EXPERIMENTAL PETROLOGY OF THE K-FELDSPAR MEGACRYST-BEARING CATHEDRAL PEAK GRANODIORITE, TUOLUMNE INTRUSIVE SUITE, YOSEMITE NATIONAL PARK, CA

Large K-feldspar megacrysts (3-10 cm in length) are common in the inner plutons of large (10³ km²) zoned Cretaceous intrusive suites that lie along the crest of the Sierra Nevada batholith, USA, but the crystallization conditions, and particularly the origin of the K-feldspar megacrysts, is controversial. Some workers argue that the megacrysts are primary phenocrysts that grew in magma at supra-solidus conditions (Kerrick, 1969; Vernon, 1986; Bateman, 1992; Vernon and Paterson, 2008; Moore and Sisson, 2008; Gagnevin, 2008; Barboni and Schoene, 2014) while others argue that megacrysts are analogous to metamorphic porphyroblasts and attain their large size by late-stage textural coarsening at or below the solidus in the presence of abundant fluid (Higgins, 1999; Johnson and Glazner, 2010; Glazner and Johnson, 2013).

We present new experimental data for crystallization conditions of the K-feldspar megacrystic Cathedral Peak Granodiorite, Tuolumne Intrusive Suite, California. H₂O-saturated experiments were performed with natural rock in hydrothermal cold-seal bombs between 700-800°C and at 100 and 200 MPa with ƒO2 buffered at Re-ReO₂. Run durations were between two to three weeks. Compositions of the run products were determined by scanning electron microscope (SEM) and electron microprobe.

K-feldspar is present at 100 MPa and 750°C (but absent at 775°C), whereas at 200 MPa K-feldspar does not appear until 720°C and is absent above 730°C. Experiments conducted at 700 and 750°C at 1 kb yield alkali feldspar between Or_83-92 and Or_69-88. Experiments at 705, 710 and 720°C and 2 kb show alkali feldspar between Or_59-87, Or_63-91 and Or_75-90, respectively. Mineral and glass modes are subject to appreciable uncertainty because of overall small crystal size for most minerals. However, the experiments show that K-feldspar either co-crystallizes with quartz or saturates before quartz with crystallinity increasing significantly near K-feldspar saturation. K-feldspar typically forms “mega”-crysts that are at least an order of magnitude larger than the largest plagioclase crystal, suggesting that large K-feldspars can grow quickly under the experimental (magmatic) conditions.