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The revised dates plot close to a single time scale line, except for Rb Sr dates on acid volcanic rocks.

We conclude that the base of the Devonian is at 410 Ma, the base of the Silurian at 437 Ma, and the base of the Ordovician (Tremadoc Series) at 519 Ma.

Timing and geometries support synchronous but opposing divergence along Cordilleran and Atlantic rifts with a junction in Southern California–Sonora.

A reassessment of published Ordovician, Silurian and Devonian isotopic age determinations available has necessitated the correction of several calculated dates and revision of several stratigraphic conclusions in the light of later work.

Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the 200-km-long northwest-aligned Big Creek-Beaverhead belt. Dating delimits two discrete magmatic pulses at ca.

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic–Paleozoic western Laurentian rift margin in the northern U. 665–650 Ma and 500–485 Ma at the western and eastern ends, respectively, of this belt.

A comparison with other time scales shows that our new scale is similar to many previously published, except for those which rely heavily on acid volcanics.

A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites.In the Trans Antarctic Mountains coarse sediments of a probable Ordovician age and a dated volcanism were recognized. Pensacola and Patuxent Mounts comprise the Neptune Group, which consists largely of conglomerate to quartzose sandstone. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lower–middle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites.A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve.It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean. Ordovician rocks were recognized in the Trans Antarctic Mountains, Ross orogen, inside the sector claimed by Argentine, and close to this area, in the Ellsworth Mountains. The end of the Ordovician marks one of the greatest of the Earth's mass extinctions.One hypothesis explains this mass extinction as the result of a short-lived, major glaciation preceded by episodes of increased volcanism brought on by the Taconic orogeny.The aim of this study is to obtain more precise U-Pb zircon ages from biostratigraphically constrained bentonites which will lead to better correlation of the Upper Ordovician and Lower Silurian relative time scales, as well as time the pulses of eruption.Zircon grains were extracted from the samples by heavy mineral separation and U-Pb dated using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer at the University of Texas-Austin.

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  1. Cial epoch begin, and how correctly are individual glacial events dated ? In the latter context the late Ordovician Hir- nantian event seems to be most complicated.

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