Secondary K-feldspar at the Precambrian–Paleozoic unconformity, southwestern Ontario

Abstract

The Precambrian–Paleozoic boundary in the subsurface of southwestern Ontario commonly is characterized by secondary K-feldspar. In the weathered and altered Precambrian granitoid gneisses at the unconformity, secondary K-feldspar has replaced preexisting minerals, and also occurs as discrete crystals of adularia, overgrowths on altered minerals, and microcrystalline veinlets. The K-feldspar is chemically pure (Or99–100) and has high δ18O values (+18.9 to +21.4‰ Vienna Standard Mean Ocean Water), features that indicate crystallization at low temperatures. Secondary K-feldspar also occurs in Cambro-Ordovician clastic and carbonate rocks that immediately overlie the Precambrian 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 Precambrian weathering or early diagenesis of the immediately overlying Cambro-Ordovician strata. Estimated crystallization temperatures for the secondary K-feldspar (≥100 °C) exceed presumed burial temperatures for the Precambrian–Paleozoic boundary in southwestern 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 of secondary K-feldspar of Late Ordovician–Silurian age throughout mid-continental North America at the Precambrian–Paleozoic boundary records the regional extent of this process. Some Cambro-Ordovician rocks elsewhere in the mid-continent also contain secondary K-feldspar and illitic 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 originated as connate (sea) water trapped in lower Paleozoic strata, and was modified by rock–water interaction at elevated temperatures, and by mixing with meteoric water.

Multiple Episodes of Clay Alteration at the Precambrian/Paleozoic Unconformity, Appalachian Basin: Isotopic Evidence for Long-Distance and Local Fluid Migrations

“…Claymineral alteration in the Precambrian rocks has not received similar attention. In SW Ontario, plagioclasereplacing chlorite and albite are the initial alteration minerals, formed prior to K-rich feldspar, whereas illite is one of the last alteration products (Harper et al, 1995). Such illite in the altered Precambrian rocks of New York, to the east of our study area, was dated at 355 Ma (Tiller and Selleck, 1992).…”

“…Ontario (average age of 444 million years, Ma ;Harper et al, 1995) show that K-rich feldspar formation is not related to Precambrian subaerial weathering. Instead, secondary K-rich feldspar crystallization probably indicates rock/water interactions driven by Paleozoic tectonic processes (Harper et aL, 1995).…”

“…The secondary K-rich feldspar within the Precambrian rocks has been studied extensively (e.g., Mensing and Faure, 1983;Duffin, 1989;Hay and Liu, 1991;Harper et al, 1995). Radiometric dates for such Krich feldspar in uppermost Precambrian rocks of SW , Present address: Department of Geology, University of California, Santa Barbara, California 93106, USA.…”

“…Instead, secondary K-rich feldspar crystallization probably indicates rock/water interactions driven by Paleozoic tectonic processes (Harper et aL, 1995). Claymineral alteration in the Precambrian rocks has not received similar attention.…”

Early Mesozoic (?Triassic) Landscapes in Australia: Evidence, Argument, and Implications

“…Certainly, they are recorded in unconformities, some of which occasion comment either because of their extent and smoothness, as in the Grand Canyon section (Powell 1875;Sharp 1940), or because of the problems presented by the occurrence of altered bedrock at the geological hiatus (e.g., Van Hise 1896;Weidman 1903;Thwaites 1931;Harper et al 1995;Sutton and Maynard 1996). Yet very old surfaces are recorded both in the landscape and in the literature.…”