The Petrology and Structure of the El Pinal Tonalite, Baja California, Mexico

Abstract

The El Pinal tonalite is one of the many plutons which comprise the southern California batholith of the Peninsular Range province. This pluton crops out on the western slope of Sierra Juarez in northwestern Baja California, Mexico, at elevations ranging from 850 to 1600 m, and is located 45 km from the Pacific coast and 35 km from the international boundary; its outcrop is roughly oval-shaped and covers about 190 square km. The surrounding country rocks include metasedimentary rocks, gabbro, diorite, tonalite, granodiorite, and adamellite. The El Pinal tonalite is very uniform in appearance. It is a pale grayish rock with an average grain diameter of 4 mm, and contains about 4 percent of ubiquitous, dark inclusions which average 30 cm in diameter. The pluton was mapped in three zones, concentric about the approximate geographic center of the body, on the bases of structural and textural features. Rocks of the inner zone are hypidiomorphic granular and exhibit a foliation defined by somewhat inequidimensional, dark inclusions. Textures of the middle zone are hypidiomorphic granular to protoclastic, and foliation is defined by discoidal inclusions and platy biotite grains. Some of the inclusions there are triaxial and, together with hornblende prisms, define a lineation that plunges down the foliation. Rocks of the outer zone are protoclastic and locally cataclastic. In this zone, discoidal inclusions and platy biotite grains define a foliation which is parallel to the contact of the pluton with country rocks. Foliation and lineation are everywhere concentric about the approximate geographic center of the pluton; they are interpreted as primary flow structures. Their dips change gradually and continuously, from gently toward the center of the pluton near its center, to vertical in the middle zone, and to steeply outward in the outer zone. Nowhere is the foliation horizontal. Modes were determined from 142 stained slabs. Twenty of the modes plot within the granodiorite field, and the remainder plot within the tonalite field. Sixteen of the 20 granodiorite samples are from the middle zone. The color indices for the entire pluton increase from an average value of 17 near the center of the pluton to 26 near contacts with country rocks. This may be the result of assimilation of mafic xenoliths stoped near contacts. The concentration of granodiorite in the middle zone is thought to be related to ring fracturing. Seventy-four oriented thin sections were prepared from samples distributed over the entire pluton. From these, poles to {010} and a axes in plagioclase grains were measured for 24 localities. When plotted stereographically, these data indicate that {010} is everywhere parallel to foliation as mapped in the field. Crystallographic a axes form a lineation which everywhere plunges down the dip of foliation. Measurements on etched slabs and thin sections show that most plagioclase grains are essentially biaxial tablets whose longer axis is from 2 to 5 times the length of the shorter axis. The plagioclase fabric is thought to have formed as the result of viscous flow during emplacement of the magma-crystal mixture. Plagioclase grains were presumably elongate parallel to a or c at that time. The total structural pattern of the pluton is best explained by a 360° ring fracture which extended from the top of the magma chamber to the surface of the earth. Although the ring dike is eroded away, its base is thought to correspond to the middle mapped zone, which shows clear evidence of upward convergence of flow lines and planes. Additional geomorphologic, mineralogic, and petrographic data also support the ring-fracture hypothesis.