GEOLOGY OF THE TALLY POND GROUP, NEWFOUNDLAND: NEW GEOCHRONOLOGY AND GEOCHEMICAL DATA FROM THE EXPLOITS SUBZONE, DUNNAGE ZONE

Recent mapping, geochronological and geochemical studies in the Tally Pond-Burnt Pond area, Newfoundland, has resulted in new interpretations of the local geology and a redefinition of the Tally Pond group. The group comprises Cambrian island-arc felsic pyroclastic rocks with intercalated mafic volcanic rocks and epiclastic volcanic and sedimentary rocks. In the study area, arc plutonic rocks of the Crippleback Lake Quartz Monzonite form the basement to Tally Pond group. A sequence of dominantly pillowed mafic volcanic rocks, mafic to andesitic volcaniclastics, and intercalated felsic volcanic rocks are nonconformable on the Crippleback Lake Quartz Monzonite. U-Pb zircon geochronology indicates that the felsic volcanic rocks were erupted at 509 ± 1 Ma. Dykes and stocks of medium-grained gabbro-diorite intrude all of the rocks of the Tally Pond group. These mafic intrusions yielded a U-Pb zircon age of 465 ± 2 Ma. Major, trace and rare earth element geochemistry indicates that the volcanic rocks have geochemical affinities consistent with a volcanic arc paleotectonic environment. Mafic intrusive rocks show no evidence of arc-related volcanism. Pb isotope data from VMS occurrences in the Tally Pond group contrast with those from the Notre Dame Subzone and are comparable to data from the Exploits subzone. The youngest rocks in the study area are conglomerates and coarse-grained sandstones of the Silurian Rogerson Lake Conglomerate which unconformably overlies rocks of the Tally Pond group; the conglomerate is dominated by volcanic clasts derived from the underlying Exploits subzone sequences. The Early Cambrian rocks of the Tally Pond group are analogous to rocks of the Brookville and Bras d’Or terranes in New Brunswick and Nova Scotia. Previous workers have suggested that the Brookville and Bras d’Or terranes represent the eastern margin of the Central Mobile Belt of the Appalachians (Gander Zone). Correlation of the Tally Pond group with these terranes, suggests that the Tally Pond group formed along the peri-Gondwanan margin of Iapetus. The Silurian Rogerson Lake Conglomerate that unconformably overlies the Tally Pond group indirectly indicates that these terranes were accreted to Laurentia by Late Ordovician.