mg numbers. This process is most likely related to underplating and fractionation of basaltic magma in the shallow mantle, which also produced olivine–clinopyroxene cumulates found among the Tok xenoliths.