Evolution of the southern Abitibi greenstone belt based on U–Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation

Abstract

New mapping and U–Pb zircon geochronological results in the southern Abitibi greenstone belt (SAGB) support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages, rather than the collage of allochthonous terranes proposed in recent publications. Based on lithological and geochronological criteria these supracrustal assemblages also encompass the Swayze, Shining Tree and Montcalm greenstone belts indicating that, despite separation by large granitic intrusions, they are in fact part of one extensive greenstone belt. In conjunction with the mapping, the geochronological results support coherent, upward-facing stratigraphic sections with about 20% of the samples containing inherited zircons with ages similar to those found in underlying assemblages. The seven oldest assemblages represent semi-continuous volcanism from 2750 to 2697 Ma. The volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc–alkaline mafic to felsic lavas. Intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section. Regional fault control on the distribution of the volcanic assemblages provides evidence for early dip–slip movement associated with volcanic extension dating back to at least 2725 Ma. The two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults. The earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 Ma, and the youngest one of subaerial conglomerates, fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 Ma. The sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons, regional folding and reactivated movement on the regional faults probably related to accretion of the Abitibi to the Superior Province craton. Autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic ± komatiitic assemblages and two dominantly calc–alkaline assemblages, as well as the extensive intermingling of these different magma clans in a number of the assemblages, appears to be unique to the Archean. It suggests complex, large-scale and long-lived interaction between mantle plumes and subduction zone magmas. The Nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with ε Nd=2.5±0.5 demonstrating a lack of contamination by ancient enriched components.