PRELIMINARY PETROGRAPHIC AND GEOCHEMICAL ANALYSES OF THE BASALT FROM THE SEVEN DEVILS OPHIOLITE

The Seven Devils Mountains extend approximately 40 miles along the Oregon and Idaho border, and have a highest peak of 9,393 feet on He Devil. The Permian to Triassic Seven Devils basalt is located in the Wallowa Terrane, an ophiolitic terrane, emplaced in western Idaho during the Cretaceous. The Wallowa- Seven Devils Terrane consists primarily of basaltic and basaltic- andicite volcanic rocks with lesser gabbroic rocks, diacitic volcanic rocks, and late Triassic to Early Jurassic platform carbonates. These rocks are intruded by the late Jurassic granodioritic plutons of the Wallowa batholith.

During the Late Jurassic and into the middle Cretaceous, extensive deformation affected the rocks of the Wallowa Terrane. Along a complex suture zone, the rocks in the Wallowa Terrane are juxtaposed on cratonic strata. Strike-slip faulting along the southern and eastern edges of the area in response to left oblique plate convergence in the Triassic and Jurassic marks the indication of movement. The final accretion of the terrane to the western part of North America occurred during the end of the Early Cretaceous.

Preliminary petrographic study has shown preservation of primary textures (interlocking, intergranular, and trachytic, along with Carlsbad twinning) and the primary minerals include: hornblende (30-60%), plagioclase (30%), clinopyroxene (20%), orthopyroxene (15%), orthoclase (10%), microcline (5-10%), and approximately 20-30% groundmass. The secondary minerals that have been found suggest pervasive greenschist metamorphisim and include epidote (8%), chlorite (3-5%), quartz (2-5%), and very little prehnite (1%). There is no evidence for fresh glass in the basalt samples. For the minerals present, it is likely that the Seven Devils basalt formed in an island arc environment.