P. Valverde-vaquero et al., The Margaree orthogneiss: an Ordovician, peri-Gondwanan, mafic-felsic igneous complex in southwestern Newfoundland, CAN J EARTH, 37(12), 2000, pp. 1691-1710
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.