MAJOR, TRACE, NOBLE AND RARE-EARTH ELEMENT DISTRIBUTION IN POLYMETAMORPHIC ULTRAMAFIC ROCKS (AAR MASSIF, CENTRAL ALPS, SWITZERLAND)

In this contribution major, minor, trace, rare earth and noble element s are investigated in order to constrain the chemical evolution of fou r representative ultramafic rock enclaves within the Aar massif. The p resence of the ultramafic rocks in the Aar massif is related to one of the earlier, but poorly understood tectonic events. The ultramafic ro cks are associated with metagabbros and/or polymetamorphic mafic rocks . Mineralogically the ultramafic rocks now consist mainly of secondary phases. Crustal (serpentinization) and mantle (melt/rock interaction) processes are characterized on geochemical grounds. During serpentini zation several elements were probably remobilized. Rare earth elements (REE) preserved a chondritic concentration and were possibly not (or only slightly) affected by serpentinization. It is suggested that plat inum group elements (PGE) were essentially immobile during serpentiniz ation. Since PGE, Ni, Sc, Co were not remobilized, their concentration s are used to determine the magmatic petrogenesis of the ultramafic ro cks. The Ni/Co ratio appears to be a valuable criteria to recognize th e magmatic evolution of the ultramafic rocks. There is no covariance b etween REE and PGE concentrations. On the basis of PGE concentrations, the investigated rocks are interpreted as residues from partial melti ng (group I; depletion of PPGE) and cumulate (group II; depletion of I PGE) which have later been contaminated by a Pt-Pd and light rare elem ents enriched melt. Other trace element data support this model. A che mical affinity with the ultramafics occurring in the Gotthard massif i s demonstrated. Lithostratigraphic and chemical information helps us t o understand the evolution of polymetamorphic ultramafic rocks from oc eanic genesis to accretion into continental crust along an active marg in.