SECONDARY K-FELDSPAR AT THE PRECAMBRIAN-PALEOZOIC UNCONFORMITY, SOUTHWESTERN ONTARIO

The Precambrian-Paleozoic boundary in the subsurface of southwestern O ntario commonly is characterized by secondary K-feldspar. In the weath ered and altered Precambrian granitoid gneisses at the unconformity, s econdary K-feldspar has replaced preexisting minerals, and also occurs as discrete crystals of adularia, overgrowths on altered minerals, an d microcrystalline veinlets. The K-feldspar is chemically pure (Or(99- 100)) and has high delta(18)O values (+18.9 to +21.4 parts per thousan d Vienna Standard Mean Ocean Water), features that indicate crystalliz ation at low temperatures. Secondary K-feldspar also occurs in Cambro- Ordovician elastic and carbonate rocks that immediately overlie the Pr ecambrian basement. K/Ar (453 +/- 9 to 412 +/- 8 Ma) and Rb/Sr (440 +/ - 50 Ma) dates obtained for secondary K-feldspar from the Precambrian host rocks suggest that its crystallization is unrelated to Precambria n weathering or early diagenesis of the immediately overlying Cambro-O rdovician strata. Estimated crystallization temperatures for the secon dary K-feldspar (greater than or equal to 100 degrees C) exceed presum ed burial temperatures for the Precambrian-Paleozoic boundary in south western Ontario during Late Ordovician - Silurian time. We infer that secondary K-feldspar formed from a hot brine that moved preferentially along the Precambrian-Paleozoic unconformity. The wide distribution o f secondary K-feldspar of Late Ordovician - Silurian age throughout mi d-continental North America at the Precambrian-Paleozoic boundary reco rds the regional extent of this process. Some Cambro-Ordovician rocks elsewhere in the mid-continent also contain secondary K-feldspar and i llitic clay of Late Pennsylvanian - Early Permian age, suggesting more than one episode of fluid movement. Major pulses of orogenic activity may have initiated brine migration. We speculate that the brine origi nated as connate (sea)water trapped in lower Paleozoic strata, and was modified by rock-water interaction at elevated temperatures, and by m ixing with meteoric water.