USE OF AUGITE TO CHARACTERIZE TRACE ELEMENT CONCENTRATIONS IN CONTEMPORANEOUS JURASSIC ARC AND REMNANT ARC MAGMAS: THE CHETCO COMPLEX AND GRAYBACK PLUTON, KLAMATH MOUNTAINS, OREGON

The Jurassic Chetco complex, Josephine ophiolite and Grayback pluton were interpreted as an arc-backarc-remnant arc assemblage. U-Pb (zircon) ages of the Chetco complex and Grayback pluton (155-160Ma & 157±2Ma, respectively; Yule, 1996, Ph.D thesis, Cal. Tech.) indicate that these arc and remnant arc plutons were coeval and were separated by the intra-arc or back-arc Josephine ophiolite (Harper & Wright, 1984, Tectonics 3, 759). Trace element compositions of augite from the Chetco complex and Grayback pluton were determined by LA-ICPMS methods. Augite from the most primitive Grayback sample contains higher Mg# (85-77), Cr (6515-960ppm) and lower Zr (13-1ppm) concentrations, more evolved samples have lower Mg# (77-71) and Cr (743-49ppm) concentrations, and generally higher Zr (112-13ppm) abundances. Augite from the Chetco decreases in Cr content (601 to below dl) with decreasing Mg # (82–68) and contains lower concentrations of Cr than the Grayback. Sc concentrations are higher for some Chetco augites (455-138ppm), these concentrations overlap Grayback augite Sc concentrations (248-34pmm). The two intrusions have similar ranges of Ti and Zr; however, in the Grayback Ti and Zr are positively correlated, whereas they are negatively correlated in Chetco augites. Total REE concentrations are similar in the intrusions, with the exception of two Chetco samples containing higher total REE. The REE patterns of Grayback augites have negative slopes and are straight to slightly concave up, whereas Chetco augites have concave-down patterns. The more evolved Grayback and Chetco samples show negative Eu anomalies, whereas they are weak or absent in primitive samples. Some augites from the most primitive Grayback sample have cores with as much as 4-5x higher total REE and lower Ti contents relative than the rims. This zoning is probably related to magma mixing. The trace element compositions of melts in equilibrium with augite were calculated using partition coefficients from Fujikami et al. (1984, JGR 89, B662). Calculated liquids for both intrusions have REE patterns with negative slopes. Although the data have not yielded diagnostic information about tectonic setting, they do indicate that parental magmas to the Chetco complex and the Grayback pluton were distinct in Cr contents and differentiation processes.