Quantifying crustal thickness over time in magmatic arcs
Authors
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.
Figure 1. Whole-rock (A) La/Yb and (B) Sr/Y correlations with crustal thickness for Quaternary and modern subduction-related volcanic arcs. La/Yb ratios are normalized to chondritic values. Gray circles represent individual arc segments used for the global correlations (dotted line and empirical fits with R2 shown on each diagram). Data are from GEOROC and various geophysical estimates of crustal thickness (See Methods). Red dots are average values for individual volcanoes of the northern part of the Southern Volcanic Zone of the Andes, where crustal thickness varies regionally by almost a factor of two; standard deviations are represented by the size of the individual data points. The range of slab-derived melts La/Yb and Sr/Y is shown in light blue assumes that the mafic oceanic crust id the only part of the slab that melts and is calculated based on typical MORB trace elemental concentrations and partition coefficients for these elements involving intermediate melt and an eclogitic residue. Pink boxes represent a predicted range
of La/Yb and Sr/Y for thin versus thick sub arc crust, using partition coefficients from reference (20) and average residual sub arc assemblages for thin (gabbronorite residues from the Ordovician Famatinian arc, Argentina23) and thick (pyroxenites with and without garnet from the Sierra Nevada arc, California22) arcs respectively. La/Yb throughout this paper are normalized to chondritic values29.
