Description
Prebatholithic rocks are exposed over a >200 km2 area in the Bahia de Los Angeles region at the southern end of the Peninsular Ranges batholith. Mapping at 1: 12,500 over a small portion of this area has documented four distinct map units, three metasedimentary and one granitic gneiss unit. All of the prebatholithic rocks in this area are steeply dipping and NW-striking. The granitic gneiss unit (here designated the Cantera Gneiss) is a mylonitic gneiss exposed over ~20 km2 that is characterized by a strong down-dip mineral stretching lineation defined by elongated quartz aggregrates and biotite concentrations. The Cantera Gneiss is relatively homogeneous internally and has the basic mineralogy of quartz, biotite, alkali feldspar, and plagioclase ± muscovite. The mineralogy, relative internal homogeneity and local preservation of relict igneous textures in thin section are all consistent with an igneous protolith for the gneiss. Stained slabs were prepared from six widely separated samples of the Cantera Gneiss to investigate modal mineralogy. The modes were determined from the stained slabs by digital image analysis of scanned slab images in Adobe Photoshop. Modes from 5 of the 6 samples cluster tightly in the monzogranite field in the standard QAP classification scheme of plutonic rocks. The sixth sample plots in the quartz monzonite field with slightly lower normative quartz and high alkali-feldspar relative to the other samples. The protolith of the Cantera Gneiss is inferred to have been a weakly peraluminous monzogranite based on the presence of muscovite in most of the samples. The gneiss and surrounding metasedimentary country rock was deformed by a NNE-SSW directed shortening produced the mylonitic fabrics in the gneiss. This compressional deformation may be related to a similar pattern of deformation recognized farther north in the Peninsular Ranges Batholith.
