Bimodal volcanism in Archean greenstone belts exemplified by greywacke composition, Lake Superior Park, Ontario

Abstract

In the Archean Gamitagama Lake greenstone belt, a metagreywacke–metasiltstone formation at least 3.6 km thick largely had an easily eroded, unconsolidated, subaerial, pyroclastic felsic volcanic provenance. The remains of this provenance are now represented by a 0–600 m thick felsic formation sandwiched between mafic flow formations in an adjacent and laterally equivalent 4.5 km thick metavolcanic sequence. The felsic formation includes flows plus various pyroclastic units including crystal-rich ash-flow tuff. Volcanism, erosion, transportation, and sedimentation appear to have been essentially contemporaneous with subaerially erupted ash that was being transported rapidly by rivers to the sedimentary basin where it was deposited in a submarine fan.Angular to rounded framework sand grains form 39% of the greywacke in the proportion 49% plagioclase, 36% quartz, and 15% lithic clasts (dominantly felsic volcanic). Despite the moderately high quartz content, a felsic pyroclastic volcanic provenance can be shown by the absence of plutonic and metamorphic clasts, the felsic nature of the volcanic clasts, the volcanic characteristics such as subhedral shape, oscillatory zoning, and high-temperature structure of some of the plagioclase, and facies relationships. The quartz/plagioclase ratio of the greywacke, when compared with that of the preserved volcanic units, is most consistent with a volcanic source that was silicic rhyodacite to rhyolite in composition and Plinian in nature. A western plutonic–metamorphic cratonic provenance is postulated for some detritus to the upper part of the metasedimentary formation.The volume of felsic volcanic material preserved in the volcanic sequence is small, and was apparently small at any given time during the development of the volcano. However, because of concomitant eruption and erosion the total volume of felsic magma erupted and dispersed to the sedimentary basin during the felsic volcanic event was several times larger than the volume of greywacke and equal to or larger than the volume of preserved mafic metavolcanics. The volcanism was thus bimodal, producing basalt and rhyodacite + rhyolite. The high silicic component implies a thick crust that presumably developed by extensive mafic volcanism loading the Archean crust.