Collections > Electronic Theses and Dissertations > Formation of K-feldspar megacrysts in granodioritic plutons by thermal cycling and late-stage textrual coarsening
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K-feldspar megacrysts are common in many granite and granodiorite plutons. They are generally inferred to be early crystallizing phases owing to their large size and euhedral form and to features that suggest deposition by magmatic sedimentation. However, all phase equilibrium experiments and natural examples of crystallization and partial melting in magmas of similar composition demonstrate that K-feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50% crystallization and is thus incapable of flow and sedimentation. Megacryst size distributions, compositions and textural relationships from the Tuolumne Intrusive Suite, California reveal that the gradational transition from equigranular to megacrystic granodiorite occurs via textural coarsening in which material from small crystals is transferred to developing megacrysts. Many other examples of K-feldspar megacrystic plutons exist. However, detailed mapping (inter-pluton K-feldspar size distributions) and high-precision multiple location geochronologic studies of many suites are seldom available. Using the TIS as an analog and the implications of the model presented here, we suggest that zoned intrusive suites containing K-feldspar megacrysts are long-lived and emplaced incrementally through multiple magmatic pulses.