THE PICURIS OROGENY IN TIME AND SPACE

Recent U-Pb detrital zircon studies of Proterozoic metasedimentary rocks and U-Pb dating of metatuffs in New Mexico and Arizona show that significant sections of the Proterozoic lithostratigraphy were deposited between about 1500 Ma to 1450 Ma. These include metasedimentary rocks of the Trampas Group and the synorogenic Marqueñas Formation in north-central New Mexico and the Yankee Joe Group and Hopi Springs Shale, Tonto Basin, Arizona. To date, no angular unconformity has been recognized between these Mesoproterozoic supracrustal rocks and the Paleoproterozoic rocks that directly underlie them. The regional metamorphism and polyphase deformation of these rocks was previously attributed to the ca. 1650 Ma Mazatzal, however that model is no longer tenable.

A compilation of U-Pb chemical and isotopic ages of metamorphic monazite from the Mesoproterozoic and underlying Paleoproterozoic rocks of CO and NM yields ages that fall largely between about 1435 Ma and 1350 Ma. Monazite from underlying Paleoproterozoic rocks do contain older ages ≥ 1700 Ma, some of which are interpreted as detrital and others as metamorphic. Notably, there are very few monazite ages between about 1700 Ma and 1650 Ma. Deposition of Mesoproterozoic sediments and volcanic rocks, the subsequent greenschist to near–granulite facies regional metamorphism in New Mexico and Arizona, and polyphase contractional deformation expands the Picuris orogeny to ca. 1450–1350 Ma. Contractional deformation features such as the Hondo syncline, New Mexico and the Four Peaks syncline, Arizona are manifestations of Mesoproterozoic tectonism. Furthermore, the timing of the Picuris Orogeny in the southwestern US is contemporaneous with deposition of the Missoula Group of the upper Belt Supergroup. Given that the age of detritus in the Missoula Group is similar to the age of the Paleoproterozoic bedrock of the southwest, the Picuris orogenic welt is a reasonable sediment source for rocks of the upper Belt Supergroup.