PETROLOGY AND GEOCHEMSTRY OF PILLOW BASALTS IN THE EASTERN ELK OUTLIER OF SOUTHWESTERN OREGON

Recent work in the western Elk outlier (WEO) of the western Klamath terrane near Port Orford, Oregon revealed a sheeted dike complex having island arc tholeiite (IAT) affinities tentatively correlated with the Josephine ophiolite (JO) as well as MORB pillows as blocks in mélange. New mapping in the eastern EO, ~ 20 km east of the sheeted dikes, reveals numerous pillow localities separated by sheared serpentinite. We examined in detail pillows from two quarries. The northern quarry is located ~ 11 km south of Powers, Oregon, and is 117,400 m³ in volume. Pillows range from 17- 23 cm in diameter. Sample 65A has vitrophyric texture in the core and altered glassy rinds at the rim. Glass abundance ranges from 100% in a 1 mm rind to 20% in the core. Microphenocyrsts sit in a very fine to altered glassy matrix. Amygdules with an average size of ~ 0.2 mm compose < 2% of the sample. Minerals include primary sector-zoned augite and plagioclase (altered to albite) and secondary quartz and fine grained sphene. Chorite-pumpellyite-calcite veins cut the pillow. The southern quarry lies 10 km south of the first and is 91,324 m³ in volume. Pillows range in diameter from 4 cm to 1 m, have glassy rinds, and are intruded by a sheeted dike complex. Sample 28M ranges in grain size from 3 mm (core) to 1 mm (near the rind) and has sub-ophitic to intergranular texture. Vesicles are rare to absent. Primary igneous phases include augite and plagioclase (altered to albite). Additional secondary minerals include chlorite, epidote, pumpellyite, sphene, and zeolite(?).

XRF analyses of sample 65A (northern), and 28M (southern) were performed by Washington State University. On a Cr-Y diagram, 28M (72 ppm Cr, 48 ppm Y) plots in the IAT field and is consistent with greater fractionation from the same parent that produced two of the dikes that cut the pillows. Sample 28M plots near the most evolved sheeted dikes from the WEO, but is distinct from the WEO pillows which plot in the MORB field. 65A (27 ppm Cr, 30 ppm Y) plots in the MORB field on a Cr-Y diagram, lies within a field defined by pillows from the WEO, but can't be related to 28M by fractionation. Ti-V ratios for both 28M (28) and 65A (36) are MORB-like, suggesting 28M is transitional, likely a back-arc basin basalt. The high FeO* (10.2 wt %) and TiO₂ (2.1 wt%) in 65A is similar to some pillows in the western EO and the upper pillow sequence of the JO.