Metabasites from the Ruhla Crystalline Complex: Evidence for distinct pre-variscan, plate-tectonic environments within the Mid-German Crystalline Rise

Geochemical characteristics of metabasites from different units of the Ruhl a Crystalline Complex (RCC) point to Volcanic activities in different geote ctonic environments, comparable with other units within the Mid-German Crys talline Rise (MGCR). Amphibolites of the Truse Formation, intercalated with in staurolite-bearing metapelites and -psammites, show a subalkaline, thole iitic character conforming to modern MOR-basalts. The geochemical signature of these amphibolites and their host rocks agree well with the geological situation known from the Horstein-Huckelheim metabasite belt (Mombris - Gei selbach formations) of the Spessart Crystalline Complex. In contrast, metabasites from the Central Gneiss Unit of the RCC, intercala ted within granitic orthogneisses of Late Silurian/Early Devonian age, cove r a wide range from subalkaline-andesitic to dacitic compositions. The calc alkaline character of these metabasites provides evidence for magmatism in an island-are environment, related to subduction processes after 410 but be fore 360 (330) Ma. Similar metabasite-orthogneiss associations are known fr om the Aschaffenburg-Feldkahl metabasite belt in the Spessart Crystalline C omplex and from the Bollstein Odenwald. Metabasites of the Brotterode Formation of the RCC reveal MORE and island-a re affinities. Due to unknown emplacement ages, a comparision of these meta basites with others within the MGCR is speculative, so far. Nevertheless, t he metabasite-host rock associations of the: (i) Truse Mombris - Geiselbach (Horstein-Huckelheim) formations and (ii) the Central Gneiss Unit Haibach/ Rotgneiss Complex - Bollstein Gneiss clearly indicated that the MGCR is a c omposite terrane of units, formed in different plate-tectonic environments at least from the Silurian onward.