TECTONIC STRESS REGIMES RECORDED IN ZIRCON GEOCHEMISTRY

We investigate the relationship between tectonic stress regime and zircon geochemistry by comparing zircon compositions to the synmagmatic stress regime using igneous zircon U-Pb ages (n=2008) from western North America, northern Argentina, and southeast China. Zircons formed in melts associated with extensional magmatism contain variable Th/U (0.3 to >3.5), including significant zircon populations with Th/U>1.0. Zircons formed in compressional magmatism exhibit low variability and low Th/U (<0.8). Higher temperature, more-fractionated, short-duration, bimodal magmatism in extensional magmatic systems favors highly variable and elevated Th/U in zircon. Lower temperature, long-lived, granitoid compressional magmatism is more conducive to low Th/U zircon crystallization. Therefore, Th/U in zircon may be associated with tectonic stress regimes that favor distinct magmatic conditions. To test this technique in the sedimentary record, we evaluate the zircon Th/U ratio of published North American detrital zircon (<2.0 Ga; n=30,587) over time and note that increases in zircon Th/U are coeval with (1) final phases of orogenesis and (2) large-scale extension in Laurentia. Times of low Th/U correspond to overall compressional processes (terrane accretion, orogenesis). This approach may be applicable in resolving the evolution of tectonic stresses in magmatic systems and provide a supplemental tool for interpreting sedimentary provenance.