Blair Valley is located approximately 120 km (75 mi) east northeast of San Diego, and is in the eastern zone of Peninsular Ranges Batholith (PRB). The pegmatite dikes at this location cut across gneissic granitic rocks with a ziron U-Pb age of about 94 ma. Other pegmatite dikes in the PRB range between 94 ma to 100 ma. The dikes show a random distribution within the study area. The focus of this study is to determine the nature of the source rocks that were melted to create the pegmatite dikes at Blair Valley. The dikes consist of quartz, potassium feldspar, plagioclase, biotite, muscovite,garnet,myrmekite, and are mainly syenogranite. Trace minerals include chlorite, allanite, and zircon. Rare earth element (REE) analysis of fine-grained portions of the pegmatite dikes shows either a steep slope with light REE depletion or a horizontal pattern with strong negative Eu anomaly, and in comparison to the data of Parrish (M.S.1990) and unpublished data from Tule Mountain near Jacumba, suggests that two melts were involved. The source region appears to have been a metagraywacke consisting mainly of feldspar, quartz, biotite and muscovite. The reaction muscovite + plag + Q ---> melt + sill + Kfs left a bte-fs-sill-Q residue and melt with a strong(-) Eu anomaly. A second reaction bte + sill + plag + Q ---> melt + garnet + Kfs used up the remaining feldspar to produce dikes and small bodies of granite with a garnet signature.