Identification of magma compositions using trace element partitioning in augite from the chetco complex, josephine ophiolite and grayback pluton located in the Klamath Mountains, Oregon and the implications for tectonic setting
Weiss, Rachel B.
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The Jurassic Chetco complex, Josephine ophiolite, and Grayback pluton, all located in the Klamath Mountains, Oregon were interpreted as an arc-backarc-remnant arc assemblage (Harper & Wright, 1984). The Klamath Mountain province is a well-exposed and well-preserved region in which to study subduction-related crust-forming processes, an important mechanism by which new continental crust is generated (Snoke and Barnes, 2006; Condie, 1989; Gill, 1981). Ages of the Chetco complex (155-160Ma; Yule, 1996), Josephine ophiolite (162-165Ma; Harper et al. 1994), and Grayback pluton (157 to 160Ma; Hotz, 1971; Barnes, et al., 1992; Allen and Barnes, 2006) indicate that these magmatic units were coeval and were separated by the intra-arc or back-arc Josephine ophiolite above a single west-facing subducting plate (Harper & Wright, 1984). Many of the rocks from the two complexes are cumulates, making it difficult to determine whether the distinct chemical trends are due to accumulation or to differences in parental magma compositions (e.g., McBirney and Hunter, 1995). Petrographic observations and geochemistry indicate that augite is an early crystallizing phase in all three units. Because augite can incorporate a number of trace elements into its crystal lattice (Blundy & Wood, 2003; Claeson et. al 2007; Cherniak and Liang, 2012; Coint, 2012), it is possible to characterize melt compositions using published partition coefficients and major and trace element data. Rare earth element patterns of calculated melts in equilibrium with augite show three distinct patterns. The Chetco complex has low-K tholeiitic to MORB patterns, the Josephine ophiolite has MORB to E-MORB patterns, and the Grayback pluton has calc-alkaline patterns. These patterns are similar to magma compositions erupted in the volcanic arc, back arc basin, and remnant arc of the modern Tonga-Kermadec Ridge-Lau Basin-Lau-Colville Ridge suprasubduction zone complex. The bulk rock geochemistry of the Chetco-Josephine-Grayback samples in terms of Hf:Th:Ta, Ba/La, La/Nb and Ba and Ta ratios are consistent with emplacement in a suprasubduction zone setting and show similarities to the Tonga-Kermadec Ridge-Lau Basin-Lau-Colville Ridge. Zoning of compatible and incompatible trace elements in augite indicates that the Chetco complex melts evolved by fractional crystallization. The Rogue Formation, previously thought to be the volcanic equivalent of the plutonic Chetco complex, is not related to the Chetco complex based on the differences in compatible element contents in augite; however incompatible element concentrations suggest a common mantle source for these two units. Zoning in some high-Mg# augite from the Grayback pluton suggests possible recharge and mixing of primitive magma during fractional crystallization.