Author: wrchurch
Company: UWO

CONTRIBUTION OF THE
DEPARTMENT OF GEOLOGY
UNIVERSITY OF WESTERN ONTARIO
LONDON, CANADA       NO.  175
PRELIMINARY RPT.  April, 1970
MANTLE PERIDOTITE AND THE EARLY PALEOZOIC OPHIOLITE COMPLEXES
OF THE NEWFOUNDLAND APPALACHIANS by
W. R. Church and R. K. Stevens

The early Ordovician ophiolite complexes of the Newfoundland Appalachians are considered to be remnants of a once extensive ophiolite sheet composed of a basal peridotite, an intermediate gabbroic layer, and an upper layer of sheeted diabase, pillow lava, and chert and greywacke sediment (Stevens, Church and St. Julien, 1969).  The basal peridotite of the allochthonous Bay of Islands complex has a lower Iherzolite-hartzburgite layer with thin magnesian amphibolite-pyroxenite layers containing kaersutite, titaniferous phlogopite, ceylonite, aluminous pyroxenes, and pyropic garnet.  The chemistry of these minerals closely resembles that of the same minerals in the upper mantle Iherzolites and amphibole-ariegites of the type-locality at Lherz French Pyrenees.  The gabbroic rocks are intrusive into the peridotite which occurs as feldspar-metasomatised xenoliths in the gabbro.  The sheeted dia-bases cut the gabbros and pillow lavas, and locally the peridotite.  The volcanic rocks are tholeiitic and spilitic.  The Newfoundland ophiolite complexes are comparable to those of the Oman (Rheinhardt, 1969), the Sesia Lanzo zone of the Western Alps (Nicolas, 1968), Papua (Davies, 1968) and New Caledonia (Avias, 1967).  They are however unique in having high-pressure mantle assemblages, and well preserved high-temperature garnet-pyroxenite contact aureoles which are in places superposed on a garnet-biotite grade regional metamorphic fabric.  These features relate the Newfoundland peridotites to intrusive ultramafic rocks such as those of Mount Albert (Smith and MacGregor, 1960), the Lizard (Green, 1964), Beni Bouchera (Kornprobst, 1969), La Ronda (Dickey, 1969) and the Ivrea Zone (Nicolas, 1968).  The high-temperature aureoles of the Newfoundland complexes also distinguish them from the Papuan and New Caledonian allochthonous ophiolite sheets (Avias, 1967) considered to be 'cold' oceanic plates, and the Franciscan ultramafic rocks emplaced into Benioff fault zones during tectonism and low-temperature blueschist metamorphism of underlying oceanic sedimentary and volcanic rock (Ernst, 1969; Blake et al., 1969).
The existence of early Paleozoic ophiolite complexes in the Appalachians contradicts recent assertions (Moore, 1969) that ophiolites are restricted to the Mesozoic, and lends little support to recent models on the evolution of the Caledonian/Appalachian orogen (eg. Dewey, 1969) whereby the peridotites are regarded as stratiform cumulates and intrusive serpentines of several ages derived from a hypothetical basaltic root beneath the Fleur de Lys-Moine-Dalradian orthotectonic belt (Church, 1969).  The present interpretation of the ophiolite complexes of the Newfoundland Appalachians suggests that they are a feature of considerable tectonic significance in the evolution of the Caledonian/Appalachian orogen, their emplacement separating a late-Cambrian phase of tectogenic deformation and metamorphism from the mid-Ordovician epeirogenic movements responsible for the development of the Western and Central Newfoundland flysch troughs, and the emplacement of the Western Newfoundland Taconic klippen (Stevens, 1970).
Along with the alpine occurrences, the Paleozoic ophiolite complexes of the Appalachians afford one of the best opportunities to resolve the question of whether ophiolites in older terrains represent allochthonous Atlantic/Pacific type oceanic crust which once separated independent continental masses growing by accretion, or pseudo-oceanic floors extruded directly over sialic crust along zones of distension within an intra-cratonic orogen undergoing basification, as is perhaps represented by the Sea of Japan type oceanic crust. The apparent absence of ophiolite complexes from the Precambrian of North America, in particular from the early Proterozoic linear erogenic belts such as the Penokean-Labrador Trough system, which in some respects are comparable to the Appalachians, also has important implications with regard to the application of plate tectonic theory to the Precambrian.

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