The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure ∼15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1–2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50–200 μ) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 ± 7 and 1197 ± 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium TDM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, foredeep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200–1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.
- Received 3 May 2010.
- Revision received 17 October 2010.
- Accepted 21 October 2010.
- © 2011 Geological Society of America