AUGITE AND AMPHIBOLE COMPOSITIONS REVEAL TWO MAGMA REGIMES: THE IRONSIDE MOUNTAIN PLUTON AND WESTERN HAYFORK ARC, NORTHERN CA

Past workers (Lanphere et al., 1968, Fratieclli et al., 2012, Mankinen et al., 2013) hypothesized the ca. 170 Ma Ironside Mountain pluton (IMP) to be the intrusive equivalent of the Western Hayfork (WH) arc. However, Barnes et al (2006) showed that the IMP intrudes the Wilson Point thrust fault, which juxtaposes WH rocks under the Eastern Hayfork mélange during the Siskiyou Orogeny, thus indicating that the IMP postdated the Western Hayfork arc. In this study, mineral assemblages and compositions, and calculated melt compositions of the IMP are compared to those of the WH to further distinguish between the two magmatic events.

The IMP is characterized by two- and three-pyroxene assemblages + ilmenite +magnetite, assemblages extremely uncommon in the province. Moreover, amphibole and biotite are scant in the most mafic IMP rocks. Where present, they are late-stage interstitial phases. The pyroxene + oxide assemblage and paucity of hydrous phases, indicate IMP parental magmas had low fO₂ and fH₂O. In contrast, Barnes & Barnes (2020) showed that the WH arc was dominated by two magma types, one calc-alkaline and one adakitic. Both types were H₂O-rich and relatively oxidized.

Augite and amphibole were analyzed from samples collected along the 70-km length of the IMP and from satellite bodies, including the coeval Wildwood pluton (WP). The data for IMP and WP reveal (1) large negative Eu anomalies and low Sr/Y ratios in augite, amphibole, and their calculated melt compositions and (2) low Mg# and Cr concentrations. These data are consistent with early plagioclase fractionation and significant pyroxene ± chromite fractionation prior to IMP and WP emplacement. All of these features are distinct from augite and amphibole from WH arc samples

Magmatism of the WH arc was shut down by the Siskiyou Orogeny, which imbricated the WH arc rocks under the Eastern Hayfork terrane. Shortly after thrusting, the IMP and satellite plutons intruded into the thickened, probably cool crust. The distinctive petrochemical features of IMP magmas are probably the result of extensive fractional crystallization in the lower crust prior to emplacement.

This work was partially supported by NSF/GSA Graduate Student Geoscience Grant # 12714-20, which is funded by NSF Award # 1949901