A zone of aplite and pegmatite dikes were emplaced along the contact between the metavolcanic and plutonic rocks in the Alabama Hills. These dikes are hosted by both plutonic and metamorphic rocks, but the dike network seems to be more extensive in the plutonics. Also a minor aplite dike swarm is associated with the margins of an alkali feldspar dike that roughly parallels the contact. The generalized distribution of the aplite, pegmatite zone is shown in Plate I, by use of a pattern. Mappable aplitic, pegmatitic dike swarms were mapped as either aplites (ap) or pegmatites (peg).
The aplite dikes are generally flat lying and range in thickness from less than one inch to several feet in width. These dikes are characterized by an allallotrio-morphic-grannular "sacchoraidal" texture, and crystals rarely exceed 1mm. Greater than 98% of the minerals in these dikes are the light colored minerals: quartz, alkali feldspar, and muscovite.
The aplites and pegmatites are interrelated which is indicated by the fact that some aplites have segregations of coarse textured pegmatites. As a general rule, the pegmatites cut the aplite dikes and dike swarms. The pegmatite dikes range in size from a few inches to several yards in width. The pegmatites are usually crudely zoned and are generally barren of pneumatolytic minerals.
Texturally, the pegmatites vary from hypidiomorphic granular to the coarse grained pegmatic textures (see Figure 22). Locally, graphic myrmekitic and perthite textures can be seen between intergrowths of alkali feldspars and quartz. Compositionally, quartz, alkali feldspar (usually perthite, micro-perthite, or microcline), sodic plagioclase (An25 rim to An27 core) and muscovite comprise over 95% of the minerals in the pegmatites. Accessory minerals that were found rarely in the pegmatites of the Alabama Hills include: beryl, rutile, tourmaline (var. schorl), garnet, epidote, and apatite (see Figure 23).
A rock that is physically separated from the pegmatite and aplite dikes by greater than a mile is the felsophyre dike in the north end portion of the hypabyssal rocks. Because of the concordant nature of this dike with the other hypabyssal dikes, it was initially believed to have formed as part of the early hypabyssal intrusions. The composition and textures both microscopic and macroscopic are not related to the intruded rocks. The composition is very similar to the aplite dikes along the intrusive contact. The crystals in the felsophyre could have formed during devitrification of a glass.