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 Al, and lower Si than those from the western side. This variation in composition yields geologically unreasonable differences in hornblende geobarometry and reflects hornblende equilibration 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 Fel(Fe + Mg) than those in the marginal facies. 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). INrnooucrroN The eastern side of the Peninsular Ranges batholith is dominated by a series of large, concentrically zoned plutons of intermediate composition that exhibit distinct similarities in age, rock type, and geochemistry (Walawender et al., 1989). The largest of these, the 94 Ma La Posta pluton, is located approximately 65 km east of San Diego, California, and straddles the international border (Fig. l). Although the northern portion of this pluton and the section south of the international border have been mapped only in reconnaissance fashion, the approximately 750 km'zthat have been studied in detail (Kimzey, 1982; Sinfield, 1983; Clinkenbeard, 1987) reveal a more or less single magmatic pulse that crystallized inward to form a lithologic succession ranging from hornblendebiotite tonalite to muscovite-biotite granodiorite. Textural evidence indicates that this pluton experienced little subsolidus deformation or alteration and thus provides an excellent opportunity to examine compositional relationships between coexisting silicate minerals that have retained their primary magmatic character. This paper describes these compositional relationships and interprets them with respect to various parameters that affect cooling magmatic systems.