Studies of ~ 2.72 Ga mafic-ultramafic volcanic sequences from the Lake of the Woods belt in the Wabigoon Subprovince and the Shebandowan Belt of the Wawa Abitibi Subprovince provide new context for the Neoarchean evolution of the Superior Province Craton’s southern margin. For clarity, we refer to rock types here in the framework provided by Pearce and Reagan (2019). In the Lake of the Woods area, sampled outcrops were part of the ~ 2720 Ma “Upper Keewatin” Assemblage where the ultramafic-bearing sequences occur conformably within dominantly calc alkaline sequences. Well preserved primary textures are found in intermediate and mafic rocks in the Lake of the Woods volcanic, including pillows and bedding. Ultramafic rocks, however, are generally schists consisting mainly of talc, tremolite and chlorite and no spinifex textures were observed. Possible spinifex texture has previously been reported, but only in samples with less than 16% MgO. Two chemically distinct ultra-mafic groups (MgO > 22 wt%) are recognized. One is boninitic with Al2O3/TiO2 ≈ 25, while the other displays Al2O3/TiO2 ≈ 21. Picrites are common Mg-rich rock types in the region but SHMB and boninites also occur. Gaps on the trends observed on element-element plots between the ultramafic and mafic rocks indicate the various rock types do not represent crystal fraction of komatiite magmas. Instead, the ultramafic rocks likely contain accumulated olivine. In the Shebandowan area, mafic-ultramafic intrusions occur in broadly contemporaneous 2720 Ma tholeiitic and calc alkaline volcanic sequences. The intrusions may contain locally well-preserved olivine and pyroxene cumulates. Pillowed flows are commonly preserved in the mafic rocks, but most ultramafic rocks are strongly recrystallized. No differentiated komatiite flow structures have been observed and spinifex textures are not observed in ultramafic rocks, although random acicular secondary amphibole may occur. Genuine clinopyroxene spinifex and randomly distributed clinopyroxene locally consist of partially preserved elongate, zoned, blades. The textures may be present in a variety of non-komatiitic rocks (e.g., MgO = 11.5 wt%) including SHMB, picrites, LOTI and boninites. The ~ 2720 Ma volcanic sequences of the SW Superior Craton document the repetition and close association of tholeiite, calc alkaline and rifted margin – proto-arc volcanism along the craton’s margin. Collectively, the evidence from along the southern Superior Craton suggests crustal growth near the temporal transition to modern-style plate tectonics involved episodic subduction at rifted cratonic margins. The occurrence of plume(s) near the margin, as recorded near the top of the Abitibi belt’s Pacaud Assemblage and in younger assemblages, points to comparatively small mantle upwellings that were intrinsically linked to active tectonics in the crust and upper mantle.

Reference(s)

Pearce, JA and Reagan, MK, 2019, Identification, classification and interpretation of boninites from Anthropocene to Eaoarchean using Si-Mh-Ti systematics: Geosphere, v. 15, p. 1008-1037.