40Ar/39Ar dates for the Spanish Peaks intrusions in south-central Colorado

Abstract

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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.

A rootless rockies—Support and lithospheric structure of the Colorado Rocky Mountains inferred from CREST and TA seismic data

“…Most of these rocks were emplaced between 33 and 20 Ma, spanning the Oligocene flare-up and initial extension of the Rio Grande rift system [Gibson et al, 1993]. These areas include the Navajo Volcanic Field , the Dulce area , the Spanish Peaks [Penn and Lindsey, 2009], and the Middle Park lamproite [Thompson et al, 1997] (Figure 1). An additional period of ultrapotassic volcanism occurred in northwest Colorado at 10-8 Ma [Leat et al, 1991;Thompson et al, 1990] during a renewed period of rift activity Ingersoll, 2001;Landman and Flowers, 2013].…”

Using Stochastic Point Pattern Analysis to Track Regional Orientations of Magmatism During the Transition to Cenozoic Extension and Rio Grande Rifting, Southern Rocky Mountains

“…Estimates of the magnitude of extension in the northern Rio Grande rift range from approximately 1 to 13 km, or 8%–17% extension (Chapin & Cather, 1994; Cordell, 1982). Magmatism that developed during this time period is compositionally diverse, including a suite of broadly alkaline magmas that intruded along the Front Range and Sangre de Cristo mountains in areas such as Cripple Creek, the Spanish Peaks, and Cerrillos Hills (Bachman & Mehnert, 1978; Kelley et al., 2020; Penn & Lindsey, 2009). Large volumes of calc‐alkaline, intermediate to silicic magmas intruded the upper crust resulting in the formation of numerous caldera‐forming magma centers in the San Juan mountains (Curry et al., 2021; Lipman & Bachmann, 2015; Steven & Lipman, 1976; Wotzlaw et al., 2013).…”

Multiscale Spatial Patterns in Giant Dike Swarms Identified Through Objective Feature Extraction

“…The nearby Dike Mountain or Silver Mountain lies 50 km NW of the Spanish Peaks, and its associated dikes are syenodiorite (Johnson, 1961). Although the exact relationship between the two stocks and Dike Mountain is unclear, the Dike Mountain is dated as older than the Spanish Peaks intrusions (Penn & Lindsey, 2009). We chose to include these dikes in our database to demonstrate the algorithm's ability to differentiate between two closely oriented radial swarms.…”

“…Spanish Peaks dikes exhibit three major components, first a radial structure centered on West Peak, a linear trend that strikes N. 60°E, and a secondary radial structure centered on Dike Mountain also known as Silver Mountain. Each intrusive body and the associated dikes represent distinct magmatic phases and compositions (Penn & Lindsey, 2009). The radial swarm of the Spanish Peaks is diffusely centered on West Peak although some dikes intercept outside the Peaks or in East Peak.…”

“…The radial swarm of the Spanish Peaks is diffusely centered on West Peak although some dikes intercept outside the Peaks or in East Peak. West Peak is a quartz syenite dated to 24.6 ± 0.13 Ma while the East Peak is composed of granite and granodiorite porphyry dated to 23.9 ± 0.08 Ma (Penn & Lindsey, 2009). The dike compositions range from gabbro lamprophyre to granite porphyry (Johnson, 1961).…”