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Description
Prebatholithic metamorphic rocks of the southern part of the Peninsular Ranges Batholith near Bahia de Los Angeles, Baja California, comprise a NW striking, steeply dipping package of regionally metamorphosed, strongly foliated schist and gneiss. New mapping at a scale of 1: 12,500 over a 14 km2 area documents four distinct lithologic units: granitic gneiss, metavolcanics, schist, and quartzite plus conglomerate. Thin section analysis of 16 samples reveal that they are all rich in quartz. Quartz, plagioclase, biotite, and muscovite is the most common mineral assemblage in the schist unit, which is strongly quartzofeldspathic. The quartzite plus conglomerate map unit contains an interval of unusually pelitic rich schist which contains the mineral assemblage staurolite + garnet + biotite + quartz which is diagnostic of lower amphibolite facies conditions of metamorphism (520-580°C and 2-3 kbar). The pressure conditions of metamorphism indicate ~6-9 km of uplift and erosion to expose these rocks at the surface. Less diagnostic assemblages from other protoliths are all consistent with this grade of metamorphism. Relict quartz phenocrysts in the metavolcanic tuff document the volcanic origin of this unit. Stained slab and thin sections from the granitic gneiss unit show that the protolith for the gneiss was a monzogranite. The mineralogy of these metasedimentary rocks indicates that the protolith probably consisted of quartz rich sandstones including some very pure quartz arenite sandstone, with lesser amounts of conglomerate and shale, and some interbedded carbonate. The likely depositional setting is a shallow nearshore continental setting.
