274 D35. Petrology, Igneous (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

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

Paper number: 274-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 Or83-92 and Or69-88. Experiments at 705, 710 and 720°C and 2 kb show alkali feldspar between Or59-87, Or63-91 and Or75-90, 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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308054

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