Archean cratonization, emergence and red bed development, Lake Shebandowan area, Canada

Abstract

The Archean sequence of Lake Shebandowan area consists of Keewatin group rocks, overlain with probable unconformity by volcanic and sedimentary rocks of the “Timiskaming’ The Keewatin volcanic rocks are predominantly pillowed metabasalts chemically analogous to modern island arc tholeiites. The Timiskaming consists of calc-alkaline volcanic rocks and alluvial-fluvial rudites and arenites. The Timiskaming volcanic rocks are hornblende and feldspar phyric basalt to rhyolite. Deposits are massive unsorted volcanic breccias and rare graded tuff-breccias. The fragments are calc-alkaline and include a high-K shoshonite association, and, thus, are analogous to calc-alkaline suites of mature island arcs. The Timiskaming sedimentary rocks represent immature detritus from proximal Timiskaming volcanic centres. They were deposited in an alluvial-fluvial environment as evidenced by mudcracks, debris flow, sheet flood and traction deposits. Localized hematitte pigmentation in the Timiskaming strata is interpreted as evidence of red bed development. The Fe 2O 3/FeO ratio is higher in rocks pigmented by hematite than in equivalent samples without pigmentation. This increase in the oxidation state of iron is probably the result of emergence and interaction of Timiskaming strata with interstitial oxidizing fluids analogous to those of contemporary red beds. The transition from Keewatin tholeiitic pillow basalts to an unconformably overlying Timiskaming calc-alkaline volcanic suite suggests progressive cratonization accompanying stratigraphic build-up. The presence of a high-K shoshonite association in Timiskaming volcanic rocks implies that orogeny, crustal thickening and stabilization of a Keewatin basaltic platform occurred prior to Timiskaming volcanism. The eventual emergence and erosion of a landmass led to deposition of alluvial-fluvial sedimentary facies in which red beds subsequently developed.