ABSTRACT The Mesoproterozoic is a controversial time within the Earth’s history, and is characterized by high temperature/pressure ratios in metamorphic rocks, a large volume of extensional plutons, very few economic mineral deposits, and possibly a slowdown in plate tectonic processes. In Laurentia, ca. 1.48–1.35 Ga is well known as a time of voluminous ferroan magmatism, which led to conflicting tectonic interpretations that range from continental extension to convergent margin settings. Recently, a ca. 1.50–1.35 Ga orogenic belt was proposed that spanned Laurentia from present-day eastern Canada to the southwestern United States. Unlike the preceding Paleoproterozoic Yavapai/Mazatzal orogenies and the subsequent late Mesoproterozoic Grenville orogeny, the early–mid-Mesoproterozoic Picuris orogeny in the southwestern United States was relatively unrecognized until about two decades ago, when geochronology data and depositional age constraints became more abundant. In multiple study areas of Arizona and New Mexico, deposition, metamorphism, and deformation previously ascribed to the Yavapai/Mazatzal orogenies proved to be part of the ca. 1.4 Ga Picuris orogeny. In Colorado, the nature and extent of the Picuris orogeny is poorly understood. On this trip, we discuss new evidence for the Picuris orogeny in the central Colorado Front Range, from Black Hawk in the central Colorado Front Range to the Wet Mountains, Colorado. We will discuss how the Picuris orogeny reactivated or overprinted earlier structures, and perhaps controlled the location of structures associated with Cambrian rifting, the Cretaceous–Paleogene Laramide orogeny, and the Rio Grande rift, and associated mineralization. We will also discuss whether and how the Picuris orogeny, and the Mesoproterozoic in general, were unique within the Earth’s history.

Abstract Proterozoic Al 2 SiO 5 “triple-point” metamorphic rocks of north-central New Mexico are examined to honor the career contributions of Lincoln Hollister to petrology and tectonics, and to promote discussion of outstanding problems in metamorphic petrology. Hollister’s career began with studies of compositional zoning in garnet and staurolite, and interpreting the occurrence of coexisting Al 2 SiO 5 polymorphs in British Columbia. These studies emphasized the kinetic and bulk composition controls of metamorphism. Hollister’s interest in the Al 2 SiO 5 polymorphs led to his graduate student, Professor Jeff Grambling’s pioneering work that proposed the equilibrium occurrence of coexisting kyanite, sillimanite, and andalusite in the Truchas Peaks, New Mexico. Subsequently, polymetamorphism and the disequilibrium coexistence of kyanite, sillimanite, and andalusite have been proposed as alternate explanations for the occurrence of the polymorphs across the region. Field stops in the Picuris Mountains and Tusas Mountains will visit classic metamorphic rock localities, which are the subject of debate regarding the equilibrium/disequilibrium nature of the regional “triple-point” metamorphism. The trip will examine Al 2 SiO 5 -bearing mineral assemblages to demonstrate the regional distribution of the polymorphs. Stops in the Picuris Mountains will examine andalusite ± cordierite, andalusite ± chloritoid, and kyanite + sillimanite + andalusite–bearing rocks. Field stops in the Tusas Mountains will show kyanite and sillimanite ± garnet–bearing assemblages. Garnet + biotite ± staurolite–bearing rocks are common in both areas and will be examined. Results of garnet, monazite, and zircon geochronology and their bearing on the P-T-t-D paths for the region and the timing of orogenesis will be discussed.