Multiple episodes of clay alteration at the Precambrian/Paleozoic unconformity, Appalachian basin: Isotopic evidence for long-distance and local fluid migrations

Chlorite and illite are commonly associated with ubiquitous secondary K-ric h feldspar in the rocks located immediately above and below the Precambrian -Paleozoic unconformity in southwestern Ontario, and elsewhere in the mid-c ontinent of North America. This alteration assemblage is attributed to long -distance migration of hot brines driven westward by orogenic processes ori ginating along the eastern seaboard of North America. The delta D and delta (18)O values of chlorite and illite, plus K-Ar dates for secondary K-rich f eldspar and illite, were used to determine the nature, origin, and timing o f the fluids that altered Precambrian granites and their overlying rocks in southwestern Ontario. The delta(18)O values of the chlorite-forming fluids are best explained by initial hot brines (greater than or equal to 150 deg rees C) evolved mostly from seawater. Secondary K-rich feldspar formation f ollowed shortly thereafter, as the fluids cooled and perhaps mixed with met eoric water. Regional migration of the brines was induced by Taconic orogen ic events to the east. The hydrogen and oxygen isotopic compositions for th e secondary illite of the early to mid-Carboniferous indicate its crystalli zation from local meteoric water at low temperatures (40-55 degrees C). Inf iltration of local meteoric water into the Paleozoic and uppermost altered Precambrian rocks occurred during uplift, erosion, and subaerial exposure o f local arches in southern Ontario. The local basement reactivation and ass ociated secondary illite formation in this portion of the North American hi nterland was likely a distal expression of east-coast Acadian and Alleghani an orogenic activity.