The Embudo Granite of the southern Sangre de Cristo Mountains intrudes Precambrian metasedimentary and metavolcanic rocks. Rb-Sr whole-rock isotopic analyses indicate that the Embudo Granite crystallized 1,673 ± 41 m.y. ago. This intrusive event is temporally related to igneous and metamorphic events in Arizona, Colorado, New Mexico, and adjacent areas.
Chemical differentiation shows a typical calc-alkaline trend, and suggests that analyzed samples of the Embudo Granite are comagmatic. A plot of Rb/Sr ratios versus differentiation index (normative albite + orthoclase + quartz) also suggests that the rocks are comagmatic and that the granite has not been significantly metasomatized. An initial Sr87/Sr86 ratio of 0.7012 ± 0.0013 for the granite indicates a mantle-derived, original magma that underwent fractional crystallization with little if any contamination by very old crustal rocks.
Rb-Sr mineral–whole-rock ages of 1,210 + 50 m.y. are similar to published mineral ages for this region and indicate a thermal event at about this time.