Orogenic to postorogenic (1.20-1.15 Ga) magmatism in the Adirondack Lowlands and Frontenac terrane, southern Grenville Province, USA and Canada

Magmatism in the southern Grenville Province records a collisional and postcollisional history during the period 1.20–1.15 Ga in the Adirondack Lowlands (New York State, USA) and the Frontenac terrane (Ontario, Canada). The 1.20 Ga bimodal Antwerp-Rossie suite of the Adirondack Lowlands was produced by subduction in the Trans-Adirondack backarc basin. This was followed by intrusion of the 1.18 Ga alkalic to calc-alkalic Hermon granite, which may have been generated by melting of metasomatized mantle during collision of the Adirondack Lowlands and Frontenac terrane during the Shawinigan orogeny. The Hyde School gneiss plutons intruded the Adirondack Lowlands at 1.17 Ga, and Rockport granite intruded into the Adirondack Lowlands and Frontenac terrane, stitching the Black Lake shear zone, which marks the boundary between these terranes. Subsequent extensional collapse and lithospheric delamination caused voluminous anorthosite-mangerite-charnockite-granite plutonism. In the Frontenac terrane, this event is represented by the 1.18–1.15 Ga Frontenac suite, which is composed predominately of ferroan granitoids produced from melting of the lower crust by underplating mafic magmas. The Edwardsville, Honey Hill, and Beaver Creek plutons are newly recognized members of this suite in the Adirondack Lowlands. High oxygen isotope ratios of this suite in the central Frontenac terrane and western Adirondack Lowlands point to the presence of underthrust altered oceanic rocks in the lower crust. Oxygen isotopes of the Frontenac suite in both terranes preclude its derivation from mantle melts alone.