The intrusive rocks of the Spanish Peaks, south-central Colorado, range in age from 22 to 25 m.y., as shown by four K-Ar dates. These rocks as well as other intrusions in the Sangre de Cristo Range and southern high plains appear to be correlated in time with the early Miocene maximum of igneous activity in the Basin and Range province. The volcanic fields of northeastern New Mexico are products of a distinct 8-m.y.-to-Holocene phase of activity. Widespread olivine basalt 2 to 5 m.y. in age was erupted in these fields during approximately the same period that similar basalt was erupted west of the Sangre de Cristo Range in the Rio Grande depression. Feldspathoidal basanite and nephelinite were erupted about 1.9 m.y. ago in the Raton-Clayton field. The Capulin basalt flows are of Holocene age, and are characterized by the presence of resorbed plagioclase phenocrysts. These rocks resemble the Hinsdale basalt of a much earlier period that are located west of the Rio Grande depression. The relation of basalt magma type and position relative to the depression is complex, or asymmetric in time and space.

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