Mineralogy of the La Posta pluton: Implications for the origin of zoned plutons in the eastern Peninsular Ranges batholith, southern and Baja California

The 94 Ma La. Posta pluton is located in the eastern zone of the Peninsular Ranges batholith and is exposed over an area of approximately 1400 km2. It is zoned from a sphene-hornblende-biotite tonalite rim inward to a core of muscovite-biotite granodiorite with internal contacts gradational over distances of several tens to hundreds of meters. This pluton is texturally distinct from the older synkinematic plutons of the western zone of the batholith in that it is undeformed and contains sharply euhedral mafic minerals. Microprobe analyses indicate that hornblendes from the eastern side of the pluton have higher AIT and lower Si than those from the western side. This variation in composition yields geologically unreasonable differences in hornblende geobarometry and reflects hornblende equilibratipn under different thermochemical conditions on either side of the pluton. Compositional variations within individual mineral suites are consistent with a general model involving inward crystallization of a static magma chamber. However, biotites in the interior facies are more oxidized and have abruptly higher Fe/(Fe + Mg) than those in the marginal fabes. These data suggest that the muscovite-bearing core facies represents the tail of a diapir that intruded a ballooned magma chamber prior to its complete solidification. A longer residence time for the tail adjacent to a cratonal source resulted in its enrichment in, at least, Fe3+ and Fe/(Fe + Mg).