A trace element and chemical Th–U total Pb dating study in the lower Belt-Purcell Supergroup, Western North America: Provenance and diagenetic implications

Abstract

Comprehensive geochemical and monazite chemical age data on the Mesoproterozoic Belt-Purcell Supergroup reveals information on provenance age(s), and the sources of interbedded argillites and sandstones. Compliance of rare earth element (REE) and multi-element patterns of argillites to post-Archean upper continental crust (PA-UCC), and Cr–Ni, and Th/Sc–Sc systematics are consistent with a dominantly post-Archean source area. Sandstones have the same geochemical fingerprint as argillite for the Appekunny and Grinnell formations in the lower Belt-Purcell Supergroup sequence, albeit variably depleted by detrital quartz. Sandstone developed in a separate high-energy environment, and argillite and sandstone became interbedded during storms, accounting for the sharp interbedding of the two facies. Detrital monazite chemical Th–U total Pb ages from the Appekunny and Grinnell sandstones, and argillites of the former, cluster at ∼ 1700 Ma. Paleocurrents and chemical ages support a Laurentian Paleoproterozoic provenance, likely the Yavapai, Mazatzal and Central Plains provinces. Other potential sources could have been terranes at ∼ 1875–1750 Ma that cooled through the blocking temperature of monazite ∼ 1700 Ma such as the Wyoming Province. Rare > 2500 Ma monazite ages in argillite and sandstone stem for minor contributions from Archean terrane(s). Diagenetic overprint in the Belt-Purcell rocks is widespread and displayed geochemically as heavy REE enrichment relative to light REE, normalized to PA-UCC. There are two populations of monazites: the first has detrital textures, with chemical ages > ∼ 1400 Ma, higher ThO 2 and Y, and lower LREE/HREE content than < ∼ 1400 Ma monazites with euhedral texture, ages up to ∼ 900 Ma post-deposition, with lower ThO 2 and Y, and higher LREE/HREE content. The second population is interpreted as the product of protracted migration of basinal brines that mobilized HREE, as documented in several Proterozoic sedimentary basins.