HYDROTHERMAL FORMATION OF EPIDOTE IN FELSIC COMPOSITIONS: EXAMPLES FROM SWEDEN AND NORTHERN MICHIGAN

Hydrothermal epidote is widespread in granitic rocks but has been the subject of relatively few scientific studies. We used a scanning electron microscope and electron microprobe to study mineral assemblages and chemistry in epidotized hydrothermal zones developed along late joints and fractures in high-grade granitic gneisses from four quarries in Halland Province, Sweden; and in arkosic conglomerates rich in rhyolite clasts from the Centennial Mine in the Keweenaw Peninsula, Michigan. In Halland, epidote is associated with chlorite, prehnite, and the replacement of Fe-Ti oxides by titanite. In the Centennial Mine epidote is associated with calcite, pumpellyite, titanite, and rare hydrogarnet (grossular-andradite). These assemblages are consistent with upper prehnite-pumpellyite to lower greenschist facies conditions. Zoning was observed in epidote grains from both localities and commonly occurs as polygonal zones within which Fe/(Fe+Al) is relatively constant but changes suddenly at the boundary. REE-enrichment in epidote occurs most commonly at the margins of grains or along fractures. Modal analysis shows an inverse relationship between the abundance of alkali feldspars and epidote, suggesting a replacement reaction in which Ca is added, and Na + K is removed by hydrothermal solutions. In the Centennial Mine, the source of Fe for epidote formation appears to be dispersed hematite in oxidized sediments, and magmatic Fe-Ti oxides may have played the same role in in Halland. Epidote formation at the Centennial Mine is part of the ca1.06 Ga hydrothermal metamorphism and Cu mineralization in the Keweenaw Peninsula. Epidotization in Halland postdates the ca1.0 Ga Sveconorwegian high-grade metamorphism. Although there is considerable scatter, the predominant direction of veins associated with epidote is NE-SW which is the same direction as an extensional event of inferred Late Carboniferous-Permian age recorded elsewhere in southern Sweden. Despite the difference in geological settings, epidotization in both areas displays common features which may be characteristic of low-grade hydrothermal alteration of felsic igneous rocks in general.