Apparent oxygen isotope equilibrium during progressive foliation development and porphyroblast growth in metapelites: implications for stable isotope and conventional thermometry

摘要: The assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite-grade pelitic schists that have undergone sequential porphyroblast growth multiple episodes recrystallization matrix minerals foliation development. Textural relationships are used to infer a metamorphic history involves garnet followed by staurolite growth, with each event at least one period minerals. Conventional geothermobarometry Qtz–Grt–Pl–Ms–Bt ± St equilibria yields peak P–T conditions c. 625 °C 9–11 kbar, consistent KMnFMASH petrogenetic grid predictions for stability the assemblage Grt + St + Bt. Qtz–Grt fractionations yield apparent temperatures 590 °C Qtz–St an temperature 595 °C. Diffusional modelling indicates quartz isotopic compositions were reset 30 °C via retrograde diffusional exchange micas. appear be in excellent agreement another, suggest attained between 625 °C. However, Qtz–Str is not petrographic observations (garnet grew before staurolite) constraints predict grows over interval 525–550 °C. Given that: (i) diffusion rates slow enough render individual effectively closed after it forms; (ii) able isotopes deformation; could simultaneously achieved other or recrystallized matrix. Thus, fractionations, which cannot resulting from exchange. Instead, they porphyroblasts formed different times P–T–D path, exchanged micas plagioclase several phases deformation.