TILTED PLUTONS AND MAGMATIC PLUMBING

In an attempt to understand the behavior of tonalitic magmas as they traverse the crustal column, we have combined petrologic data from three tilted plutons to create a 30 km structural profile. We utilize the Bell Island, Wooley Creek, and Duncan Hill plutons to construct this profile. The Bell Island pluton (Sinha et.al. 2002), in southeastern Alaska, has a recorded depth profile from ~10.5Kb to 6.5 Kb, and has been interpreted to be emplaced dominantly as a liquid across a garnet isograd. Although three mapable igneous facies are recognized, the mineralogic and geochemical data support in-situ crystallization without significant mineral fractionation or mixing. The Wooley Creek Complex (Barnes et.al., 1990), with a depth profile of ~ 6.5 to 3 Kb is represented by two compositionally diverse parts of the same pluton (upper tonalite-granite, and a lower gabbro- tonalite). Compositional diversity is modeled through crystal-melt segregation, mixing of water rich basaltic pulses, and in situ assimilation of host rocks. The Duncan Hill pluton with an emplacement depth of ~ 4 to <1 Kb, records vertical zoning through five mapable zones. From deeper to shallower sections, the zones are leucogranites, granodiorite, and heterogeneous mafic tonalite to heterogeneous appinitic rocks. The extreme lithologic heterogeneity observed at Duncan Hill has been attributed to both extensive mixing of mantle and crustal components associated with the protodiapir stage, as well as fractional crystallization and dynamic compaction (Dellinger, 1996; Hopson et.al, 1991).The progressive increase in diversity of lithologies associated with the three plutons, especially in the abundance of mafic rocks, could be either the result of local processes, or more likely the result of basaltic fluxing of anatectic magmas to thermally drive the magma bodies to higher structural levels. Our model suggests that the mineralogic, geochemical and isotopic properties of high level magma bodies with significant basaltic (heat) fluxing, which facilitates to keep the system mobile and chemically reactive, is likely to result in extensive mixing during both transport and emplacement. Such plutons are unlikely to yield a clean petrologic signal regarding the primary source of the magma.