THE KTB SUPERDEEP BOREHOLE - PETROGRAPHY AND STRUCTURE OF A 9-KM-DEEPCRUSTAL SECTION

The KTB boreholes that were drilled from 1989 to 1994 near Windischesc henbach, NE Bavaria, penetrated rocks of the Zone Erbendorf-Vohenstrau ss (ZEV). The upper and lower sections of the 9101m-deep profile are m ainly composed of gneiss units (former greywacke sediments) and varieg ated units (alternating gneisses and metabasites, probably of volcanos edimentary origin). Metabasic units (amphibolites, metagabbros) consti tute mainly the middle section. Geochemical data suggest their derivat ion from oceanic crust. All rocks are probably pre- to early Ordovicia n in protolith age. They underwent early Devonian MP metamorphism. Rel ies of preceeding HP metamorphism are preserved in metagabbros. Felsic to mafic dykes (dominantly Upper Carboniferous lamprophyres) crosscut the metamorphic rocks. The ZEV units are steeply inclined down to the final depth. The main structures (foliation, folds, faults) trend NW- SE. Semibrittle to brittle deformation plays an unexpected important r ole. In contrast to the conditions of the Mesozoic (mainly Cretaceous/ Tertiary) faulting the P-T conditions of MP metamorphism, the cooling ages and the late-Variscan graphite-bearing prehnite-actinolite facies mineralization show no significant depth-dependent gradients. This ca n be explained by Mesozoic supracrustal stacking. A bundle of reverse faults between 6850 and 7300 m belongs to the Franconian lineament and can be correlated with the most prominent seismic reflector (SE1) in the area.