Rubidium-strontium geochronology of Proterozoic rocks from the Pecos and Truchas metamorphic terranes, north-central New Mexico
David B. Ward
Rubidium-Sr isochrons obtained from seven suites of supracrustal rocks (volcanic and sedimentary rocks now metamorphosed to amphibolite facies), four from the Pecos terrane and three from the Truchas terrane, yield a bimodal distribution of ages ranging from 1232 to 1730 Ma. The older ages, averaging near 1650 ma, are consistent with U-Pb ages from metarhyolites and pre-kinematic plutons and may represent formational ages. The younger ages range from 1384 Ma downward and are the product of one or more late thermal events superimposed on thorough resetting of Rb-Sr systematics by regional metamorphism. A mineralseparate age (1285.6 ± 8.7 Ma) from feldspar-muscovite schist may precisely date one late event. In micaceous schistose rocks, Rb-Sr systematics are dominated by the Rb-rich micas, producing whole-rock isochrons that are essentially identical to mineral isochrons. In micas, the blocking temperature for resetting Rb-Sr systematics appears to be similar to that for K-Ar. Rocks exhibiting reset Rb-Sr systematics are distinguished by the presence of a differentiated cleavage defined by abundant mica.
- Ward, David B., 1990, Rubidium-strontium geochronology of Proterozoic rocks from the Pecos and Truchas metamorphic terranes, north-central New Mexico, in: Tectonic development of the southern Sangre de Cristo Mountains, New Mexico, Bauer, Paul W.; Lucas, Spencer G.; Mawer, Christopher K.; McIntosh, William C., New Mexico Geological Society, Guidebook, 41st Field Conference, pp. 211-218.