AMPHIBOLES FROM SUPRASUBDUCTION AND INTRAPLATE LITHOSPHERIC MANTLE

Abstract

Geochemical features of amphiboles, mainly from mantle xenoliths, were investigated for a number of intraplate and suprasubduction localities, with the aim of fingerprinting the metasomatic signatures for the two different geological settings. Amphiboles generated in the mantle wedge above subduction zones (suprasubduction amphibole, S-Amph) are depleted in Nb, with suprachondritic Ti/Nb and Zr/Nb ratios, whereas intraplate amphibole (I-Amph) is enriched in Nb, with subchondritic Ti/Nb and Zr/Nb ratios. These complementary features can be reconciled by Nb-depleted fluids coming off the subducted oceanic crust, leaving a rutile-bearing eclogite residuum. Rutile is a major repository for High Field Strength Elements (mainly Nb, Ta and Ti), with a preference to retain pentavalent elements. During the subduction process, rutile-bearing eclogite will continue its descent ,into the lower part of the upper mantle (or even below), generating a subchondritic Ti/Nb or Zr/Nb reservoir. The partial incorporation of this material in an asthenospheric plume will ultimately contribute to the genesis of intraplate alkaline basalts, characterized by high Nb contents. The link between the complementary geochemical features of suprasubduction and intraplate amphiboles suggests a relationship between calc-alkaline and intraplate magmatisms. This is also in agreement with the temporal sequence of subduction, calc-alkaline volcanism and intraplate magmatism that can be observed in several localities around the Mediterranean areas and in most subduction zones worldwide.