ORTHOGNEISSES IN THE SPESSART CRYSTALLINE COMPLEX, NORTH-WEST BAVARIA- SILURIAN GRANITOID MAGMATISM AT AN ACTIVE CONTINENTAL-MARGIN

The Spessart Crystalline Complex, northwest Bavaria contains two ortho gneiss units of granitic to granodioritic composition, known as the Ro tgneiss and Haibach gneiss, respectively, which are structurally confo rmable with associated metasediments. The igneous origin of the Rotgne iss is apparent from field and textural evidence, whereas strong defor mation and recrystallization in the Haibach gneiss has obscured most p rimary textures. New geochemical data as well as zircon morphology pro ve the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a g enetic link between those two rock units. Both gneiss types have chemi cal compositions typical of anatectic two-mica leucogranites. They sho w characteristics of both I- and S-type granites. Rb-Sr whole rock dat a on the Haibach gneiss provide an isochron age of 407+/-14 Ma (IR=0.7 077+/-0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439+/-15 Ma; IR=0.7048+/-0.0026; MSWD 4.9). Si ngle zircon dating of six idiomorphic grains, using the evaporation me thod, yielded a mean (207)pb/(206)pb, age, of 410+/-18 Ma for the Haib ach gneiss and 418+/-18 Ma far the Rotgneiss. Both zircon ages are wit hin analytical error of the Rb-Sr isochron dates and are interpreted t o reflect the time of protolith emplacement in Silurian times. Three x enocrystic zircon grains from the Rotgneiss yielded (207)pb/(206)pb ag es of 2278+/-12, 2490+/-13 and 2734+/-10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the gene ration of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were form ed through anatexis of older continental crust, the relatively low Sr- 87/Sr-86 initial ratios of both gneisses may also indicate the admixtu re of a mantle component. The Rotgneiss and the Haibach gneiss thus do cument granitic magmatism at an active continental margin during late Silurian times.