Geodynamic significance of Neoarchean metasedimentary belts in the Superior Province: Detrital zircon U-Pb LA-ICP-MS geochronology of the Opinaca and La Grande subprovinces

Abstract

Metasedimentary subprovinces within the Superior Province and many Archean cratons worldwide occur as prominent belts separating volcano-plutonic subprovinces. The stages of their development mark major, distinct geodynamic events. Deposition ubiquitously follows a restricted period of voluminous, regional, felsic pluton emplacement. Their geodynamic setting of deposition is not well understood, and despite similar sedimentological, structural and post-deposition metamorphic characteristics, metasedimentary subprovinces have been variously interpreted as developing in accretionary prism, syn-orogenic, and back-arc basin settings. We present the first synthesis of detrital zircon U-Pb LA-ICP-MS geochronology from paragneiss and migmatite of the Opinaca and La Grande subprovinces. This study constrains detrital zircon age populations of the sedimentary protoliths, assesses their provenance, and evaluates the geodynamic setting of deposition. Including detrital geochronology compiled from regional literature, nine paragneiss samples from the Laguiche Complex (Opinaca) and eleven from the Rivière Salomon, Thor, Keyano, Magin, Marbot, Low and Ekomiak formations (La Grande) provide dominant modal age peaks similar to regional Neoarchean felsic plutonic suites, and lesser age peaks consistent with regional Neoarchean and Mesoarchean greenstone belts and Mesoarchean gneiss. The units have comparable zircon age populations; therefore we interpret a shared provenance from the La Grande and Minto subprovinces, suggesting that none of the metasedimentary units represent exotic accreted terranes. Mean ages of youngest LA-ICP-MS zircon age clusters range from 2716 to 2695 Ma, thus defining the maximum ages of deposition. ID-TIMS analyses of select youngest grains confirm the efficacy of the LA-ICP-MS constraints. Regional intrusive suites ranging from ca. 2710 to 2678 Ma further constrain deposition to a short time interval. Our detrital geochronology results are similar to all other Superior Province metasedimentary subprovinces, which consistently exhibit large proportions of detrital zircon ages close to their depositional age. This would conventionally lead to the interpretation that the extensional environment is a back-arc basin to an active convergent margin. However, as the sequence of events preceding formation of the Opinaca subprovince may not reflect uniformitarian tectonic processes, we discuss alternate models of basin formation and how the similar patterns observed in Superior Province detrital geochronology could be consistent with a variety of geodynamic environments.