High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland

INTRODUCTION The redox state of sillimanite zone (650–700°C, 5–6 kbar) metasediments of the Barrovian type area, Scotland, was investigated The chemical and mineralogical evolution of the crust using estimates of metamorphic oxygen fugacity (fO2 ), sulfur fugacity during orogenesis is critically dependent upon the redox (fS2 ), and fluid chemistry based on new determinations of mineral state of metamorphic fluids ( James, 1955; Chinner, 1960; and rock compositions from 33 samples. A total of 94% of the Buddington & Lindsley, 1964; Miyashiro, 1964; French, samples lack graphite, contain both ilmenite–hematite solid solutions 1966; Eugster & Skippen, 1967; Thompson, 1970; Harte, (RHOMOX) and magnetite, and had metamorphic fO2 about 2 1975; Ohmoto & Kerrick, 1977; Rumble, 1978; Lamb log10 units above the quartz–fayalite–magnetite (QFM) buffer. The & Valley, 1985; Frost, 1991a, 1991b; Brenan et al., 1995; regional variation in metamorphic fO2 for these rocks was minimal, Harlov et al., 1997; Ague, 1998). The classical Barrovian about ±0·3 log10 units, reflecting either a protolith that was type locality in Glen Clova, Angus, Scotland, is a superb homogeneous with respect to redox state, or an initially variable example of redox controls on metamorphism (Chinner, protolith whose redox state was homogenized by metamorphic 1960). Based on field relations, petrography, and the fluid–rock interaction. RHOMOX inclusions in garnet porchemical systematics of minerals and bulk rocks, Chinner phyroblasts that become richer in ilmenite from the interior to the (1960) showed that large differences in bulk-rock oxedge of the host porphyroblast suggest that at least some synidation ratio (OR) existed at local and regional scales metamorphic reduction of rock occurred. Significant variations in between metasediments during amphibolite facies metabulk-rock oxidation ratio (OR) that are probably inherited from morphism in Glen Clova [OR = molecular 2Fe2O3 × sedimentary protoliths are found from one layer to the next; OR 100/(2Fe2O3 + FeO)]. These differences were attributed ranges mostly between >20 and >50 [OR = molecular 2Fe2O3 to chemical contrasts inherited, at least in part, from the × 100/(2Fe2O3 + FeO)]. These OR variations are uncorrelated sedimentary protoliths, and were taken as evidence that with fO2 and do not indicate that large, order-of-magnitude gradients significant differences in fO2 and fluid composition existed in fO2 and redox state existed or were preserved between layers during between layers during metamorphism (Chinner, 1960). metamorphism. The other 6% of the samples contain ilmenite, lack Rock oxidation ratio variations in Glen Clova have magnetite, and had low fO2 0–1 order of magnitude below QFM been unequivocally established, but important questions in the stability field of graphite. They are characterized by comremain regarding the metamorphic fluids. For example, binations of the following: large fluid HF/H2O; metasomatic, what were the quantitative variations in the fugacities of tourmaline-bearing veins; absence or rarity of primary organic O2 and sulfur species such as S2 in the region? Did large matter; and crosscutting late metamorphic shear zones rich in variations in fO2 and fluid composition exist between carbonaceous material. Such observations suggest that locally low intercalated metasedimentary layers? Could metafO2 conditions may have been related to the influx of reducing fluids morphic fluid–rock interaction between layers have overfrom elsewhere in the area. come characteristic buffer capacities inherited from