Geology, Petrology, and Geochemistry of the Viejas Mountain Gabbro Pluton San Diego County, California

Thesis, (M.S.) Geological Sciences.\ud \ud Coordinates were derived from base map, Plate 1, at 1:17,000 and the USGS Viejas Mountain 7.5 minute topographic quadrangle.The Viejas Mountain Gabbro pluton is located about 2 km northeast of Alpine, San Diego County, California, and is one in a series of gabbroic plutons in the Peninsular Ranges batholith. The rocks of the pluton are divided into two groups. Group 1 consists of troctolite, plagioclase porphyry, and two-pyroxene olivine gabbronorite and Group 2 consists of two units of leuconorite plus hornblende gabbro. Group 1 has plagioclase and olivine as cumulus minerals, whereas Group 2 has plagioclase and orthopyroxene as the cumulus phases. Clinopyroxene is a cumulus phase in Group 2 and rarely in Group 1. The rocks display mineral layering, comb structures, and local flow banding. Overall, the pluton has a crude basinal shape with all of the rocks dipping inward. Geochemical analyses show that a process of fractional crystallization from a cogenetic parent melt produced the two groups of gabbro. Group 1 is less fractionated than Group 2 and has correspondingly lower values for the Iron Index (??Fe203/??Fe203 + MgO) and lower values for total alkalis. Each group displays a general iron enrichment trend. The petrographic and geochemical information suggests the rocks had a parent magma of hydrous high-alumina basalt that began crystallizing at greater than 5kb pressure and was emplaced in a series of pulses to produce the relationships between Group 1 and Group 2 at their present erosional level