THE ENIGMA OF THE PICURIS OROGENY IN COLORADO

The ~1.4 Ga Picuris orogeny in Colorado is enigmatic, because (1) it is a broad ~400-800 km wide orogen of which the architecture, including location of an arc, is unknown, (2) it shows almost exclusively ferroan magmatic rocks, which generally reflect an extensional setting, and (3) it formed within the Statherian-Mesoproterozoic global period of anomalies. We present an overview of geochemical and structural characteristics of Mesoproterozoic plutons of Colorado and discuss the results in the context of the larger conundrum.

The Picuris orogeny is part of the Pinware-Baraboo-Picuris orogenic belt, which is attributed to a convergent or accretionary plate margin across the southern margin of Laurentia. Between ~1.48 Ga and ~1.36 Ga, granitoids were emplaced in a broad belt spanning from the southwestern U.S. through the Baltic shield. These were initially interpreted as anorogenic. However, episodes of significant deformation and metamorphism are now recognized in parts of New Mexico, Arizona and Colorado that are coeval with pluton emplacement, consistent with a convergent setting.

In Colorado, most ~1.4 Ga granitoids are ferroan alkali-calcic, or ferroan peraluminous, which is typical for an extensional tectonic setting. The peraluminous ferroan batholiths exist primarily along a ~200 km wide NE-trending zone, between primarily alkali-calcic ferroan plutons to the northwest and southeast. The peraluminous belt may be a result of assimilation of metasedimentary rocks, although similar metasedimentary rocks are also present in the alkali-calcic belts. The Mount Evans Batholith, within this zone, is the only known magnesian pluton, consistent with an arc.

Tectonic foliations in the ~1.4 Ga granitoids, and locally folds and shear zones in the host rocks, show evidence for generally NW-directed convergence. High-grade metamorphic rocks in Colorado, clockwise P-T paths in New Mexico and Arizona, and crustal thickness estimates of ≥50 km in ~1.4 Ga granitoids in the SW US also suggest convergence. Conversely, the ubiquitous ~1.4 Ga ferroan granitoids suggest widespread extension. The observations remain difficult to reconcile.