Cambrian-Ordovician rocks in New Mexico and Colorado; Evidence for a failed rift?

Abstract

A suite of alkaline and carbonatite rocks define a N-S trending band through New Mexico and southern Colorado. Although volumetrically insignificant, these rocks constitute some of the only evidence for geological activity in the region during the early Paleozoic era. Rocks from this suite, composed of syenites, episyenites, carbonatites, and mafic/ultramafic cumulates, have been dated through a variety of methods including zircon (ZrSiO4) U-Pb, whole rock Rb-Sr, and 40Ar/39Ar using biotite (K(Mg,Fe)3(AlSi3O10)(OH)2) ages. All ages fall within 664 ± 35 Ma (whole rock K-Ar)1 to 427 ± 22 Ma (whole-rock K-Ar)2. Thus, this suite has been interpreted as forming during incipient stages of continental rifting, with extension being attributed to differential motion across the southwest Iapetan transform margin. There are multiple sub-parallel rift features in eastern North America with similar ages (Reelfoot Rift, Rough Creek graben, Rome Trough, etc)3. Furthermore, the observation that alkaline igneous rocks often form as low-degree partial melts of the lithospheric mantle, combined with the high REE concentrations throughout the suite, support the interpretation that these rocks result from rifting, which never reached completion: an aulacogen. However, most of the research conducted on these rocks has been for their economic mineralization potential, with very little attention paid to their petrogenesis. Other than the loose geographical association, similar ages, and broad geochemical similarities, there is little evidence that the rocks share a common magmatic source or even that they formed in the same tectonic setting. Obtaining precise U-Pb zircon dates for these distributed bodies will help to answer this question. Accordingly, six localities spanning the entire compositional and geographical range were sampled for future zircon U-Pb dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). These localities, listed from north to south, include Democrat Creek syenite and mafic/ultramafic cumulates (CO), Spring Mountain syenite (NM), Lobo Hill episyenite (NM), Lemitar Mountains carbonatite (NM), Caballo Mountains episyenite (NM), and the Florida Mountains syenite (NM). All samples were observed to be intrusive bodies within diverse Paleoproterozoic terranes. None of the samples have deformation textures, indicating limited post-emplacement deformation. Importantly, this shows that the Laramide orogeny and initiation of the Rio Grande rift have not affected these rocks. Primary igneous rocks (syenites, cumulates, and carbonatites) were prioritized for sampling, as episyenitization may have disrupted or destroyed zircon crystals. Carbonatites and some syenites have associated fenites (a metasomatic alteration product), however, indicating a fluid phase exsolved from the magmas during their crystallization. Future work will provide U-Pb ages from these samples and, by utilizing literature whole-rock geochemical data, probe questions about the source of magmatism and metasomatizing fluid. 1. Setter & Adams, New Mexico Geological Society, Guidebook, 36, 147–149 (1985). 2. Armbrustmacher, U.S. Geological Survey, Professional Paper 1269 (1984). 3. McMillian & McLemore, NM Bureau of Geology and Mineral Resource Bulletin, 160, 1-12 (2004). A suite of alkaline and carbonatite rocks define a N-S trending band through New Mexico and southern Colorado. Although volumetrically insignificant, these rocks constitute some of the only evidence for geological activity in the region during this time. Composed of syenites, episyenites, carbonatites, and mafic/ultramafic cumulates, this suite has been dated producing poorly constrained ages within 664±35 Ma (whole-rock K-Ar)(1) to 427±22 Ma (whole-rock K-Ar)(2). Thus, this suite has been interpreted as forming during incipient stages of continental rifting which never reached completion: an aulacogen(3). Six localities spanning the compositional and geographical range were sampled for future zircon U-Pb dating using laser ablation inductively coupled-plasma mass spectrometry. Primary igneous rocks were prioritized for sampling, as metasomatic alteration may have disrupted zircon crystals. Carbonatites and syenites have associated alteration, however, indicating a fluid phase exsolved from the magmas during their crystallization. Future work will provide U-Pb ages from these samples.

1. Setter & Adams, New Mexico Geological Society, Guidebook, 36, 147–149 (1985). 2. Armbrustmacher, U.S. Geological Survey, Professional Paper 1269 (1984). 3. McMillian & McLemore, NM Bureau of Geology and Mineral Resource Bulletin, 160, 1-12 (2004).