Taapaca Volcano, Chile

K-feldspar megacrysts are common in granitoids, but rare in chemically equivalent volcanic rocks, of which the Taapaca, Chile, dacites are the best known. Crystal size distributions (CSD) of sanidine megacrysts are hump-shaped. Large crystals are euhedral and small crystals are rounded. Growth of the megacrysts engulfed plagioclase and amphibole crystals. All these data show that megacrysts developed from the host magma by coarsening: this was enabled by cycling of the temperature of the magma around the sanidine liquidus temperature in response to injections of more mafic magmas and subsequent magmatic overturns. Hence, the megacrysts are not xenocrysts, but phenocrysts. Chemical zonation of the megacrysts reflects heterogeneity of the host magma and not transfer of crystals from other magmas. Plagioclase crystals enclosed in the megacrysts are small and have straight CSDs, which contrasts with the hump-shaped CSDs of plagioclase in the matrix.  This shows that plagioclase was also coarsened, starting at the same time as the growth of the sanidine crystals and continuing afterwards.

Higgins, M.D. Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano, Chile. Contrib Mineral Petrol 162, 709–723 (2011). https://doi.org/10.1007/s00410-011-0620-9