POTASSIUM FRACTIONATION ACROSS OSCILLATORY ZONING IN A K-SPAR MEGACRYST, TUOLUMNE INTRUSIVE COMPLEX CA

Abstract

Intrusively cooled igneous rocks, such as granites and granodiorites, are interpreted on basis of their geochemistry, geochronology, and geology. Prevalent in these rock types, potassium feldspar (KAlSi3O8) megacrysts—singular mineral grains that have grown to approx 5-10 cm in length and are larger than other nearby grains—are thought to act as long-lived recorders of a pluton’s magmatic history. A single K-spar megacryst from the Tuolumne Intrusive Complex, California is analyzed for potassium isotopes and mineral geochemistry across oscillatory zoning. The analyses show variability in δ41K similar to patterns of major and trace element concentrations. Done as a proof-of-concept, the methodology can be improved for further work on K-spar megacrysts, to reduce sample contamination by mineral inclusions and to increase spatial resolution for the analyses. Incorporation of plagioclase and biotite material does not seem to significantly alter the measured K isotopic trend, though this can be investigated further.