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Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ~5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery. Keywords: U-Pb geochronology, conodont, biostratigraphy, Permian, Triassic, extinction.

Biostratigraphic, carbon isotope, and U-Pb zircon geochronological data from the Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542.0 [+ or -] 0.3 Ma, consistent with previous age constraints from Siberia and Namibia. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary. Keywords: extinction, Proterozoic, Cambrian, Oman, U-Pb, geochronology, carbon isotopes.

Quartzite, conglomerate, and shale of the Mazatzal Group record the filling of a Proterozoic intra-arc basin in central Arizona. U-Pb ages of zircons from rhyolite ash-flow tuff indicate that deposition began at 1701 [+ or -] 2 Ma. Basal deposits of the newly defined Pine Creek Conglomerate formed in an alluvial-fan setting, synchronous with the final phase of extrusive rhyolite volcanism and active faulting. After volcanic activity ceased, shallower-slope braided-stream environments developed in early Deadman Quartzite time. Subsequent marine transgression produced shallow subaqueous deposits of the upper part of the Deadman Quartzite. These were overlain by prodelta sediments of the Maverick Shale, indicating a phase of basin deepening. Finally, the Mazatzal Peak Quartzite was deposited as the basin shoaled again, first to shallow-marine and subsequently to fluvial conditions. The quartzite is diagenetic quartz arenite, originally deposited as lithic arenite and lithic arkose. The provenance was initially restricted, and earliest sediment was derived mainly from the subjacent Red Rock Group. When tectonic activity ceased, however, the surrounding highlands were planed down by erosion, and detritus from a wider variety of source rocks was funneled into the basin. This included contributions from arc-related supracrustal rocks of the Payson Ophiolite and East Verde River Formation, and finally a granitic basement input. Detrital quartz in the lower part of the Mazatzal Group is largely monocrystalline, and volcanic in origin. Polycrystalline quartz, associated with detrital feldspar and of probable continental affinity, is concentrated in the uppermost parts of the sequence. The life span of the intra-arc basin was on the order of 30 m.y., from the formation of the Payson Ophiolite at 1.73 Ga to the deposition of the upper Mazatzal Peak Quartzite sometime after 1.70 Ga. The pre-Mazatzai Red Rock Group represents the last stages of volcanic arc activity, and the Mazatzal Group records the transition from orogenic to nonorogenic sedimentation in central Arizona. Keywords: arc, Arizona, orogeny, Proterozoic, U-Pb correlation.

The Devonian Solund Basin in SW Norway is the only Old Red Sandstone basin in the Scandinavian Caledonides from which volcanic rocks have been described. These rhyolitic lavas yield a concordant ID TIMS U-Pb zircon age of 439 [+ or -] 1 Ma (Early Silurian) interpreted as dating the crystallization of the rhyolite, and are thus too old to be syndepositional with the Devonian sedimentary rocks. This new age constraint, combined with field observations, suggests that the volcanic rocks form part of a 30 [km.sup.2] landslide of rhyolite, granite, gabbro and metasedimentary rocks, rather than extension related volcanism as previously suggested. The landslides show an inverted stratigraphy with respect to the nearby Solund-Stavfjord Ophiolite Complex and its overlying volcanic arc, which was formed immediately prior to obduction of Caledonian exotic terranes onto Baltica. The dated rocks thus appear to record the youngest pre-collisional volcanism in the Scandinavian Caledonides. Tectonic models involving Devonian Old Red volcanic rocks in the Scandinavian Caledonides must thus be revised. Keywords: Devonian, Silurian, U-Pb, western Norway, landslides

The Vindhyan Supergroup of central India, the focus of many paleontological studies, has been reported to contain Cambrian small shelly fossils, Ediacaran fossils, trace fossils, and Proterozoic microfossils and carbonaceous megafossils. New U-Pb zircon and [sup.87]Sr/ [sup.86]Sr isotopic data from the Lower Vindhyan Supergroup require that the rocks are latest Paleoproterozoic to earliest Mesoproterozoic in age. Two rhyolitic volcanic horizons from the Deonar Formation, between the Kajrahat and Rohtasgarh Limestones and below the unit containing trace fossils, yield U-Pb zircon ages of 1631 [+ or -] 5 Ma and 1631 [+ or -] 1 Ma. The Kajrahat and Rohtasgarh Limestones of the Semri Group that are below and above the reported Mesoproterozoic trace fossils have [sup.87]Sr/[sup.86]Sr ratios of 0.70460 and 0.70479, respectively. The Bhander Limestone from the Upper Vindhyan Supergroup has an [sup.87]Sr/ [sup.86]Sr ratio of 0.70599, consistent with a Neoproterozoic age for this formation. These results indicate that the Kajrahat Limestone is of latest Paleoproterozoic age and the Rohtasgarh Limestone is of probable Mesoproterozoic age. These findings are in conflict with the report of Cambrian small shelly fossils and fossils of articulate brachiopods in the Rohtasgarh Limestone and argue for a Mesoproterozoic age for the formation that contains the alleged trace fossils. Reports of an Ediacaran fossil Spriggina(?) from the Lower Vindhyan Supergroup from the northern margin of the Vindhyan Basin suggest either incorrect stratigraphic correlation of units or misidentification of this fossil. Keywords: Vindhyan Supergroup, metazoan evolution, U-Pb zircon dating, Sr isotope stratigraphy.

U-Pb geochronological data from the Connemara region of Ireland indicate that continental arc magmatism along the southern margin of Laurentia was short-lived, lasting only from ca. 475 to 463 Ma. Previous work has demonstrated that intrusive activity in Connemara was broadly synchronous with Grampian mid-crustal deformation and upper amphibolite facies metamorphism of Neoproterozoic - lower Paleozoic Dalradian Supergroup rocks. The two oldest intrusions, the Currywongaun and Cashel-Lough Wheelaun gabbros, have U-Pb zircon ages of 474.5 [+ or -] 1.0 and 470.1 [+ or -] 1.4 Ma, respectively, whereas the U-Pb xenotime age of the postdeformational Oughterard granite is 462.5 [+ or -] 1.2 Ma. Thus, the implied age of the Grampian orogeny in Connemara is substantially younger than generally acknowledged, but consistent with other age constraints from Ireland and Scotland. A revised age for the Grampian orogeny helps to solve a long-standing controversy about the age of contractional deformation and arc magmatism in Dalradian Supergroup rocks of the British Isles. This event now demonstrably postdates Neoproterozoic-lower Paleozoic rift deposits and Laurentian passive margin sediments, a finding consistent with deposition of Dalradian Supergroup rocks in an extensional environment at or near the Laurentian margin. The Grampian orogeny is also now demonstrably synchronous with the Taconian orogeny of the northern Appalachians. In Middle Ordovician time, the Laurentia-Iapetus plate boundary was characterized by ophiolite obduction, arc-continent collision, and associated short-lived subduction beneath Laurentia from at least the New England Appalachians to the Irish Caledonides.

We report a U-Pb zircon age of 1105 [+ or -] 2 Ma for the extensive Umkondo dolerites in eastern Zimbabwe, which are part of a large igneous province that can be traced over much of southern Africa and originally contiguous parts of East Antarctica. Other members of the province include widespread tholeiitic intrusions in Botswana and South Africa, bimodal volcanic rocks in Botswana and Namibia, and dolerites and flood basalts in Dronning Maud Land, Antarctica. Available data indicate that substantial parts of the province were emplaced in a restricted interval at ca. 1.1 Ga and originated from a large-scale mantle thermal anomaly inboard of a coeval continental-margin orogen. Striking similarities in age and tectonic setting between the Umkondo igneous province and widespread 1.1 Ga within-plate magmatism in Laurentia are consistent with reconstructions of the early Neoproterozoic Rodinia supercontinent that place southern Africa and Dronning Maud Land off the southern tip of Laurentia.

The central Mojave metamorphic core complex is defined by a belt of Miocene brittle-ductile extension and coeval magmatism. The brittle-ductile fault zone defines a basin-and-dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes. One fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km. The axial surfaces of these folds can be traced from the footwall mylonites, through the brittle detachment, and into hanging-wall strata. However, folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment, suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime, possibly after movement across the fault zone ceased. Mesoscopic fabrics related to the transport-parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction. We interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large-magnitude extension. The other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km. Mesoscopic fabrics related to these folds include northwest-striking joints, kink bands, and en echelon tension-gash arrays. These fabrics formed after mylonitization and record both layer-parallel extension and northeast-side-up subvertical shear. The postmyionitic fabrics are kinematically compatible with rolling-hinge-style isostatic rebound of the footwall following tectonic denudation. The relative timing of extension-related magma intrusion and ductile deformation varies through the central Mojave metamorphic core complex. On the scale of the small mountain ranges that make up the central Mojave metamorphic core complex, no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension-related igneous rocks. This suggests that models that invoke a single upper-crustal genetic relationship, such as magmatism triggering extension or vice versa, do not apply to the central Mojave metamorphic core complex. Systematic variation in the relative timing of dike emplacement and mylonitization suggests that, at the time of dike emplacement, rocks in the Mitchel Range were below the brittle-ductile transition while those in the Hinkley Hills were above it. The Hinkley Hills and Mitchel Range are separated by [approximately]2 km in the dip direction of the fault zone, which suggests that the vertical thickness of the brittle-ductile transition probably was between 100 and 950 m.

The geologic history of the outer continental margin (eugeoclinal) rocks in the El Paso Mountains and northern Mojave Desert has long been important in models for the development of the active continental margin in the western Cordillera. Current interpretations call for either strike-slip or thrust juxtaposition of eugeoclinal rocks against miogeoclinal/cratonal (platformal) rocks, or some combination of both strike-slip faulting and thrusting. Two broad and interrelated aspects of the history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the eugeoclinal rocks? and (2) When were the eugeoclinal rocks thrust against platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures restore to a position structurally beneath the eugeoclinal rocks when Tertiary extension is restored, implying stacking by a thrust. New U-Pb zircon geochronology and whole rock geochemistry and Sr, Nd, and Pb isotopic data for plutons in the El Paso Mountains and northern Mojave Desert lend insight into the timing of this thrusting. The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and [[Epsilon].sub.Nd(t)] isotopic values from these plutons are distinct ([less than or equal to]0.704 and [greater than or equal to] +2, respectively) from continental lithosphere isotopic signatures ([greater than or equal to]0.705 and [less than or equal to]-2) of both Middle Jurassic plutons in the same area and Triassic plutons in the southern and eastern Mojave Desert. Feldspar common lead data for the Permian and Triassic plutons within the eugeoclinal outcrop belt also indicate limited crustal involvement and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Proterozoic basement is widespread. The observations and data reported here indicate that Late Permian-Early Triassic plutons in the northern Mojave Desert and El Paso Mountains were generated within or passed primarily through oceanic lithosphere, but later Jurassic plutons were derived from and/or interacted extensively with continental lithosphere. We hypothesize that the eugeoclinal rocks were deposited on oceanic crust that was thrust eastward over Precambrian cratonal basement and overlying strata between approximately 240 Ma and 175 Ma, a time of little documented tectonic activity in the Mojave Desert region. Postulated Permian or late Middle Jurassic east-directed thrusting is incompatible with our data and observations. The data also fill an important gap in palinspastic reconstructions of the early Mesozoic arc and indicate that this arc was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.

Stratigraphic correlations presented here for the ductilely deformed and metamorphosed rocks exposed in the Shadow Mountains indicate that they formed on the North American continental margin and are not exotic or significantly displaced from their site of origin. These strata represent a depositional history that spans Late Proterozoic and Paleozoic passive margin development, late Paleozoic transitional passive to active plate margin tectonics, and late Paleozoic-early Mesozoic establishment of a convergent margin along the western edge of the North American craton. The stratigraphic sequence in the Shadow Mountains is represented by a basal siliceous and calcareous section that is correlated with upper Proterozoic and Cambrian miogeoclinal strata. These strata are overlain unconformably(?) by a calcareous sequence correlated with rocks of Pennsylvanian and Permian age of borderland affinity. The uppermost sequence comprises hornfels and calcareous rocks that rest unconformably across older strata and are correlated with Permian and Triassic rocks that likely record uplift and erosion of a magmatic arc. These stratigraphic correlations have several important paleogeographic and tectonic implications for southwestern North America. First, passive margin, Late Proterozoic and early Paleozoic miogeoclinal facies extend as far west in the Mojave Desert as the Shadow Mountains. Second, the presence of Pennsylvanian and Permian borderland basin sediments suggests that the area was affected by late Paleozoic tectonics, probably associated with the transition from a passive to an active plate-margin setting. Third, inferred Late Permian-Triassic rocks record the onset of convergent-margin tectonism and magmatism in this region.

In the southwestern United States, two early Miocene extensional terranes, one along the Colorado River trough and the other in the central Mojave Desert, have been inferred to be part of an en echelon rift system linked by a ductile detachment at depth. Evidence from pre-Tertiary surface geology in the central and western Mojave Desert suggests that a zone of east- to northeast-trending right-lateral strike-slip faulting may have transferred extension between these two areas. These data include the apparent offsets of the Late Jurassic age Independence dike swarm, a Middle to Late Jurassic age deformation belt, Triassic(?)-Jurassic volcanic rocks, Lower Triassic sedimentary rocks, and late Paleozoic age stratigraphic and tectonic elements. This inferred fault zone probably served as the main shallow-crustal kinematic link between extension in the central Mojave Desert and in the Colorado River trough.

Data supporting the existence of Proterozoic basement in the central and western Mojave Desert include U-Pb zircon geochronology and Nd, Sr, and Pb isotopic values of quartzofeldspathic gneisses, detrital zircon provenance ages, and the presence of basement clasts in Paleozoic and Mesozoic conglomerates. These data corroborate existing isotopic data from Mesozoic and Tertiary intrusive rocks that suggest involvement of Proterozoic crust in their genesis. Exposures of Proterozoic basement and Late Proterozoic and Paleozoic transitional miogeoclinal-cratonal facies trends in the central and western Mojave Desert consistently imply that cratonal North America continues westward uninterrupted through this region to the San Andreas fault. These data place geographic limits the position of several pre-Tertiary tectonic elements speculated to exist in the Mojave Desert.

The presence of rocks from the Proterozoic and early Paleozoic ages in the Mojave Desert, CA, is not proof that there is a miogeoclinal-cratonal hingeline in the area. This is the contention of some observers who maintain that stratigraphic data shows that no such hingeline exists in the region. On the other hand, observers say that the hingeline in the area applies to those areas of the Mojave Desert that are not properly preserved. Furthermore, these same observers interpret the stratigraphic data for the region as sufficient evidence that a hingeline is present in the area.

The Connemara region of the western Irish Caledonides records the evolution of a short-lived mid-Ordovician (c. 475-463 Ma) continental magmatic arc that intruded into Dalradian metasedimentary rocks during regional ductile deformation. New U-Pb geochronological data for intermediate intrusive phases and anatexis in southern Connemara are consistent with the relative age succession suggested by field relationships. Calc-alkaline plutons (quartz diorites) were emplaced into the Dalradian sequence and into the Connemara Gabbros at about 467 [+ or -] 2 Ma, coeval with anatexis, which was manifested by a 467 [+ or -] 2 Ma granitic pegmatite and 468 [+ or -] 2 Ma leucosome formation in a metapelite. Metasomatic diopside rocks from west-central Connemara and a calcsilicate rock from southern Connemara yield U-Pb titanite dates of c. 462 [+ or -] 2 Ma and 463 [+ or -] 2 Ma, respectively. These dates postdate the metamorphic peak by at least five million years and indicate titanite crystallization at a late stage during the metamorphic history. On the basis of U-Pb and [sup.40]Ar/[sup.39]Ar geochronological data we conclude that the cooling history of southern Connemara following anatexis was simple and brief, at a minimum rate of 35 [degrees] C [Ma.sup.-1] between 468 and 460 Ma, and 14 [degrees] C [Ma.sup.-l] between 460 and 450 Ma. Keywords: U-Pb, [sup.40]Ar/[sup.39]Ar, Connemara, magmatism, metamorphism, deformation.

Research is presented concerning the use of 40Ar/39Ar dating and U-Pb dating of monazite to investigate the main anatectic midcrustal ultraductile top of the orogenic thickening occurring in the shear zones of the east Greenland Caledonides.

Research is presented concerning the relationship between the tectonics of the noncollisional deformation of hinterland and foreland in the fold-and-thrust orogen formation of central North America.

Field and geochronologic data from the late Paleozoic paired metamorphic belt and magmatic arc is presented, exposed in the Lake District of the Andean foothills and Coastal Ranges of southcentral Chile. The data brings tighter age constraints on the initiation of pre-Andean subduction processes along this area. Regional exhumation and juxtaposition of high pressure temperature and low pressure temperature assemblages took place during a time of quiescence in arc magmatism, during or prior to dextral oblique convergence over the margin between the Middle Permian and Late Jurassic.