DECOUPLED OXYGEN ISOTOPE AND AFM EQUILIBRIA IN METAMORPHIC ROCKS

Petrographic, O isotope, and microprobe analyses were undertaken to test the assumption of major element and O isotope equilibrium in staurolite grade schists that have undergone sequential porphyroblast growth and multiple episodes of matrix recrystallization. Textural relations among minerals in the Littleton Formation at Bolton Notch, CT permit the inference of Grt growth followed by Str growth; each porphyroblast growth event was followed by a period of matrix recrystallization. A pervasive, planar foliation developed during the final phase of deformation (extensional shear). Conventional geothermobarometry using Qtz-Grt-Pl-Mus-Bt±St equilibria yield P-T of ~625 ^oC at ~8 kbar, consistent with KMnFMASH pretrogenetic grid predictions for stability of Grt+St+Bt (+Mus+Qtz+H₂O). Qtz-Grt O isotope fractionations yield apparent T’s (T_a) = 600±18 (1 sig.) ^oC and Qtz-Str fractionations yield T_a = 555±55 ^oC, anomalous results given that: (1) based on inclusion and microstructural relations, Grt grew before Str; and (2) grids predict that the Grt isograd is ~500 and the Str isograd is ~ 580 ^oC.  A second anomalous result is that, except for one sample, Qtz-Mus T_a are 550 to 900 ^oC, whereas Qtz-Bt T_a’s are in the range expected for retrograde cooling and exchange of Bt and Ms (360-440 ^oC). Given that: (1) O diffusion rates in Str and Grt are extremely slow, rendering an individual porphyroblast closed to exchange upon formation; and (2) matrix phases are able to exchange with one another during each period of deformation, Grt and St could not have achieved O isotope equilibrium with each other or minerals in the recrystallized matrix. Thus, the Qtz-Grt fractionations, which yield T_a that are apparently close to true peak T, and the Qtz-Str fractionations, which in some cases agree with Qtz-Grt T_a, cannot be fractionations resulting from equilibrium isotopic exchange. They are apparent fractionations between porphyroblasts formed at different T in the prograde P-T-deformation path, and Qtz that recrystallized and exchanged with micas and Pl in the final stages of deformation. The scatter of Qtz-Str T_a and the scattered and erroneously high Qtz-Mus T_a must result from variable, late, open system exchange of Mus with fluid.