New Mexico Geological Society Annual Spring Meeting — Abstracts

The Proterozoic metamorphic rocks surrounding the Eldora/Bryan Mountain stock, Nederland Quandrangle, Colorado, yield Rb-Sr whole-rock ages of 1775 ± 130 MYBP for hornblende gneiss, 1744 ± 47 MYBP for microcline gneiss, and 1778 ± 57 MYBP for biotite gneiss. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.7026 ± 0.0003 for hornblende gneiss, 0.7033 ± 0.0004 for microcline gneiss, and 0.7106 ± 0.0020 for biotite gneiss. At least three generations of pegmatites crosscut the gneisses. The first, consisting of a single sample, plots with the biotite gneiss and is assumed to be of comparable age. The second generation is the most common and yields an age of 1488 ± 27 MYBP, with an initial ⁸⁷Sr/⁸⁶Sr of 0.7044 ± 0.0002. The third gives an age of 1338 ± 19 MYBP and an initial ⁸⁷Sr/⁸⁶Sr of 0.7056 ± 0.0003.

Major and trace element analyses by instrumental neutron activation reveal element concentrations in hornblende gneiss are similar to average basalt, element concentrations in microcline gneiss are similar to average high-calcium granite, and element concentrations in biotite gneiss are similar to average shale. This agreement extends to both major and trace elements and applies to elements that behave quite differently in most geological environments. For this reason, it is believed that the regional metamorphism affecting these rocks did not disturb their whole-rock geochemistry. Gneiss whole-rock ages are thus those of deposition. The interbedded nature of the biotite, hornblende, and microcline gneisses suggest that the present sequence once represented an interlayering of shale and volcanoclastics, the volcanoclastics consisting of basalt and a felsic member such as dacite or rhyodacite.