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 Ruhla Crystalline Complex (RCC) point to Volcanic activities in different geotectonic environments, comparable with other units within the Mid-German Crystalline Rise (MGCR). Amphibolites of the Truse Formation, intercalated within staurolite-bearing metapelites and -psammites, show a subalkaline, tholeiitic 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 - Geiselbach formations) of the Spessart Crystalline Complex. In contrast, metabasites from the Central Gneiss Unit of the RCC, intercalated within granitic orthogneisses of Late Silurian/Early Devonian age, cover a wide range from subalkaline-andesitic to dacitic compositions. The calcalkaline character of these metabasites provides evidence for magmatism in an island-are environment, related to subduction processes after 410 but before 360 (330) Ma. Similar metabasite-orthogneiss associations are known from the Aschaffenburg-Feldkahl metabasite belt in the Spessart Crystalline Complex and from the Bollstein Odenwald. Metabasites of the Brotterode Formation of the RCC reveal MORE and island-are affinities. Due to unknown emplacement ages, a comparision of these metabasites with others within the MGCR is speculative, so far. Nevertheless, the 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 composite terrane of units, formed in different plate-tectonic environments at least from the Silurian onward.