Date of Award

5-2014

Document Type

Thesis open access

Abstract

The Sanak-Baranof plutonic belt (SBPB) includes a series of 61 – 48 Ma biotite tonalite, granodiorite, and granite near-trench plutons that intrude the Campanian to Eocene flysch of the Chugach-Prince William terrane for 2100 km along the southern Alaskan margin (Hudson et al., 1979; Hill et al., 1981; Bradley et al., 2003; Haeussler et al., 2003). The time-transgressive nature of these forearc plutons makes them central to tectonic reconstructions of the North American Cordillera through the Paleocene-Eocene (Hudson et al., 1979; Bradley et al., 2003; Haeussler et al., 2003; Cowan, 2003).

The coeval Crawfish Inlet (53 – 47 Ma) and Krestof Island (52 Ma) plutons on Baranof Island mark the eastern limit of the SBPB. The plutons are dominated by medium-grained granodiorites and tonalites, which are host to fine-grained magmatic enclaves. The enclaves exhibit a wide range in composition, from gabbrodiorite to granite. The least evolved enclaves (< 62 wt% SiO2) in the Crawfish pluton are geochemically distinct from enclaves in the Krestof pluton. With the exception of Pb, Y and the heavy rare earth elements, least evolved Crawfish enclaves are enriched in trace elements relative to the Krestof enclaves. In addition, initial Sr and Nd isotopic ratios are distinct between Crawfish and Krestof enclaves. All the isotopic analyses for both enclaves and their host granitoids lie within or near the mantle array.

Krestof enclaves plot in the field for typical mid-ocean ridge basalt (MORB) on a Sr/Y versus Y diagram, where as the least evolved Crawfish enclaves plot in fields for adakites and island arc basalts, along with Krestof and Crawfish granitoids. These adakitic characteristics (Sr/Y > 25 and La/YbCN > 5) are shared by ~50 Ma intrusive rocks occurring in the eastern SBPB and the Coast Plutonic Complex.

Assimilation-fractional crystallization (AFC) models assuming a variety of mafic parental magmas and accretionary wedge assimilants were generally unsuccessful in producing Crawfish and Krestof compositions. The most plausible model for the origin of Krestof enclaves and granitoids involves MORB parental magma assimilating isotopically evolved sediment. Isotopic compositions of all Crawfish and Krestof rocks may be explained by mixing between mantle and sedimentary endmembers.

Minor and trace element and isotopic compositions for the Krestof enclaves, which are MORB-like, and the least evolved Crawfish enclaves, which are similar to ocean island basalts, suggest distinct mantle sources and/or parental magmas. The spatial and temporal proximity of these two enclave types suggests mantle heterogeneity on a relatively local scale.

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