IMPLICATIONS OF OPEN-SYSTEM PROCESSES AND THE COMPOSITIONS OF LATE-STAGE MELTS FOR THE ORIGIN OF ALKALI-FELDSPAR MEGACRYSTS IN GRANITIC MAGMAS
Because Af is a late-crystallizing phase in granitic magmas Glazner and Johnson argue megacrysts are unlikely to occur in rheologically “unlocked” magmas with >50% melt. These authors also imply that individual batches of magma undergo largely closed-system fractionation during the incremental growth of plutons: transferring heat—which drives the textural coarsening that produces megacrysts—but not crystals among successive increments. Petrologic data and modeling suggest, however, that warming of the upper crust during growth of the MWIS led to a transition from a composite outer member to a compositionally-zoned inner one in which intrusive increments mixed, exchanging both heat and mass. Textures preserved in coeval porphyry dikes show that some crystals grown in early increments (including megacrysts) survived such mixing events and were incorporated into melt-rich (≈60%) magmas.
Af megacrysts in the MWIS (and similar Tuolumne Intrusive Suite) have low An contents (0.0-0.5 mol %) which Glazner and Johnson attribute to growth at temperatures as low as 400°C. Sanidines crystallized from F-rich rhyolitic melts at ≥700°C, however, have similar An contents. Leucogranites that sample late-stage melts segregated from the MWIS plutons contain F-rich biotite, suggesting these melts—from which the megacrysts grew—were also F-rich and had low KD for Ca in Af.