MAGNETOSTRATIGRAPHY, GEOCHRONOLOGY AND GEOCHEMICAL EVOLUTION OF THE PLIOCENE COSO VOLCANIC FIELD, CA AND CONTEMPORANEITY WITH SIERRA NEVADA DELAMINATION

Detailed stratigraphic, magnetostratigraphic, geochronologic and geochemical study of the Coso volcanic field, CA shows that onset of the Pliocene pulse of lavas coincided with the proposed timing for delamination of the Sierra Nevada batholithic root at ca. 3.5 Ma (Manley et al., 2000). The Coso volcanic field is located just east of the Sierra Nevada, at the edge of the Basin and Range and within the westernmost strand of the Eastern California shear zone-Walker Lane belt. Thus, tectonics is usually cited as the cause of the volcanic field. However, new magnetostratigraphy and Ar/Ar geochronology places the most voluminous pulse of volcanism in the region at the time of the proposed foundering of the Sierran ecologite root. Our new data indicate the voluminous activity in the Wild Horse Mesa part of the Coso Range commenced around 3.6 Ma with andesitic stratovolcanoes and polygenetic eruptive centers. A pulse (~10 km3 as calculated by Duffield et al., 1980) of primitive basalts between 3.5 and 3.2 Ma from monogenetic vents was followed by minor dacites and andesites, ending this phase of volcanism by about 3 Ma. This stratigraphy and geochemical evolution is a substantial revision of the previous model (Novak and Bacon, 1986) based on K-Ar geochronology (Duffield et al., 1980) that called upon mixed andesite, basalt, monogentic and polygenetic eruptions throughout 3-4 Ma. While the Sierran delamination is also associated with some highly potassic lavas (e.g. shoshonite), to date no such lavas have been identified in the Coso range. Rather Pliocene Coso volcanics are alkalic to subalkalic.