Rare earth geochemistry of fused ophiolitic and alpine lherzolites—I. Othris, Lanzo and Troodos

Abstract

Lherzolites from two Mediterranean peridotite masses have major and trace element data compatible with an origin as a fragment of relatively undepleted mantle. Field observations indicate a close association with in situ basaltic melt (gabbroic dikes and segregations) and a barren refractory residue (harzburgite) produced by the removal of the melt fraction. Two lherzolites Othris (ophiolite) and Lanzo (alpine ∗ ∗ Alpine peridotites will be referred to as ‘mantle peridotite massifs’ throughout this text. Such peridotites may be cumulate or tectonite in nature and exist in either the plagioclase. spinel or garnet Iherzolite fields. Those stable in the plagioclase Iherzolite field (<9kbar) also occur as mantle sections in ophiolites. periodotite) have approximately chondritic rare earth abundances with a slight depletion in light rare earths. The refractory material is moderately to heavily depleted in light REE dependent on the efficiency of removal of basaltic melt. Lherzolite xenoliths from the Massif Central probably contain an interstitial light REE enriched fraction as the recalculated lherzolite is depleted and not light REE enriched like the actual whole rock. These basaltic xenoliths are similar in major, trace and REE profile to the Lanzo and Othris mantle lherzolites, giving some indication of source homogeneity in the Mediterranean area. Partial fusion calculations on the Othris and Lanzo peridotites reveal that tholeiitic liquids could be generated by 10–30% partial melting. Such tholeiitic liquids separated from the Othris mantle section and probably formed early sea floor in a small ocean basin. Alkalic basalts are also associated with the Othris ophiolite as an early rifting sequence, and such liquids could have been generated from the source Iherzolite but difficulties would occur in removing such a liquid from the refractory residue.