GEOCHEMICAL CHARACTERISTICS OF THE APPEKUNNY-GRINNELL FORMATION: MESOPROTEROZOIC SILICICLASTIC ROCKS OF THE BELT SUPERGROUP, WESTERN MONTANA

Siltstones and sandstones have been analyzed for major and trace elements, to establish: (1) if there is one or two formations; (2) the age of the provenance area; (3) its intensity of weathering; (4) oxidation state of the depositional environment; (5) primary mineralogy of the sediments, and (6) post depositional diagenetic events.

Near primary red siltstones have two REE populations. The first has flat REE patterns at 0.7 to 1.4 PA-UCC, whereas the second shows flat LREE but a negative slope from Pr to Lu. There are two populations of sandstones defined by low or high Al2O3. The former appears compositionally to be siltstones highly diluted with detrital quartz. LREE patterns are fractionated, but HREE flat to positively fractionated.

No systematic geochemical differences exist between the Appekunny and the Grinnell formations that were defined on color differences: accordingly they are here considered one formation. Th/Sc, (Gd/Yb)cn, Eu/Eu* and LREE/HREE ratios are all in keeping with a dominantly Proterozoic provenance of a craton with K rich granites. Negative Eu anomalies are consistent with recycled sedimentary rocks in the source area. A Proterozoic source terrane places constraints on reconstructions of Rodinia. Negative Sr anomalies, and K/Cs systematics, stems from intensive weathering. Uranium covaries negatively with Ce/Ce* in the low-Al sandstone population, recording a weathering trend. CIA corrected for K addition ranges from 55 to 86 for the siltstone population. The occurrence mixed of clays and K-feldspar could indicate a large substantially weathered source area located farther away and a proximal terrane dominated by mechanical erosion in a warm, dry environment. In siltstones, U/Th averages 0.28, and V/Sc averages 4.9, consequently the depositional environment involved oxic surface waters, with locally anoxic bottom waters. There was post-depositional K metasomatism, which locally transported REE. Redistribution of elements would have been the result of the transformation of montmorillonite into illite, and K enrichment with a loss of SiO2, Ca, Na, Fe and Mg. Color differences between the "formations" are attributed to post depositional basinal fluids.