PROVENANCE, DEPOSITION AND AGE OF GNEISS UNITS FROM THE KTB DRILL HOLE (GERMANY) - EVIDENCE FROM SHRIMP AND CONVENTIONAL U-PB ZIRCON AGE-DETERMINATIONS

In order to investigate the provenance, deposition age and metamorphis m of metasedimentary gneisses from the pilot and the main holes of the German Continental Deep Drilling Program (KTB), zircons from volcano- metasedimentary (v-unit) and monotonous greywacke (g-unit) gneisses we re dated by ion microprobe (SHRIMP) and conventional U-Pb zircon metho ds. The zircons were found to consist of anhedral detrital or subhedra l cores surrounded by one or two metamorphic rims. Analysis of growth zones in 37 zircons indicated ages for the detrital cores between 2.25 and 2.05 Ga in a v-unit gneiss (sample K52), and 1.8-2.35 Ga in two g -unit gneisses (K747 and K798). Neo-Proterozoic (650-545 Ma) subhedral igneous cores were identified in all investigated KTB gneiss samples, and an additional two zircon growth episodes in the source area of th e sediments at 712+/-15 and 525+/-11 Ma were also recognised. Zircon o vergrowths dated at 487+/-13 Ma are interpreted to have formed during an early metamorphic episode following incorporation of the zircons in to their host metasediments. A second metamorphic overprint indicated by a date of 381+/-22/-11 Ma was determined for zircon overgrowth in t he g-unit gneisses. The dates determined for zircon growth phases sugg est that the KTB metasediments were derived from northern Gondwana. Hy pothetical provenance areas and a new model of terrane positions that is consistent with the geochronological, geodynamic and palaeo-geograp hic preconditions at the northern border of Gondwana in Late Precambri an to Early Ordovician times, are proposed.