Chapter 18 - Reconnaissance Geochemistry of Devonian Island-Arc Volcanic and Intrusive Rocks, West Shasta District, California

Abstract

Major- and minor-element geochemistry of Middle Devonian Copley Greenstone and Balaklala Rhyolite of the West Shasta district, eastern Klamath Mountains, California, confirms earlier worker's conclusions that these rocks formed in an island-arc environment. Four greenstones have 54.2-62.7 percent SiO 2 (as recalculated H 2 O-free); 0.07-0.86 percent K 2 O; 0-3.7 ppm Rb; 48-124 ppm Sr, 5-11 times chondrites La; 8-17.5 times chondrite Lu, and moderate depletion of light rare earth elements (REE's). These rocks are metabasaltic andesite and metaandesite, probably formed by fractionation of low-K or arc-type tholeiite. Four high-silica dacites (defined as having SiO 2 >70 percent and K 2 O <2 percent) of the Balaklala Rhyolite have 76.4-79.7 percent SiO 2 ; low values of K, Rb, and Sr, which in part are not primary but were produced by alteration during emplacement, diagenesis, and metamorphism; 7-8 times chondrites La; 13-23 times chondrites Tb-Lu, and pronounced negative Eu anomalies. Light and intermediate REE's decrease with increasing SiO 2 , perhaps by fractionation involving plagioclase, sphene, and quartz. The high—silica dacites are similar to modern arc dacites. The absence of rocks of intermediate composition is problematic and may be due to inadequate sampling. Trondhjemite of the Mule Mountain stock is nearly identical in composition to the Balaklala high-silica dacite: two samples contain 74.0-75.2 percent SiO 2 ; 0.76-1.22 percent K 2 O; 7-32 ppm Rb; 53-57 ppm Sr; and REE's about 10 percent greater than values of the Balaklala sample of 76.4 percent SiO 2 . This trondhjemite, which was metamorphosed with the Balaklala Rhyolite to greenschist facies, probably is cogenetic with the Balaklala extrusives. It approaches some ophiolite-related plagiogranites in composition.