The geology, geochemistry, geochronology and regional setting of the Annieopsquotch Complex and related rocks of southwest Newfoundland

Dunning, Gregory Ralph (1984) The geology, geochemistry, geochronology and regional setting of the Annieopsquotch Complex and related rocks of southwest Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Annieopsquotch Complex is an ophiolite which forms the Annieopsquotch Mountains of southwest Newfoundland. It contains rocks of the critical zone, gabbro zone (2.3 km thick), sheeted dyke zone (1.5 cm) and pillow lava zone of a typical ophiolite. The zones trend northeast, face and dip southeast at approximately fifty to seventy degrees and are offset by faults. -- Cumulate rocks of the critical zone preserve graded layers, trough structures and slump folds and elsewhere are metamorphosed and deformed. The gabbro zone is heterogeneous and contains many textural varieties of gabbro, pegmatitic pods, layering, trondhjemite pods and amphibolite near the base. It passes through a transition zone to a sheeted dyke zone which extends the full length of the ophiolite. Dykes trend northwest and are aphyric or plagioclase-phyric diabase. The pillow lava zone, besides pillow basalt, contains minor pillow breccia, hyaloclastite and chert. -- Major, trace and rare earth element geochemistry and clinopyroxene chemistry indicate that the suite is most similar to Normal-typ mid-ocean ridge basalt, low Zr contents being the only unusual feature. Trondhjemites have variable chemistry but elevated contents of trace elements consistent with differentiation from this basalt. -- The Annieopsquotch Complex is faulted against an Ordovician tonalite terrane to the northwest across the Lloyds River fault and against the Victoria Lake Group to the southeast. It is cut by dykes and sills correlated with both these units. The ophiolite is cut by two late Ordovician gabbro-diorite intrusions and a granite intrusion of presumed Devonian age and is unconformably overlain by early Silurian terrestrial sedimentary and volcanic rocks. -- Two U-Pb ages for zircon from trondhjemite (477.5, 481.4 Ma) indicate that formation of the Annieopsquotch Complex occurred in Arenigian time. Zircon ages for the Bay of Islands (485.7 Ma) and Betts Cove Complexes (488.6 Ma) show them to be time equivalents, likewise Arenigian, refuting previous interpretations. A survey of ages of other ophiolites suggests that only oceanic crust (s.l.) formed over a limited time range is preserved in the Appalachian-Caledonian Mountain Belt. -- Other ophiolitic fragments, most intruded by or included in the tonalite, are preserved in a belt from Shanadithit Brook to King George IV Lake. Because of lithologic and chemical similarities, they are included in the Annieopsquotch ophiolite belt. Basalts of the Victoria Lake Group also have very similar chemistry suggesting a genetic link. Common orientations of sheeted dykes in the fragments suggest that they may have comprised one allochthon of Iapetus oceanic crust. This allochthon was emplaced over the Ordovician continental margin of North America during the Taconic Orogeny and intruded by tonalitic melts at that time.

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