METAMORPHISM OF THE CANADIAN SHIELD, ONTARIO, CANADA. II. PROTEROZOIC METAMORPHIC HISTORY

Abstract

This paper is a complement to the Tectonometamorphic Map of the Canadian Shield, and contains a brief description of the metamorphic history of Proterozoic orogenic belts and rock sequences in the Canadian Shield in Ontario, with a focus on rocks of the Southern and Grenville provinces. Metamorphism in the Southern Province has generally been attributed to the Penokean orogeny (~1879–1820 Ma), although the timing of this metamorphism is poorly constrained by either relative or absolute ages. In the Sault Ste. Marie – Sudbury area, previously described regional Na- and K-metasomatism has likely altered the original assemblages of metamorphic minerals, making unravelling of the metamorphic history of these rocks problematic. Metamorphism appears to postdate the emplacement of the Sudbury Igneous Complex at 1850 Ma; however, it is unclear if regional metamorphism in the Sault Ste. Marie – Sudbury area is related to the early (1870–1820 Ma) or late-stage (1740–1700 Ma) events of the Penokean orogeny. Three main metamorphic events are associated with the Grenville (1300–950 Ma) orogeny. These events are not necessarily orogen-wide in their effects. The first, at 1250–1240 Ma, affected rocks of the Composite Arc Belt, and is similar in style to that found in Superior Province granite–greenstone belts. It consists of a dominant subgreenschist- to greenschist-facies event, with amphibolite-facies assemblages developed near larger plutonic bodies. The next event (1190–1170 Ma) is restricted mainly to the southern part of the orogen (southern Laurentian margin, Composite Arc and Frontenac–Adirondack belts), and is responsible for regional upper-amphibolite- to granulite-facies metamorphism. In parts of the orogen, metamorphic rocks formed at this time are well preserved ( e.g., Parry Sound and Frontenac domains), whereas in other areas ( e.g., Adirondack Highlands), the rocks are overprinted by younger, high-grade metamorphism. Pan-Grenville greenschist- to granulite-facies metamorphism in Ontario in the period 1070–1050 Ma was associated with major thrusting. In deeper structural levels, this event persists to 990 Ma. Tectonic unroofing after 1070 Ma likely played a major role in the current distribution of metamorphic rocks within the Grenville Province in Ontario.