Trace element distribution in eclogites and their clinopyroxene/garnet pair: a case study from Soazza (Switzerland)

Rare earth element (REE) abundances have been determined in six samples of metabasites from the Pennidic Adula nappe in the area of Soazza (Mesolcina Valley, Switzerland). The samples cover a broad range of mineralogical comp osition from eclogite to epidote amphibolite through symplectitic garnet am phibolite. The REE patterns do not vary significantly and indicate a common origin of these rocks as "normal" mid-ocean ridge basalts (N-MORB). Two main eclogitic phases, clinopyroxene (cpx) and garnet (grt), have also been analysed for REE and other trace elements (Sc, V, Cr, Co, Ni, Cu, Zn, Sr, Y, Zr). The incorporation of REE and most trace elements in structural sites of cpx and grt is in accordance with that expected mainly from an eva luation of site geometry, tonic radius and charge balance and is therefore dependent on the major-element composition of the host phases. The changes in cpx/grt partition coefficients (D-i) for HREE are particularly associate d with the temperature condition of the equilibration of the pair.