The Magdalena District in central New Mexico comprises the Magdalena Range and Granite Mountain which are block mountains of the Basin Range type. They consist of Carboniferous sedimentary formations on a pre-Cambrian basement and are covered by Tertiary volcanic rocks.
Volcanism in the District was preceded by considerable faulting, tilting, and uplifting very probably in early Tertiary time. The initial volcanic extrusion consisted of latite flows and tuffs now resting unconformably on the Granite Mountain fault-block in the northern part of the District. Subsequent extrusions consisted of alternations of andesites, rhyolites, and latites, and have a minimum thickness of approximately 4,000 feet. Graben faulting, probably in compensation for the effusion of the large volumes of lava, followed and some of the major faults were subsequently the sites of the intrusion of monzonitic and granitic stocks. Late lamprophyre and white rhyolite dikes in large numbers intruded the intrusive as well as earlier extrusive and sedimentary rocks.
