Circular 141—Petrography and petrogenesis of Tertiary camptonites and diorites, Sacramento Mountains, New Mexico

By G. B. Asquith, 1974, 6 pp., 4 tables, 6 figs.

Dominant igneous rocks are Tertiary dikes and sills of diorite and minor camptonite intruding Paleozoic sediments. Though comagmatic, these rocks differ in texture and composition as a result of flow differentiation. The dikes and sills of diorite and minor camptonite of Tertiary age intruded into Paleozoic sediments are the dominant igneous rocks of the Sacramento Mountains. The diorite and camptonite are similar mineralogically, but differ texturally. The camptonites have large phenocrysts of euhedral hornblendes and altered diopsidic augite with minor phenocrysts of plagioclase. The dominant phenocrysts of the diorite are plagioclase up to 5 mm long accompanied by minor euhedral hornblende and altered diopsidic augite. Both rocks have a groundmass of plagioclase laths, interstitial chlorite, magnetite, minor orthoclase, and apatite.

These rocks are comagmatic. The parageneic sequence is similar to that described by Yoder and Tilley for alkaline basaltic magmas with H₂O pressure between 2-5 kilobars. After total olivine resorption, but before total pyroxene resorption, the pressure dropped due to intrusion. Differences in mineralogy and texture are the result of flow differentiation. With a high rate of discharge from the magma chamber, strong magma currents picked up the large hornblende and augite crystals concentrated at the base of the magma chamber, and intruded them along with smaller crystals of hornblende, augite, and minor plagioclase-forming a camptonite.
The Sacramento Mountains are located in south-central New Mexico a few miles east of Alamogordo. A steep fault scarp marks the western boundary of the mountains with the Tularosa Basin. The range is composed mostly of gently dipping limestones and dolomites, with subordinate sandstones and shales, of early Ordovician to Permian age. The sedimentary rocks are intruded by numerous thin sills and dikes. The geology of the Sacramento Mountains has been described by Pray.

The Tertiary igneous rocks are predominately fine-grained diorite porphyries with a few closely associated comptonites. These rocks occur as numerous thin dikes and sills along the northern and central parts of the escarpment. The steeply dipping dikes trend northeast; the sills are concentrated in the shaly, sandy strata of the Devonian, lower Mississippian, and basal Pennsylvanian.

This study is concerned with the petrography and petrogenesis of the diorites and associated camptonites in the central Sacramento Mountains. Mineralogically, both rocks consist of phenocrysts of diopsidic augite, euhedral green hornblende, and plagioclase in a groundmass of plagioclase laths, interstitial chlorite, minor orthoclase, magnetite, and apatite. The rocks differ by the proportions of the various phenocrysts and by the size of the phenocrysts. The camptonites have larger phenocrysts; the mafic minerals predominate with only minor plagioclase. In the diorites the phenocrysts are smaller and plagioclase phenocrysts predominate with only minor pyroxene and hornblende.