The Margaree orthogneiss: an Ordovician, peri-Gondwanan, mafic-felsic igneous complex in southwestern Newfoundland

The igneous protoliths of the Margaree orthogneiss intruded the peri-Gondwanan rocks of the Port-aux-Basques Complex, in the southwestern corner of the Hermitage Flexure, prior to regional deformation and metamorphism. Field relationships and U–Pb geochronology indicate that the Margaree orthogneiss represents a 20 km long, mafic-felsic intrusive complex formed by amphibolite, dioritic orthogneiss, 474+14-4 Ma hornblende-bearing tonalitic orthogneiss with mafic enclaves, 472 ± 2.5 Ma and 465 ± 3 Ma biotite-bearing granitic orthogneiss, and minor ultramafic rocks. Most amphibolite have the chemistry of volcanic-arc tholeiite and are interpreted to be coeval with the intrusion of tonalitic and granitic magma with volcanic-arc geochemical signatures. Locally, mafic magmatism continued after the intrusion of the 465 Ma granite. The Margaree orthogneiss and its country rock were overprinted by upper amphibolite-facies metamorphism and deformation associated with the final closure of the Iapetus Ocean. A late-synkinematic granite dates the late stages of the high-temperature regional deformation at 417+7-4 Ma, while metamorphic titanite (411 ± 2 Ma) is interpreted to date extensive recrystallization under amphibolite-facies conditions as Early Devonian. The Margaree orthogneiss is broadly coeval with the plutons that postdate the Early Ordovician Penobscottian arc – passive margin imbrication in central and southern Newfoundland. It also coincides with the extensive late Arenig – early Llanvirn back-arc rifting event along the entire peri-Gondwanan margin of the northern Appalachians. The external position of the Port-aux-Basques Complex with respect to the back-arc elements in the Hermitage Flexure, coupled with the rock types and geochemistry of the orthogneiss, suggest formation of the Margaree orthogneiss in an arc and (or) back-arc transitional setting.

The geology and geochronology of the Makkovik Bay area, Labrador

The sedimentary and volcanic formations of the Aillik series in the Makkovik Bay area of the Labrador coast are about 25 000 ft (~7620 m) in thickness. They have been folded into a series of northerly-trending folds and are intruded by gabbro, diorite, syenite, and abundant granite. Potassium–argon age determinations range from 1730 to 1830 million years for two gneisses, and from 1500 to 1600 million years for the metamorphosed sedimentary and volcanic rocks and intrusive granite-gneiss and granite. The structural features of the area indicate a single cycle of orogenic deformation for the sedimentary and volcanic rocks, in which the emplacement of synkinematic granite gneiss as domes and other bodies played an important role. One of the gneisses is considered a part of the migmatized Archean basement on which the Aillik series was laid down, and was subsequently involved in the Hudsonian orogenic cycle about 1600 million years ago. The area lies to the northwest of the Grenville Front, and has been intruded by diabase dikes corresponding in age to the Grenville orogeny, and by lamprophyre dikes of Cambrian age.