Abstract

Two occurrences of a distinctive red syenite in central New Mexico east-southeast of Albuquerque have yielded a whole-rock Rb-Sr isochron age of about 469 ± 7 m.y. Maximum possible ages of 496 and 604 m.y. are calculated for the two different occurrences. These Ordovician and possibly Cambrian syenites intrude foliated and regionally metamorphosed Precambrian rocks but are themselves unfoliated and unmetamorphosed.

The New Mexico syenites are lithologically similar to Ordovocian red syenite dikes of the Wet Mountains and Powderhorn area of southern Colorado, which cut but are related to Cambrian alkalic intrusive complexes. It appears that alkalic igneous activity affected southern Colorado, central New Mexico, and perhaps south-central New Mexico in early Paleozoic time.

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