Structure and petrology of the alpine-type peridotite at Burro Mountain, California, U.S.A.

The alpine-type peridotite at Burro Mountain is a partially serpentinized harzburgite-dunite body approximately 2 km in diameter. It lies in a chaotic mélange derived from the Franciscan Formation (Upper Jurassic to Upper Cretaceous) of the southern Coast Ranges of California. The peridotite is bounded on the east by a vertical fault in the Nacimiento fault zone that brings sedimentary rocks of Taliaferro's (1943b) Asuncion Group (Upper Cretaceous) into contact with the peridotite. The peridotite appears to be one of a number of tectonic lenses, having a wide range in size, that make up the mélange. These lenses include metagraywacke, metachert, greenstone, amphibolite, and blueschist, as well as ultramafic rocks, and represent a wide range of pressure-temperature environments.

The outer shell of the peridotite is a sheared serpentinite zone 10–15 m thick. The peridotite was tectonically emplaced at its present level as a cold solid mass and had little effect on the mineral assemblages of the Franciscan Formation. Local development of lawsonite and aragonite in shear zones may be related to the peridotite emplacement.

Foliated harzburgite forms approximately 60 per cent of the peridotite. It is a lithologically uniform rock that has an olivine: orthopyroxene ratio of approximately 75:25. Accessory clinopyroxene and chromian spinel generally make up less than 5 per cent of the harzburgite. Dunite, composed of olivine, accessory chromian spinel (< 5 per cent), and trace amounts of pyroxene, makes up approximately 40 per cent of the peridotite and occurs as dikes, sills, and irregular bodies in the harzburgite.

Olivine and pyroxene show small but significant compositional variations and chromian spinel shows a large range in the cation ratio Cr/(Cr+Al+ Fe³⁺). The compositional variations in these minerals are related to original differences in bulk chemical composition. The following compositional ranges were determined for minerals in the harzburgite: olivine, Fo_91.1−Fo_91.4; orthopyroxene, En_89.8−En_91.1; clinopyroxene, Ca_47.0Mg_50.0Fe_3.0−Ca_48.7Mg_48.2Fe_3.1; chromian spinel, Cr/(Cr+Al+Fe³⁺) 0.37−0.55. The pyroxenes have a range in A1₂O₃ content of 1.3−3.0 wt per cent. Olivine from dunite ranges from Fo₉₁ to Fo_{92 7} and the chromian spinel has a range in the Cr/(Cr+Al+Fe³⁺) ratio of 0.30−0.75. Although all the dunites are lithologically similar, three distinct types are recognized on the basis of composition of coexisting olivine and chromian spinel. Structural relations between the three types of dunite suggest three periods of emplacement (possibly overlapping) of dunite into harzburgite. The evidence indicates that the dunite, and probably also the harzburgite crystallized from an ultramafic magma, probably in the upper mantle.

After the magmatic episode and crystallization, the peridotite was subjected to a deep-seated plastic deformation and recrystallization. The first phase of the deformation produced a pervasive, planar structural element (S₁) that crosscuts many harzburgite-dunite contacts. It is probable that some of the dunite sills were emplaced during this deformation. The foliation, S₁, is defined by layers of different orthopyroxene content in harzburgite, and by discontinuous layers of chromian spinel in dunite. Flow or slip along S₁ produced slip folds in harzburgite—dunite contacts with axial planes parallel to S₁. At a later stage, isoclinal folds developed in S₁, and the present olivine microfabric was probably formed by recrystallization in the stress field that produced the isoclinal folding. In the olivine microfabric, X tends to be perpendicular to the axial planes (S₂) of the isoclinal folds and Y and Z tend to form double maxima in S₂ approximately 90° apart. Mg−Fe²⁺ distribution between coexisting mineral pairs yields a calculated temperature of formation of approximately 1200 °C. Although this temperature is only a nominal value, it indicates that the mineral pairs equilibrated at a significantly high temperature. In view of the deformation and recrystallization, the calculated temperature possibly represents subsolidus re-equilibration of the minerals during this event. The deformation and recrystallization probably occurred shortly after crystallization while the peridotite was still at a high temperature.

A later deep-seated deformation produced small scattered kink folds in S₁ that tend to disrupt the major olivine microfabric. The kink folding was accompanied or followed by the development of kink bands in olivine that reflect intragranular gliding on the system T = [Okl], t = [100]. The kink bands probably formed at a minimum temperature of 1000 °C.

Following the deep-seated deformation, which probably took place in the mantle, the peridotite mass was tectonically detached and moved upward to its present level in the crust. Cleavages, joints, and faults provided channels for water to pervade the peridotite and allow alteration of the primary minerals.