The ophiolites of southern Quebec: oceanic crust of Betts Cove type

Abstract

The Thetford and Asbestos ophiolites of the Eastern Townships of Quebec (Internal Domain) resemble the Betts Cove ophiolite of Newfoundland in three significant respects: (1) ultramafic cumulate sequences of the ophiolites include units with cumulus orthopyroxene; (2) the 'gabbro' unit is composed of a relatively thin yet compositionally and structurally complex clinopyroxenite–gabbro member which unconformably overlies the ultramafic cumulate sequence; and (3) basaltic rocks associated with the ophiolites have extremely low Ti contents. In contrast, in the Bay of Islands region of western Newfoundland (External Domain) basaltic rocks of the ophiolites of the Humber Arm Allochthon have Ti contents typical of normal oceanic crust; the cumulate sequences rarely contain cumulus orthopyroxene; and the 'gabbro' unit includes a major sequence of cumulus plagioclase-bearing rocks which appear to conformably follow the ultramafic cumulates. The Baie Verte ophiolite of Newfoundland is intermediate in character and location between the Betts Cove and Bay of Islands complexes. All of the ophiolites of the Internal Domain are overlain by sequences having conglomerate, olistostrome, and greywacke units containing abundant clastic material derived from ophiolitic rocks.The homologous nature of the Betts Cove and Thetford ophiolites lends some support to the view that the Fleur de Lys and Caldwell - Chain Lakes orthotectonic zones of the Appalachian system are laterally correlative, and that all ophiolites of the Internal Zone of the Appalachians are allochthonous rather than collapsed in situ small ocean basins. The variation exhibited by the ophiolites of the Appalachian system, even within the Internal Zone, cautions however against a too simplistic view of the role of ophiolites in plate tectonic models of orogenic belts of Appalachian type.