Geology of the Alkaline Precambrian Rocks at Pajarito Mountain, Otero County, New Mexico

Abstract

Pajarito Mountain is an oval-shaped Laramide dome involving Permian carbonates and red beds with a core of Precambrian rocks. The melanocratic rocks of the core have until recently been thought to be a Tertiary intrusive and perhaps laccolithic. The Permian beds lie on the core rocks unconformably above marked soil profiles of weathering, and radiometric dating shows the Precambrian to be near 1170 m.y. The rocks are predominantly melanocratic types high in amphibole. Fine- to medium-grained syenite is the principal terrane. Locally it is granitized or invaded by a coarser-grained granitoid syenite, and both of these are very locally invaded or converted to hornblende syenite pegmatite. Although hornblende is the predominant amphibole, riebeckite is common, and locally in the pegmatite late-stage aegirine has formed and commonly replaces the hornblende or riebeckite.