Time-temperature-fluid evolution of migmatite dome crystallization: Coupled U-Pb age, Ti thermometry, and O isotopic ion microprobe depth profiling of zircon and monazite

Fluid infiltration of high-grade terrains is commonly difficult to track; however, the use of secondary ionization mass spectrometry (SIMS) depth profiling analysis of zircon in high-grade metamorphic rocks allows acquisition of micron-scale U-Pb geochronology, Ti in zircon thermometry, and oxygen isotope measurements that can be used to decipher temporal and physicochemical records of orogens. This investigation focuses on a migmatite dome in a Cordilleran metamorphic core complex that records a long history of partial melting, deformation, and fluid flow during crustal thickening and/or rapid orogenic collapse. SIMS depth-profiling analyses of the outermost rims of zircon and monazite from samples representing a range of crustal levels elucidate the nature of the deep crustal environment during the final stages of extension and orogenic collapse from 58 to 50 Ma, following a major episode of granitoid emplacement and partial melt crystallization. The results from ultrathin (0.25 to 5.00 μm) outer rims of zircon demonstrate that fluid infiltration driven by amphibolite facies metamorphic devolatilization and/or igneous crystallization persisted at deep crustal levels until just before the time that the rocks were rapidly exhumed to upper crustal levels. Collectively, the results indicate that metamorphic fluids continued to infiltrate the gneiss dome from ca. 58 to 50 Ma. The overlap in ages from the zircon rims and monazite with argon cooling ages suggests that the dome cooled rapidly from > 650 °C to 300 °C from 51-47 Ma. The results further demonstrate that the evolution of high-grade terrains, including the fluid history, can be revealed in unprecedented detail through the combination of depth profiling analyses of U-Pb age, Ti thermometry, and O isotopes.