Reconstructing the evolution of the Ironside Mountain batholith, northern California through textural and single-mineral compositional analyses of pyroxene, amphibole, and plagioclase

The ~170-168 Ma Ironside Mountain batholith in northern California encompasses the Ironside Mountain pluton, quartz diorite of Happy Camp Mountain, Wildwood pluton, West China Peak complex, and Denny complex. The Ironside Mountain batholith was emplaced within ca. two m.y. after regional thrusting attributed to the Siskiyou orogeny. Single-mineral compositions in clinopyroxene, hornblende, and plagioclase were measured in-situ by electron-probe micro-analyses and laser ablation inductively coupled plasma mass spectrometry and coupled with bulk rock X Ray Fluorescence analyses. Mineral data revealed two compositionally diverse magmas within the Ironside Mountain batholith: A) those of the Ironside Mountain pluton, quartz diorite of Happy Camp Mountain, and Wildwood pluton, which are characterized by two- and three-pyroxene assemblages + ilmenite +magnetite, scant amphibole and biotite, low Mg number, low Cr concentrations, and rare earth element abundances in clinopyroxene and amphibole to 300 times chondrites with deep negative europium anomalies; and B) those of the Denny and West China peak complexes, which are characterized by 2-pyroxene assemblages, have rare earth element abundances in clinopyroxene and amphibole to 50 and 100 times chondrites, respectively, with small to absent europium anomalies. Europium anomalies in clinopyroxene and amphibole of group A suggest crystallization from plagioclase-fractionated melts. Early plagioclase crystallization indicates magmas with low f(H2O). Small to absent europium anomalies in clinopyroxene and hornblende of group B suggests these minerals were liquidus phases without plagioclase, which indicates higher f(H2O). Lower Cr abundances and Mg numbers in clinopyroxene and amphibole from group A suggests their magmas were evolved. Rimming relationships between clinopyroxene, orthopyroxene, and inverted pigeonite, and the absence of clear fractionation trends in major and trace element data of pyroxene and amphibole indicates magmas of the Ironside Mountain pluton did not evolve as a single closed system. Iron-Mg exchange coefficients higher than equilibrium values in amphibole, clinopyroxene, and orthopyroxene; Ab-An exchange coefficients higher than equilibrium values in plagioclase; and the fact that melts calculated from augite compositions display deep negative europium anomalies, but such anomalies are absent in bulk rock rare earth element patterns, indicates partial accumulation of Ca-rich plagioclase, orthopyroxene, and clinopyroxene occurred. Clinopyroxene and amphibole compositions from the Ironside Mountain pluton and coeval Wildwood pluton are distinct from those of the Western Hayfork arc. Barnes and Barnes (2020) showed the predominant magma types erupted from the Western Hayfork arc were calc-alkaline or high-Mg andesites and dacites, the latter of which experienced fractionation of garnet at depth. The lack of negative europium anomalies in Western Hayfork augite and hornblende suggests plagioclase was absent early in the crystallization history, which is further evidence of H2O-rich and oxidized magmas (Muntener and others, 2001). Along with the geologic relationships reported by Barnes and others (2006), the compositional distinctions between Western Hayfork arc and Ironside Mountain pluton parental magmas refute the hypothesis that the Ironside Mountain pluton is the intrusive equivalent of the Western Hayfork arc.

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