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A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of this transition and the inception and tectonic development of the Permo-Triassic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America, in successions in the distal back-arc region on the Colorado Plateau, and in the more proximal back-arc region in the rocks of the Buckskin Formation of southeastern California and west-central Arizona (southwestern North America). The Buckskin Formation is correlated to the Lower--Middle Triassic Moenkopi and Upper Triassic Chinle Formations of the Colorado Plateau based on stratigraphic facies and position and new detrital zircon data. Calcareous, fineto medium-grained and locally gypsiferous quartzites (quartz siltstone) of the lower and quartzite members of the Buckskin Formation were deposited in a marginal-marine environment between ca. 250 and 245 Ma, based on detrital zircon U-Pb data analysis, matching a detrital-zircon maximum depositional age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An unconformity that separates the quartzite and phyllite members is inferred to be the Tr-3 unconformity that is documented across the Colorado Plateau, and marks a transition in depositional environments. Rocks of the phyllite and upper members were deposited in wholly continental depositional environments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz wacke) in the phyllite member indicate deposition in fluvial systems, whereas the fine- to medium-grained beds of quartzite (quartz arenite) in the upper member indicate deposition in fluvial and shallow-lacustrine environments. The lower and phyllite members show very strong age and Th/U overlap with grains derived from Cordilleran arc plutons. A normalized-distribution plot of Triassic ages across southwestern North America shows peak magmatism at ca. 260-250 Ma and 230-210 Ma, with relatively less activity at ca. 240 Ma, when a land bridge between the arc and the continent was established. Ages and facies of the Buckskin Formation provide insight into the tectono- magmatic evolution of early Mesozoic southwestern North America. During deposition of the lower and quartzite members, the Cordilleran arc was offshore and likely dominantly marine. Sedimentation patterns were most strongly influenced by the Sonoma orogeny in northern Nevada and Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism and the "shoaling" of the arc. The phyllite and upper members were deposited in a sedimentary system that was still influenced by a strong contribution of detritus from headwaters far to the southeast, but more locally by a developing arc that had a far stronger effect on sedimentation than the initial phases of magmatism during deposition of the basal members. [ABSTRACT FROM AUTHOR]

Background: Many states in the U.S. have changed how they regulated medicinal and/or recreational marijuana. Understanding the impact of these policy changes on marijuana use and perceptions of acceptability and safety of use is needed. The current study examined perceptions and patterns of marijuana use among college students, a population at increased risk for initiation and heavy use. Method: Data include Colorado university students and national sample averages. We used repeated cross-sectional analyses to examine responses to the American College Health Association National College Health Assessment and added marijuana measures. We examined differences in use and norms pre- to post-legalization among Colorado college students. Colorado data were also compared to national summary statistics. Results: Social norms, use initiation, and use recency and frequency in Colorado increased significantly across changes in recreational legislation. Descriptive norms and rates of ever users in Colorado increased faster than national averages. Conclusions: Results suggest recreational legalization increases how quickly social norms become more approving. As marijuana policy continues to change, implementing social norms interventions alongside legislative changes may offset increases in risk for use initiation and frequency by countering increased social norms favorable to use. [ABSTRACT FROM AUTHOR]

The Upper Triassic Chinle Formation in southwestern Laurentia is the oldest distinctive record of Early Mesozoic Cordilleran arc magmatism, in the form of detrital zircons and volcanic clasts. Initial deposition of the basal Shinarump and Mesa Redondo members, herein collectively called the Shinarump conglomerate, began in Late Triassic time, yet the earliest known arc magmatism is older by as much as 40 m.y. Analysis of detrital zircons from eight sites in southeastern Nevada, southern Utah, and northeastern Arizona and volcanic-clast zircons from four of these sites provides a basis for understanding the evolution of the Early Mesozoic arc. Most Permian and Triassic detrital zircons from the Shinarump conglomerate have ages from ca. 260 to 220 Ma with rare grains as old as 280 Ma. These ages are compatible with derivation from sources in the magmatic arc to the west and southwest, including plutons of corresponding age in the Mojave Desert. Volcanic clasts are uniformly in the range 232-224 Ma; their age and zircon geochemistry argue against a source in currently exposed Mojave Desert Triassic plutons. As a further test, we compared Th/U ratios of clast and detrital zircons with those of possible sources to the west. Th/U values of many detrital grains support their derivation from Triassic Mojave Desert plutons. Some detrital grains and those from the clasts, however, have Th/U values that are uniformly higher than those in Permo-Triassic Mojave Desert plutons and therefore argue for a different, unexposed source. We propose that the early arc lay offshore of western Laurentia. Over time, plutons were emplaced across a range of continental crustal thicknesses that likely increased toward the east. At approximately 235-230 Ma, a land connection between the arc and retro-arc areas was established and fluvial sedimentation began. The observation that the youngest grain ages in our detrital samples are variable suggests that this land connection was tenuous for perhaps 10 m.y. until well into Chinle Formation sedimentation. [ABSTRACT FROM AUTHOR]

Upper Triassic sandstones in diverse locations in eastern California, southern Arizona, and on the Colorado Plateau (USA) yield detrital zircons that are remarkably similar in age and geochemistry, leading to the hypothesis that they are temporally related and were derived from similar sources. Volcaniclastic sandstone from the lowest Vampire Formation in eastern California, the Sonsela Member of the Chinle Formation at Petrified Forest National Park, northeastern Arizona, and the herein-named Waterman formation in southern Arizona yield zircons that range in age from ca. 205 to ca. 235 Ma. Together with the similar range of ages, these zircons uniformly have Th/U ratios between ∼0.2 and 2. In addition, the Waterman formation contains zircon grains with an age range from ca. 225 to 250 Ma, but with markedly lower Th/U ratios of 0.1-0.2, and a distinctively older group with ages to ca. 280 Ma. In a general sense, variations in Hf concentrations and Yb/Gd ratios support the discrimination of grains based on age and Th/U. We use age and geochemical data from the zircons to infer that these units capture a slice of time during development of the early Mesozoic Cordilleran magmatic arc along western North America. Plutonic rocks that record magmatism in the arc are Permian-Triassic in age, and match zircon ages in the detrital grains, thus providing a view of which parts of the arc were actively eroding into the stream systems that deposited the three units. Streams diverged from a common source that maintained a relatively uniform magma composition over time, as indicated by a narrow range of Th/U values, as well as tapping a somewhat different source evidenced by a grouping in which Th/U ratios are lower. Once the streams left the highlands of the arc and the depocenter of the lowest Vampire Formation, they diverged, such that one flowed to the area of the Colorado Plateau while the second flowed toward southern Arizona. At the same time, a stream system originating in the older, Sonoran part of the arc flowed from the south into southern Arizona. [ABSTRACT FROM AUTHOR]

Trace-element analyses of zircons from ash-flow tuffs and granitic rocks in the eastcentral Sierran Nevada, combined with U-Pb zircon ages and trace-element abundances in whole rocks, allow us to interpret a shared magmatic heritage for widespread eruption of ash-flow tuffs and assembly of a typical Sierran granodioritic to granitic intrusive suite. The tuffs have 5%-15% phenocrysts of quartz, feldspar, biotite, and amphibole, silica contents ranging from 70% to 74%, and have variable alkali abundances but are consistently light rare-earth enriched with weak negative Eu anomalies, suggesting chemical classification as hydrothermally altered, arctype, low-silica rhyolites. The tuffs yielded U-Pb zircon crystallization ages of 232, 224, and 219 Ma, indicating that explosive volcanism began before and continued throughout emplacement of granodioritic to granitic plutons in the underlying 226-218 Ma Scheelite Intrusive Suite, one of the oldest and largest intrusive suites in the Sierra Nevada batholith. Zircon crystals from ash-flow tuffs and contemporaneous felsic granodiorite and granite show distinct chemical zoning and extensive overlap in trace-element abundances, indicating crystallization of zircons from very similar melt compositions despite dissimilar late cooling histories. These data provide evidence for a unified volcano-plutonic model for Sierran arc magmatism that describes a longlived, multi-stage magma system composed of an incrementally emplaced, granodioritic to granitic intrusive suite in the upper crust, overlain by widespread deposits of ignimbrite eruptions generated by thermal rejuvenation of felsic crystal mushes. [ABSTRACT FROM AUTHOR]

Granitic and volcanic rocks in the eastcentral Sierra Nevada, western United States, record the earliest stages of magmatism in the eastern Sierra Nevada magmatic arc, allowing us to examine magma sources and connections between plutonic and volcanic processes in the initial stages of arc construction. The Scheelite Intrusive Suite is one of the largest in the Sierra Nevada region, and is composed of the Wheeler Crest Granodiorite, granite of Lee Vining Canyon, and Pine Creek Granite. The Pb/U zircon ages from each unit of the suite suggest assembly between 226 and 218 Ma. The Scheelite Intrusive Suite is a high-K calcic or calc-alkalic suite, compositionally broadly similar to the nearby Late Cretaceous Tuolumne and John Muir Intrusive Suites, though plutons of the Scheelite Intrusive Suite are consistently Ca and Fe rich and lower in Na. Although Triassic granodiorites are isotopically quite similar to nearby Late Cretaceous intrusive suites, the trend toward more isotopically primitive granites is in contrast to the constant or more whole-rock radiogenic Sr trends observed in younger intrusive suites. Along the western margin of the Scheelite Intrusive Suite, the basal Mesozoic volcanic section in the Saddlebag Lake pendant includes silicic volcanic rocks that are in part coeval and potentially comagmatic with Triassic plutonic rocks. Widespread quartz-phyric ash-flow tuffs of Black Mountain, Saddlebag Lake, and Greenstone Lake yield Pb/U zircon ages of 232, 224, and 219 Ma, indicating that felsic ignimbrite volcanism commenced earlier and continued during emplacement of the 226-218 Ma Scheelite Intrusive Suite. Ash-flow tuffs are hydrothermally altered but have high fi eld strength element abundances and Nd isotopic compositions, suggesting affinity to the relatively felsic parts of the Wheeler Crest Granodiorite and the granite of Lee Vining Canyon. [ABSTRACT FROM AUTHOR]