Abstract

The Garnet Hill roof pendant, located in Calaveras County, California, contains sparse lenses and beds of limestone within metasedimentary rocks, which have been converted to marble or zoned skarn in response to thermal metamorphism by intruding granodiorite of the Sierra Nevada batholith. Skarn near the granodiorite contact is composed of iron-rich minerals in disordered bands parallel to bedding in adjacent metasedimentary rocks, whereas skarn farther from the contact contains iron-poor minerals arranged in sequential bands. Occurring within these metasomatic rocks are phases stable over a range of different temperatures, which formed in response to cooling of the adjacent pluton. Banding in the skarn is most easily explained by considering direct chemical reaction between the metasedimentary rock and the marble.

During emplacement of the pluton, marble-pelite chemical interactions produced diffusion skarn composed of sequential monomineralic bands parallel to the contact between the two rock types. Subsequently, an aqueous phase emanating from the cooling pluton permeated the iron-poor skarn, converting it to infiltration skarn. These magmatically derived solutions introduced Fe, Si, and base metals, but the preponderance of Si, Al, and Mg utilized in skarn development was supplied by directly adjacent metasedimentary rocks, thereby offering an explanation for the lack of endoskarn in contiguous granodiorite. Interaction between skarn and aqueous solutions continued until the granodiorite was below temperatures at which zeolites develop, and chemical disequilibrium caused by falling temperatures resulted in complex mineralogy and intricate replacement textures in the skarn.

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