Formation of Wollastonite by Chemically Reactive Fluid Flow During Contact Metamorphism, Mt. Morrison Pendant, Sierra Nevada, California, USA

Quartz–calcite sandstones experienced the reaction calcite+ quartz = wollastonite + CO₂ during prograde contact metamorphismat P = 1500 bars and T = 560°C. Rocks were in equilibriumduring reaction with a CO₂–H₂O fluid with XCO₂ = 0·14.The transition from calcite-bearing, wollastonite-free to wollastonite-bearing,calcite-free rocks across the wollastonite isograd is only severalmillimeters wide. The wollastonite-forming reaction was drivenby infiltration of quartz–calcite sandstone by chemicallyreactive H₂O-rich fluids, and the distribution of wollastonitedirectly images the flow paths of reactive fluids during metamorphism.The mapped distribution of wollastonite and modeling of an O-isotopeprofile across a lithologic contact indicate that the principaldirection of flow was layer-parallel, directed upward, withany cross-layer component of flow <0·1% of the layer-parallelcomponent. Fluid flow was channeled at a scale of 1–100m by pre-metamorphic dikes, thrust and strike-slip faults, foldhinges, bedding, and stratigraphic contacts. Limits on the amountof fluid, based on minimum and maximum estimates for the displacementof the wollastonite reaction front from the fluid source, are(0·7–1·9) x 10⁵ cm³ fluid/cm² rock. Thesharpness of the wollastonite isograd, the consistency of mineralthermobarometry, the uniform measured ¹⁸O–¹⁶O fractionationsbetween quartz and calcite, and model calculations all arguefor a close approach to local mineral–fluid equilibriumduring the wollastonite-forming reaction. KEY WORDS: contact metamorphism, fluid flow, wollastonite, oxygen isotopes, reaction front