EVIDENCE OF MESOZOIC MULTIPLE HYDROTHERMAL ACTIVITY IN THE BASEMENT AT NONNENMATTWEIHER (SOUTHERN SCHWARZWALD), GERMANY

Two botryoidal hematites from the qtz-fl-hem veins al the Nonnenmattwe iher cirque (zone of Badenweiler-Lenzkirch, Sudschwarzwald) were dated using the (U + Th)-He method. One hematite gives an Early Triassic ag e (234 +/- 5 Ma) and the second straddles the Cretaceous/Jurassic time boundary (126 +/- 4 Ma). The ages bracket the similar to entire age s pectrum reported to date for Mesozoic vein-type mineralizations in the Schwarzwald. The maximum sedimentary overburden and associated palaeo -temperatures (>130 degrees C) in the basement of the southern Schwarz wald during the Jurassic did not cause severe age-resetting of the hem atites. This implies that both ages probably are formation apes rather than ages of cooling, provided that the He-4 retentivities of both he matites are similar. The fact that two hematites from the veins at Non nenmattweiher yield highly discordant Mesozoic ages suggests multiple hydrothermal activity during the Mesozoic. A previously published age of two adularia (similar to 155 Ma), intermediate between the hematite ages, supports this view.