The four Nathrop Domes (∼30 Ma) are located near Buena Vista, Colorado, within the Arkansas Valley Graben segment of the Rio Grande Rift. The domes are largely composed of sparsely to moderately porphyritic, flow-banded rhyolite, with local vitrophyric margins. Phenocrysts include sanidine, plagioclase, quartz, biotite, and Fe–Ti oxides. Bright red Mn-garnet locally occurs in vapor-phase cavities. Two-feldspar and two-oxide temperatures assuming 1 kb pressure were, respectively, 670°C and 653°C.

All four domes erupted rhyolite of essentially identical major-element chemistry, although with substantial trace-element variations, high incompatible trace-element contents (Rb up to 364 ppm; Nb up to 67 ppm), and extreme depletion of Ba, Sr, P, Eu, and Ti. These depletions are consistent with fractionation of observed phenocrysts. A rare-earth element diagram shows parallelism of the rhyolite plots, with sloping light REE values, flat heavy REE values, and a prominent, negative Eu anomaly. The narrow range of major-element compositions within the Nathrop Domes’ rhyolites precludes any fractionation modeling amongst them. Nonetheless, their elevated Rb/Sr ratios (as great as 75–120) strongly suggest that these magmas have undergone extensive fractional crystallization.

Neodymium isotope analysis shows ∊ Ndt values of −10.1 for a Bald Mountain sample and −13.9 from a Precambrian granite sample (t = 29 Ma). The similarity of the ∊ Ndt of the Bald Mountain rhyolite and the Precambrian granite suggests that the Nathrop rhyolite magmas were initially formed through partial melting of Precambrian rocks. Neodymium Crustal Index (NCI) calculations were performed using the Precambrian granite to estimate the contribution of crustal sources, assuming lithospheric mantle contributions of basalt with ∊ Nd of 0 to 4, or asthenospheric mantle contributions with ∊ Nd of 5 or greater. The resulting calculations indicate NCI values of 0.727 to 0.789, assuming lithospheric mantle, and 0.799 assuming an asthenospheric mantle contribution with ∊ Nd = 5. Thus, 72.7 to 79.9 percent of the Nd present in the Nathrop rhyolite sample is likely from crustal sources, depending upon what type of mantle contribution was involved.

The Nathrop rhyolites may represent the earliest phase of magmatism associated with the northern segments of the Rio Grande Rift.

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