RARE-EARTH ELEMENT REDISTRIBUTION DURING HIGH-PRESSURE-LOW-TEMPERATURE METAMORPHISM IN OPHIOLITIC FE-GABBROS (LIGURIA, NORTHWESTERN ITALY) - IMPLICATIONS FOR LIGHT REE MOBILITY IN SUBDUCTION ZONES

To unravel the rare earth element (REE) redistribution during high-pre ssure-low-temperature metamorphism, we have analyzed by ion microprobe all the minerals from representative Fe-gabbros from Ligurian metaoph iolites (northwestern Italy). Contrary to what is observed for fresh F e-gabbros, the clinopyroxene contribution to the whole-rock REE invent ory of blueschists and eclogitized Fe-gabbros is minor or negligible. In both blueschists and eclogites, REE are redistributed among newly f ormed minerals, In blueschists, lawsonite is the major REE carrier and concentrates almost all the light REE (LREE) in the rock, whereas tit anite plays an important role for middle and heavy REE (MREE, HREE). I n eclogites, LREE and HREE mainly reside in accessory allanite and gar net, respectively. The development of blueschist and eclogite facies m ineral assemblages in mafic oceanic crust, due to subduction to 35-65 km depths, was not accompanied by release of significant amounts of LR EE to the overlying mantle wedge. Owing to the stability of lawsonite and epidote at ultra-high-pressure conditions, subduction of mafic roc ks is considered an effective mechanism to bring LREE to great depth w ithin the mantle.