GEOCHRONOLOGY OF THE MOUNT READ VOLCANICS, TASMANIA, AUSTRALIA

The late Middle Cambrian Mount Read Volcanics, Tasmania, Australia, comprise submarine rhyolite to basalt lavas, intrusions, and volcaniclastic rocks which host significant polymetallic massive sulfide deposits. Isotopic U-Pb and 40Ar/39Ar dating of magmatic zircons and hornblendes, respectively, from different parts of the Mount Read Volcanics yields a concordant age of 502.6 +/- 3.5 Ma (1sigma), which constrains the time of volcanogenic massive sulfide mineralization. The Comstock Tuff from the Tyndall Group in the upper part of the volcanic sequence and the Mount Black Dacite from the Central Volcanic Complex have weighted mean Pb-206/U-238 ages of 494.4 +/- 3.8 Ma and 494.9 +/- 4.3 Ma, respectively, and may be slightly younger than the other rocks. Fauna in the Mount Read Volcanics is diagnostic of a late Middle Cambrian (middle to late Menevian stage) age, and the isotopic dates therefore imply that the Late Cambrian is younger than 500 Ma. Inherited zircons in the Mount Read Volcanics have ages similar to those found in granites and sediments in the Tasman fold belt, southeastern Australia, which suggests similar source material for the two areas.