Susanne Seitz, Guilherme Augusto Rosa Gualda , Lydia Harmon 

The mechanisms whereby alkali feldspar megacrysts form have been debated for several decades, with both magmatic and subsolidus processes having been invoked to explain their origin. We take advantage of glacially polished outcrop surfaces from the Cathedral Peak Granodiorite in the Tuolumne Intrusive Complex, CA to quantitatively characterize alkali feldspar textures. On the glacially polished surfaces, we trace alkali feldspar crystals >10 mm in the field. We also determine CSD for smaller alkali feldspar crystals using large, stained slabs collected from the same localities. We scan the resulting tracings and slabs to quantify CSDs using image processing techniques with the software ImageJ. The CSDs from glacially polished outcrop surfaces and complementary polished and stained rock slabs reveal two stages of crystallization. Crystals >20 mm show log-linear CSDs with shallow slopes, suggesting magmatic nucleation and growth on timescales of thousands of years. Crystals <20 mm define a second stage of crystallization, with much steeper slopes, suggesting a period of enhanced nucleation leading to formation of a groundmass during the final stages of solidification on timescales of decades to centuries. We do not find any evidence for CSD affected by textural coarsening, neither of any effects of subsolidus processes. Our data suggest that these megacrysts form in large, slowly cooling magma, where low nucleation rates dominate. These crystals are not special in their magmatic formation – only in their size. A change in solidification conditions led to the formation of a groundmass, which warrants further study to better understand this crystallization stage in a plutonic environment.