New Mexico aulacogen explained

New Mexico Aulacogen
Period:	Cambrian
Country:	United States
Coordinates:	35°N -106°W.

The New Mexico Aulacogen is a failed rift, or failed rift arm (aulacogen), that may have formed during the Cambrian and Ordovician Periods in the area from central Colorado through southern New Mexico. Its presence is inferred from pervasive alkaline and carbonatite intrusions of this age in this area. These include the Iron Hill carbonatite complex of central Colorado.^[1] A smaller alkali-carbonatite complex at Lobo Hill returns an Ar-Ar age of 518 ± 5.7 million years. The Florida Mountains uplift took place at about this same time and may be associated with this aulacogen.

Among the dikes associated with the postulated aulacogen are episyenite dikes in southern New Mexico believed to have formed from circulation of alkali-rich fluids through the host rock. Clasts of episyenite are found in the Bliss Formation, suggesting the dikes formed prior to late Cambrian. Some^[2] of the dikes contain heavy rare-earth elements in sufficient concentrations to be potentially exploitable.^[3] Episyenites found in the Zuni Mountains^[4] and the alkaline-carbonatite complex at Lobo Hill^[5] both lack the valuable concentrations of rare-earth elements.

The New Mexico aulacogen may be associated with a rift extending from Oklahoma to Colorado.^[6]

Notes and References

1. McMillan . Nancy J. . McLemore . Virginia T. . Cambrian-Ordovician magmatism and extension in New Mexico and Colorado . New Mexico Bureau of Geology and Mineral Resources Bulletin . 2004 . 160 . 130142252 .
2. McLemore . Virginia T. . Episyenites in the Sevilleta National Wildlife Refuge, Socorro County, New Mexico: preliminary results . New Mexico Geological Society Field Conference Series . 2016 . 67 . 255–262 . 11 June 2020.
3. McLemore . V.T. . Smith . A. . Riggins . A.M. . Dunbar . N. . Frempong . K.B. . Heizler . M.T. . Characterization and origin of episyenites in the southern Caballo Mountains, Sierra County, NM . New Mexico Geological Society Field Conference Series . 2018 . 69 . 207–216 . 21 May 2020.
4. McLemore . Virginia T. . Episyenites in the Zuni Mountains, Cibola County, New Mexico -- New interpretations . New Mexico Geoloical Society Special Publication . 2020 . 14 . 29–35 . 24 October 2020.
5. McLemore . Virginia T. . Cambrian alkaline rocks at Lobo Hill, Torrance County, New Mexico: More evidence for a Cambrian-Ordovician aulacogen . New Mexico Geological Society Field Conference Guidebook . 1999 . 50 . 247–253 . 25 January 2021.
6. Amato . J. M. . Mack . G. H. . Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources on opposite sides of the Transcontinental Arch . Geological Society of America Bulletin . 1 November 2012 . 124 . 11–12 . 1826–1840 . 10.1130/B30657.1.