Sedimentology and metamorphism of a proterozoic volcaniclastic turbidite suite that crosses the boundary between the Flin Flon and Kisseynew belts, File Lake, Manitoba, Canada
Bailes, A. H.
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The Kisseynew sedimentary gneiss belt is near the southeast corner of the Churchill Province in Manitoba. It is a large, east-trending Proterozoic sedimentary basin composed largely of greywacke-, siltstone-, and mudstone-derived paragneisses and migmatites. It is bounded on the south by the Flin Flon volcanic-sedimentary belt. In the File Lake area, well preserved, weakly recrystallized pebbly greywacke, greywacke, siltstone and mudstone of the Aphebian Amisk Group of the Flin Flon volcanic-sedimentary belt have been traced northwards across a steep metamorphic gradient directly into migmatitic Kisseynew belt paragneisses. The Amisk Group sedimentary rocks are mainly turbidites, with minor debris and fluidized sediment flow deposits. They are 1 km thick; consist almost entirely of volcanic detritus, which is mainly felsic in composition; and overlie a thick accumlation of Amisk Group mafic subaqueous flows. The felsic volcanic detritus is texturally and compositionally variable. This indicates a level of mixing which requires subaerial transport. The detritus was probably derived from easily eroded pyroclastic deposits of contemporaneous Amisk volcanoes, rather than by dissection of an older volcanic terrain by stream activity. This is indicated by: (i) only slight rounding of clasts; (ii) local intercalation of volcanic and sedimentary rocks; (iii) direct input of some strata into the sedimentary basin from their volcanic source without reworking; and (iv) absence of plutonic or metamorphic clasts... The metamorphic gradient of 21oC/km is too steep to be accounted for by selective uplift of more deeply buried and metamorphosed strata in the Kisseynew belt. There was probably a higher geothermal gradient in the Kisseynew belt than in the Flin Flon belt. This could have been caused by lower thermal conductivity of the volcanic rocks and consequent impedance of upward movement of heat in the Flin Flon belt relative to the Kisseynew belt. This mechanism could explain why Precambrian volcanic belts are invariably much lower grade than associated sedimentary belts, and does not require special tectonic conditions for development of this difference.