New Mexico Geological Society Annual Spring Meeting — Abstracts

We report petrographic and geochemical data from granitic phase within the Hermit’s Peak batholith, a Proterozoic plutonic-metamorphic complex in the southern Sangre de Cristo Mountains northwest of Las Vegas, New Mexico. We recognize at least three distinct intrusive phases. Early granitoid intrusions formed centimeter- to meter-wide coarse-grained tabular sheets and layers within Paleoproterozoic host rock gneisses. The intrusions were deformed during isoclinal folding along with their host rocks, suggesting that these early granites are preor syntectonic with contractional deformation associated with Yavapai-Mazatzal collision. These granites show a fine- to medium-grained anhedral granular texture with quartz microstructures indicative of dynamic strain and solid state deformation, including undulose extinction, serrated grain boundaries, and ribbon texture. The Hermit’s Peak granite, a supposed 1.4 Ga "anorogenic" granite, is a foliated fine- to medium-grained anhedral granular granite. Aligned interstitial minor biotite and magnetite define the foliation, which is variable at the outcrop-scale. The relationship between the granite foliation and host rock foliation has not been concluded. Strain-related microstructures are rare, suggesting that the Hermit’s Peak granite fabric developed syntectonically within a local or more regional deformation event. Discordant dikes and stocks of pegmatitic alkali granite intruded the other units. The pegmatites are megacrystic with a preponderance of alkali feldspar and quartz. All of the granitic phases are nearly saturated with respect to alumina. The granitic sheets and foliated granite have overlapping major and trace element values, while the pegmatites have higher weight percent SiO2, higher elemental Ba and Rb, and lower elemental Zr and Zn. The Hermit’s Peak batholith is located in the transition zone between the Yavapai and Mazatzal Precambrian provinces. Radiometric age determinations are needed to constrain the timing of magmatism and fabric development and help resolve whether the Mazatzal orogeny was a discrete or protracted tectonic event.