Thermal History of the Flinton Group, Mazinaw Terrane, Grenville Province, Ontario

Abstract

The eastern section of the Flinton Group, from Cloyne to Clyde Forks, is a long, narrow NE-SW trending belt of metasedimentary rocks belonging to the Mazinaw Terrane of the Grenville Province. Metamorphic grade increases southwest to northeast across the Flinton Group from greenschist to amphibolite facies, with metamorphic isograds perpendicular to the strike of bedding. Rocks from various members within the Flinton Group contain abundant biotite and muscovite and very little evidence of alteration. Detailed petrography, electron-microprobe analysis, and 40Ar/39Ar dating of biotite and muscovite were used to investigate the thermal history of the Flinton Group. 40Ar/39Ar ages were obtained on eleven muscovite/biotite pairs and two muscovites from pelitic schists, and on a muscovite/biotite pair from a pegmatite. Biotite ages show a broad variation across the Flinton Group. Biotite ages in the western half of the study area range from 979 ± 6 Ma to 939 ± 6 Ma with an average pooled age of ~953 Ma, while the ages from the eastern half are much younger, from 908 ± 7 Ma to 885 ± 6 Ma with an average pooled age of ~900 Ma – reflecting a ~50 Ma age difference. Muscovite ages across the Flinton Group are uniform (with one exception) and range from 909 ± 8 Ma to 880 ± 7 Ma (±2σ) with an average pooled age of ~899Ma. The observed age jump appears to coincide with increasing metamorphic grade and a change in quartz microtextures from grain boundary area reduction in the west to grain boundary migration in the east, suggesting that the eastern Mazinaw Terrane was buried deeper than the west due to tilting of the Mazinaw Terrane and movement along the Robertson Lake Shear Zone.