The Burro Mountain body is a crudely equidimensional block of unusually fresh ultramafic rock. This block, along with numerous smaller and more elongate serpentinite bodies, has been emplaced in a highly sheared Franciscan terrane immediately west of the Nacimiento fault. This fault separates two major structural units: (1) on the east, a 40- by 300-mile block with basement of granitic rock, schists and gneisses, and (2) on the west, a 10- by 100-mile tract of Franciscan Formation.
Within the Burro Mountain body the primary rock types and their approximate percentages are: peridotite (65), dunite (35), and pseudopyroxenite (<1). Variations among these lithologic types produce a well-defined internal structure, the two major elements of which are: (1) pervasive compositional layering whose attitude throughout the body varies little from N. 5° W., 80° E., and (2) superimposed discordant structures which may cut the earlier layering at any angle. The first set is thought to have been formed by metamorphic differentiation associated with internal deformation, the second set by metasomatic replacement, possibly a succeeding phase of the earlier metamorphic differentiation.
Serpentinization is nearly complete at the margins but progressively decreases toward the center of the body. Contact metasomatism, related to Serpentinization, is responsible for the sporadic presence of rodingites around the margins. An arm-like extension of the main body and linear trains of serpentinite bodies extending for miles parallel to the regional structure may represent previous marginal material sheared out during emplacement. The tectonic nature of the emplacement is demonstrated by the discordance of internal versus regional structure, discrepancy of metamorphic grade between ultramafic and country rock, extreme marginal shearing, lack of igneous contacts, and association with other tectonic inclusions.
Several lines of evidence suggest a subcrustal origin for the primary ultramafic rock.