304 Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts (Posters)

This session focuses on deciphering magmatic, metamorphic, depositional and deformational events recorded in orogenic belts via the integration of geochemistry and geochronology. We welcome contributions from modern applications of analytical methods aimed at unraveling orogenic events worldwide, fostering interdisciplinary discussion.

Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Mesozoic Granulites Reveal Timescales and Conditions of Deep Crustal Processes during Cordilleran Mountain Building: A Case Study from the Syringa Embayment, Idaho.

Paper number: 304-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

High-grade metamorphic rocks, reaching granulite facies metamorphism are exposed in the Syringa embayment near Orofino, Idaho. They preserve a record of Cretaceous magmatism and metamorphism along the North American continental margin. These rocks, and nearby intrusions of mafic to intermediate composition lie outboard of the Salmon River suture zone and straddle the Sri 0.704-0.706 isopleth. Some studied samples show little to no retrogression and some samples contain SO42- and CO32--rich scapolite, suggesting relatively oxidized lower crustal conditions. These samples are well-suited for investigating orogenic processes, including the rates of orogenesis and the metamorphic conditions within the Syringa embayment. Specifically, they provide an opportunity to examine: (1) the relationship between metamorphism in the Syringa embayment and Cretaceous orogenesis in the northern U.S. Cordillera, and (2) the behavior of volatiles in the lower crust, including the potential for volatile transfer between the upper mantle and lower crust. The latter providing insights into subduction-derived mass-transfer to the lower and middle crust. Here, we present geochemical and isotopic data from rocks and minerals collected within a 4-7 km wide mylonitic shear zone in the hanging wall of the Ahsahka thrust fault, located eight miles east of Orofino, Idaho to provide insight into the tectono-thermal history of the region. U-Pb zircon dates from a scapolite-bearing hornblende granulite yields two distinct date populations at ca. 110 Ma and ca. 83 Ma. The older date population is contemporaneous with the emplacement age of deep-seated plutonic rocks of intermediate composition in the region, while the younger date population may have recorded deformation and partial melting in the deep crust within the Ahsahka shear zone. The aluminum concentrations of amphibole and plagioclase feldspar pairs from this sample yield equilibration temperatures of up to 750 °C at 7-8 kilobars, which may record equilibration during deformation. We are currently conducting geochemical and isotopic characterization of samples from additional lithologies of the high-grade metamorphic zone including garnet-bearing paragneisses and marbles to reconstruct the P-T-t pathway of this metamorphic package. These results and our future work may provide greater clarity on the metamorphic processes and conditions of the middle-to-lower crust during terrane accretion and offer fundamental insights into the research questions outlined above.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414194

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