PETROGENESIS OF PERALUMINOUS RHYOLITE OF THE NATHROP DOMES, CENTRAL COLORADO

The four Nathrop Domes (~28-30 Ma) are located near Buena Vista, Colorado, within the Arkansas graben segment of the Rio Grande Rift. The domes are composed of sparsely to moderately porphyritic, flow banded rhyolite, with local vitrophyric margins. Phenocrysts include sanidine, plagioclase, biotite, Fe-Ti oxides and apatite. Bright red Mn garnet locally occurs in vapor-phase cavities. Feldspar and two oxide temperatures assuming 1 Kb pressure were, respectively, 670 and 653^oC.

All four domes erupted high silica rhyolite of essentially identical major element chemistry, although with substantial trace element variation - 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 REE diagram shows parallelism of the rhyolite plots, with sloping LREE values, flat HREE values, and a prominent, negative Eu anomaly. The narrow range of major element compositions within the rhyolites precludes fractionation modeling amongst them. Nonetheless, their elevated Rb/Sr ratios (as great as 75-120) strongly suggests that these magmas have undergone extensive fractional crystallization.

Neodymium isotope analysis yielded ENdt values of -10.1 for a Bald Mountain sample and -13.9 from the Precambrian granite sample. The similarity of the ENdt of the Bald Mountain rhyolite and the Precambrian granite suggest that the Nathrop rhyolite magmas were largely initially formed through partial melting of Precambrian rocks.

Neodymium Crustal Index (NCI) calculations were done using the Precambrian granite to estimate the contribution of crustal sources, assuming lithospheric mantle contributions of basalt with ENd of 0 to 4, or asthenospheric mantle contributions with ENd 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. Thus, 72.7 to 79.9 % of the Nd present in the rhyolite is likely from crustal sources, depending upon what type of mantle contribution was involved.