U–Pb geochronology of the Mazinaw terrane, an imbricate segment of the Central Metasedimentary Belt, Grenville Province, Ontario

Abstract

The Mazinaw terrane, in the Central Metasedimentary Belt of the Grenville Province comprises, volcanic, sedimentary, and plutonic rocks that were intensely folded and faulted, and metamorphosed to as high as upper amphibolite facies. U–Pb geochronology establishes an early period of magmatism and sedimentation at about 1280–1240 Ma, probably in a marginal basin setting, and a multistage rhetamorphic evolution in the period between 1100 and 980 Ma, which was probably related to crustal thickening by imbrication during compression and wedging of the terrane. Some of the earliest magmatism formed calc-alkalic volcanic rocks of Kashwakamak Formation at 1276 ± 2 Ma. An associated sedimentary assemblage was intruded by the Helena trondhjemite stock at 1267 ± 5 Ma. A younger succession, formed between 1250 and 1240 Ma, includes the bimodal volcanic Mazinaw Lake Formation, alaskitic granites of the Norway Lake and Abinger plutons, and a granodioritic gneiss phase of the Cross Lake pluton. These units were covered unconformably by the Flinton Group sometime after 1150 Ma, as defined by published detrital zircon data. A northern segment of the Mazinaw terrane underwent deformation and metamorphism accompanied by the emplacement of granitic pegmatites in the period between 1100 and 1050 Ma. In contrast, metamorphism and deformation occurred significantly later in central domains, as shown by several metamorphic zircon populations grown at about 1040–1030, 1020, and 1010–1000 Ma, by pegmatite emplacement at about 980 Ma, and titanite ages of 1010–960 Ma. Younger rutile ages of about 915 Ma may reflect uplift of the terrane. Some rutile also appears to record the latest Proterozoic faulting and mafic diking related to formation of the Ottawa-Bonnechere graben at about 600 Ma.