GEOCHEMICAL ANALYSIS AND TECTONIC IMPLICATIONS OF THE LOVEJOY BASALTS, NORTHERN CALIFORNIA

The Mid-Miocene Lovejoy Basalt is located in Northern California and has a total volume of 150 kilometers^3 (1). The source of this thoeliitic basalt is located approximately 135 km NE of Chico and occurs on either side of the Sacramento Valley as far south as Vacaville (approximately 250 km from source). The compositions, large volumes, and timing of regional Mid-Miocene basalts suggest the Lovejoy Basalt erupted at a time of high mafic magmatism. Eruptions of thoeliitic basalts occured approximately 12-16My, east of the Cascade Volcanic Arc in the Pacific Northwest and include the Northern Nevada Rift (NNR), the Northwestern Great Basin (NGB), at Steens Mountain and the Columbia River Plateau (CRB). The NNR extends from east central Nevada to the Nevada-Oregon border and originated at the same time as the Yellowstone mantle plume(2). Both the NNR and NGB basalt eruptions are also associated with tensional stresses (2). Steens Mountain is located in eastern Oregon and northern Nevada and is thought to be the earliest eruption of the CRB (3). The CRB basalts are located in Washington, Oregon, and Idaho with suggested tectonic origins that include both plume and extensional influences (4,5). The source and petrogenesis of the Lovejoy Basalt is not well understood. With new geochemical data of Chico Lovejoy Basalt samples, and detailed study of the timing and regional tectonics and previous work we suggest that the petrogenetic history and emplacement of the Lovejoy basalt is relevant to understanding the sources, and petrogenetic histories of the regional Miocene basalts. (1) Garrison, 1996 (2) Zoback et al., 1994 (3) Hooper et al., 1995 (4) Brandon and Goles, 1998 (5) Eaton, 1984