High f(O2) during sillimanite zone metamorphism of part of the Barrovian type locality, Glen Clova, Scotland

The redox state of sillimanite zone (650-700 degreesC, 5 6 kbar) metasedime nts of the Barrovian type area, Scotland, was investigated using estimates of metamorphic oxygen fugacity (f(o)), sulfur fugacity (f(s2)), and fluid c hemistry based on new determination of mineral and rock compositions from 3 3 samples. A total of 94% of the samples lack graphite, contain both ilmeni te hematite solid solutions (RHOMOX) and magnetite, and had metamorphic f(o 2) about 2 log(10) units above the quartz-fayalite magnetite (QFM) buffer. The regional variation in metamorphic f(o) for these rocks was minimal, abo ut +/- 0.3 log(10) units, reflecting either a protolith that was homogeneou s with respect to redox state, or an initially variable protolith whose red ox state was homogenized by metamorphic fluid rock interaction. RHOMOX incl usions in garnet porphyroblasts that become richer in ilmenite from the int erior to the edge of the host porphyroblast suggest that at least some synm etamorphic reduction of rock occurred. Significant variations in bulk-rock oxidation ratio (OR) that are probably inherited from sedimentary protolith s are found from one laver to the next; OR ranges mostly between similar to 20 and similar to 50/OR = molecular 2F(2)O(3); x 100/(2Fe(2)O(3) + FeO)]. these OR variations are uncorrelated with fo., and do not indicate that lar ge, order-of-magnitude gradients in fo(2) and redox stale existed or were p reserved between layers during metamorphism. The other 6% of the samples co ntain ilmenite, lack magnetite, and had low f(o2) 0 1 order of magnitude be low QFM in the stability field of graphite. They are characterized by combi nations of the following: large fluid HF/H2O; metasomatic, tourmaline-beari ng veins; absence or rarity of primary organic matter; and crosscutting lat e metamorphic shear zones rich In carbonaceous material. Such observations suggest that locally low f(o2) conditions mar have been related to the infl ux of reducing fluids from elsewhere in the area.