EVIDENCE FOR MIXING BETWEEN SUBDUCTION AND BASIN AND RANGE MAGMAS IN THE LASSEN VOLCANO AREA, CALIFORNIA

Lassen volcano is a large stratovolcano (10,457 ft. tall) in northern California and the southernmost large, young volcano in the Cascades volcanic arc, which extends along the Pacific coast up to Canada. Like the other volcanoes in the Cascades, Lassen has formed due to subduction of oceanic lithosphere from the west beneath North America. Therefore, lavas erupted in the Lassen area are expected to have the geochemical and isotopic fingerprints typical of subduction volcanoes found around the world. These features most notably include depletions in high field strength elements. However, Lassen is also located in an area of continental extension known as the Basin and Range, which has produced numerous normal faults in the Lassen area. This crustal extension also produces lavas, but with more broadly depleted trace element characteristics than arc lavas. Previous studies have revealed calc-alkaline lavas derived from subduction and tholeiitic lavas from continental extension in the Lassen area but have not concluded that these magmas undergo mixing prior to eruption. In our study, we collected both types of lavas from a region within about 20 km of Lassen, primarily basalts that are least affected by fractional crystallization or assimilation. We analyzed the samples to evaluate their mineralogy, major and trace element and isotopic compositions, to seek evidence for hybridization of the two magma types. Our geochemical data show strong mixing trends between the two end-member magma types, which is clear evidence that the two can interact in the shallow mantle or crust beneath the Lassen area.