Metamorphism, geochemistry and origin of magnesian volcanic rocks, Klamath Mountains, California

, ABSTRACT Metabasaltic rocks in the Klamath Mountains of California with 'komatiitic' major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentrations preserve relict igneous trends and suggest that the rocks are not komatiitic basalts, but im~at~re arc rocks and within-plate alkalic lavas. Correlation of 'excess' MgO with the volume per cent hornblende (:t:clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions. Early submarine alteration produced regional 6180 values of + 10:t: 1.5%0 and shifts in AlzO3,. NazO, and KzO concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350-550° C, c. 3 kbar) southward to prehnite-actinolite facies (200-400° C, ~3 kbar) , but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclas~ + microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these hases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearl stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatibl~ with P-T conditions in a magmatic arc. Later contact metamorphism at 2-2.9:t: 0.5 kbar and at peaf temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3... -km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, m~ covite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatisrtt :during contact metamorphism produced further increases in bulk-rock 6180SMOW of as much as +6%0. Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metas~ratism and the presence of magnesian cumulus phases. :