Diverse Primitive Basalts from an Extensional Back-arc Setting: Fort Rock Volcanic Field, Oregon

Abstract

The Fort Rock Volcanic Field study area (FRVF) is situated in a highly complex volcanic and tectonic extensional back arc setting, influenced by multiple episodes involving the subduction of the Juan de Fuca Plate under the North American Plate, eruptions from the Western and High Cascades, High Lava Plains (HLPs), Newberry, and extension from the Basin & Range continental rift zone. Hydrovolcanic eruptions created tuff rings/cones and maars, while conventional eruptions created cinder cones, and lava fields in the FRVF area. These landforms contain a diverse array of primitive basalts, with an Mg# (xMgO / (xMgO + xFeO) * 100) > 60 (molar %), deducing information of mantle source regions and melting processes through geochemical analyses of major and trace elements to achieve a greater understanding of the complex tectonic framework and eruptive history of the FRVF. Geochemical data was acquired through X-ray fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for 75 mafic basaltic samples. FRVF basalts analyzed four diverse primitive magma types; low-K tholeiites (LKTs), calc-alkaline basalts (CABs), high Mg# basaltic andesites (BAs), and ocean island basalts (OIBs). These magma types are further separated into western/central and eastern sections of the FRVF based on their major and trace element variations. These variations suggest the subduction enrichment, mantle fertility, and the depth and degree of partial melting trends of their primary mantle sources depend on their distribution across the FRVF.