Temporal variations in the composition of Cretaceous Cordilleran arc volcanism through a high-frequency record of bentonites

Abstract

Continental arc volcanism generates a wide diversity of magma compositions, but the tempos of compositional variation are unclear. Here, we investigate a 7-million year record of volcanic ash layers in the Cretaceous Eagle Ford Group, which was deposited in the Western Interior Seaway in western North America. Low oxygen conditions prevented significant bioturbation, allowing for the preservation of hundreds of thin volcanic ash layers in the form of bentonites. A drill core with an established high-resolution age model provides an opportunity to investigate temporal changes in ash composition on <100 kyr timescales. Because of intense alteration during diagenesis, ash protolith compositions were reconstructed from Ti/Zr. We first show that Ti and Zr had limited mobility during diagenesis. We then show, based on a compilation of data from unaltered volcanic rocks from six continental arcs, that the relationship between Ti/Zr and SiO2 is similar across continental arcs from different times and geographic locations, confirming that Ti/Zr is a robust differentiation index and indirect measure of SiO2. Applying an empirical Ti/Zr-SiO2 relationship to Ti/Zr measurements of 52 Eagle Ford bentonites allowed for reconstruction of ash protolith SiO2. Ash compositions vary from basalt/basaltic andesite to dacite/rhyolite, but the variations are not random. Ash compositions fluctuate between periods of high silica (>60 wt. % SiO2) and low silica (<60 wt. %) volcanism over ∼100 kyr timescales. If the temporal variability of these ashes represents broad snapshots of the Cordilleran continental arc, these results suggest that continental arc systems may undergo episodic changes in the extent of magmatic differentiation or the nature of eruption on rapid (<100 kyr) timescales.