THE MIDDLE ORDOVICIAN TO EARLY SILURIAN VOYAGE OF THE DASHWOODS MICROCONTINENT, WEST NEWFOUNDLAND; BASED ON NEW U/Pb AND ~(40)Ar/~(39)Ar GEOCHRONOLOGICAL, AND KINEMATIC CONSTRAINTS

摘要

The Dashwoods microcontinent is an important tectonic segment in the peri-Laurentian setting of the Newfoundland Appalachians. In order to better understand the tectonic history of Dashwoods during the Ordovician Taconic orogeny, we have undertaken field mapping, microscopic studies, and U-Pb and ~(40)Ar/~(39)Ar geochronological studies along the northern (Little Grand Lake Fau LGLF) and western (Baie Verte Brompton Line - Cabot Fault Zone; BCZ) boundaries. Oblique-dextral ductile deformation in the BCZ occurred from late Middle Ordovician into the Early Silurian, based on the presence of a late syn-tectonic pegmatite dike (455 +- 12 Ma) and a foliated granodiorite sheet (445.8 +- 0.6 Ma). Deformation is coeval with oblique-sinistral accretion along the eastern margin of Dashwoods, which means that Dashwoods and its Notre Dame Arc had a southward translation with respect to the Laurentian margin and the then-present Iapetus Ocean during the Late Ordovician. Dextral movement along the BCZ continued after the collision of Dashwoods with the Laurentian margin. Deformation along the Little Grand Lake Fault is bracketed between 463 +- 5 Ma and 440 +- Ma. These ages combined with other geological arguments indicate that motion probably took place during the Late Ordovician to earliest Silurian contemporaneous with the southward translation of Dashwoods. A possible explanation is that the Snooks Arm arc moved independently from and faster southwards than the Notre Dame Arc with its Dashwoods infrastructure, thereby underthrusting the Dashwoods along the Little Grand Lake Fault. Our new U-Pb geochronological data, including a muscovite granite (463 +- 5 Ma), a schistose muscovite granite (459~(+17)/_(-21) Ma), and a tectonized tonalite (458 +- 20 Ma), add to the geochronological database of the voluminous second phase of the Notre Dame Arc. Additionally, in all-but-one of our U-Pb samples, inherited grains of Mesoproterozoic (circa 1.0 Ga) age have been obtained. Their regional presence fortifies the possible relationship of the Dashwoods microcontinent with the Long Range Inlier in western Newfoundland. Furthermore, it introduces a potential link with the Blair River Inlier in Cape Breton Island.