K-FELDSPAR AND COOTIES; OR WHY K-FELDSPAR LIKES TO GROW INTO BIG CRYSTALS

The common occurrence of 5-10 cm K-feldspar (Kfs) phenocrysts, commonly termed megacrysts, in calc-alkaline granitic rocks has led to the common assumption that these crystals form early in a magma’s history, when there is lots of space for them to grow into huge, euhedral crystals. However, this interpretation conflicts with virtually all experimental, observational, and analytical information about how Kfs fits into the crystallization sequence of such magmas, leading to the question: how does a late crystallizing phase form big crystals?

Kfs is typically the last major phase to crystallize in magmas of this composition, entering when there is ~50% liquid left. This remaining liquid is about 1/3 each Kfs, sodic plagioclase (plag), and quartz, all of which compete for room in an ever-diminishing, tortuous liquid volume. To understand how Kfs nevertheless forms megacrysts we have examined grain-grain contacts across the transition in Yosemite where equigranular granodiorite with dispersed, anhedral Kfs gradually turns into megacrystic granodiorite with 10-cm megacrysts. The modal proportion of Kfs is constant across this transition, so the textural change involves gathering the little bits of anhedral Kfs into large islands separated by a sea of tonalite. Using image analysis of stained slab surfaces we have calculated the proportions of the three main types of grain-grain contacts: Kfs-plag, Kfs-quartz, and plag-quartz. In the equigranular rock these three types each make up about 1/3 of the total contact length, but in the megacrystic rocks the Kfs-quartz contact length shrinks to half this as plag-quartz length grows to nearly 50%. Kfs and plag are nearly everywhere separated by a sharply defined zone of albite ~20 um thick, a texture that is ubiquitous in granitic rocks.

We interpret these changes in texture to reflect local migration and recrystallization of Kfs into more equant shapes as it steadily decreases its contact area with other phases owing to high surface energies. Kfs-plag contacts are tolerable if the phases are separated by albite, which is a mutual friend. Thus, among the three phases growing in the last volumes of liquid, Kfs has the greatest energetic need for compact crystal forms, leading to idiomorphism—not unlike that of garnet or staurolite in a schist.