INTERMEDIATE TO ULTRAMAFIC PLUTONIC ROCKS EXPOSED ON JOHNSON MOUNTAIN, EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON: ARC OR OPHIOLITE?

The Western Klamath terrane (WKT) exposes the 162 Ma Josephine ophiolite (JO) overlain by metasediments of the Galice Formation and intruded by ~150 Ma calc-alkaline intrusives. This study concerns intrusives exposed in the eastern Elk outlier (EO) of the WKT, ~40 km NW of the WKT near the Oregon coast. The EO exposes Galice metasediments intruded by arc-related dikes and plutons, unconformably overlain by Lower Cretaceous strata. Ophiolitic dikes cutting gabbro in the EO are tentatively correlated with sheeted dikes of the JO.

The upper slopes and summit of Johnson Mountain consist of, from west to east, intermediate to ultramafic intrusives (the Johnson Mountain intrusives or JMI), peridotite tectonite, and serpentinite-matrix mélange. We examined the JMI along a 6 km stretch of Forest Service Rd 5560. Southern exposures include an outcrop of diorite dikes cutting serpentinized peridotite and another of layered mafic to ultramafic cumulates; the remainder is gabbro and diorite. JMI diorite, gabbro, and ultramafic rocks contain augite, plagioclase, and blocky brown hornblende and secondary green amphibole, epidote, chlorite, and albite. Diorite dikes cutting peridotite contain euhedral augite and brown hornblende microphenocrysts and show little alteration. Two layers in the cumulate ultramafics contain olivine.

We compared JMI to ophiolitic pyroxene gabbros cut by diabasic dikes. The gabbro contains blocky, commonly poikilitic, brown hornblende, augite, albitized plagioclase and secondary green amphibole, epidote, chlorite, and prehnite. Diabasic dikes contain uralitic green amphibole and albitized plagioclase and have chilled margins.

The geology of the JMI strongly suggests arc rather than ophiolite affinity as evidenced by: 1) the presence of primary euhedral hornblende in diorite dikes cutting peridotite suggests calc-alkaline geochemistry; 2) the paucity of alteration in these dikes as compared to the ophiolitic rock; 3) relict olivine in the layered cumulates is a unique in the EO; 4) diabase dikes with chilled margins are present only in the ophiolitic rocks; 5) the JMI contains abundant diorite. The green schist facies assemblage of uralitic green amphibole, epidote, chlorite, and albitized plagioclase indicates shared metamorphic conditions with the ophiolitic rocks.