(U-HE EVIDENCE OF JURASSIC CONTINUOUS HYDROTHERMAL ACTIVITY IN THE SCHWARZWALD BASEMENT, GERMANY(TH))

(U + Th)-He model and isochron ages of 21 botryoidal hematites collect ed from the Hohberg base-metal vein system in the Triberg granite comp lex, Mittelschwarzwald (Germany), cover most of the Jurassic period. T he possibility of quantitative He-4 loss triggered by successive hydro thermal events, although conceivable for individual hematites - based on a He-4 retentivity study and calculated opening temperatures of hem atite for He-4 of > (90-180)degrees C - can largely be ruled out. Thus the range of model and isochron ages indicates that the majority of t he hematites are not cogenetic. This is supported by a broad range of Pb-total concentrations in the Hohberg botryoidal hematites (150-2700 ppm), by comparison with virtually identical Pb-total concentrations f ound in cogenetic hematites from four different vein systems in the Sc hwarzwald. The Th/U ratios of the hematites vary by a factor of simila r to 700 (0.01-7.25) mainly as a result of the range of Th concentrati ons (0.1-74 ppm). The incorporation of substantial amounts of Th in he matite is thought to result from increased Th transport mobility, poss ibly caused by association with colloids and/or complexing in the ore solution. delta(18)O analyses of a similar to 160 Ma old cogenetic hem atite-quartz pair indicate that precipitation occurred from a fluid of magmatic origin at a temperature of similar to 180 degrees C. The reg ional temperature peak in the basement at the present erosional surfac e during the Jurassic (similar to 130 degrees C) provides a lower temp erature limit of hematite formation. This implies that hematite emplac ement occurred at temperatures roughly ranging between similar to 130 and similar to 180 degrees C. The Jurassic continuous hydrothermal act ivity suggests that the pre-rifting European craton is characterized b y local thermal fluxes during a long time span.