GEOCHEMISTRY OF POLYMETAMORPHIC ULTRAMAFICS (MAJOR, TRACE, NOBLE AND RARE-EARTH ELEMENTS) - AN EXAMPLE FROM THE HELVETIC BASEMENT, CENTRAL ALPS, SWITZERLAND

Polymetamorphic ultramafic rocks in orogenic terranes rarely preserve relic structures or minerals from their former mantle stages. The dete rmination of their protoliths and their tectonic evolution by chemical discrimination methods is often difficult due to possible metasomatic processes. Ultramafics of the pre-Variscan Helvetic basement (Central Alps, Switzerland) have been investigated geochemically to address th ese problems. These ultramafics are partially to completely serpentini sed. According to field observations several ultramafic lenses were pa rt of an ophiolite suite, but distinct cumulate ultratmafic lenses wer e also recognized. CIPW norms indicate that large parts of the ultrama fics are harzburgites, but metasomatic CaO depletion may have produced an overestimation of the importance of the harzburgite protoliths. Ma jor element distributions suggest a depleted mantle protolith. Close t o chondritic or slightly depleted REE patterns are characteristics of the studied samples. The REE normalized patterns confirm the presence of harzburgites, lherzolites and cumulates. In some samples light REE enrichment processes have occurred. The noble metal concentrations are both affected (Pt-Pd-Au) and unaffected (Ir-Os-Ru) by melt infiltrati on processes. They suggest the presence of undepleted or slightly enri ched harzburgites and more differentiated, probably cumulate ultramafi cs. Information obtained by different chemical elements leads to contr asting results. REE and noble metals show enrichment inconsistent with the major element depletion. Refertilization of depleted ultramafics is proposed.