Petrogenesis and geodynamic setting of the Neoarchaean Haines Gabbroic Complex and Shebandowan greenstone belt, southwestern Superior Province, Ontario, Canada

Abstract

The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma.Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas.The initial εNd values of the Haines Gabbroic Complex (εNd = +3.0 to +4.1) and the Greenwater (εNd = +1.8 to +4.1) and Burchell (εNd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces.