Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado

Epidote phenocrysts (Ps_19-24) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO₂ = 64%-70%). Phenocryst assemblages consist of combinations of epidote, plagioclase (An_53-19), biotite [Fe³⁺/ Fe^total = 0.25; Mg/(Mg + Fe²⁺) = 0.42-0.55], quartz, aluminous amphibole (Al_total = 2.4-3.0 per formula unit), igneous garnet (And_2-6 Gros_13-27Spess_2-13Pyr_7-26Alm_51-60), igneous muscovite, and sanidine. Holocrystalline, aphanitic groundmasses consisting mainly of quartz, plagioclase, and alkali feldspar comprise 60%-75% of the dike rocks and require that epidote grew early in the crystallization sequence.

Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrogenetic connotations implied by each type. Type I epidote occurs as euhedral phenocrysts, up to 8 mm long, with low content of allanite component (<0.02 total REE cations per 12.5 oxygen formula unit); many have allanitic cores upon which they nucleated epitaxially. Type II epidote forms smaller (0.2-2 mm) phenocrysts which have a significant amount of allanite component (from 0.01 to 0.30 total REE cations p.f.u.) in oscillatory zones not confined to crystal cores. Types I and II epidote both show a continuous solid solution between epidote and allanite. Type III epidote, interpreted as relict type I phenocrysts, occurs as skeletal, optically continuous inclusions inside plagioclase phenocrysts. A reaction relationship, in which epidote becomes unstable relative to plagioclase due to depressurization, is inferred.

Thermobarometric calculations provide empirical evidence that epidote is stable in silicic, calc-alkaline magmas at high total pressure, high water content, and high oxygen fugacity. The dacitic magmas originated by partial melting at temperatures above 800 °C. Temperatures (±50 °C) ranged from 800 to 880 °C during early phenocryst growth down to 620 to 670 °C during groundmass crystallization. Phenocrysts formed at pressures between 7.2 ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was within the magnetite stability field at two log units (±1.0) below the hematite-magnetite buffer. H₂O fugacity calculations give values ranging from 3.4 to 16 kbar, depending on how annite activity in biotite is modeled. The occurrences of types I and III epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8.0 ± 1.0 kbar. These results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large, Cordilleran plutons. Phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow-level dikes.