PROTEROZOIC PLUTONISM AND REGIONAL DEFORMATION – NEW CONSTRAINTS FROM THE SOUTHERN SANGRE DE CRISTO MOUNTAINS NORTHERN NEW MEXICO

Field studies and U-Pb geochronology of granitoid intrusions in the Las Vegas Range, southern Sangre de Cristo Mountains, northern New Mexico, provide new constraints on the Proterozoic tectonomagmatic evolution of southern Laurentia. The studied plutons intrude basement quartzofeldspathic gneisses and amphibolites >1.70 Ga. The oldest pluton, the Evergreen Valley tonalite is a coarse equigranular strongly foliated garnet-bearing biotite-hornblende tonalite that crystallized at 1.736±0.014 Ga. The tonalite exhibits a penetrative syntectonic foliation that trends N-NW and dips moderately to the E-NE. The pluton also includes abundant mafic dikes that parallel the plutonic fabric. The Hermit’s Peak granite displays a fairly regular steeply dipping N-NW trending foliation and abundant synplutonic pegmatite sills. Two samples from the Hermit’s Peak granite yielded U-Pb zircon crystallization ages of 1.700±12.1 Ga and 1.705±0.017 Ga. The syntectonic fabric in both plutons indicates that they were intruded during at least 30 million years of arc formation and accretion across northwest-striking tectonic boundaries. The northeast-striking tectonic grain that was developed regionally during the culmination of the Yavapai orogeny is not recognized in this area. The youngest studied plutons belong to the Evergreen Valley granite-gabbro complex which yielded crystallization ages of 1.448±0.012 Ga (granite) and 1.450±11.9 Ga (gabbro). The granite-gabbro complex is layered on the macroscale but lacks a tectonic foliation. Our results illustrate the episodic nature of magmatism in the Las Vegas Range at circa 1.7 and 1.4 Ga as well as the predominance of pre-1.45 Ga deformation.