BIRCHY COMPLEX, BAIE VERTE PENINSULA, NORTHWEST NEWFOUNDLAND: A PRODUCT OF LATE EDIACARAN HYPER-EXTENSION OF THE LAURENTIAN IAPETAN MARGIN AND IMPLICATIONS FOR THE OPENING OF IAPETUS AND FORMATION OF PERI-LAURENTIAN MICRO-CONTINENTS

The Birchy Complex of the Baie Verte Peninsula, northwest Newfoundland comprises an assemblage of mafic schist, ultramafic and meta-sedimentary rocks, which are structurally sandwiched between overlying ca. 490 Ma ophiolite massifs of the Baie Verte oceanic tract and underlying metasedimentary rocks of the Fleur de Lys Supergroup of the Appalachian Humber margin. Birchy Complex gabbro yielded a Late Ediacaran U-Pb zircon ID-TIMS age of 558 ± 1 Ma, whereas LA-ICPMS concordia zircon ages of gabbro - and an intermediate tuffaceous schist gave ages of 564 ± 7.5 Ma and 556 ± 4 Ma respectively. These ages overlap the last phase of rift-related magmatism observed along the Humber margin of the northern Appalachians (565-550 Ma), yet the mafic rocks have MORB-like characteristics. The associated ultramafic rocks shed detritus into the interleaved sedimentary rocks and hence were exhumed by Late Ediacaran. Psammite in the overlying Flat Point Formation yielded a detrital zircon population typical of the Laurentian Humber margin in the northern Appalachians. Age relationships and characteristics of the Birchy Complex and adjacent Rattling Brook Group suggest that the ultramafic rocks represent slices of continental lithospheric mantle exhumed onto the seafloor shortly before or coeval with magmatic accretion of MORB-like mafic rocks. Hence, they represent the remnants of an ocean-continent transition (OCT) zone formed during hyper-extension of the Humber margin prior to establishment of a mid-ocean ridge further outboard in the Iapetus Ocean. We propose that ribbon-shaped microcontinents such as Dashwoods and the Rattling Brook block formed as extensional crustal allochthons riding on a convex (concave-downwards) lithosphere-scale detachment, analogous to the isolation of the Brianconnais block during the opening of the Alpine Tethys (Ligurian-Piemonte) oceanic seaway.