GEOCHEMISTRY AND PETROGRAPHY OF THE ASHLAND PLUTON, KLAMATH MOUNTAINS, NORTHERN CALIFORNIA AND OREGON

The Ashland pluton is a Middle to Late Jurassic plutonic complex intruding rocks of the western Paleozoic and Triassic belt. The pluton consists of an early gabbroic stage, an intermediate (~161 Ma) stage comprised of tonalite and quartz monzodiorite suites, and younger (~152-156 Ma) granite that is intruded by dioritic dikes. Abundant mafic enclaves are present within the quartz monzodiorites.

Geochemical data including bulk-rock compositional analysis and hornblende and titanite mineral analysis from new samples builds upon existing data presented by Gribble et al (1990). Analysis of mineral compositional trends and zoning patterns was guided by backscatter electron imaging.

Bulk-rock major element analyses define broadly linear trends when plotted against silica with the exception of P₂O₅, Na₂O, and Al₂O₃, which are characterized by significant changes in slope. Mafic enclaves do not plot within the overall major or trace element compositional arrays. The sample suite is magnesian and calcic to calc-alkalic and generally metaluminous; only the most evolved samples are peraluminous. Chondrite-normalized bulk-rock REE patterns have negative slopes and total REE abundances fluctuate from high to low as the pluton becomes more evolved with the exception of the mafic enclaves which are anomalously high.

Zoning patterns in titanite include well-defined sector zoning, fine-scale oscillatory zoning, complex zoning patterns attributed to resorption/ recrystallization and/or modification by melt inclusions. Hornblende is primarily unzoned with sparse low-z rims. Chondrite normalized REE patterns in hornblende and titanite are bimodal. The first mode is defined by an overall decreasing trend from core to rim. The second mode is defined by fluctuation in total REE abundances from core to rim, sometimes accompanied by anomalously enriched rims. Calculated temperatures and pressures in hornblende range from ~760-850 ºC (Putirka, 2016) and ~2.1-5.4 kbar (Schmidt, 1992). Hornblende Mg# is utilized to determine that hornblende is in disequilibrium with the melt phase and, therefore, may represent a cumulate phase. [Gribble et al. (1990) J. Petrology, 31, p. 883; Putirka (2016) Am. Min., 101, p. 841; Schmidt (1992) Cont. Min. Petrology, 110, p. 304]