TECTONIC STRESS REGIME RECORDED BY ZIRCON TH/U

Detrital zircon geochronology is a powerful tool for investigating sedimentary provenance, but U-Pb zircon ages alone cannot distinguish between source terranes with similar age signatures. We integrate recent developments in petrochronology with sedimentary provenance analyses by investigating the relationship between tectonic stress regime and zircon Th/U. In the North American Cordillera, zircons that 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, whereas zircons that crystallized from compressional magmatism exhibit low variability and low Th/U (<1.0). Higher temperature, more-fractionated, short-duration, bimodal magmatism in extensional magmatic systems may produce highly variable and elevated zircon Th/U. In compressional magmatic systems, lower temperature, long-lived, granitoid, oxidizing melts are more conducive to low Th/U zircon crystallization. Therefore, zircon Th/U may be correlated with end-member tectonic stress regimes (continental extension vs continental arc) that correlate with distinct magmatic conditions. To test the utility of this correlation, we evaluate U-Pb ages and Th/U ratios from the North American and southern Gondwanan zircon record.

North American detrital zircon (<2.0 Ga; n=30,587) contain large components of zircon with elevated Th/U (>1.0) that are temporally associated with (1) the final phases of orogenesis (orogenic collapse?), and (2) large-scale extension in Laurentia, while low Th/U ratios (0.1-1.0) correspond to overall compressional phases (i.e., terrane accretion and orogenesis). U-Pb ages and zircon Th/U ratios from southern Gondwanan volcaniclastic and igneous rocks display a time transgressive trend that suggests advancing extensional magmatism associated with the unzipping of the Gondwanan margin from Australia to South America from ~340-250 Ma. This along-strike migration of extensional magmatism may correlate to Carboniferous-Permian slab-rollback along the eastern Gondwana (Australia and Antarctica) margin that culminated in Permo-Triassic slab break-off beneath western Gondwana (South America).