MICROPROBE ANALYSIS OF SILICIC AND INTERMEDIATE LAVAS FROM THE ARNES CENTRAL VOLCANO, NORTHWESTERN ICELAND

The ~13 Ma Árnes central volcano lies on the eastern edge of the Westfjords peninsula in northwest Iceland. It erupted in the Skagi-Snaefellsnes rift zone which formed as a plume-centered rift at 15 Ma and was abandoned once it had drifted off of the plume at 7 Ma. This project focuses on silicic and intermediate lavas from the Árnes central volcano, sampled during a Keck Geology Consortium sponsored undergraduate research expedition in 2007. Electron microprobe analysis was conducted on plagioclase and clinopyroxene phenocrysts from six samples: two basaltic andesites, one andesite, and three rhyolites. The electron-microprobe data was analyzed within the context of previously acquired whole-rock major and trace element geochemical data to explore the origin of intermediate and silicic magmas in a fundamentally basaltic province. The data for the three intermediate samples suggests that these samples have undergone some amount of magma mixing during their petrogenesis. Both basaltic andesites show evidence for magma mixing in their origin. One (SiO2=52.2 wt%) has plagioclase with a high-An core (An86) with a distinctly lower An65 rim, and another grain has a transect from core to rim that first rises and then falls to an An69 rim. A clinopyroxene from the same sample also exhibits an irregular zoning pattern. The other basaltic andesite (SiO2=53.0 wt%) features plagioclase with weakly oscillatory zoning, An63-An66, and one transect has a low-An rim (An58). The andesite (SiO2=58.4 wt%) has a plagioclase that exhibits a small degree of oscillatory zoning, An59-An62, and two other grains exhibit reverse zoning with An62 cores that rise to An70 rims, again reflecting mixing. Two less evolved rhyolites (SiO2=70.4 & 71.5 wt%) have plagioclase with overlapping core compositions (~An41) but distinct rims (An41 and An34); clinopyroxenes are identical, either unzoned or more Wo-rich rims. A more evolved rhyolite (SiO2=73.3 wt%) has higher An plagioclase (An47-An50) than the less evolved rhyolites, as well as lower Wo-content pyroxenes, indicating it cannot be related to the others by fractional crystallization. Further study using oxygen isotope analysis may help better distinguish between crustal melting or fractional crystallization as an initial origin of the rhyolitic magmas.