SHALLOW MESOZOIC LAYERED GABBROS OF THE SHADOW MOUNTAINS, SAN-BERNARDINO COUNTY, CALIFORNIA

Jurassic hornblende gabbros intrude as a conspicuous 9 km(2) arcuate c omplex within Paleozoic metasedimentary pendants of the Shadow Mountai ns, San Bernardino County, California. This complex is significant bec ause it: (1) displays evidence of processes active in magma chambers w hich are crystallizing cumulus phases; (2) resembles many mid-Mesozoic plutons described in the western and central Sierra Nevada and wester n Mojave desert which may all be petrologically related; and (3) may b e used as a diagnostic tectonic marker for Mesozoic tectonic reconstru ctions of the Mojave Desert and western Cordillera. The gabbroic body is unusual for its compact, near-circular plan, conspicuous banding an d layering and reverse geochemical zonation. The mafic complex was int ruded as a hydrous magma and was emplaced as a concentric epizonal plu ton. Oxygen isotopic data indicate the melt was mantle derived but has been contaminated by assimilation of metasediments. The gabbro was Li kely generated by reaction of olivine with fractionated melt from the melting of a low MgO basalt under water-saturated conditions. The band ing and layering is ascribed to the cyclic ascent of felsic rejected s olute along the walls of the chamber and its accumulation near the roo f of the complex. Cooling was facilitated by conduction and the presen ce of pendant rocks at uppermost levels. The Shadow 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 ar e useful as markers of subsequent large-scale tectonic crustal displac ements affecting the western Mojave Desert and Sierra Nevada. In parti cular, the Shadow Mountains gabbros show age, mineralogic and textural affinities with correlative gabbroic complexes in the central Sierra Nevada 400 km to the north. All these plutonic bodies may represent su bvolcanic sources of the Jurassic-Cretaceous Independence dike swarm i n the western Mojave Desert and eastern Sierra Nevada.