MID-MIOCENE THOLEIITIC BASALT IN THE ANCESTRAL CASCADES ARC: GEOLOGY, VOLCANOLOGY AND GEOCHRONOLOGY OF THE LOVEJOY BASALT

The Lovejoy Basalt is a 150 km3 quartz-normative tholeiite that is geochemically similar to the Columbia River Basalt, but erupted within the ancestral Cascades arc in the present-day northern Sierra Nevada. The Lovejoy Basalt is coeval with the main phase of the Columbia River Basalt Group and the development of the McDermitt Caldera and the Northern Nevada rift, with Ar40/Ar39 ages of 16 Ma. The trend of the inferred Lovejoy Basalt vent (N20-25°W) is parallel to that of coeval basalt feeder dikes for these features in Washington, Oregon and Nevada, suggesting exploitation of a regional stress field that affected the ancestral Cascades magmatic arc as well as the backarc region.

The Lovejoy Basalt consists of up to 13 exposed flows of aphyric olivine basalt. It erupted from the Thompson Peak area near Susanville, CA, and flowed down a paleovalley cut into Sierran basement that trended south 30 km to Red Clover Creek before bending southwestward and flowing 65 km to the Sacramento Valley. This rugged paleogeography is in marked contrast with that of the Columbia River basalts, which flowed over extremely low gradients. The Lovejoy Basalt ponded in the ancestral Sacramento Valley where individual flows inflated up to 60 m thick with N-S trending pressure ridges on their upper surfaces. Mapping and dating at Red Clover Creek show that ongoing dissection of the ancestral Sierra Nevada produced a narrow, steeper-walled paleocanyon cut into the basalt and filled with plagioclase andesite block-and-ash flow tuffs and lahars with an Ar40/Ar39 age of 14.1 Ma . These in turn are overlain by a landslide megabreccia wedge of Oligocene ignimbrite blocks, in turn overlain by a hornblende andesite block-and-ash flow tuff with an Ar40/Ar39 age of 11.1 Ma.

The physical volcanology of the Lovejoy Basalt contrasts with the Columbia River basalts in a several ways, aside from smaller volume: it is much glassier, even though its flow thicknesses are comparable; the well-exposed basal parts of flows lack pahoehoe toes, and although tops do not outcrop well, we have not found a single pahoehoe rope preserved in float; and two of the flows show agglutinated clastogenic textures at their base. It is similar to Columbia River basalts in that clinker and ragged vesicles typical of Aa flows are absent.