Petrology and Alpine metamorphic evolution of the metabasic and ultrabasic rocks in the Berisal crystalline complex (Switzerland)

Relict diopsides have been found in meta-ultrabasic rocks from the Berisal crystalline complex in the Pennine nappe system of the Alps. They show optical and chemical zonation. The idiomorphic cores are enriched in chromium. During the Eo-Alpine high-pressure metamorphic event the originally magmatic clinopyroxene suffered substantial replacement of Ca by Na. New diopside, poor in chromium grew around the borders of the idiomorphic Cr-rich cores. During the subsequent main stage of Alpine metamorphism the diopside was mainly altered into tremolite. Some of the diopsides survived but alteration induced exsolution of Na out of the rims, creating the observed zoning in the pyroxenes. Field and microscopic investigations of the metabasic rocks support the existence of an Eo-Alpine high-pressure metamorphic event. The amphibolites reflect a typical retrograde evolutionary stage between “eclogite” amphibolite and garnet amphibolite with strongly corroded kelyphitic garnet and the symplectitic, radially fibrous growth of amphibole, plagioclase, epidote and quartz. However, the surprisingly low pyrope and high almandine contents in the garnets are not typically representing a high-pressure metamorphic event.