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Epidote phenocrysts in dacitic dikes, Boulder County, Colorado

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Abstract

Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H2O and O2 activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.

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  1. Department of Geological Sciences AJ-20, University of Washington, 98195, Seattle, WA, USA

    Bernard W. Evans & Joseph A. Vance

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  1. Bernard W. Evans
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  2. Joseph A. Vance
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Evans, B.W., Vance, J.A. Epidote phenocrysts in dacitic dikes, Boulder County, Colorado. Contr. Mineral. and Petrol. 96, 178–185 (1987). https://doi.org/10.1007/BF00375231

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