Contributions to Mineralogy and Petrology

, Volume 162, Issue 4, pp 709–723 | Cite as

Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano, Chile

  • Michael D. Higgins
Original Paper


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.


K-feldspar Megacryst Crystal size distribution Plagioclase Lava Petrography 



I would like to thank Jorge Clavero and Georg Zellmer for helping me to plan the project. Judit Ozoray accompanied me in the field and made editorial comments. Georg Zellmer and an anonymous reviewer helped to improve the manuscript. UQAC awarded me a sabbatical which enabled this project. The ‘Departamento de geología, Universidad de Chile’ hosted me during my sabbatical. Funding was provided by a Discovery grant from NSERC (Canada).


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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Sciences de la TerreUniversité du Québec à ChicoutimiChicoutimiCanada

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