Manikewan: An early proterozoic ocean in central Canada, its igneous history and orogenic closure

Abstract

During Early Proterozoic time (Aphebian Period) much of the southwestern Churchill province of the Canadian shield was covered by a sea, called here the Manikewan Ocean. Although the original dimensions of this ocean are unknown, the highly deformed and metamorphosed rocks that represent its evolution and final closure as the Manikewan mobile belt now crop out over an area of at least 6 × 10 5 km 2. Along strike (NE—SW) these rocks are buried at both ends by flat-lying Phanerozoic sediments, whereas, across strike, they are bounded by Archaean cratons, the Western craton to the northwest and Superior province to the southeast. Part of the Western craton and Manikewan mobile belt is overlain by the flat-lying, Middle Proterozoic, Athabasca sandstone, a remnant of a once wide-spread, predominantly fluvial group of rocks. Manikewan rocks can be divided into two major zones: 1. (1) The Cree Lake zone in the northwest consists of a predominantly meta-sedimentary succession which overlies unconformably the southeastern extension of the Western craton. This region appears to have been an ensialic stable platform to miogeosyncline during Aphebian time. 2. (2) The Reindeer Lake zone in the southeast consists of thick sequences of mainly mafic, island-arc type, meta-volcanic rocks (e.g., Amisk volcanics) and their meta-sedimentary equivalents. Locally these are overlain unconformably by thick molasse deposits (e.g., Missi Fm.). This region appears to have been an ensimatic eugeosyncline during at least part of Aphebian time. Numerous stocks and batholiths intrude the Manikewan mobile belt and form an integral part of it. They are mainly granitic and predominantly syn-tectonic. The largest, the Wathaman batholith is over 900 km long, making it the largest known Precambrian batholith. It lies along the junction of the Reindeer Lake and the Cree Lake zones. Radiometric dating is scanty, but what has been done indicates that closure of the Manikewan Ocean was well advanced by 1865 Ma and complete by 1800 Ma (Hudsonian orogeny).