Abstract
We present global and regional correlations between whole-rock values of Sr/Y and La/Yb and crustal thickness for intermediate rocks from modern subduction-related magmatic arcs formed around the Pacific. These correlations bolster earlier ideas that various geochemical parameters can be used to track changes of crustal thickness through time in ancient subduction systems. Inferred crustal thicknesses using our proposed empirical fits are consistent with independent geologic constraints for the Cenozoic evolution of the central Andes, as well as various Mesozoic magmatic arc segments currently exposed in the Coast Mountains, British Columbia, and the Sierra Nevada and Mojave-Transverse Range regions of California. We propose that these geochemical parameters can be used, when averaged over the typical lifetimes and spatial footprints of composite volcanoes and their intrusive equivalents to infer crustal thickness changes over time in ancient orogens.
Highlights
Sr/Y and La/Yb are commonly used in petrology to qualitatively infer depths of magmatic diversification
Magmatism at subduction zones originates in the asthenosphere above the downgoing slab and is a result of partial melting of the mantle in the presence of aqueous fluids released from the slab[1]
As an extension of that work, here we show that two trace elemental ratios, Sr/Y and La/Yb can be used as crustal thickness proxies in low magnesium intermediate calc-alkaline rocks (55–68 wt% SiO2) covering the compositional range of andesites and dacites and their intrusive equivalents, when larger global and regional datasets are averaged out
Summary
Sr/Y and La/Yb are commonly used in petrology to qualitatively infer depths of magmatic diversification They can distinguish deep from shallow fractionation processes due to differences in the partition coefficient between these elements in various residual phases and intermediate melt[11,14,18]. A larger Sr/Y ratio in the volcanic and batholithic parts of an arc signifies a greater pressure or depth Predicted values for this ratio are shown, based on typical concentrations in residual sub-arc rocks[22,23] and known residue-intermediate melt partition coefficients from the GERM (Geochemical Earth Reference Model) online database of partition coefficients between various minerals in equilibrium with liquids (http://earthref.org/KDD/). We use averages of partition coefficients for dacitic and andesitic liquids referenced on that online database
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