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In central Newfoundland, metamorphic rocks of felsic pyroclastic origin occur in the southern part of the Robert's Arm volcanic belt adjacent to the Red Indian Line suture between the Gondwanan and Laurentian realms of the Iapetus Ocean. These supracrustal rocks are typically associated with black cordierite pelite, variegated spotted metachert, amphibolitized alkaline and calc-alkaline metabasite, and garnet-andalusite schist derived from volcaniclastic wacke and debrite. In the study area, these strata are overthrust by a tectonostratigraphic sequence predominantly composed of Early Ordovician volcanic and plutonic rocks, which formed on the peri-Laurentian side of the oceanic Dunnage Zone.
Igneous crystallization ages of felsic volcanic schists lying in several thrust sheets within the structurally lowest part of the Robert's Arm thrust stack were determined using the U/Pb TIMS method. They range from as old as ca. 460 Ma (Powderhorn Lake tract) to as young as ca. 456 Ma (Baker Brook tract). A folded thrust sheet of sillimanite-bearing metasedimentary gneiss having ubiquitous intrusions of garnetiferous amphibolite (Burnt Pond tract) lies tectonically adjacent to these Upper Ordovician pyroclastic rocks. Delicate zircon needles preserved within migmatitic lits in paragneiss indicate that an early phase of metamorphism affected these rocks about 460 myr ago after the emplacement of the amphibolites. This indicates that migmatization was broadly coeval with felsic volcanism in the southern part of the Robert's Arm volcanic belt.
Pyrenean-type metamorphic rocks were intruded by schistose or recrystallized gabbro bodies injected along the crests of domes and basins that refold the lower nappes in the Robert's Arm thrust belt. The highest grade rocks were reworked in ductile shear zones and intruded by orthogneiss sheets on the flanks of these structural features. In the Powderhorn Dome, a syn-tectonic granodiorite vein hosted by orthogneiss yielded a U/Pb zircon age of ca. 434 Ma for igneous crystallization. Later plutons of gabbro and granodiorite comprise parts of post-tectonic batholiths that invade through these tectonic culminations and depressions and fragment the country rocks. A high-level isotropic gabbro crosscutting the Burnt Pond, Powderhorn Lake and Baker Brook tracts yielded a U/Pb zircon age of ca. 429.5 Ma for igneous crystallization. This implies very rapid uplift of the metamorphic tectonites in the southern part of the Robert's Arm volcanic belt.
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