A combined petrographical—geochemical provenance study of the Newland Formation, Mid-Proterozoic of Montana

Abstract

A provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation.Sandstones of the Newland Formation are typically arkosic sands and arkoses with very-wellrounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographical features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions.Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th rations, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small Tio 2 /Al 2 O 3 ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO 2 contents and large K 2 0/Na 2 0 ratios reflect derivation from stable cratonic areas and tectonic quiescence.Thus, in general, the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a large variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.

Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

“…5. Schieber (1992) showing that all sandstones from the Adaminaby Group plot on or above Post Archean Australian Shale (PAAS). The PAAS and Upper Continental Crust (UCC) lines are from Taylor and McLennan (1985) and Rudnick and Goa (2003) respectively.…”

Geochemistry of shales from the Paleoproterozoic to Neoproterozoic Vindhyan Supergroup: Implications on provenance, tectonics and paleoweathering

“…Besides the major oxides, concentration of trace elements is also used to characterize provenance in shales. For example, abundance of zircon, vanadium, lanthanum and barium in shales provides significant information about nature of source rocks through different binary and ternary plots in combination with other major and trace element concentrations (Nesbitt andYoung, 1984, 1989;Roser and Korsch, 1986;Schieber, 1992;Fedo et al, 1995Fedo et al, , 1996Fedo et al, , 1997Hayashi et al, 1997;Zhang et al, 1998;Campo and Guevara, 2005;Sugitani et al, 2006;Deru et al, 2007). The present paper describes the major and trace element geochemistry of shale samples from the Proterozoic Vindhyan Supergroup of central India and discusses inferred source rocks, paleoweathering pattern and paleotectonics.…”

Anomalous iron distribution in shales as a manifestation of ?non-clastic iron? supply to sedimentary basins: relevance for pyritic shales, base-metal mineralization, and oolitic ironstone deposits

“…REEs, Th, Hf etc. ; Taylor and McLennan 1985) show little variation in their ratios (Schieber 1986a(Schieber , 1992b. Small La/Th ratios and large Hf contents of shales suggest derivation from a crust of largely granitic/granodioritic composition (Bhatia and Taylor 1981;Condie and Martell 1983).…”

“…A petrographic study of sandstones in the Newland Formation suggests that granitoid gneisses and granites were dominant in the source area of the Newland Formation throughout its deposition (Schieber 1992b).…”