VOLCANISM IN THE SOUTHERNMOST CASCADES: TECTONIC CONTROLS, MANTLE SOURCES, AND CRUSTAL INTERACTION

In the past 12 Ma, the Cascade arc in the Lassen region has migrated from near the CA-NV border to its present position, probably by steepening of the slab. Simultaneously, the width of the arc has contracted, and northward migration of the Mendocino Triple Junction and the southern edge of the slab has caused the southern terminus of volcanism to migrate north to 40° 20' at present. Two types of primitive mafic magma have erupted in the Lassen region. Calc-alkaline arc basalt displays regular cross-arc variation and is produced by melting of compositionally heterogeneous deeper mantle under the influence of a variable proportion of slab-derived component. This calc-alkaline volcanism has built a broad platform of small volcanoes having short lifetimes and composed primarily of basalt to andesite variably evolved by crystal fractionation and interaction with the crust. Impingement of the extensional regime of the Basin and Range province has resulted in normal faulting within the arc. Low-potassium olivine tholeiitic basalt generated in the uppermost mantle and displaying very little interaction with the crust erupted as widespread flows in conjunction with Basin and Range extension. Superimposed on the regional volcanism are a few larger, long-lived (up to 1 Ma) volcanic centers, each consisting of an andesitic stratocone and younger flanking silicic lavas. Volcanic centers form where prolonged focussing of mantle-derived basalt heats the crust and promotes extensive interaction between the magmatic system and the crust. At least 5 volcanic centers younger than 3.5 Ma are recognized in the Lassen area. The Latour, Yana, Dittmar and Maidu volcanic centers are extinct. The still-active Lassen Volcanic Center (LVC) serves as the model for center-based volcanism. Eruption of andesite built the 80 km3 andesitic Brokeoff Volcano from 600-385 ka. From ~350-0 ka, 3 sequences of dacite with abundant magmatic inclusions and a 4th sequence of ol-qtz hybrid andesite produced a 50 km3 domefield and lava plateau. The silicic volcanism provides the heat source for the modern hydrothermal system. The complex petrologic evolution of LVC is dominated by mixing at all levels: in the mantle source region, with the crust, and within the magmatic system itself.