Polycyclic evolution of the Late Neoproterozoic basement in the Hermitage Flexure region (southwest Newfoundland Appalachians): New evidence from the Cinq-Cerf gneiss

Abstract

The Cinq-Cerf gneiss represents one of the oldest tectonic elements yet documented within the “Avalonian” basement complex of the Hermitage Flexure region of the Newfoundland Appalachians. This gneiss is a composite unit that includes both supra-crustal and granitic gneisses, the latter having an igneous crystallization age of 675 + 12/−11 Ma. The timing of the gneiss-forming event (D 1 polyphase deformation) is constrained by the protolith age of the orthogneiss and the 584 + 7/−6 Ma crystallization age of granodiorite intrusive into gneiss. The emplacement of the granodiorite is coeval with the previously dated 585 Ma eruption of felsic tuff within nearby low-grade stratified elements of the pre-Silurian basement, north of the Grand Bruit Fault Zone (GBFZ), adding support to models of a Late Neoproterozoic basement-cover relationship between the gneiss and the low-grade volcano-sedimentary sequence. Younger Neoproterozoic magmatism in the gneissic basement is recorded by emplacement of 557 + 14/−5 Ma gabbro and the intrusion of 547 + 2/−7 Ma diorite dykes. The latter have a recrystallized tectonic fabric that hosts 448 + 9/−3 Ma titanite. Peak Appalachian-cycle overprinting of the Cinq-Cerf gneiss occurred in mid- to late Silurian time, and commenced with high-temperature deformation (D 2) of the gneiss, associated with synkinematic intrusion of Western Head granite at 431.5 ± 1 Ma. Mafic dyke swarms that intrude the gneiss and the granite contain 420 ± 3 Ma titanite. This age reflects new growth during Silurian retrogression and mylonitization of the mafic dykes. The D 3 mylonitization is widespread and linked to thrusting of the Cinq-Cerf gneiss along the GBFZ during the 420–418 Ma closure of the northerly adjacent Silurian volcano-sedimentary cover basin. New data bearing on the Late Neoproterozoic record of the basement block of the western Hermitage Flexure further demonstrates similarities in magmatic, stratigraphic and metallogenic record documented in Avalonian rocks of east Newfoundland's Connaigre, Burin and Avalon peninsulas, pointing to a Neoproterozoic linkage with the rocks in these areas. The early to mid-Paleozoic history of the Hermitage Flexure basement indicates that, during that time, the basement block evolved as a terrane separate from the Avalon Zone (sensu stricto).