Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano,Chile |
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Authors: | Michael D Higgins |
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Institution: | 1.Sciences de la Terre,Université du Québec à Chicoutimi,Chicoutimi,Canada |
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Abstract: | K-feldspar megacrysts are common in granitoids, but relatively rare in chemically equivalent volcanic rocks. Dacites from
Taapaca volcano have euhedral sanidine megacrysts up to 5 cm long. Small crystals, where present, are rounded. Growth of the
megacrysts engulfed plagioclase and amphibole crystals. Crystal size distributions (CSD) of sanidine megacrysts are hump shaped.
All these data show that megacrysts developed from the host magma by coarsening: this was enabled by the cycling of magma
temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic
overturns. Plagioclase crystals enclosed in the megacrysts are small and have short, steep, straight CSDs, which contrasts
with the CSDs of plagioclase in the groundmass which are shallower and extend to larger sizes. This shows that plagioclase
was also coarsened approximately synchronously with sanidine, in response to the same temperature conditions. |
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