Origin and significance of spinel–pyroxene symplectite in lherzolite xenoliths from Tallante, SE Spain

Abstract

We found spinel–pyroxene symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) symplectites. The symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type symplectite. The Tallante spinel–pyroxene symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.