The Genesis of Zoned Skarns in the Sierra Nevada, California

Zoned skarns occur at plutonic-metamorphic contacts, in veins cutting marble, and at contacts between marble and interlayered amphibolite and biotite-rich rocks. For P = 2 kb, fluid inclusions and P-T-XCO2 stability relations of calc-silicate assemblages suggest T< 650 °C and a H2O-rich fluid (XCO2 < 0.1). Small-scale, Ca-rich endoskarns are common near exoskarns. Mass balance calculations suggest that: (a) the formation of exoskarn requires the influx of solute in an aqueous solution from uncontaminated magma in addition to material derived from the endoskarn, (b) some ‘limestone assimilation’ is required to form endoskarns, and (c) skarn formation was essentially a constant-volume process. Applying chromatographic theory, compositional profiles of garnet and pyroxene across zoned skarns suggest that infiltration metasomatism was an important process, although diffusion metasomatism appears to have produced local compositional gradients at the infiltration ‘fronts’. Fluid flow calculations show that thick exoskarns could readily form by intergranular in filtration of aqueous solutions. Reciprocal diffusional exchange is suggested as a dominant mechanism in the formation of zoned skarns formed at contacts between interlayered metamorphic lithologies.