Cambrian alkaline rocks at Lobo Hill, Torrance County, New Mexico: More evidence for a Cambrian-Ordovician aulacogen
— Virginia T. McLemore, Nancy J. McMillan, and Christopher McKee


Alkali-feldspar syenite, monzonite, quartz syenite, monzogranite, lamprophyre, and carbonatite dikes that intrude Proterozoic metamorphic rocks at Lobo Hill, a small hill in the Estancia basin, southeast of Moriarty in Torrance County, are part of a widespread alkaline magmatic event that occurred throughout New Mexico and southern Colorado. The dikes are unfoliated, unmetamorphosed, fine to medium grained, 1-2 m wide, and vary in attitude from nearly flat-lying to nearly vertical. Chemically, the dikes are metaluminous and can be differentiated into two groups based on differences in mineralogy and chemistry: high-K alkali-feldspar syenites and high-Na monzonites, quartz syenites, and monzogranites. An 40Ar/39Ar biotite-plateau age of 50 ± 5.7 Ma records the age of monzonite emplacement due to the rapid cooling of this high-level intrusion. Unlike similar alkaline rocks elsewhere in New Mexico, the Lobo Hill alkaline rocks do not have significant economic potential, except for aggregate. The magmatic compositions at Lobo Hill are consistent with those generated in a continental rift system, although geologic data such as rift-basin sediments and geophysical signatures are absent for this time period in New Mexico. Recognition of widespread Cambrian–Ordovician magmatic activity in New Mexico, evidence of relatively rapid uplift and erosion in the Florida Mountains, and the presence of carbonatites suggest that New Mexico was not a simple passive margin during the Cambrian–Ordovician, but rather experienced sufficient extension to perturb the mantle and initiate magmatism. Thus, we propose that an aulacogen, similar to the southern Oklahoma aulacogen, existed in New Mexico during Cambrian and Early Ordovician time.

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Recommended Citation:

  1. McLemore, Virginia T.; McMillan, Nancy J.; McKee, Christopher, 1999, Cambrian alkaline rocks at Lobo Hill, Torrance County, New Mexico: More evidence for a Cambrian-Ordovician aulacogen, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 247-253.

[see guidebook]