Pumpellyite from zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone,Hokkaido, Japan |
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Authors: | Hideo Ishizuka |
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Institution: | (1) Department of Geology, Kochi University, 780 Kochi, Japan |
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Abstract: | Pumpellyite occures in zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan, filling
veins or amygdules, replacing igneous plagioclase or clinopyroxene or olivine, and occupying the matrix. Its composition and
pleochroism vary greatly even within a single sample, but appear to be related to its mode of occurrence. Thus, the most Al-rich
pumpellyite with pale green to green pleochroism develops in pseudomorphs after plagioclase, whereas the most Fe*-rich variety with deep green to brown pleochroism occurs in the matrix. In low-grade metamorphic rocks which commonly contain
relict minerals, chemical equilibrium is attained only locally. This results in the correlation of the composition of pumpellyite
with its mode of occurrence, such as the precursor phases which are replaced by pumpellyite. On the other hand, among pumpellyites
occurring in similar mode and coexisting with Ca-zeolite (laumontite or wairakite), epidote, chlorite and quartz, the Al content
tends to be enriched in the wairakite-bearing metabasites over the laumontite-bearing metabasites. It follows that the composition
of pumpellyite is also dependent upon the temperature of metamorphism. |
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