First find of coesite in the ultrahigh-pressure metamorphic area of the central Erzgebirge, Germany

Abstract

In the ultrahigh-pressure metamorphic area of the Saidenbach reservoir, Erzgebirge, where microdiamond- bearing gneiss occurs, coesite was found as inclusion in garnet and omphacite in an unusually well preserved eclog- ite. Most of the eclogite bodies of this area are thoroughly altered, with all omphacite transformed to plagioclase + amphibole symplectites. The coesite-bearing eclogite contains abundant dolomite in the matrix and as surprisingly large inclusions in garnet, which is another unusual feature compared to the many eclogites of the Saxonian Erzgebirge. Coesite is at least partly replaced by quartz showing a palisade texture. This pattern is accompanied by abundant radial cracks in the host minerals. Polycrystalline inclusions of equigranular quartz are also related to former coesite because some of them preserve minor coesite relics. In such cases, cracks in the host mineral are not more pronounced than around moderately transformed coesite. No K-feldspar was observed in inclusions consisting of partly replaced coesite. Thus, inclusions in omphacite of K-feldspar-quartz symplectites with abundant cracks around, which occur as well in eclogites near the Saidenbach reservoir, cannot be coesite pseudomorphs.