GEOCHEMISTRY OF PLIOCENE AGE BASALT IN THE BASIN-AND-RANGE/CASCADE TRANSITION ZONE: SUGARPINE MOUNTAIN QUADRANGLE, KLAMATH COUNTY, OREGON

Lower Pliocene to lower Pleistocene basaltic lava flows are separated into five units on the basis of geochemistry, texture, mineralogy, and field relations. The oldest (late Miocene?) are basalt to basaltic andesite flows that underlie Sugarpine Mountain, a 425 m composite volcano (SiO2=50.16-52.24 wt %; K2O=0.61-0.74 wt %; TiO2=1.199-1.403 wt %, Ba=325-411 ppm; Sr=532-720 ppm, Zr=77-98 ppm). South and east of the volcano three flow units of olivine basalt form a broad low-relief platform. 1) Weakly to moderately diktytaxitic olivine phryic basalt with olivine phenocrysts (1 mm) are dated at 4.94±0.68 Ma. K2O=0.47 to 0.51 wt % (n=2); TiO2=1.841 to1.876 wt %, Mg# = 54; Ba=222 to 232 ppm; Sr=407 to 412 ppm, Zr=129-131 ppm. Chrondrite normalized La:Lu = 2.5, the lowest ratio for the basalt flows. 2) Phyric olivine basalt with abundant plagioclase (5 mm) and common olivine phenocrysts (1 mm) are distinctly diktytaxitic. K2O=0.65 wt % (n=1); TiO2=1.590 wt %, Mg# = 53; Ba=402 ppm; Sr=607 ppm, Zr=118 ppm. Chondrite normalized La:Lu = 4.06. 3) Phyric olivine basalt is ophitic and aphyric olivine basalt contains interstitial olivine. Principal component analysis indicates Ba and Sr (P1) accounts for 85% of the variation and Ba, Sr, and Cr (P2) account for 11%. K2O=0.44 to 0.61 wt % (n=5); TiO2=1.338 to 1.507 wt %, Mg# = 53 to 61; Ba=296 to 308 ppm; Sr=388 to 541 ppm, Zr=90 to 107 ppm. Chondrite normalized La:Lu ranges from 3.0 to 3.4. The Jack Creek valley was cut into these flows starting approximately 3.5 Ma. This valley is partially infilled by olivine phyric basalt flows erupted from vents located east of the quadrangle. These flows have K2O=0.40 to 0.43 wt % (n=2); TiO2=1.241 to 1.338 wt %, Mg# = 54 to 57; Ba=292 to 334 ppm; Sr=540 to 544 ppm, Zr=86 to 93 ppm. Chondrite normalized La:Lu = 3.1.

Geochemistry of the four basalt units shows little variation over approximately 2 million years. Subtle variations exist between flows erupted at approximately 5 Ma but the overall impression is one of relatively low volume batches that have experienced similar magmatic processes.