A SILURIAN U-PB AGE FOR THE CAPE ST. MARYS SILLS, AVALON PENINSULA, NEWFOUNDLAND, CANADA - IMPLICATIONS FOR SILURIAN OROGENESIS IN THE AVALON ZONE

Mafic sills from Cape St. Mary's on the Avalon Peninsula of Newfoundla nd give an U-Pb baddeleyite age of 441 +/- 2 Ma. This age corresponds with the earliest ages recorded for the climactic Silurian orogenic ev ent that dominantly affected rocks of the Central Mobile Belt in Newfo undland. The age is consistent with but in no way necessitates that th e Avalon and Gander zones were juxtaposed during the Silurian. Because sills tend to form in poorly lithified and undeformed sedimentary roc ks, it is unlikely that Cambrian sediments hosting the sills were affe cted by Ordovician orogenic events that strongly affected central Newf oundland. Negative Nb and Ti anomalies on mid-ocean-ridge basalt norma lized diagrams show that the sill geochemistry is consistent with form ation in a transpressional tectonic environment. Mafic magmas clearly associated with the Silurian event share these chemical and tectonic a ffinities. Thus both the age and geochemical data are consistent with but do not require a link between the Gander and Avalon zones during t he Silurian. If the two zones were joined prior to the Silurian then t he Avalon must have been distal to both the Ordovician and Silurian or ogenic activity. Further, considerable post-Silurian movement would ha ve had to occur along the bounding Hermitage-Dover fault to account fo r contrasts in the intensity of metamorphism, plutonism, and deformati on between the Gander and Avalon zones.