The North American Cordilleran Anatectic Belt

The North American Cordilleran Anatectic Belt (CAB) is a ~3,000 km long region in the hinterland of the Cordillera that comprises numerous exposures of Late Cretaceous to Eocene intrusive rocks and anatectic rocks associated with crustal melting. As such, it is comparable in size and volume to major anatectic provinces including the Himalayan leucogranite belt. The CAB rocks are chiefly peraluminous, muscovite-bearing leucogranite produced primarily by anatexis of Proterozoic to Archean metasedimentary rocks. The CAB rocks lack extrusive equivalents and were typically emplaced as thick sheets, laccoliths, and dike/sill complexes. The extent, location, and age of the CAB suggests that it is integral to understanding the tectonic evolution of North America, however, the belt is rarely considered as a whole. This paper reviews localities associated with crustal melting in the CAB and compiles geochemical, geochronologic, and isotopic data to evaluate the melt conditions and processes that generated these rocks. The geochemistry and partial melting temperatures (ca. 675–775 °C) support water-absent muscovite dehydration melting and/or water-deficient melting as the primary melt reactions and are generally inconsistent with water-excess melting and high-temperature (biotite to amphibole) dehydration melting. The CAB rocks are oldest in the central U.S. Cordillera and become younger towards both the north and south. At any single location, partial melting appears to have been a protracted process (≥10 Myr) and evidence for re-melting and remobilization of magmas is common. End-member hypotheses for the origin of the CAB include decompression, crustal thickening, fluid-flux melting, and increased heat flux from the mantle. Different parts of the CAB support different hypotheses and no single model may be able to explain the entirety of the anatectic event. Regardless, the CAB is a distinct component of the Cordilleran orogenic system

Criteria for Recognition of Subducted (Orocopia) Schist in Western Arizona

INVITED POSTER: The Orocopia Schist is a latest Cretaceous low-angle subduction channel, part of a larger subduction complex, the Pelona-Orocopia-Rand Schist (PORS), that underlies much of southern California and southwest Arizona (Fig. 1; Jacobson et al. 1988, 2007, 2011; Haxel et al. 2002; Chapman 2016). The principal locus of Orocopia Schist, the Chocolate Mountains anticlinorium extending from the Orocopia Mountains east to Neversweat Ridge, has been known since the mid 1970s (Haxel and Dillon 1978). Recently, two more exposures of the oceanic Orocopia Schist have been found at isolated localities farther inland in southwest Arizona: Cemetery Ridge (Haxel et al. 2015, 2018b, 2021; Jacobson et al. 2017) and northern Plomosa Mountains (Strickland et al. 2017, 2018; Seymour et al. 2018). These discoveries raise the possibility that additional inboard areas of Orocopia Schist have yet to be found. Hence a review of criteria for recognition of Orocopia Schist is warranted.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Mantle peridotite and associated metasomatic rocks in the Orocopia Schist subduction channel (latest Cretaceous) at Cemetery Ridge, southwest Arizona

This geologic map and report document peridotite—harzburgite, olivine orthopyroxenite, and serpentinite after dunite—entrained in a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Bedrock Geologic Map of Organ Pipe Cactus National Monument and Vicinity, Southwest Arizona

Organ Pipe Cactus National Monument preserves an area of 1300 km² typical of the Lower Sonoran Desert ecosystem of southern Arizona, encompasses part of the ancestral lands of the Tohono O’odham Nation, and adjoins the international border between United States and Mexico. In 1976, the Monument was declared a UNESCO Biosphere Reserve; in 1977, 95 percent of the Monument area was designated Wilderness. The hallmark species of OPCNM is the organpipe cactus (Stenocereus thurberi). This columnar cactus, though common in northwest Mexico and widely scattered through parts of southwest Arizona, is most abundant in the United States in OPCNM. Another arboreal Mexican cactus, the senita (Lophocereus schottii), is found in the U.S. only in one small area within OPCNM. OPCNM geology is characterized by northwest-southeast trending mountain ranges separated by broad alluvial plains, typical of the southern Basin and Range tectonic province. Diverse volcanic, intrusive, metamorphic, and sedimentary rocks exposed within and near the Monument record nearly two billion years of Earth history, weather to picturesque geomorphic features, and support several distinctive plant communities. Rocks near the town of Ajo (18 km north of the Monument) host ores of copper, molybdenum, and gold. This report is a much-revised version of data originally compiled for the National Park Service Geologic Resource Inventory. The report and accompanying map collect decades-long work in geologic mapping of OPCNM and analytical studies of its rocks; document the geologic history of the Monument; and provide a complete, current geologic map for scientific, educational, and public use.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Geologic Map and Geochronology of the Picacho, Picacho NW, Picacho SW, and Hidden Valley 7.5-Minute Quadrangles, Arizona and California

This report accompanies a digital geologic map of the Picacho, Picacho SW, Picacho NW, and Hidden Valley 7.5-minute quadrangles — the four quadrants of the Picacho 15-minute quadrangle. The map area is located within the central and south-ern Trigo Mountains, Arizona, and southeastern Chocolate Mountains, California and is dominated by stratigraphically and structurally complex Mesozoic and Cenozoic rocks relevant to understanding the geologic evolution of the U.S. Southwest more broadly. The report provides descriptions of the geologic map units, their environments of formation, age constraints, deformational history, and regional correlations. It also includes a brief summary of metallic ore deposits within the map area, as well as a selective bibliography of publications on ore deposits in and adjacent to the map area.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134-79Ma). Sediment was likely underthrust into the active arc by circa 74-65Ma, as soon as 7Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79-60Ma) and may indicate a link between the burial of sediments and pluton emplacement