The Los Pines gabbro is located about 60 km east of San Diego, California and has a K-Ar cooling age of 143 Ma with Ar-Ar integrated ages of 115 Ma and 111 Ma. Field relationships indicate that the gabbros have been intruded by younger rocks of tonalitic composition with ages ranging from 108 Ma to 102 Ma. A geochemical modeling analysis was performed to determine if the tonalites could have been produced by partial melting of a gabbroic source rock similar to Los Pines gabbro. The gabbroic source rock was modeled in a spreadsheet program, using the batch melting equation, to see if it produce rare earth element (REE) patterns within the tonalite compositional range. The parameters established by the REE modeling were tested by modeling other trace elements (i.e., Rb, Sr, Ni, Cr, Ba, Zr, Y) to see if their content fell within one standard deviation (1 cr) of the tonalite trace element content. Modeling showed that it is possible to produce a tonalitic composition by partially melting 5% to 30% of the gabbroic source rock. The trace element model results agreed with those of the REE models suggesting that the tonalites near Los Pines Mountain could have been produced by partial melting of a source rock similar to the Los Pines gabbro.
