EARLY PALEOZOIC CHRONOLOGY - A REVIEW IN LIGHT OF NEW U-PB ZIRCON AGES FROM NEWFOUNDLAND AND BRITAIN

The time scale for the Cambrian through Early Devonian periods is revi sed using new U-Pb ages from volcanic and pyroclastic rocks in sequenc es that are stratigraphically well defined. A rhyolite flow from the l atest Precambrian, dated at 551.4 +/- 5.8 Ma, gives a maximum age for the base of the Cambrian (Phycodes pe dum Zone) at Fortune Bay, Newfou ndland. The age ranges of several Ordovician and Silurian series are n ow established to within 2-3 Ma based on stratigraphically controlled samples from Britain. A basal Llandeilo ash has an age of 460.4 +/- 2. 2 Ma; a basal Caradoc lava has an age of 456.1 +/- 1.8 Ma; and K-bento nites from the latest Llandovery and latest Ludlow have ages of 430.1 +/- 2.4 and 420.2 +/- 3.9 Ma, respectively. Using other data from the literature, we interpolate the base of the Cambrian to be near 545 Ma, the base of the Ordovician as ca. 495 Ma, and the base of the Siluria n as ca. 443 Ma. While we have a good estimate for the base of the Mid dle Devonian (at ca. 391 Ma), the base of the Devonian is less precise , probably around 417 Ma.