Shallow Mesozoic layered gabbros of the Shadow Mountains, San Bernardino County, California

Jurassic hornblende gabbros intrude as a conspicuous 9 km2 arcuate complex within Paleozoic metasedimentary pendants of the Shadow Mountains, San Bernardino County, California. This complex is significant because it: (1) displays evidence of processes active in magma chambers which are crystallizing cumulus phases; (2) resembles many mid-Mesozoic plutons described in the western and central Sierra Nevada and western Mojave desert which may all be petrologically related; and (3) may be used as a diagnostic tectonic marker for Mesozoic tectonic reconstructions of the Mojave Desert and western Cordillera. The gabbroic body is unusual for its compact, near-circular plan, conspicuous banding and layering and reverse geochemical zonation. fic complex was intruded as a hydrous magma and was emplaced as a concentric epizonal pluton. Oxygen isotopic data indicate the melt was mantle derived but has been contaminated by assimilation of metasediments. The gabbro was likely generated by reaction of olivine with fractionated melt from the melting of a low MgO basalt under water-saturated conditions. The banding and layering is ascribed to the cyclic ascent of felsic rejected solute along the walls of the chamber and its accumulation near the roof of the complex. Cooling was facilitated by conduction and the presence of pendant rocks at uppermost levels. adow Mountains gabbros resemble other gabbroic bodies of similar Jurassic age throughout the Sierra Nevada, Mojave Desert and Transverse Ranges, California (Lahren and Schweickert, 1994; Miller and Glazner, 1995). These intrusions are useful as markers of subsequent large-scale tectonic crustal displacements affecting the western Mojave Desert and Sierra Nevada. In particular, the Shadow Mountains gabbros show age, mineralogie and textural affinities with correlative gabbroic complexes in the central Sierra Nevada 400 km to the north. All these plutonic bodies may represent subvolcanic sources of the Jurassic-Cretaceous Independence dike swarm in the western Mojave Desert and eastern Sierra Nevada.