Unimpeded growth of K-feldspar megacrysts in granitic liquids

Though K-feldspar megacrysts are generally regarded as normal phenocrysts, some petrologists have suggested that megacrysts nucleate too late in the duration of crystallization for sufficient unoccupied liquid to be available for the growth of large crystals. Instead, they have proposed that megacrysts grow by "textural coarsening", involving solution of small crystals and deposition on larger crystals, owing to "thermal cycling", in response to repeated magma chamber replenishment late in the crystallization duration. This process could contribute to the grainsize of euhedral, zoned K-feldspar megacrysts only at the stage when megacrysts are free to grow in liquid. However, this process is not necessary for the development of megacrysts because, though K-feldspar saturates typical granitic magmas late in the nucleation (saturation) sequence, plenty of liquid is available for the growth of large crystals. So, though K-feldspar typically nucleates late in the sequence of mineral nucleation, it grows throughout most of the crystallization duration. Abundant structural evidence in granitoids is consistent with free growth of K-feldspar megacrysts in liquid.15 page(s