Archean Molasse Basin Evolution and Magmatism, Wabigoon Subprovince, Canada

Abstract

The late‐orogenic 2703–2696 Ma Stormy basin of western Wabigoon Subprovince, 20 m.yr. younger than the surrounding greenstone belt, is a 1.5–2‐km‐thick volcano‐sedimentary sequence. The basin, bounded by a major fault to the north and an unconformity to the south, contains an association of subaerial felsic and mafic massive to brecciated flows, shoshonitic dykes, and coarse clastic fluvial deposits. The volcanic basement, represented by the ca. 2732–2722 Ma Wapageisi Group and Boyer Lake Group, is dominated by tholeiitic mafic flows. In addition, it contains minor intermediate to felsic components that have calc‐alkaline characteristics. The Wapageisi Group is similar to a back‐arc sequence, whereas Boyer Lake Group is typical of an oceanic island arc setting. The Stormy basin displays bimodal volcanism with calc‐alkaline porphyry stocks and felsic flows along the basin margins and tholeiitic mafic rocks in the center. The subaerial mafic flows compositionally resemble rift‐related continental tholeiites. The amphibole‐phyric shoshonites are similar to modern shoshonites as well as to other Archean shoshonitic rocks. Parental magmas of both the intrabasinal tholeiites and shoshonites were generated at a significantly greater depth (>60–80 km) than those of the older volcanic basement. The crustal thickening prior to basin formation reflects the transition from incipient arc formation to mature arc evolution. Although convergent‐style magmatism was dominant at the arc/back‐arc stage, rift‐related volcanism is characteristic of the arc fragmentation stage. Collectively, the setting and evolution is analogous to evolving modern oceanic island and continental arcs, in which terminal arc stages were affected by rifting.