LASER ABLATION U-TH-PB GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM THE BOTWOOD AND INDIAN ISLANDS BELTS: ACCRETIONARY TECTONIC EVOLUTION OF THE NORTHEAST EXPLOITS SUBZONE, DUNNAGE ZONE, NEWFOUNDLAND APPALACHIANS

Recent developments at Memorial University of Newfoundland in Laser Ablation Microprobe-Inductively Coupled Plasma-Mass Spectrometry have demonstrated the effectiveness of this technique as a U-Pb geochronometer. This work has allowed the determination of ages for in situ minerals (zircon, monazite, perovskite) in rocks, typically as polished thin sections and also for age determinations of detrital zircons in sedimentary rocks for sedimentary provenance, depositional history and tectonic evolution studies.

The Early Paleozoic tectonic history of the Newfoundland Appalachians is recorded in the Dunnage Zone which comprises a collection of Cambro-Ordovician ophiolites and marine volcanic/sedimentary sequences that record numerous accretionary events in a complex series of island arc and back arc basins. The Dog Bay Line (DBL) lies within the Exploits Subzone of the Dunnage Zone and separates different Silurian rock groups. West of the line, subaerial volcanic rocks and sandstones (Botwood Group) overlie marine greywackes and conglomerates of the Badger Group which were deposited on Ordovician rocks already accreted to Laurentia. East of the line, red sandstones overlie shallow marine shales and limestones of the Indian Islands Group that were deposited on Ordovician rocks already amalgamated with the continental Gander Zone.

Detrital zircon samples were collected from both sides of the DBL and at different stratigraphic levels to study temporal contrasts in the collision. Five samples from the Botwood Belt, east of the DBL are dominated by Grenvillian zircons (0.9-1.2 Ga) at the base of the belt, whereas samples from the top of the belt consist of mostly Late Cambrian to Ordovician (540-440 Ma) zircon from the Exploits arc/back arc volcanic rocks. Rocks from east of the DBL are dominated by Avalonian age detritus. One sample each from the Davidsville Group and overlying Indian Islands Group consist of zircons that are mainly Ordovician to Neoproterozoic (450-650 Ma) with minor Mesoproterozoic (0.8-1.6 Ga) and Archean (2.7 Ga) grains present. Silurian overlap sequences from opposing sides of the DBL consist of detritus from contrasting source areas. Laurentian influences are present west of the DBL while east of the DBL, Gondwanan sources dominate. Therefore the DBL may mark the terminal Iapetus Ocean.