Olivine tholeiitic basalt
Olivine tholeiitic basalt
middle to early Pleistocene
Low-K olivine tholeiitic basalt; typically erupted from fissures as widespread lava flow sheets; commonly has ropy (pahoehoe) surfaces; notably chemically (48-50% SiO2) and petrographically (sparse olivine phenocrysts) homogeneous (Clynne and Muffler, 2010). Co-located with (but chemically and petrologically distinct from) the calc-alkaline Cascade arc volcanic units in the northern map area (map units mcy, mco). These primitive low-K tholeiitic lavas differ from the calc-alkaline arc rocks by having lower concentrations of large ion lithophile elements (LILE), higher concentrations of heavy rare earth elements (REE), lower LILE/high field strength element ratios, and lower ratios of light to heavy REE (Borg and others, 2002). Clynne and Muffler (2010) reported several K-Ar and Ar-Ar ages ranging from late Pleistocene (e.g. 65 ± 45 ka) to middle Pleistocene (e.g. 610 ± 22 ka) and note that older tholeiitic lavas are present to the east and north of the Lassen region (north of map area). This unit is broken out where adequate data are available but probably occurs more extensively than depicted.