Geology and petrogenesis of magmatic veins at El Porticito volcanic vent, Catron County
Robert R. Horning, Nelia W. Dunbar, W. Scott Baldridge, and Philip R. Kyle


El Porticito is the approximately 70-m high eroded remnant of a 7.08 ± 0.25 Ma volcanic vent located in the transition zone between the Colorado Plateau and the Basin and Range province in west-central New Mexico. It is distinguished by the strikingly white, vertical and horizontal, centimeter- to meter-scale, magmatic veins that cut across the black faces of the edifi ce. At nearby Tejana Mesa, a latticework of centimeter-scale subvertical veins merges upward into a single 10-m thick white sill that extends some 200 m along the cliff face. El Porticito consists primarily of two basanites. The late basanite has more Al2O3 and less MgO and compatible trace elements, indicating that it is more evolved than the early basanite. Residuals calculated during mass balance modeling of crystal fractionation suggest that the second basanite probably did not evolve from the early basanite. Instead, similar rare earth element concentrations and identical εNd (+3.2) suggest that the two basanites evolved from a common mantle parent. Although concentrations are near detection limits, the lower calcium contents of the cores of olivine phenocrysts found in the late basanite suggest that equilibration between olivine cores and surrounding melt occurred at greater depth than in the early basanite.

Geochemical modeling, using major and trace element data, suggests that vein magma probably evolved at shallow depth through crystal fractionation from the older, less-evolved basanite. However, the solutions of the models are close enough that the possibility remains that the veins evolved from the later-erupted, less evolved basanite. Veins occur only within the late basanite, except in one small volume at Tejana Mesa where they extend less than a meter into the early basanite. Partially and entirely detached xenoliths of wall rock found in veins are compatible with forceful intrusion of vein magma into its host. The existence of both smooth trajectories and abrupt, right angle changes in vein orientation suggest that vein magma intruded the younger basanite at presently exposed levels while the basanite cooled through the rheological critical melt percentage and behaved as both a ductile and brittle host, depending upon its degree of crystallinity at a particular location.


  1. Horning, Robert R.; Dunbar, Nelia W.; Baldridge, W. Scott; Kyle, Philip R., 2003, Geology and petrogenesis of magmatic veins at El Porticito volcanic vent, Catron County, in: Geology of the Zuni Plateau, Lucas, Spencer G.; Semken, Steven C.; Berglof, William R.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 54th Field Conference, pp. 139-154.

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