Late Cenozoic Basaltic Volcanism and Development of the Rio Grande Depression in the Southern Rocky Mountains

In the Southern Rocky Mountains, upper Cenozoic basalt flows were erupted widely in areas characterized in middle Tertiary time by predominantly intermediate-composition volcanism. Initiation of basaltic volcanism coincided approximately with the beginning of extensional block faulting that resulted in development of the Rio Grande depression, a major rift structure that separates the stable platforms of the High Plains and the Colorado Plateau. Time relations are especially clear along the San Luis Valley segment of the Rio Grande depression in southern Colorado and northern New Mexico, where 16 basalt flows have been dated by K-Ar methods. Along the west margin of the San Luis Valley, silicic alkalic basalt flows as old as 26 m.y. rest unconformably on a pediment cut on middle Tertiary andesitic and related rocks (35 to 27 m.y. old), and similar basalt 20 to 0.24 m.y. old interfingers with and overlies volcaniclastic alluvial fan deposits (equivalent to Santa Fe Group) that accumulated in the subsiding depression. Basalt erupted during late Cenozoic block faulting varies in composition with distance from the axis of the northern Rio Grande depression. Tholeiitic rocks are largely confined to the depression, and the basalt types become more alkalic to the west and east. Relatively silicic alkalic basalt, including both undersaturated and saturated types, occurs throughout the region, but very undersaturated alkalic basalt flows were erupted only on the Colorado Plateau and the High Plains. The lateral change from tholeiitic to alkalic basaltic volcanism probably reflects different conditions of magma generation in the mantle that are related to changes in crustal thickness and thermal gradients across the depression. The compositions and compositional ranges of basalt of the Southern Rocky Mountain region are similar to those of many Pacific islands, but the proportions of basalt types are markedly different. These relations are tentatively interpreted as reflecting origin of both continental and oceanic basalt types under similar P-T conditions, with generation of the Southern Rocky Mountain basalt from lithospheric mantle and the oceanic basalt from relatively shallow asthenosphere.