A diverse suite of spatially and temporally juxtaposed igneous rocks ranging from alkaline lamprophyres to granites intruded south-central Colorado during late Oligocene and early Miocene time. In addition to the stocks of the East and West Spanish Peaks, there are three types of dikes exposed in the region, based on orientation: radial, subparallel (striking approximately east–west), and independent dikes. The most striking features of this area are the numerous dikes radiating out from West Spanish Peak, some rising several tens of meters above the surrounding terrain and discontinuously exposed for tens of kilometers. New results from 40Ar/39Ar dating indicate that magmatism in the Spanish Peaks region began about 26.6 Ma and continued until about 21.8 Ma. Field evidence suggests that the initial intrusions were subparallel alkaline lamprophyre dikes south of the Spanish Peaks. A subsequent period of sub-alkaline magmatism occurred, producing West Spanish Peak (24.6 ± 0.13 Ma), East Spanish Peak (23.9 ± 0.08 Ma), and the radial dikes focused on West Spanish Peak. The final phase of magmatism included subparallel sub-alkaline lamprophyre dikes northeast of the Spanish Peaks. The 40Ar/39Ar results of this study substantiate the intrusive history derived from field relationships and establish the order of intrusion as West Spanish Peak, East Spanish Peak, and radial dikes, respectively. This study has implications for both the timing and style of the initiation of the Rio Grande rift, as well as the petrogenetic relationship between alkaline and sub-alkaline rocks in relatively stable cratonic areas.

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