Crust and mantle evolution in the late Archean; evidence from a Sm-Nd isotopic study of the North Spirit Lake greenstone belt, northwestern Ontario, Canada

Sm-Nd data are presented for volcanic, plutonic, and sedimentary rocks from the Late Archean North Spirit Lake greenstone belt in northwestern Ontario. The greenstone belt is divided into six lithotectonic assemblages emplaced between 2700 and 3000 Ma. The earliest assemblage, indicative of an island arc setting, is characterized by positive initial epsilon (sub Nd) values (0 to +3), whereas shales and granitoids in succeeding assemblages yield increasingly negative epsilon (sub Nd) values (as low as -3). This argues strongly that the assemblages formed in close proximity to each other because the younger assemblages likely include recycled crustal material from earlier assemblages through erosion (into younger basins) and remelting. However, even the earliest assemblage exhibits decreasing epsilon (sub Nd) values from basaltic to intermediate-felsic volcanic strata within the same assemblage. This suggests that the initial arc was itself contaminated by either a sedimentary component or remelting of older oceanic crust. Granitoid samples yield epsilon (sub Nd) values similar to the shales in the belt, indicating a common evolutionary history. These granitoids may thus represent the roots of the greenstone belt. Mantle-derived rocks such as basalts and komatiites are derived from a variably depleted mantle source with values as high as +3.8 at 2925 Ma. A 2925+10/-8 Ma U-Pb zircon age obtained for a tonalitic pluton on the northern margin of the North Spirit Lake belt is similar to the age of a volcanic-plutonic cycle in the adjacent Favourable Lake greenstone belt, implying that the two belts may have had common orogenic episodes. The North Spirit Lake greenstone belt appears to have nucleated from an initial primitive volcanic arc and through continued volcanism was transformed into a stable platform exhibiting continental features such as intracrustal differentiation and sedimentary recycling to produce the current array of assemblages. In this respect the evolution of the North Spirit Lake greenstone belt resembles the evolution of Proterozoic and Phanerozoic continents and suggests that Archean microcontinents grew in much the same fashion as present-day continents.