Potassium Feldspar Megacrysts in Granites: Passive Markers of Magma Dynamics or Products of Textural Coarsening?

JOHNSON, Breck R., GLAZNER, Allen F., and COLEMAN, Drew S., Department of Geological Sciences, Univ of North Carolina at Chapel Hill, CB# 3315, Mitchell Hall, Chapel Hill, NC 27599-3315, breckj@email.unc.edu

Megacrysts of potassium feldspar (K-spar) in granitic rocks are commonly interpreted as early-crystallizing phases whose textural relationships record flow, settling, and diapirism within evolving magma chambers. However, experimental studies on granitic magmas show that K-spar does not begin to nucleate until the system is at least 30 % crystalline, and that much of the final crystallization history records co-crystallization of K-spar, quartz, and sodic plagioclase. These data require that the megacrysts cannot have reached large sizes until the magma was largely crystallized and incapable of flow.

We have made chemical and textural observations of K-spar megacrysts from the Tuolumne Intrusive Suite (TIS), California . Cathodoluminescence images show sawtooth oscillatory zoning in K-spars, albite-rich rims on plagioclase, reaction zones at boundaries between plagioclase and K-spar, and almost no perthite. Electron microprobe analyses of the sawtooth zones reveal a sharp outward increase in Ba concentration at each zone boundary.  Plagioclase core compositions follow whole-rock compositions, becoming increasingly albitic toward the center of the TIS, but K-spar in all units is highly potassic (Or_80-95). A three-feldspar assemblage (An_15-35, An_1-7, and Or_80-95) occurs in several megacrystic samples. Stained rock slabs reveal tentacles of interstitial K-spar radiating from megacryst edges far into the adjacent matrix, and a deficit of smaller K-spar crystals in megacrystic units.  K-spar size measurements across the contacts of the TIS from the 10 largest crystals within a 1 m² area show a steady increase in the average megacryst area from 0.2 to 30 cm².  In contrast, bulk rock K²O and K-spar mode (vol %) are constant across this same transect (at 3.7 ± 0.5 wt % and 22 ± 5 vol % respectively).