THE 2.00 GA PURTUNIQ OPHIOLITE, CAPE SMITH BELT, CANADA: MORB-LIKE CRUST INTRUDED BY OIB-LIKE MAGMATISM

Abstract

The Québec-Baffin segment of the Paleoproterozoic Trans-Hudson Orogen comprises a collage of tectonostratigraphic elements that accumulated on, or were accreted to, the northern margin of the Archean Superior craton throughout more than 200 million years of divergent and subsequent convergent tectonic activity. The Cape Smith Belt, on the Ungava peninsula in northern Québec, preserves a series of thrust imbricates of >2.04-1.87 Ga sedimentary and mafic volcanic rocks that record the subsidence and rifting of the Superior craton and subsequent development of an oceanic basin. The structurally highest component of the Cape Smith Belt, the Watts Group, consists of ultramafic and mafic cumulate rocks, gabbros, sheeted mafic dykes, pillowed and massive basalts that are interpreted as the igneous crustal components of a 2.00 Ga ophiolite. Although the rocks have been deformed and metamorphosed, primary igneous features such as cumulate textures, intrusive contacts, chilled dyke margins and pillow selvedges are locally well preserved. The ophiolite is a composite of two physically, chemically and isotopically distinct suites of tholeiitic rocks; the older consists of gabbroic cumulate rocks, pillowed basalts and sheeted mafic dykes, and is similar to suites formed at modern mid-ocean spreading ridges, whereas the younger suite consists of ultramafic and mafic cumulate rocks and mafic dykes, and is analogous to modern hotspot-related oceanic-island complexes such as Hawaii. Reconstructions of the pre-deformation crustal configuration suggest that the mid-ocean ridge suite was at least 5 km thick, and the younger, oceanic-island suite was greater than 4 km thick, a total thickness of this composite oceanic crust of >9 km.