The Spanish Peaks are located in south-central Colorado, 30 km southwest of the town of Walsenburg. Physiographically, the two peaks (West Spanish Peak, 4152 m; East Spanish Peak, 3873 m.) lie on the upland portion of the far western edge of the Great Plains. Numerous intrusive features, including stocks, dikes, sills, laccoliths, plugs, and sole injections dominate the landscape. East and West Spanish Peaks (ESP and WSP, respectively) are the largest stocks, with several smaller stocks including Silver Mountain (previously known as Dike Mountain), and North, Middle, and South White Peaks. The well-developed and exposed systems of radial dikes of the Spanish Peaks region are often cited in geologic textbooks (Best, 1982; Hyndman, 1985; Philpotts, 1990). There are actually two sets of radial dike systems in the area, both with loci just west of the axis of the La Veta Syncline. The best known radial dike system is focused on WSP. The greatest concentration of radial dikes is found just west of WSP (figure 1b; Johnson, 1961). The second radial dike system is focused on Silver Mountain 27 km NNW of West Spanish Peak. The outline of the WSP radial dike system is elliptical, while the system around Silver Mountain is more oval. Both dike systems have their long axes perpendicular to the La Veta Syncline (Johnson, 1961). It is noteworthy that none of the more than 500 (Knopf, 1936) radial dikes actually contact WSP (Johnson, 1968). Two other dike systems are also exposed here: a regional subparallel dike set and various independent dikes which are oriented neither radially to WSP or Silver Mountain, nor with the regional trend of subparallel dikes. The subparallel dikes generally strike N80E and are some of the longest dikes in the region with discontinuously exposed lengths extending upwards of 20 km. The dikes range in width from .5 m to 30 m with the majority being 1-3 m; however, most of the dikes exhibit variable thicknesses with abrupt thinning and bulging common (Johnson, 1961). None of the radial dikes are composite and only one is known to be multiple (Johnson, 1968), while there are several examples of composite and multiple dikes in the subparallel system. Several of the dikes (mostly independent and subparallel dikes) grade into sills (Johnson, 1968; Crelling, 1973).
Individual dikes are up to 30 m wide with the majority being 1-3 m. Most of the dikes vary in thicknesses with abrupt thinning and bulging common (Johnson, 1961). None of the radial dikes are described as composite and only one is known to be multiple (Johnson, 1968). Composite and multiple dikes in the subparallel system are numerous. Composite dikes are multiple injections of magma of different compositions, while multiple dikes are dikes with several injections of magma of the same composition. Several of the dikes, mostly from the independent and subparallel systems, grade into sills (Johnson, 1968; Crelling, 1973).
The Spanish Peaks intrusives were emplaced during late Oligocene to early Miocene, roughly synchronous with the initiation of the Rio Grande rift on the west side of the Sangre de Cristo Mountains in southern Colorado (Tweto, 1979). Until the recently absolute age data for the area were minimal and inconsistent, leading to controversial conclusions regarding the intrusive history.
Compositionally, the intrusive rocks range from lamprophyres to granite porphyries. It is the spatial, and apparent temporal, juxtaposition of these lithologies that has eluded explanation until now. Knopf (1936) suggested that a follower of Rosenbusch's lamprophyre theories would perceive the mixture of alkalic and subalkalic lamprophyres in the Spanish Peaks region as a realm of petrographic anarchy.
Dramatic exposures, diverse mineralogies, range in silica content, proximity to the Rio Grande Rift, and high alkali and titanium contents have made the Spanish Peaks region the subject of numerous studies over the last century (Hills, 1899, 1900, 1901; Knopf, 1936; Johnson et al., 1958; Johnson, 1960, 1961, 1964, 1968, 1969; Larson and Strangway, 1969; Podwysocki and Dutcher, 1971; Crelling, 1973; Jahn, 1973; Arnold, 1977; Gibson, 1977; Smith, 1975, 1979, 1987; Jahn et al., 1979; Utada and Vine, 1983; Hutchinson and Vine, 1987; Gibson et al., 1993).