Chapter 1: Origin and evolution of the zoned La Posta-type plutons, eastern Peninsular Ranges batholith, southern and Baja California

The Peninsular Ranges batholith has been subdivided into two zones based on geochemical, geophysical, and lithologic parameters. Plutons in the eastern zone (La Posta–type) are typically larger and inwardly zoned from hornblende-bearing tonalite margins to muscovite-bearing monzogranite cores. U-Pb ages on zircon are generally in the 100 to 90 Ma range. They tend to be more discordant in the cores of the plutons and have upper concordia intercepts near 1300 Ma. Rb-Sr systematics on mineral separates yield an Sri range from 0.7030 to 0.7044, although one pluton is reported to have a rim-to-core variation from 0.7043 to 0.7074. Whole-rock δ18O is lowest in the hornblende-bearing facies (8.3 to 10.9 per mil) and highest in the muscovite-bearing facies (10.2 to 11.8 per mil); the level of variation is pluton dependent. δ18O for quartz separates indicate an eastward-directed asymmetry toward heavier oxygen rather than the facies control observed in the whole-rock data. REE patterns from two plutons have nearly identical LREE enrichment and lack any Eu anomaly. Associated with the La Posta–type plutons are a series of small, compositionally restricted, garnet-bearing monzogranites. They are 1 to 5 m.y. younger than the surrounding La Posta–type plutons and contain zircons inherited from a 1200- to 1300-Ma source. Whole-rock δ18O values between 12.5 and 13.2 per mil and Sri = 0.706 reflect a continental contribution to these magmas. La Posta–type melts were generated by subduction-related anatexis of amphibolite-or eclogite-grade oceanic crust. The relatively short emplacement interval and large size of the plutons suggest rapid separation of large volumes of melt from the source region under elevated PH2O. Rise toward the present erosional level occurred along the juncture between oceanic lithosphere and the older (ca. 1300 Ma) continental margin. Interaction with the continental crust produced the present-day eastward bias toward higher δ18O and zircon discordance.