U–Pb geochronology of Archean metasedimentary rocks in the Pontiac and Abitibi subprovinces, Quebec, constraints on timing, provenance and regional tectonics

Abstract

U–Pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the Pontiac subprovince and from the Cadillac, Kewagama and Lac Caste Groups within the southern Abitibi subprovince. Youngest detrital zircon ages from these sequences are 2685, 2686, 2687, and 2693 Ma, respectively. The Lac Fournière pluton, which intrudes Pontiac Group metasedimentary rocks, is dated at 2682±1 Ma. A similar age was previously measured from a pluton that cuts the Kewagama Group. These data bracket deposition of greywacke in the Pontiac and Abitibi subprovinces to 2685±3 Ma. In contrast, the youngest zircon found in a volcaniclastic unit interbedded with Timiskaming Group metawacke is 2673±2 Ma, while the age of a pluton that intrudes the Timiskaming is 2672±1 Ma. Thus, the Timiskaming Group is distinctly younger than the other metasedimentary rocks. The normalized probability distributions of detrital zircon ages from the Pontiac and the Abitibi metasedimentary rocks, as well as of dated igneous rocks from the southern Abitibi subprovince, all show that their major zircon populations formed in the age range 2685–2710 Ma. The distributions for the Abitibi metaturbidites and dated igneous rocks are very similar, suggesting that metasedimentary rocks in the Abitibi belt were mostly derived from the surrounding granite–greenstone terrain. The largest peak of the Pontiac distribution is a few million years younger than that from the Abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older, pre-Abitibi components (2750–2850 and 3020–3030 Ma). Deposition of the ca. 2685 Ma sediments was virtually syn-deformational. Based on previous field, geochemical and isotopic studies, as well as the detrital zircon age data, the Pontiac Group may represent a foreland basin that may have developed from an accretionary prism. The source of much of the sediment may have been an uplifted extinct continental arc to the north. The arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin, now partly preserved in the southern volcanic zone of the Abitibi subprovince. Metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones. These were brought up by high angle faults, which also localized deposition of non-marine (Timiskaming-type) sediments. It is tentatively suggested that Pontiac-type metasedimentary rocks extend most of the way beneath the Abitibi subprovince, the oldest part of the basin emerging as the Quetico Group in the western Superior province.