RE-EVALUATION OF PHENGITE PETROLOGIC DATA AND 40AR/39AR AGE SPECTRA TO DETERMINE PRESSURE-TEMPERATURE-TIME PATHS

Phengite is commonly utilized to resolve P-T-t-D paths of metamorphic terranes because it often occurs as a fabric forming mineral, has P-T dependent chemistry, and can be dated using the ⁴⁰Ar/³⁹Ar method. Interpretation of phengite ⁴⁰Ar/³⁹Ar data, especially for samples that yield complex age spectra, requires determining the mineral’s petrogenesis and ⁴⁰Ar^* loss mechanism(s). This study uses ⁴⁰Ar/³⁹Ar, geochemical, and thermobarometric data for phengites from the Baja California high-pressure terrane to reassess post-crystallization ⁴⁰Ar^* loss mechanisms and to model thermal histories. Phengite from a blueschist block in mélange yielded pressures of 11.5±1 kbar based on Si barometry. Core to rim [Ti] variations are consistent with a temperature decrease from 635-570°C, above the closure temperatures for ⁴⁰Ar^* in white mica. ⁴⁰Ar/³⁹Ar age gradients from 115-98 Ma are interpreted to result from ⁴⁰Ar^* loss by volume diffusion subsequent to recrystallization during blueschist facies metamorphism. Phengite from an epidote-amphibolite facies metasedimentary block preserves prograde major and trace element zoning. Rim [Si] and [Ti] indicate metamorphic conditions of 7±1 kbar and 575°C respectively. Data imply that phengite recrystallization did not occur during exhumation. We infer ⁴⁰Ar/³⁹Ar age gradients from 168-153 Ma result from ⁴⁰Ar^* volume diffusion. Arrhenius plots for these phengites support an interpretation of ⁴⁰Ar^* loss via volume diffusion. Inverse thermal history modeling and forward modeling of age spectra yield continuous thermal histories in agreement with published results. Multi-diffusion domain models indicate an activation energy (E_a) of 68 ± 2 kcal/mol for Ar in phengite from the blueschist. Best fit thermal histories indicate cooling from 500°C at 120 Ma to 360°C at 114 Ma (35°C/Ma), reheating to 385°C at 108 Ma, cooling to 350°C at 104 Ma (8.75°C/Ma), and cooling to 250°C by 98 Ma (32°C/Ma). Phengite from the epidote-amphibolite block has an E_a of 66.5 ± 2 kcal/mol. Best fit thermal histories indicate cooling from 425°C at 171 Ma to 345°C at 153 Ma (5°C/Ma), then to <120°C by 147 Ma (36°C/Ma). The integrated geochemical, thermobarometric, and ⁴⁰Ar/³⁹Ar datasets suggest meaningful modeled thermal histories may be resolved from white micas with complex age spectra to discern P-T-t paths.