ABSTRACT An accretionary tectonic model for the Mesoproterozoic ca. 1500–1340 Ma tectonic evolution of the southern Laurentian margin is presented. The tectonic model incorporates key observations about the nature and timing of Mesoproterozoic deposition, magmatism, regional metamorphism, and deformation across the 5000-km-long southern Laurentian margin. This time period was one of transition in the supercontinent cycle and occurred between the breakup of Columbia and the formation of Rodinia, and the southern Laurentian margin was a significant component of a much greater accretionary margin extending into Baltica and Amazonia and possibly parts of Antarctica and Australia. However, fundamental questions and contradictions remain in our understanding of the tectonic evolution of Laurentia and paleogeography during this time interval.

ABSTRACT The discovery of multiple deformed and metamorphosed sedimentary successions in southwestern Laurentia that have depositional ages between ca. 1.50 and 1.45 Ga marked a turning point in our understanding of the Mesoproterozoic tectonic evolution of the continent and its interactions with formerly adjacent cratons. Detrital zircon U-Pb ages from metasedimentary strata and igneous U-Pb zircon ages from interbedded metavolcanic rocks in Arizona and New Mexico provide unequivocal evidence for ca. 1.50–1.45 Ga deposition and burial, followed by ca. 1.45 and younger deformation, metamorphism, and plutonism. These events reflect regional shortening and crustal thickening that are most consistent with convergent to collisional orogenesis—the Mesoproterozoic Picuris orogeny—in southwestern Laurentia. Similar metasedimentary successions documented in the midcontinent of the United States and in eastern Canada help to establish ca. 1.45 Ga orogenesis as a continent-scale phenomenon associated with a complex and evolving convergent margin along southern Laurentia. Metasedimentary successions of similar age are also exposed across ~5000 km of the western Laurentian margin and contain distinctive 1.6–1.5 Ga detrital zircon populations that are globally rare except in select cratonic provinces in Australia and Antarctica. The recognition of these distinctive detrital zircon ages provides a transient record of plate interactions prior to breakup of Nuna or Columbia ca. 1.45 Ga and provides key constraints on global plate reconstructions.

ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT It is proposed that the Pinware orogen of eastern Canada, the Baraboo orogen of the midcontinent, and the Picuris orogen of the southwestern United States delineate a previously unrecognized, ~5000-km-long, ca. 1520–1340 Ma trans-Laurentian orogenic belt. All three orogenic provinces are characterized by Mesoproterozoic sedimentation, magmatism, metamorphism, and deformation—the hallmarks of a tectonically active plate margin. Tectonism was diachronous, with the earliest stages beginning ca. 1520 Ma in eastern Canada and ca. 1500 Ma in the southwest United States. Magmatic zircon age distributions are dominated by Mesoproterozoic, unimodal to multimodal age peaks between ca. 1500 and 1340 Ma. The onset of magmatism in the Pinware and Baraboo orogens was ca. 1520 Ma, and onset for the Picuris orogen was ca. 1485 Ma. Detrital zircon age distributions within each orogenic province yield maximum depositional ages between ca. 1570 and 1450 Ma. Minimum depositional ages generally fall between ca. 1500 and 1435 Ma, as constrained by crosscutting intrusions, metatuff layers, or the age of subsequent metamorphism. Metamorphic mineral growth ages from zircon, garnet, and monazite yield peak ages between ca. 1500 and 1350 Ma and tend to be older in the Pinware and Baraboo orogens than in the Picuris orogen. The 40 Ar/ 39 Ar cooling ages for hornblende, muscovite, and biotite yield significant peak ages between ca. 1500 and 1350 Ma in the Baraboo and Picuris orogens. We propose that the Pinware-Baraboo-Picuris orogen formed in a complex, diachronous, convergent margin setting along the southern edge of Laurentia from ca. 1520 to 1340 Ma.

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 New detrital zircon data from deformed metasedimentary rocks of the Mazatzal Group in the northern Mazatzal Mountains, Arizona, indicate that formation of a regional fold-and-thrust belt occurred after ca. 1570 Ma. Regional correlations with pelites within the syncline at Four Peaks and deformed and intruded sediments in the upper Salt River Canyon allow us to revise the timing of deformation to ca. 1470–1444 Ma, contemporaneous with the Picuris orogeny in New Mexico. Fold- and thrust-style deformation of the Mazatzal Group was previously interpreted to be Paleoproterozoic and was a hallmark of the ca. 1650 Ma Mazatzal orogeny in the southwestern United States. However, recognition that protoliths of the deformed rocks formed in the Mesoproterozoic requires reconsideration of the age and regional tectonic significance of the orogenic event in its type locality. Our new findings are incompatible with published tectonic models invoking a regional ca. 1650 Ma Mazatzal orogeny and localized, pluton-enhanced deformation across the region ca. 1450 Ma. This field trip visits and reviews three localities across the Tonto Basin of central Arizona: (1) the northern Mazatzal Mountains; (2) Four Peaks of the southern Mazatzal Mountains; and (3) exposures of the early Mesoproterozoic Yankee Joe Group in the upper Salt River Canyon. At each location, deformation previously attributed to ca. 1650 Ma is, instead, demonstrably younger and represents a different episode of regional orogenesis. Thus, the nomenclature and tectonic significance of ca. 1650 Ma versus 1450 Ma regional orogenic events must be reconsidered and revised to reflect our present data and understanding, with implications for the tectonic evolution of Proterozoic rocks of southwestern North America.

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.

Abstract The early Mesoproterozoic (ca. 1400 Ma) is an enigmatic time in the tectonic evolution of southern Laurentia. Circa 1400 Ma granites within Laurentia and multiple other continents have distinctive geochemistry consistent with crustal extension or mantle upwelling. In the southwestern United States, these granites are commonly foliated and are often spatially associated with km-scale ductile shear zones. Deformation is attributed to intracontinental tectonism driven by active convergence along the distal southern margin of Laurentia. The recent discovery of deformed and metamorphosed, ca. 1450 Ma sedimentary rocks in northern New Mexico has strengthened the case for regional deformation and orogenesis. However, important questions remain about the tectonic significance of these events and how to reconcile tectonic models with granite petrology at the regional to global scale. This trip focuses on the protolith age of Proterozoic metasedimentary rocks and the kinematics, timing, and tectonic significance of deformation, magmatism, and metamorphism for the Mesoproterozoic across different crustal levels in the southern Rocky Mountains to highlight the ongoing questions and controversies regarding the Mesoproterozoic tectonic setting of Laurentia. This field trip will examine some of the diverse and most recently discovered evidence for ca. 1400 Ma orogenesis in the southern Rocky Mountains. We hope this trip will promote new interest and discussion about the Mesoproterozoic tectonic evolution of Laurentia. We will visit multiple outcrops in the Wet Mountains of southern Colorado and the Picuris Mountains of northern New Mexico. Stops in the Wet Mountains are arranged from north to south to examine contrasting styles of ca. 1400 Ma deformation with increasing paleodepth across the tilted Proterozoic crustal section. In the Picuris Mountains, we focus on detrital zircon geochronology and revisions to the lithostratigraphy of Paleoproterozoic and recently documented Mesoproterozoic metasedimentary rocks, the nature of regional metamorphism, and the style of deformation, ca. 1450–1400 Ma.