SIGNIFICANCE OF K-FELDSPAR MEGACRYST SIZE AND DISTRIBUTION IN THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA

Potassium feldspar (K-spar) crystals in granitic rocks grow to huge sizes (> 10 cm) in some granites and are termed megacrysts. Many workers interpret these megacrysts as having grown from relatively few nuclei in a largely liquid magma, and they infer that alignment and concentration of megacrysts reflect magma flow, crystal sorting, and debris avalanching. However, experimental studies indicate that K-spar crystallizes late in most granites (Swanson, 1979; Clemens and Wall, 1981; Whitney, 1988). Estimates of the amount of melt present during initial K-spar growth range from 65-70%, with a majority of K-spar growth occurring within a few degrees of the solidus. These data indicate that magmas would be choked with crystals and unable to flow by the time the K-spar crystals grew to significant size.

To determine the origin of these intriguing megacrysts, we studied K-spar size distribution in the Tuolumne Intrusive Suite, Yosemite National Park, California. On a 12 km traverse from the equigranular Half Dome Granodiorite into the megacrystic Cathedral Peak Granodiorite, the average area of the 10 largest crystals within a 1 m² area steadily increases from 0.2 to 28 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). Backscattered electron images of megacrysts show highly variable and truncated concentric compositional zones that correlate with Ba content.

The compositional features and especially the spectacular increase in K-spar size at constant K-spar vol % are interpreted to reflect growth of megacrysts at the expense of smaller crystals. These data are consistent with Higgins's (1999) crystal size distribution measurements in the same area that show trends indicative of textural coarsening and are inconsistent with other explanations of Higgins's data as reflecting crystal accumulation. Instead, we suggest that these textures are the result of remelting and textural coarsening/crystal aging, consistent with a protracted cooling history.