U-PB ZIRCON AGE CONSTRAINTS ON THE LATE CAMBRIAN AND LOWER ORDOVICIAN FAUNA OF SIBUMASU

Siliciclastic strata of the Molohein Group, of the southern Shan State in Myanmar, and the Tarutao Group of Ko Tarutao island in the Satun province of southernmost peninsular Thailand were deposited on a peri-Gondwanan terrane which became the microcontinent of Sibumasu with the Triassic opening of the Meso-Tethys Ocean. The Molohein Group and the Tarutao Group contain a rich trilobite faunal record from the early Paleozoic, and they are interbedded with rhyolitic tuffs putatively sourced from the subduction zone volcanic belt that rimmed Gondwana’s northern margin. We will present high-precision CA-ID-TIMS U-Pb zircon geochronology from the interbedded tuffs. In the Molohein Group, four tuff beds over about 12 meters of a measured stratigraphic section yielded indistinguishable ²⁰⁶Pb/²³⁸U dates implying near-instantaneous deposition of the section in geologic time, corresponding to the Furongian (end-Cambrian) Eosaukia buravasi biozone in Myanmar. In the Tarutao Group, a Cambrian faunal assemblage that spans the late Jiangshanian to the middle of Cambrian Stage 10 is interbedded with tuffs erupted in a similarly short timeframe. These represent the first biostratigraphically calibrated absolute dates from the late Cambrian-early Ordovician timeframe outside of Avalonia. Preliminary zircon geochronology from a single sample of the Bawdwin rhyolite, a potential source for the tuffs, yielded a middle-Ordovician youngest population of zircons. This date may be spurious due to undetected Pb loss, and will be better constrained by CA-ID-TIMS.