Detrital zircon grains in the ∼1740–1750 Ma Vishnu Schist and similar rock units in northwestern Arizona consist of up to 30% grains dated by U-Pb isotopic analysis at 2470–2490 Ma. These zircon grains are distributed over ∼40,000 km2 and define an age peak at 2480.0 ± 27.3 Ma (2SE). These grains have yielded unusually consistent 207Pb/206Pb dates, with generally smaller analytical uncertainty and greater concordance to ideal U-Pb evolution than grains of other ages. A weighted mean age of 2480 ± 0.9 Ma (2SE) for this zircon population reflects consistent analytical results and high analytical precision but not the accuracy of the age. The source of these zircons has not been identified. To better characterize the unidentified source, we analyzed 45 of these grains for trace and rare-earth elements by laser-ablation mass spectrometry and scanned 16 grains with an electron microprobe to identify mineral inclusions. Mass spectrometer determinations of Sc/Yb and Nb/Sc support derivation from an oceanic-island igneous source. Electron microprobe scans revealed quartz in 5 of 16 grains, indicating a felsic source. The low variability in 207Pb/206Pb dates and a generally linear relationship between U and Th support zircon derivation from a single igneous unit or closely related set of units without xenocrystic zircons. A literature search for other zircon populations with similar age and U/Th ratios identified ∼2480 Ma zircons in a Mesoproterozoic(?) metapsammite and conglomerate in southwestern Montana. This sandstone was deposited near the margin of the Wyoming craton and contains almost entirely 2400–3600 Ma zircons, unlike zircon grains in Vishnu Schist which include a large population of 1730–1900 Ma zircons. From this relationship we infer that the 2480 Ma zircons in both areas were derived from a source in the Wyoming craton. We conclude that the 2480 Ma Vishnu zircons were derived from a felsic batholith that formed above and from hotspot magma related to the ∼2450–2480 Ma Matachewan Large Igneous Province, that this batholith formed by mixing between a mantle-derived hotspot magma and assimilated Archean continental crust, and that the source rock was emplaced during initial rifting between the Wyoming craton and the Superior province.

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 CO2, 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 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.

This presentation is designed to draw attention back to the type area where new geochronologic constraints warrant revising the timing of the Mazatzal orogeny to ca. 1471-1436 Ma.The term "Mazatzal orogeny" is widely used to describe regional deformation across southwest Laurentia roughly between ca.1695-1580 Ma (Karlstrom and Bowring, 1988;Duebendorfer et al., 2015).The type area for the Mazatzal orogeny is located in the Mazatzal Mountains of central Arizona's Tonto Basin (Wilson, 1922(Wilson, , 1937(Wilson, , 1939)).Thought to have closely followed the northwestdirected shortening of the ca.1700 Ma Yavapai orogeny (Karlstrom and Bowring, 1991), the coaxial deformation of the Mazatzal orogeny occurred after deposition of the Mazatzal Group quartzite and shale, and long before deposition of the Middle Mesoproterozoic Apache Group (Wilson, 1939).The early estimates on the timing of the Mazatzal orogeny in its type area, and starting in the middle 1960's, bracketed timing of deformation roughly between 1715 and 1650 Ma (Silver, 1965(Silver, , 1967)).However, emphasis on this stage of deformation has shifted from Arizona to New Mexico, Colorado, and southern Wyoming where the Mazatzal orogeny was expanded spatially and temporally to ca. 1580 Ma (for example Bauer and Williams, 1994 and others).

For more than 25 yr, the Mazatzal orogeny has been a central component of virtually all tectonic models involving the Proterozoic rocks of the southwestern United States. Recent recognition that some sedimentary sequences and some major structures are Mesoproterozoic rather than Paleoproterozoic has led to new questions about the nature, even the existence, of the Mazatzal orogeny. This study aims to clarify the relationship between Mazatzal (ca. 1.65 Ga) and Picuris (ca. 1.45 Ga) orogenic activity. New U-Pb geochronology of variably deformed igneous and metasedimentary rocks constrains several periods of deformation at ca. 1.68 Ga, 1.66 Ga, and 1.49–1.45 Ga in the Four Peaks area of central Arizona. Detrital zircon analyses and field relationships indicate the deposition of a rhyolite-sandstone-shale assemblage at ca. 1.660 Ga with renewed deposition at 1.502–1.490 Ga and a significant disconformity, but no recognized angular unconformity, between these episodes. Three populations of monazite growth at 1.484 ± 0.003 Ga, 1.467 ± 0.004 Ga, and 1.457 ± 0.005 Ga indicate prolonged Mesoproterozoic metamorphism. The ca. 1.485 Ga population is associated with the formation of the Four Peaks syncline during Mesoproterozoic orogenesis and subsequent amphibolite-facies contact metamorphism. Rocks in the Four Peaks area record polyphase deformation, sedimentation, and plutonism from the Paleoproterozoic to Mesoproterozoic. Hf-isotopic data suggest the involvement of older, nonjuvenile crust. In this area, effects of the Mazatzal (ca. 1.65 Ga) and Picuris orogenies (ca. 1.49–1.45 Ga) are entwined and involved sedimentation, deformation, pluton emplacement, and pluton-enhanced metamorphism.

The Arizona Geological Survey has been conducting field investigations in Arizona as part of the joint Federal-State STATEMAP program (a component of the National Geologic Mapping Act of 1992). During the course of geologic investigations many samples were collected for U-Pb geochronologic analysis as part of field-mapping studies. These samples were analyzed by the Arizona LaserChron Center at the University of Arizona (with support from NSF grant EAR 1338583). Numerous U-Pb analyses of Arizona rocks have also been produced by other laboratories over the past ~25 years. The partial compilation of data included with this publication (Fig. 1) is intended for use by those engaged in geologic investigations. Special care was taken in this report to determine exact sample locations and present both latitude-longitude and UTM coordinates for samples. Samples without accurate sample locations are not included in this database. Much of the analytical data has not been completely evaluated, and statistical techniques for analysis are still being developed. Analytical data are not reported for many samples - these were added to this database simply so that sample locations would be accurately recorded.

Mesoproterozoic sedimentary basins in western North America provide key constraints on pre-Rodinia craton positions and interactions along the western rifted margin of Laurentia. One such basin, the Belt-Purcell basin, extends from southern Idaho into southern British Columbia and contains a >18-km-thick succession of siliciclastic sediment deposited ca. 1.47-1.40 Ga. The ca. 1.47-1.45 Ga lower part of the succession contains abundant distinctive non-Laurentian 1.61-1.50 Ga detrital zircon populations derived from exotic cratonic sources. Contemporaneous metasedimentary successions in the southwestern United States-the Trampas and Yankee Joe basins in Arizona and New Mexico-also contain abundant 1.61-1.50 Ga detrital zircons. Similarities in depositional age and distinctive non-Laurentian detrital zircon populations suggest that both the Belt-Purcell and southwestern U.S. successions record sedimentary and tectonic linkages between western Laurentia and one or more cratons including North Australia, South Australia, and (or) East Antarctica. At ca. 1.45 Ga, both the Belt-Purcell and southwest U.S. successions underwent major sedimentological changes, with a pronounced shift to Laurentian provenance and the disappearance of 1.61-1.50 Ga detrital zircon. Upper Belt-Purcell strata contain strongly unimodal ca. 1.73 Ga detrital zircon age populations that match the detrital zircon signature of Paleoproterozoic metasedimentary rocks of the Yavapai Province to the south and southeast. We propose that the shift at ca. 1.45 Ga records the onset of orogenesis in southern Laurentia coeval with rifting along its northwestern margin. Bedrock uplift associated with orogenesis and widespread, coeval magmatism caused extensive exhumation and erosion of the Yavapai Province ca. 1.45-1.36 Ga, providing a voluminous and areally extensive sediment source-with suitable zircon ages-during upper Belt deposition. This model provides a comprehensive and integrated view of the Mesoproterozoic tectonic evolution of western Laurentia and its position within the supercontinent Columbia as it evolved into Rodinia.

Globally rare 1.6-1.5 Ga zircons provide an underutilized correlation tool for Precambrian supercontinent reconstructions. Detrital 1.6-1.5 Ga zircons, long known from the Belt Supergroup, have recently been discovered in multiple metasedimentary successions in southwestern North America. Few igneous or metamorphic sources in this age range are known in Laurentia, implying non-Laurentian provenance. Combined U-Pb and Hf isotopic data offer a robust test of their provenance. New U-Pb detrital zircon ages from quartzite exposed in the Defiance uplift in northeastern Arizona indicate a maximum depositional age of ca. 1476 Ma and a predominance of locally derived ca. 1655 Ma and older detritus. The quartzite contains populations at 1570, 1554, and 1519 Ma that do not have known Laurentian sources and are similar to our new and recently published data from the ca. 1474-1436 Ma Blackjack Formation in southcentral Arizona. Based on similarities between age spectra and depositional age constraints, we suggest a correlation between the two stratigraphic sections and postulate that a previously unrecognized, and now largely eroded, 1475-1450 Ma sedimentary basin or set of basins may have extended across much of southwestern Laurentia. We refer to this basin as the Yankee Joe-Defiance basin. Hf isotopic analysis of ca. 1.6-1.5 Ga detrital zircons from both localities yields positive epsilon-Hf (epsilon(Hf)) values of +0.2 to +12 indicating a near-juvenile Hf fingerprint with a mean of similar to +7. This range overlaps with, but is more juvenile than, epsilon(Hf) values of +3 to +7.5 from the Belt basin. In terms of possible non-Laurentian source regions, Hf data from both the North and South Australia cratons overlap with, but are also generally less juvenile than, epsilon(Hf) values of Laurentian zircons. Only the North Australia craton contains the full range of ages and Hf isotope values recognized in the 1.6-1.5 Ga Yankee Joe-Defiance populations. This finding supports plate reconstructions involving Australia as a source for exotic detritus in western Laurentia ca. 1480-1450 Ma. (C) 2013 Elsevier B.V. All rights reserved.

Detrital zircon data from the upper parts of the Proterozoic Hess Canyon Group of southern Arizona reveal abundant 1600–1488 Ma detrital zircons, which represent ages essentially unknown from southern Laurentia. This basinal succession concordantly overlies a >2-km-thick-section of 1657 ± 3 Ma rhyolite of the Redmond Formation. The rhyolite is intercalated with and hence contemporaneous with the lower parts of the overlying White Ledges Formation, a 300-m-thick orthoquartzite unit at the base of the Hess Canyon Group. These quartzites contain a unimodal detrital zircon age probability distribution with peak ages of 1778, 1767, and 1726 Ma, supporting regional correlation with other ca. 1.65 Ga quartzite exposures in southwestern Laurentia. However, the ∼900-m-thick argillaceous Yankee Joe and minimum 600-m-thick quartzite-rich Blackjack Formations contain younger detrital zircons, with peak ages ranging from 1666 to 1494 Ma and a maximum depositional age of 1488 ± 9 Ma. Prominent age peaks at 1582–1515 Ma and 1499–1488 Ma represent detritus that is exotic and not derived from known southern Laurentian sources. The Blackjack Formation is cut by the 1436 ± 2 Ma Ruin Granite, indicating that deposition, deformation, and intrusion occurred between 1488 and 1436 Ma. This basin likely developed before or in the early stages of the 1.45–1.35 Ga intracontinental tectonism in southwestern Laurentia. Our findings necessitate the presence of an ∼170 m.y. disconformity within the Hess Canyon Group and document a previously unrecognized episode of Mesoproterozoic basin sedimentation (>1.5 km of section) between 1488 and 1436 Ma in southern Laurentia. This new record helps to fill the 1.60–1.45 Ga magmatic gap in southern Laurentia and supports hypotheses for a long-lived Proterozoic tectonic margin along southern Laurentia from 1.8 to 1.0 Ga. The 1.6–1.5 Ga detrital zircon ages offer important new constraints for ca. 1.5 Ga Nuna reconstructions and for the paleogeography of contemporaneous basins such as the Belt Basin in western Laurentia.

New field studies combined with U-Pb zircon geochronology constrain the ages of deposition and sedimentary provenance of Paleoproterozoic quartzite successions exposed in the southwestern United States. Orthoquartzites were deposited in short-lived basins at two times (ca. 1.70 and 1.65 Ga) during crustal assembly of southern Laurentia. The more voluminous ca. 1.70 Ga successions occur in southern Colorado, northern New Mexico, and central Arizona and are interpreted here to-be time correlative, though not necessarily deposited in the same basins. Detrital zircon from quartzites and metaconglomerates exposed in southern Colorado and northern New Mexico is characterized by a single population with a relatively narrow range of ages (1.80-1.70 Ga) and minimal Archean input (< 5% of grains analyzed). Peak detrital zircon ages (1.76-1.70 Ga) vary slightly from location to location and mimic the age of underlying basement. Unimodal detrital populations suggest local sources and a first-cycle origin of the orthoquartzites within a short time interval (1.70-1.68 Ga) during unroofing of local underlying basement. The maximum age of quartzite exposed at Blue Ridge, Colorado, is constrained by the 1705-1698 Ma coarse-grained granitoid basement on which quartzite was deposited unconformably. The minimum age of Ortega Formation quartzite (New Mexico) is constrained by ca. 1680-1670 Ma metamorphic monazite overgrowths. These dates agree with direct ages on the lower Mazatzal Group, Arizona, and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca. 1.70 Ga Yavapai orogeny. Regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca. 1.66-1.60 Ga Mazatzal orogeny, thus making them important, time markers separating the Yavapai and Mazatzal orogenic events. Our model for syntectonic deposition involves extensional basin development followed by thrust closure, possibly due to opening and closing of slab rollback basins related to outboard suhduction. The first-cycle origin of orthoquartzites near the end of the arc collisions of the Yavapai orogeny seems to contrast sharply with their extreme compositional maturity. This can be explained in terms of protracted, extreme diagenesis and/or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the Proterozoic. Similarities among quartzites exposed throughout the southwestern United States and along the Laurentian margin suggest that they represent a widespread regional, and perhaps global, episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions, a combination that might make these units a signature lithology for Paleoproterozoic time.

Geologic map and cross section through the northern Matzazal Mountains of Shake Tree Canyon, Central Arizona. Scale 1:24,000. Two sheets.