HIGH-RESOLUTION U-PB DATING OF MIDDLE TRIASSIC VOLCANICLASTICS - TIME-SCALE CALIBRATION AND VERIFICATION OF TUNING PARAMETERS FOR CARBONATESEDIMENTATION

We report high-resolution single-zircon U-Pb age data for Middle Trias sic volcaniclastic intercalations in biostratigraphically calibrated p elagic successions of the Southern Alps. The results require a redefin ition of the chronometric scale for the Middle Triassic. Moreover, the y do not support current models relating cyclic sedimentation in platf orm carbonates of the Dolomites to orbital tuning. Tight concordant ag e clusters were obtained for five volcaniclastic layers in three ammon oid biozones of late Anisian to early Ladinian age. Two layers in the (Nevadites) Secedensis Zone yielded identical mean Pb-206/U-238 ages o f 241.2 + 0.8/ - 0.8 Ma and 241.2 + 0.8/ - 0.6 Ma (errors given at the 95% confidence level). A layer in the Gredleri Zone is dated at 238.8 + 0.5/ - 0.2 Ma, and two horizons in the Archelaus Zone yield similar ages of 237.9 + 1.0/ - 0.7 Ma and 238.0 + 0.4/ - 0.7 Ma. These result s are significantly older, than the age values of 233-235 Ma assigned to the Anisian/Ladinian boundary by several current time scales [1-4]. Moreover, our estimate of 240.7-241.3 Ma (depending on biostratigraph ic collocation) for the Anisian/Ladinian boundary casts doubts on the reliability of age values of 245-250 Ma proposed by most time scales f or the Permian/Triassic boundary. The occurrence of pelagic fossils in basinal sediments as well as in age-equivalent shallow marine Middle Triassic platform carbonates in the Dolomites allows the sedimentary s equences of both environments to be correlated. The 800 m thick Latema r platform (western Dolomites) is characterized by cyclic stacking pat terns, which have been interpreted as results of Milankovitch-type hig h-frequency/low-amplitude sea-level fluctuations. The 12 m.y. interval of platform growth postulated from the assignment of orbital periodic ities to the platform carbonate cycles [5,6] is in conflict with a max imum time span of 4.7 m.y. allowed by the present zircon data.