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U-Pb geochronologic and Hf isotopic analy ses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents signifi cant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early-Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2-1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7-0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with signifi cant production of juvenile crust during late Archean (3.0- 2.5 Ga) and Paleoproterozoic (1.78-1.6 Ga) time and phases of mainly crustal reworking at 2.0-1.78, 1.5-1.3, 1.2-1.0, and 0.6-0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3-0.2 Ga. [ABSTRACT FROM AUTHOR]

The article discusses a study on the age and tectonic significance of the McCoy Mountains Formation (MMF) in western Arizona. The researchers performed uranium-lead (U-Pb) isotopic analysis (IA) of detrital-zircon grains, and geochronological, geochemical and neodymium IA of mafic sills and lava flows within the clastic sequence. Their findings support the interpretation that the lower MMF was deposited during rifting within the western extension of the Sabinas-Chihuahua-Brisbee rift belt.

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature mé1ange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 4°Ar/[sup 39]Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite [sup 40]Ar/ [sup 39]Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona. [ABSTRACT FROM AUTHOR]

U–Pb geochronologic studies have been conducted on 60 detrital zircon grains from Permian(?) and Triassic metasandstones of the Taku terrane in central southeast Alaska. The resulting ages are mainly in the range 349–387 Ma, with five additional grains that yield probable ages ranging from ~906 to ~2643 Ma. These ages are similar to the ages of detrital zircons in Carboniferous and older rocks of the Yukon–Tanana terrane, which lies directly east of the Taku terrane. In contrast, these ages are different from the ages of detrital zircon grains in the Alexander terrane to the west. The data are accordingly consistent with models in which the Taku terrane is a western component of the Stikine and Yukon–Tanana terranes, and that this crustal fragment is separated by a fundamental tectonic boundary from rocks of the Alexander and Wrangellia terranes to the west.Des études géochronologiques ont été effectuées sur soixante grains détritiques de zircon provenant de métagrès (Permien(?) et Trias) du terrane de Taku dans le centre sud-est de l'Alaska. Les âges déterminés sont principalement dans la fourchette 349–387 Ma et cinq grains additionnels ont donné des âges probables allant de ~906 à ~2643 Ma. Ces âges sont similaires aux âges des zircons détritiques dans des roches datant du Carbonifère et dans des roches plus anciennes du terrane de Yukon–Tanana, lequel est situé directement à l'est du terrane de Taku. Par contre, ces âges diffèrent des âges de grains détritiques de zircon dans le terrane d'Alexander, plus à l'ouest. Les données concordent donc avec des modèles dans lesquels le terrane de Taku est une composante occidentale des terranes de Stikine et de Yukon–Tanana et que ce fragment de la croûte est séparé des roches des terranes d'Alexander et de Wrangellia, plus à l'ouest, par une frontière tectonique fondamentale.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Provides information on the extensional tectonism, ocean-chemistry fluctuations and biological diversification during the late Neoproterozoic Era, preserved by the Chuar Group, exposed only in the Grand Canyon in Arizona. Documentation of shallow-marine deposition; Geochronology of the Chuar Group.

Studies the geology and geochronology of Greenville-age rocks in the Van Horn and Franklin Mountains area, west Texas, and its implications for the tectonic evolution of Laurentia during the Grenville. Mesoproterozoic rocks of Trans-Pecos Texas; Rifting.

Presents the results of a study which examined the origin and age of the Manila high and associated sunklands in the Big Lake basin of northern Arkansas' New Madrid seismic zone. Stratigraphy; Topography; Deformation chronology of the Manila high and Big Lake basin; Position of the Little River.

A small, but perfectly exposed outcrop of Mesoproterozoic rocks in the western Llano uplift displays lithologic and structural relations from which a sequence of geologic events has been determined. Detailed geologic mapping and petrologic studies combined with new U-Pb ages of key units are used to constrain the tectonic evolution of this little-studied region and provide a basis for comparison with the previously studied southeastern uplift. The outcrop consists of layered mafic and felsic gneiss cross-cut by three generations of granitic dikes and sills. The sequence of events as deduced from cross-cutting relations and U-Pb geochronology is as follows: (1) generation of compositional layering (S[sub0]) and parallel biotite foliation (S[sub1]) in the gneisses at 1256 Ma; (2) emplacement of the first set of granitic sills and dikes at 1253 Ma; (3) transposition, isoclinal folding, amphibolite facies metamorphism, and fabric formation (S[sub2]) between 1253 Ma and 1126 Ma; (4) emplacement of pegmatitic granitic dikes at 1126 Ma; (5) open folding and boudinage between 1126 Ma and 1076 Ma; and (6) emplacement of aplitic granitic dikes at 1076 Ma. Four U-Pb ages of titanite, two each from the gneiss and pegmatitic granite, are concordant at ∼ 1114 Ma indicating that the region had been sufficiently hot to reset titanite ages in the gneiss and that the region had cooled below the titanite blocking temperature by this time. The tectonic evolution of this outcrop is remarkably similar to that of other areas of the Llano uplift, suggesting that the entire uplift experienced a similar evolution. [ABSTRACT FROM AUTHOR]