THE IRONSIDE MOUNTAIN BATHOLITH, KLAMATH MOUNTAINS PROVINCE: TWO DISTINCT MAGMAS RECORDED IN CLINOPYROXENE, HORNBLENDE, AND PLAGIOCLASE COMPOSITIONS

The ~170-168 Ma Ironside Mtn batholith (IMB) in northern CA encompasses the Ironside Mtn, Happy Camp Mtn, Wildwood, West China Pk and Denny plutons. The IMB was emplaced within ca. two m.y. after regional thrusting (Siskiyou orogeny). Single-mineral compositions in clinopyroxene (cpx), hornblende (hbl), and plagioclase (plag) were measured in-situ by laser ablation inductively coupled plasma mass spectrometry and coupled with bulk rock analyses. They revealed three compositionally diverse groups within the IMB: 1) the three-pyroxene Ironside Mtn and Wildwood plutons, which follow a bulk-rock Fe enrichment trend, lack enclaves and magmatic structures, and have total REE abundances to 200 and 1000 times chondrites in cpx and hbl, respectively, both with negative Eu anomalies; 2) The Happy Camp Mtn pluton, which is a 2-pyroxene quartz diorite that follows a bulk-rock calc alkaline trend, has abundant enclaves, and REE abundances in cpx and hbl to 300 times chondrites with negative Eu anomalies greater than group 1; and 3) the Denny and West China Pk complexes, which have 2-pyroxene assemblages, follow bulk-rock Fe-enrichment trends, and have REE abundances in cpx and hbl to 50 and 100 times chondrites, respectively, with small to absent Eu anomalies. Eu anomalies in cpx and hbl of groups 1 and 2 suggest crystallization from plag-fractionated melts. Early plag crystallization indicates magmas with low f(H₂O). Small to absent Eu anomalies in cpx and hbl of group 3 suggests cpx and hbl were liquidus phases without plag, which indicates higher f(H₂O).

Detailed study of the Ironside Mtn pluton indicates calculated mineral-bulk rock ^Fe/MgKd values of 0.53 - 0.76 and 0.88 - 1.3 for cpx and hbl, respectively, which suggests these phases were in equilibrium with melts lower in Mg than the bulk rocks. Cr concentrations in cpx are mostly below 42 ppm. Mg # in cpx and An content in plag average 63 and 47, respectively. Low Cr concentrations, Mg # s, and An contents indicate these minerals crystallized in evolved melts. Thus, the mafic bulk rock compositions are interpreted to result from the accumulation of magnesian phases (augite ± opx ± pigeonite). Moreover, the plagioclase must be a cumulate phase because Eu anomalies are absent in the bulk rock, yet calculated melts from cpx have large negative Eu anomalies.