INCREMENTAL GROWTH OF GRANITIC MAGMA BODIES: THE GUADALUPE IGNEOUS COMPLEX, SIERRA NEVADA BATHOLITH, CALIFORNIA

The 153 Ma Guadalupe Igneous Complex (GIC) is a remarkably bi-modal pluton that represents the first of a series of Late Jurassic-Early Cretaceous plutons that record the birth of the Sierra Nevada Batholith. Due to ~30^o of tilting, the GIC provides rare exposures of the mafic roots of the system (Heaussler and Paterson, 1992). Targeted, high-density sampling and new isotope data from the GIC provide clues regarding granite magma genesis and pluton assembly. For example, lower crust partial melting can be rejected because unaltered samples from the GIC fall on a single Rb-Sr isochron with an age of 152±7 Ma, matching single crystal zircon ages (Ernst et al., 2009) within error; this isochron cannot be generated by partial melting of the Mid-Jurassic and earlier metabasalts of the region. In addition, alumina saturation indices of the upper crust Mariposa Formation (the sedimentary framework into which the GIC was intruded) are far too high to explain GIC compositions. Compositional relationships and mass balance calculations, however, support Best’s (1963) idea of in situ differentiation, and especially the differentiation mechanism of Bachmann and Bergantz (2004), which explains the GIC’s bi-modality. In the latter model, crystal settling is hindered as interstitial liquids evolve to high SiO₂ compositions, and melt is segregated from its crystalline residue only when melt fractions approach 50%. Our evidence indicates, however, that granite magma chambers grow by small increments of such differentiation events: the, lower mafic part of the system is emplaced as a sequence of sills that differentiate and feed felsic magmas upward into an overlying, convective felsic magma cap. Pluton growth and convection in the felsic cap ends when basaltic inputs cease. We posit that the GIC preserves its bi-modality in part due to its smaller size and shorter lifespan, compared to larger, well-mixed systems, such as the Bass Lake Tonalite and Tuolumne Intrusive Complex.