POTASSIC AND SODIC METASOMATISM IN THE SOUTHERN PROVINCE OF THE CANADIAN SHIELD - EVIDENCE FROM THE PALEOPROTEROZOIC SERPENT FORMATION, HURONIAN SUPERGROUP, CANADA

Major element geochemistry, petrographic observations, and microprobe analyses of the Paleoproterozoic Serpent Formation, Huronian Supergrou p, Canada, suggest that these strata experienced both potassium and so dium metasomatism. Major element data, plotted in Al2O3-CaO + Na2O-K2 O compositional space, form an array nearly identical to data from sub -Huronian paleosols reported elsewhere, suggesting that Serpent fluvia l systems captured materials from entire weathering profiles. Potassiu m metasomatism was concentrated in shales, and represents the illitiza tion of kaolinite, which is a residual product of plagioclase weatheri ng. Sodium metasomatism principally affected silt-and sand-sized parti cles. Microprobe analyses of plagioclase grains indicate that they are nearly pure albite (>Ab(96)). Corresponding whole-rock major element data plot in bulk albite space on an (Al2O3-K2O)-CaO-Na2O ternary dia gram, whereas basement rocks plot in bulk andesine and oligoclase spac e. End member compositions and the absence of luminescence when bombar ded with an electron beam indicate that the albites are of secondary o rigin. Extremely pure albite cements (>Ab(99)) commonly fringe detrita l grains. There has been little replacement of detrital K-feldspars, d espite the complete albitization of plagioclase grains. The timing of metasomatism, at least the last stage, is constrained to the period be tween similar to 1700 and 1750 Ma (based on geochronologic data report ed elsewhere), when the Southern Province of the Canadian Shield exper ienced considerable felsic plutonism, possibly related to are collisio n. This plutonism may have resulted in the heating of deep basinal bri nes. Stresses associated with orogenic build-up, or gravity driven gro undwater circulation working in concert with the plutonism, expelled t hese brines through the Serpent Formation (and the entire Huronian Sup ergroup), thoroughly metasomatizing the region. Such processes associa ted with collisional tectonics are akin to the development of Mississi ppi Valley-type Pb-Zn mineralization in the mid-continent of the Unite d States. (C) 1997 Elsevier Science B.V.