PEPERITE IN THE PURCELL LAVA AND A REVISED AGE OF THE UPPER PROTEROZOIC BELT-PURCELL SUPERGROUP

In his seminal 1912 publication of the geology of the 49^thparallel, Reginald Daly described many formations of the Belt-Purcell Supergroup, including the basaltic Purcell Lava. At a measured section in the McGillivray Range he characterized a thin flow unit above the main body of the Purcell Lava as rhyolite. The rock in many respects resembled the underlying basalt but it contained grains of quartz and lath-like fragments that could be confused with large feldspar crystals. Subsequent publications have referred to the Purcell Lava as bimodal. A SHRIMP U-Pb (207Pb/206Pb) age of ca. 1.45 Ga zircons from this “rhyolite”, collected 7.5 km south of Daly’s locality, has been the accepted age of the Belt-Purcell (Evans et al., 2000, CJES 37:1287-1300).

Our investigations at Daly’s site, the first in over one hundred years, reveal that rather than being a rhyolite, this unit is a peperite formed by the volatile interaction of basalt and water-laden sediment. Exposures in the Lake Geneva locality of NW Montana comprise two main lithologies: a fragmental basalt with dark green angular clasts, which range from meters to less than a centimeter, surrounded by white to light gray veins of matrix, and a light greenish-gray, highly fragmental basalt characterized by white to light-gray chips of metamorphosed sediment. The green clasts appear to be solely basalt, but rather contain large amounts of thermally altered sediment in a groundmass of chlorite. Quartz is present as floating monocrystalline grains or as microcrystalline fragments in both peperite types. Zircon grains vary from fragments <10um to rare large >200um euhedral crystals that were originally constituents of the sediment. Basalt flows above and below the peperite are devoid of zircon.

Analysis of zircons extracted from peperite samples, including one from the sample site of Evans et al., 2000, and also their data table, yield U-Pb ages that range from ca. 1380 to 1870 Ma. The probability density plots for these samples show a ca. 1450 Ma peak and a subordinate peak at ca. 1800 Ma, which we interpret as detrital zircon populations. The calculated maximum depositional age is 1386 +/- 10 Ma. We also dated apatites from the Purcell Lava by U-Pb using ID-TIMS, and constrain its high-temperature igneous cooling age to 1338.1 +/- 7.5 Ma, thus about 110 Ma younger than previously cited.