Petrogenesis of Basaltic Volcanic Rocks from the Pribilof Islands, Alaska, by Melting of Metasomatically Enriched Depleted Lithosphere, Crystallization Differentiation, and Magma Mixing

• Published in: Journal of Petrology
• Main Authors: Chang, J. M., Feeley, T. C., Deraps, M. R.
• Format: Text
• Language: English
• Published: Oxford University Press 2009
• Subjects: Original Papers; Bering Sea; Alaska; Ocean Island
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/50/12/2249; https://doi.org/10.1093/petrology/egp075

The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of volcanic rocks from St. Paul (0·54–0·003 Ma) and St. George (2·8–1·4 Ma) Islands, the two largest Pribilof Islands. Rocks from St. George can be divided into three groups: group 1 is a high-MgO, low-SiO 2 suite composed primarily of basanites; group 2 is a high-MgO, high-SiO 2 suite consisting predominantly of alkali basalts; group 3 is an intermediate- to low-MgO suite that includes plagioclase-phyric subalkali basalts and hawaiites. Major and trace element geochemistry suggests that groups 1 and 2 formed by small-degree partial melting of amphibole-bearing to amphibole-free garnet peridotite. Group 1 rocks were the earliest melts produced from the most hydrous parts of the mantle, as they show the strongest geochemical signature of amphibole in their source. The suite of rocks from St. Paul ranges from 14·4 to 4·2 wt % MgO at relatively constant SiO 2 contents (43·1–47·3 wt %). The most primitive St. Paul rocks are modeled as mixtures between magmas with compositions similar to groups 1 and 2 from St. George Island, which subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved rocks. Plagioclase-phyric group 3 rocks from St. George are modeled as mixtures between an evolved melt similar to the evolved magmas on St. Paul and a fractionated group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts are ∼1400°C and are similar to those of mid-ocean ridge basalts (MORB). Similarly, 87Sr/86Sr and 143Nd/144Nd values for all rocks are MORB-like, in the range of 0·702704–0·703035 and 0·513026–0·513109, respectively. 208Pb/204Pb vs 206Pb/204Pb values lie near the MORB end-member but show a linear trend towards HIMU (high time-integrated 238U/204Pb). Despite isotopic similarities to MORB, many of the major and trace element characteristics are similar to those of ocean island ...