Abstract

The crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the Early Proterozoic Cape Smith belt in northern Quebec, Canada. The rocks of the ophiolite represent two magmatic suites, each with a mantle source that is Nd isotopically distinct. The older suite comprises pillowed and massive volcanic rocks, sheeted dikes, dominantly gabbroic layered cumulate rocks, and rare plagiogranite sills and dikes in the volcanic sequence. This >5-km-thick tholeiitic suite is ca. 1998 Ma, and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges. The younger suite (>4 km) comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume- generated oceanic islands, such as Hawaii. The composite section is in excess of 9 km. Although the total thickness of the Purtuniq ophiolite may not be typical of that for oceanic crust formed at Early Proterozoic spreading ridges elsewhere, it does suggest that ancient ridge-formed crust was at least as thick as that in modern oceans.

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