LINKS BETWEEN THE ADIRONDACKS AND THE MORIN TERRANE: NEW EVIDENCE FROM GEOCHRONOLOGY

The Morin terrane is part of the southwestern Grenville Province located to the north of Montreal. It has long been recognized as geologically similar to the Adirondack Highlands to the south, both having similar packages of granulite-facies rocks and hosting ca. 1155 Ma plutons of the anorthosite – mangerite – charnockite – granite (AMCG) suite. Many workers also highlight similarities to the Mt. Laurier terrane of the Central Metasedimentary Belt to the west. Geologic differences between these three terranes raises several questions, especially about the tectonic level of the Morin terrane during the 1090–1020 Ma Ottawan orogeny. Magmatism in the easternmost Morin terrane has been dated as Ottawan (i.e. the LeJeune mangerite, Shawinigan gabbronorite, and Saint-Didace batholith), but the timing of high-grade metamorphism has not until now been well constrained.

New geochronology sheds light on some of these questions. Titanite was dated from marbles that have calcite-graphite temperatures of 755±38°C. Some marbles contain titanite with Ottawan metamorphic ages, showing that the Morin terrane was at mid-crustal levels at this time. Titanite from other marbles preserve ages of ca. 1120 Ma, an age observed in the Central Metasedimentary Belt boundary thrust zone and Parry Sound domain. Ottawan zircon growth is seen in quartzites, along with a Proterozoic to Archean detrital zircon population similar to that found in Adirondack Highland quartzites (Peck et al., Am Min 2010). These lines of evidence, along with the presence of 1.3 Ga tonalites and the unusually high oxygen isotope ratios of Adirondack and Morin anorthosites connect the Morin terrane and Adirondack Highlands together in tectonic reconstructions, a continuity that persisted through the Ottawan orogeny.