LAYERED MAFIC TO ULTRAMAFIC INTRUSIVES IN THE EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE: OPHIOLITIC CRUST OR SYN-NEVADAN INTRUSIVES?

The Upper Jurassic Galice Formation conformably overlies the 164-Ma Josephine Ophiolite (JO) in the Western Klamath terrane (WKT) of the Klamath Mountains. The 700 mi² Elk Outlier (EO) of the WKT lies in the Oregon coast range, 50 km northwest of the main WKT and contains widely exposed Galice Formation and a recently recognized sheeted dike complex tentatively correlated with the JO. We recently discovered two exposures of layered mafic to ultramafic intrusive rock on the southeastern (loc. A) and western (loc. B) flanks of Johnson Mountain in the eastern EO, 23 km south of Powers, Oregon. The locales are separated by 1 km and each is in apparent fault contact with sheared serpentinite. Samples from both outcrops were examined petrographically to evaluate the hypothesis that they correlate with either the sheeted dikes or the layered intrusives of the JO.

Loc. A is 38 m. by 3 m. in dimension. Layers are commonly 0.2 to 0.5 m. thick, and contain presumably hypabyssal pyroxene hornblende gabbro, hornblende gabbro/diorite, pyroxene hornblendite, and plagioclase-bearing pyroxene hornblendite having an average grain size of 0.3 mm. Primary minerals include zoned augite, brown igneous hornblende commonly rimmed by green locally uralitic amphibole, and zoned plagioclase. Augite and hornblende occur locally as 0.7-mm phenocrysts.

Loc. B is 9 m. by 4 m. in dimension, has layers ranging from 0.05 to 1 m in thickness, and contains layers of fine grained (av. 0.4 mm) hornblendite and pyroxene hornblende gabbro and coarse grained (av. 1.5 mm) pyroxene hornblende gabbro and olivine hornblendite containing olivine rimmed by secondary fibrous antigorite. Brown igneous hornblende is ubiquitous; zoned augite and plagioclase are common. A cumulate hornblende pyroxenite contains 4 mm poikilitic augite grains. Secondary phases in both outcrops include actinolite, chlorite, epidote, and, possibly, prehnite and pumpellyite.

The abundance of igneous hornblende suggests that the rocks do not correlate with the JO and that they likely formed by fractionation from a calc-alkaline magma. The rocks may correlate with the 151 to 146 Ma syn-Nevadan calc-alkaline dikes and sills that intrude both the JO and the Galice Formation in the WKT. The coarser grained layered mafic/ultramafic rocks might possibly correlate with mafic-ultramafic rocks of the 151-to-144-Ma western Klamath plutonic suite.