167-12 Miocene ages of monazite and titanite in Orocopia Schist and included metasomatized rocks at Cemetery Ridge, southwest Arizona

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

Presenting Author:

Suzanne Craddock Affinati

Authors:

Craddock Affinati, Suzanne¹, Droubi, Omar², Hoisch, Thomas³, Bonamici, Chloe⁴, Cummings, Chloe⁵, Durica, Zoe⁶, Epstein, Gabe⁷, Haxel, Gordon⁸, Jacobson, Carl⁹

Abstract:

Orocopia Schist, trench sediments underplated during Laramide flat-slab subduction, is exposed at Cemetery Ridge, southwest Arizona. Included in the dominant quartzofeldspathic schist are peridotite bodies and several metasomatic and metamorphic rock types.  To determine the timing and nature of metasomatising fluids we analyzed U-Th-Pb isotopes and trace-elements in monazite and titanite by LASS-ICP-MS and δ18O of plagioclase and titanite using SIMS. Lower intercept Tera–Wasserburg titanite ages from one calc-silicate sample and three samples of altered quartzofeldspathic schist (albite-actinolite gneiss, “spotted gneiss”), yielded 21.0 ± 1.0 to 21.90 ± 0.96 Ma. Monazite yielded ²³⁸U/²⁰⁶Pb ages 25-21 Ma (n = 217) in ten other samples including quartzofeldspathic schist, altered harzburgite (spinel-olivine-tremolite-chlorite schist), and spotted gneiss. REE patterns on dated monazite grains display negative Eu anomalies and show LREE depletion and HREE enrichment with younger ages. Plagioclase from quartzofeldspathic schist, spotted gneiss, and metabasalt yielded δ¹⁸O values of ~2‰ to 10‰, with the quartzofeldspathic schist clustered at 9.5‰ (representing the least altered lithology). Titanite yielded 1‰ to 7‰, including a sample of highly altered Al-enriched quartzofeldspathic gneiss. Variations in titanite-plagioclase fractionation for each sample (ranging from 0‰ to -8‰) suggest titanite and feldspar are not in isotopic equilibrium, likely due to variable fluid compositions over the period of recrystallization. Fluid δ¹⁸O values, calculated from measured mineral δ¹⁸O ranges, assuming ~550°C equilibration, range from 1‰ to 10‰.  

The age of subduction-related metamorphism is constrained by detrital and metamorphic zircon to be <65 Ma and by the end of flat-slab subduction at approximately 40 Ma. The 3.5 km x 0.6 km area of Orocopia Schist at Cemetery Ridge is bounded and intruded by diorite and granite dated 22-20 Ma by U-Pb zircon. The negative Eu anomaly and LREE depletion observed in monazite are consistent with interaction with an evolving melt-equilibrated fluid. δ¹⁸O results indicate interactions between meteoric water and high-temperature, rock-equilibrated magmatic or metamorphic fluid. This evidence suggests that fluids coming from or interacting with the intruding Miocene magmas, possibly including meteoric water, mediated crystallization or recrystallization of monazite, titanite, and plagioclase. Thus, the data provide evidence of Miocene contact metamorphism and fluid interaction that pervasively overprinted subduction-related metamorphism.  

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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