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

The igneous protoliths of the Margaree orthogneiss intruded the peri-Gondwa nan rocks of the Port-aux-Basques Complex, in the southwestern corner of th e 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 amphibol ite, dioritic orthogneiss, 474(-4)(+14) Ma hornblende-bearing tonalitic ort hogneiss with mafic enclaves, 472 +/- 2.5 Ma and 465 +/- 3 Ma biotite-beari ng granitic orthogneiss, and minor ultramafic rocks. Most amphibolite have the chemistry of volcanic-arc tholeiite and are interpreted to be coeval wi th the intrusion of tonalitic and granitic magma with volcanic-arc geochemi cal signatures. Locally, mafic magmatism continued after the intrusion of t he 465 Ma granite. The Margaree orthogneiss and its country rock were overp rinted by upper amphibolite-facies metamorphism and deformation associated with the final closure of the Iapetus Ocean. A late-synkinematic granite da tes the late stages of the high-temperature regional deformation at 417(-4) (+7) Ma, while metamorphic titanite (411 +/- 2 Ma) is interpreted to date e xtensive recrystallization under amphibolite-facies conditions as Early Dev onian. The Margaree orthogneiss is broadly coeval with the plutons that pos tdate the Early Ordovician Penobscottian arc - passive margin imbrication i n central and southern Newfoundland. It also coincides with the extensive l ate Arenig - early Llanvirn back-arc rifting event along the entire peri-Go ndwanan margin of the northern Appalachians. The external position of the P ort-aux-Basques Complex with respect to the back-arc elements in the Hermit age Flexure, coupled with the rock types and geochemistry of the orthogneis s, suggest formation of the Margaree orthogneiss in an arc and (or) back-ar c transitional setting.