IMPLICATIONS OF OPEN-SYSTEM PROCESSES AND THE COMPOSITIONS OF LATE-STAGE MELTS FOR THE ORIGIN OF ALKALI-FELDSPAR MEGACRYSTS IN GRANITIC MAGMAS

Glazner and Johnson (Contrib. Min. Pet., 166, 777-799) argue that many features of alkali-feldspar (Af) megacrysts in calc-alkaline granitic rocks are more consistent with their growth from volatile-rich melts at relatively low temperatures (≈650-400°C) than exclusively from silicate melts at higher temperatures. Because granitic rocks inevitably undergo textural and compositional modification during prolonged cooling in the presence of exsolved fluids, insights into the origins of their magmatic features can come from related rocks that cooled rapidly and have weaker subsolidus overprints. Studies of dike rocks associated with the megacrystic plutons of the Mount Whitney Intrusive Suite (MWIS) in the eastern Sierra Nevada suggest that open system behavior during the growth of these plutons, combined with F enrichment of their late-stage melts, may explain both the occurrence of megacrysts in relatively melt-rich magmas and their low An contents.

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 K_D for Ca in Af.