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Volcanic clasts incorporated in the lower portion of the Tertiary Santa Fe Group sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, provide constraints on the timing of regional tectonic events by provenance determination. Based on currently exposed volcanic terrains, possible clast sources include Spanish Peaks and Mount Mestas to the east, the San Juan volcanic field to the west, and the Thirtynine Mile volcanic field, a remnant of the Central Colorado volcanic field, to the north and east of the San Luis Basin. Provenance was determined by a variety of geochemical, mineral chemical, and geochronologic data.

Large porphyritic Santa Fe Group volcanic clasts are potassic with a wide compositional range from potassic trachybasalt to rhyolite. The whole-rock chemistry of the Culebra graben clasts is similar to that of the Thirtynine Mile and San Juan volcanic fields. Culebra graben amphibole and biotite chemistry is generally consistent with that of rocks of the San Juan volcanic field, but not with Spanish Peaks samples. Trace-element data of Culebra graben volcanic clasts overlap with those of the San Juan and Thirtynine Mile volcanic fields, but differ from those of the Mount Mestas. Thermobarometric calculations using mineral chemistry suggest that many Culebra graben rocks underwent a three-stage crystallization history: ~1120 °C at 7–10 kbar, ~1100 °C at 2.3–4.6 kbar, and hornblende formation ~800 °C at 3 kbar. Within the Culebra graben clasts, zircon rim U-Pb geochronologic systematics as well as amphibole and biotite 40Ar/39Ar plateau data yield ages ranging from 36 to 29 Ma. These ages are consistent with ages of the Thirtynine Mile volcanic field (36–27 Ma) and the Conejos Formation of the San Juan volcanic field (35–29 Ma), but predate Spanish Peaks (ca. 27–21 Ma) and Mount Mestas (ca. 25 Ma). Based on these data, Spanish Peaks and Mount Mestas are excluded as potential source areas for the Santa Fe Group volcanic clasts in the Culebra graben. The San Juan volcanic field is also an unlikely source due to the distance from the depositional site, the inconsistent paleo-current directions, and the pressure-temperature conditions of the rocks. The most likely scenario is that the Central Colorado volcanic field originally extended proximal to the current location of the Culebra graben and local delivery of volcanic clasts was from the north and northeast prior to the uplift of the Culebra Range and Sangre de Cristo Mountains.

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