Geochronology of Proterozoic metamorphism in the deformed Southern Province, northern Lake Huron region, Canada

Abstract

Metamorphic geochronology and thermochronology of the deformed Southern Province north of Lake Huron in Ontario, Canada reveals a complex and protracted tectonothermal history for the Proterozoic southern Laurentide margin that is distinct from that of the Lake Superior region to the west. While Penokean-interval deformation is well-constrained in the Lake Superior area, a well-defined geon 18 geochronometric signature is lacking in the Huronian vicinity. The oldest metamorphic ages revealed by in situ monazite U-Th-total Pb electron microprobe analyses are ca. 1800 Ma, with a strong geon 17 (Yavapai-interval) and local geon 14/15 magmatic influences. Furthermore, conventional 40Ar/ 39Ar incremental step-heating of hornblende indicates Paleoproterozoic heating through 500 °C occurred only adjacent to geon 17 intrusions; regional analysis of mica cooling ages suggests that Mesoproterozoic thermal overprinting of primary geon 17 magmatism and metamorphism is limited to south of the Murray fault. The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Great Lakes region, is now recognized as only the first of several accretionary events that impinged on the southern Laurentide margin. However, the Penokean orogen is restricted to the 2100 Ma embayment of the Superior Province rifted margin. Moreover, subsequent tectonothermal pulses (Yavapai, Mazatzal, and ca. 1450 Ma) recorded in the rocks of the Southern Province in Ontario mimic that documented across much of the central and western United States. Based on the distinct lack of Penokean-interval ages and the strong geon 17 and geon 14 total-Pb and 40Ar/ 39Ar geochronometric signature, the tectonothermal history of Southern Province more closely resembles that of the southwest United States than the Lake Superior region.