MINERALS AND REACTIONS IN MIAROLITIC CAVITIES IN THE EOCENE SAWTOOTH BATHOLITH, SOUTH CENTRAL IDAHO

The Eocene Sawtooth Batholith (c. 45 Ma) is a granitic body emplaced in south-central Idaho along the bare back of the Sawtooth Mountains crest, and is associated with the Challis volcano-plutonic event. The epizonal intrusion has geochemical affinities with A-type or fractionated I-type granite, and possesses a diverse assemblage of igneous textures and associated minerals. Miarolitic cavities (MC) are widespread, and are often associated with late-stage aplite and pegmatite dikes: this observation has been used to infer a shallow depth of emplacement for the system. MC probably indicate significant upward movement of volatile phase-saturated melt. Miarolitic cavities, the best evidence that a pluton has exsolved a magmatic volatile phase, are present at scales that cover 6 orders of magnitude (tens of microns to tens of meters). When small, the MC are often elongate and asymmetric. On this small scale, isolated MC are present, but cracks sometimes extend between them, a feature that may represent paleo-fluid pathways . The mineralogy of the MC is highly variable, but comprises primarily Kfs, (smoky) Qtz, and Ab, in addition to Fa, Mt, Bt (Ann: Fe/(Fe+Mg)=0.5-0.85, with up to 4.3 wt.% F), Fl, Toz, Zrn and huttonite/thorite. Where asymmetric, the MC exhibit an asymmetry defined primarily by fibrous/platy Bt on one side, and Qtz, Kfs and Ab on the other. The Ol is fayalitic (Fe1.84Mg0.02Mn0.14SiO4) and homogeneous over the cm-scale, except around fractures where a hydrous Fe-silicate phase forms, and adjacent to the interior of the MC in contact with Mt and Qtz (as overgrowths on the Fa), where the Fa is depleted in Mn. The mineralogy suggests that near-solidus crystallization occurred at the QFM oxygen fugacity buffer. The appearance of Fa in the MC may be due to the breakdown of Ann upon decompression of the magma, (and the attendant reduction in the fugacity of water) as indicated by the simplified reaction: 3/2SiO2 + KFe3AlSi3O10(OH)2=> 3/2Fe2SiO4 + KAlSi3O8 + H2O. According to Dachs and Benisek (1995; CMP, 121:p380) this equilibrium occurs at 520o C at 200MPa; however, the substitution of fluorine into the biotite may displace the equilibrium up to the solidus temperature allowing the reduction in f(H2O) to initiate Fa crystallization, at the expense of Ann, near the solidus.