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

Abstract

Listen

Translate article

Quartz–calcite sandstones experienced the reaction calcite + INTRODUCTION quartz = wollastonite + CO2 during prograde contact metaFlow of chemically reactive fluid may control the minmorphism at P = 1500 bars and T = 560°C. Rocks were in eralogy, stable isotope composition, and trace element equilibrium during reaction with a CO2–H2O fluid with XCO2 = chemistry of rocks during contact metamorphism (e.g. 0·14. The transition from calcite-bearing, wollastonite-free to Labotka et al., 1988; Jamtveit et al., 1992a, 1992b; Nabelek wollastonite-bearing, calcite-free rocks across the wollastonite isograd & Labotka, 1993; Bowman et al., 1994; Roselle et al., is only several millimeters wide. The wollastonite-forming reaction 1999). A number of aspects of metamorphic fluid flow, was driven by infiltration of quartz–calcite sandstone by chemically however, remain the subject of debate, including the reactive H2O-rich fluids, and the distribution of wollastonite directly direction of flow (Labotka et al., 1988; Ferry & Dipple, images the flow paths of reactive fluids during metamorphism. The 1992; Ferry, 1995a; Hanson, 1995a), whether numerical mapped distribution of wollastonite and modeling of an O-isotope models that assume uniform rock properties adequately profile across a lithologic contact indicate that the principal direction predict the geometry of fluid flow in aureoles that contain of flow was layer-parallel, directed upward, with any cross-layer a variety of anisotropic structural features (Hanson, 1992, component of flow <0·1% of the layer-parallel component. Fluid 1995b; Cook et al., 1997), and the degree to which local flow was channeled at a scale of 1–100 m by pre-metamorphic mineral–fluid equilibrium is attained (Lasaga & Rye, dikes, thrust and strike-slip faults, fold hinges, bedding, and 1993; Bolton et al., 1999; Lasaga et al., 2000). These and stratigraphic contacts. Limits on the amount of fluid, based on related questions were addressed in an investigation of minimum and maximum estimates for the displacement of the contact metamorphism in the Mt. Morrison pendant, wollastonite reaction front from the fluid source, are (0·7–1·9) × eastern Sierra Nevada, California. The location is an 10 cm fluid/cm rock. The sharpness of the wollastonite isograd, exceptional site for the study of metamorphic fluid flow the consistency of mineral thermobarometry, the uniform measured for several reasons. First, the Mt. Morrison Sandstone, O–O fractionations between quartz and calcite, and model the focus of this study, is ideal for application of concalculations all argue for a close approach to local mineral–fluid tinuum models of coupled fluid flow and reaction because equilibrium during the wollastonite-forming reaction. it is unusually homogeneous in mineralogy and texture.