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 Continuous sediment, pollen, and charcoal records were developed from an 8.46-m-long sediment core taken from Hermit Lake in the northern Sangre de Cristo mountain range of Colorado. Presently, vegetation around the lake is upper subalpine forest, consisting of Picea engelmannii (Englemann spruce) with some Abies lasiocarpa (subalpine fir), and the lake lies >200 m below present tree line. We used several pollen ratios to reconstruct the relative position of the tree line and the occurrence of clay layers to infer landscape instability through time. Deglaciation of the Hermit Lake drainage began during the Bølling-Allerød interval. Between ca. 13.5 and 12.4 ka, high Artemisia (sagebrush) pollen abundance, low Picea / Pinus (spruce/pine; S/P) ratios, and sporadic occurrence of Picea macrofossils indicate alpine tundra-spruce conditions. Though the pollen record shows no transition to the Younger Dryas, the subsequent absence of Picea needle fragments suggests a lowering of tree line. By ca. 10.2 ka, a subalpine forest of Picea and Pinus grew there. Based on pollen ratios, tree line was higher than today from ca. 9.0 to ca. 3.8 ka, after which the tree line began to lower to its present elevation. Maximum expansion of the Picea-Abies subalpine forest, determined from both pollen and macrofossils, was coincident with the highest influx of charcoal particles and maximum deposition of postfire erosion (clay layers) into the lake. The period ca. 7.8–6.2 ka was the driest period, as shown by aquatic indicators, but pollen ratios suggest that ca. 6.2–3.8 ka was the warmest period of the Holocene, accompanied by high rates of burning, and consequently elevated erosion of clays into the lake. During the late Holocene, declining S/P ratios are interpreted as declining alpine tree line, while decreases in both Picea to Artemisia (S/Art) and Pinus to Artemisia (P/Art) ratios suggest climate cooling. Pollen evidence suggests expansion of the lower-elevation Colorado piñon ( Pinus edulis ), which has been documented as part of a widespread phenomenon noted by other studies.

The Wet Mountains–southern Front Range region is underlain by high-grade granitic gneiss, amphibolite, and schist of Early Proterozoic age. These rocks were intruded by granitic to granodioritic plutons during four episodes: one in the Early Proterozoic (1,660 to 1,700 Ma) and three in the Middle Proterozoic (1,485 to 1,440 Ma, 1,370 to 1,360 Ma, and about 1,060 Ma). We also report here a zircon age determination (536 ± 4 Ma) for syenite of the Cambrian McClure Mountain alkaline-mafic complex. The granitic gneiss was clearly formed before 1,700 Ma. Its protolith was probably pelitic to psammitic sedimentary rocks, in contrast to the volcanogenic rocks of this age farther west in the Gunnison and Salida areas of Colorado. The early Proterozoic plutons emplaced within the granitic gneiss are mostly somewhat younger than those emplaced within volcanogenic rocks to the west, although some are coeval. The middle Proterozoic rocks are representatives of the widespread “anorogenic granite-rhyolite suite” which is known in the St. Francois Mountains of Missouri and the subsurface of the midcontinent.