PETROLOGY AND GEOCHEMISTRY OF THE OLIGOCENE WEST ELK VOLCANIC CENTER, GUNNISON AND MONTROSE COUNTIES, COLORADO

The West Elk Volcanic Center forms a distinct volcanic field that was topographically separated from the main San Juan Volcanic Field to the south by the rocks of the Laramide Gunnison uplift. The West Elk center was active at 30 ± 0.8 Ma and is the farthest NW extension of the San Juan Volcanic Field. The preserved volcanic field covers ~1200 km² of predominantly volcaniclastic material. The volcanic field presumably represents the debris apron of a large stratovolcano. Vent facies intrusives, radial dikes and hydrothermal activity are concentrated at the northern end of the preserved volcanic field and mark the location of the volcanic center.

The south and west slopes of West Elk Peak (WEP) are just NE of the highly altered vent area and expose ~600 m of interbedded lava and coarse volcaniclastics that were deposited at the base of the aggrading stratovolcano. To study the evolution of the West Elk Volcanic center, the WEP area was remapped and 32 separate discontinuous lava flows were sampled. Individual flows are < 3m thick and can typically be traced laterally for 100-200m. The WEP lavas are mostly two pyroxene shoshonites and latites with some olivine-bearing high-K basaltic andesites. The WEP lavas are compositionally similar to other pre-caldera samples from the San Juan Mountains. The WEP samples are LREE enriched, show no Eu anomaly and have flat HREE. They show the strong Nb, Ta, and Ti depletions typical of subduction related magmas. The WEP lavas form three magmatic groups based on the degree of LREE enrichment, Ba/Rb and La/Th systematics. Wide variations in incompatible element ratios preclude a simple model of fractional crystallization with magma recharge and hint at variable assimilation and melting of heterogeneous crust or lithospheric mantle.