Chemical denudation rates of the western Canadian orogenic belt: the Stikine terrane

We report in this paper major element and Sr isotopic ratio analyses for 19 rivers draining the Stikine Province in the Western Canadian Cordillera. Dissolved solutes, suspended sediments and river sands were analyzed. The aim of the paper is to calculate chemical denudation rates for this uplifted accretionary prism, mainly dominated by volcanics and volcanic-derived sedimentary rocks. The major chemical features of the Stikine rivers are their dilute character and the excess of Ca, Mg and Sr in the dissolved load with regards to Na. Molar Ca/Na ratios from 2 to 14, mean value of 5.4, are observed although the region is dominated by silicate rocks. Suspended sediments show Ca/Na ratios close to 0.4. Isotopically, most of the rivers investigated display lower Sr isotopic ratios in the dissolved load compared to the suspended load, which is very unusual. To account for the excess of cations, three different hypotheses are tested by an inversion method allowing us to determine the origin of each major solute. The first scenario assumes that the excess of Ca and Mg is due to the dissolution of unradiogenic carbonates. The second scenario supposes that the excess of base cations is derived from the preferential dissolution of Ca- and Mg-rich minerals, such as amphiboles. In the third scenario, all solutes are assumed to derived from silicate weathering. The cationic weathering rates of silicates, although imprecise, are in the order of 2–5 tons/km2/year for the two first hypotheses. If all solutes are taken into account, denudation rates are between 9 and 15 tons/km2/year. On a global scale, these numbers are among the lowest found for rivers draining basaltic rocks. They are however found to be consistent with the global law for chemical weathering rates established for basaltic rocks showing that chemical denudation rate are linearly correlated to runoff and depend on temperature according to an Arrhenius law. The rivers draining the complex assemblage of volcanic and volcanic-derived rocks in the subduction/collision geological context of the Stikine Province thus respond similarly to the other volcanic provinces of the world. This does not seem to be true for associated CO2 consumption rates. The abundance of sulfate ions in the dissolved load, possibly derived from oxidative weathering of sulfides, may indeed provide a non-negligible part of the protons necessary for chemical weathering. Within Canada, the Stikine geologic province is one whose silicates weather at a high rate. The order of chemical weatherabilty of Canadian geological province from the lowest to the highest is established as follows: granites<shales of the rockies<volcanics of the Western Cordillera<shales of the interior platform<carbonates.