Approximately ten square miles of the Alabama Hills bedrock outcrops are metamorphic rocks. They occur as two separate bodies on the eastern margin. The southeastern body has an exposed area of less than one square mile. While the northeastern body is a nine square mile wedge. It is separated from the southern metamorphics by two miles of granitic outcrops (See Plate I).
In the north ten square miles of metamorphic rock consists of a complex of metahypabyssal rocks, metasediments, and metarhyolite tuffs. Because the field relationships between these rock units were subtle at best, the petrography of the complex was studied to develop an understanding of the pre-metamorphic petrological and structural relationships.
Initial field classification of the metamorphic rocks was based on textures and mineral compositions recognizable in hand specimen. Texturally, the blastoporphyritic rocks were differentiated using the size of the phenocrysts and whether or not the matrix was aphanitic. The darkness of the rock was used as a semi-quantitative measure of the aphanitic nature of the matrix. The more aphanitic the matrix, the darker the rock. Compositionally, the rocks were grouped on the basis of the dominant phenocryst. This classification yielded three mappable rock types:
Quartz phenocrysts dominant
Alkali feldspar phenocrysts
Plagioclase phenocrysts dominant
Of the several types of field groups, the classification based on the dominant phenocryst is the only division that was corroborated by petrography. The petrographic classification incorporates these three phenocrysts compositional groupings used infield mapping.
A significant distinction between rocks in the metavolcanic complex was made between two very similar groups of rocks with dominant quartz phenocrysts. The initial grouping of these rocks was based on field relationships and later substantiated by detailed petrography. Both of these rock groups are metamorphic: one is a metarhyolite tuff; and the other is a metaquartz porphyry. There are three field relationships that complimented the petrographic recognition of these two rock types:
A metasediment was mapped between the two rock types.
The quartz porphyries are conformable with the rest of the hypabyssal dike swarm, while the metatuff is not related to any dikes but appears to cap the porphyries and in one place the metasediments.
The metatuff has numerous xenoliths of a more mafic rock (see detailed petrographic descriptions) while the quartz porphyry has few if any.
The following petrographic descriptions were based on examination of approximately one hundred thin sections. Detailed results of the petrographic studies are presented in Appendix B and only generalized summaries are given below. Grain size data is given in terms of largest dimensions of elongate grains. Mineral compositions where given were determined by optical methods.