| Summary: | The Adirondack Mountains form the southern extension of the Grenville Province, an orogeny that formed during the formation of supercontinent Rodinia in the late Mesoproterozoic. This orogenesis can be defined by three major phases over 250 million years: the Elzevirian (1245 – 1225 Ma), the Shawinigan (1200 – 1160 Ma), and the Grenville orogeny that is further subdivided into two phases the Ottawan (1090 – 1020 Ma) and the Rigolet (1010 – 980 Ma). The Adirondacks can be defined by two geologic terranes: the Adirondack Highlands and the Adirondack Lowlands. The Highlands are composed of granulite facies metamorphic and igneous rocks (AMCG unit) while the Lowlands are characterized by amphibolite-facies metasedimentary and meta-igneous rocks. These terranes are separated by the Colton Carthage Mylonite Zone. Small bodies of metagabbro and garnetiferous amphibolites that are genetically related to the AMCG unit are exposed in the Highlands, although their timing and conditions of magmatism and metamorphism are unknown. We collected samples of coronitic metagabbro to garnetiferous amphibolite units and applied petrographic analysis coupled with U-Pb zircon geochronology. Our results show the coronitic metagabbro exhibited three age populations of 1147±8 Ma, 1036±7 Ma, and 1026±5 Ma. Zircons from the garnetiferous amphibolite sample yield an age of 1046± 54 Ma. These ages show that magmatism occurred in Shawinigan while metamorphism took place during the Ottawan phase.
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