SYNTECTONIC EMPLACEMENT OF THE 1.7 GA HERMIT'S PEAK PLUTON, SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO

The ~32 km² Hermit’s Peak pluton, a 1.70 Ga biotite-granite, perforates Paleoproterozoic crust of the Las Vegas Range in the southern Sangre de Cristo Mountains of New Mexico. We are studying this pluton to address outstanding issues regarding the timing and tectonic setting of early crust formation and assembly. The Hermit’s Peak granite is significant as it occurs at the suture zone between the Yavapai and Mazatzal Proterozoic crustal provinces. Our field and petrographic observations suggest that significant deformation accompanied pluton emplacement. The pluton has a fine- to medium-grained subhedral to anhedral granular texture. It displays a fairly regular moderately to steeply dipping N-NW trending foliation defined by aligned biotite and to a lesser extent by shape-preferred orientation of quartz and microcline. The granite contains pegmatite sills that give rise to a conspicuous compositional banding typically at the contact. In some places, the pegmatites exhibit boudinage and are folded within the main fabric. The pluton also contains numerous cross-cutting pegmatites, some of which were emplaced into late-stage shear zones. In thin section, large framework microcline crystals retain a superficial igneous appearance but show undulatory extinction and are surrounded by aggregates of recrystallized quartz, plagioclase, biotite and local myrmekite indicating solid-state deformation. Likewise, quartz crystals range from having straight extinction and planar grain boundaries to strongly undulatory extinction and sutured grain boundaries. Contacts of the pluton with the Paleoproterozoic country rock are well exposed in the northwest and southwest portions of the pluton. The host metamorphic rocks are predominantly amphibolite and quartzofeldspathic gneiss that display a strong penetrative NW-striking gneissosity. This main foliation is axial planar to tight to isoclinal, gently to moderately plunging, upright folds. The solid-state to submagmatic deformation fabrics coupled with the parallelism between the pluton foliation and metamorphic rock foliation strongly suggests that the Hermit’s Peak pluton was emplaced syntectonically during significant NE-directed shortening associated with arc formation and amalgamation of the Yavapai province.