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There is growing consensus that development of a semipermanent ice sheet on Antarctica began at or near the Eocene-Oligocene (E-O) boundary. Beyond ice-rafted debris in oceanic settings, however, direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to East Antarctica. It is unclear where glacier ice first accumulated and how extensive it was on the Antarctic continent in the earliest Oligocene. Sediments at the top of the Eocene marine shelf section on Seymour Island, Antarctic Peninsula, include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula. Dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the E-O boundary. Glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive West Antarctica ice sheet, and thus an even more dramatic response to the forcing factors that facilitated high-latitude ice expansion in the earliest Oligocene. Keywords: Oligocene, glaciation, Antarctica, till, stratigraphy.

Detrital zircons from three Neoproterozoic sandstone units from the Cadomia terrane of northern France and the Channel Islands yield ages in three broad groups: late Neoproterozoic (650-600 Ma), early Palaeoproterozoic (2.4-2.0 Ga) and Archaean (>2.5 Ga). The hack of Mesoproterozoic zircon crystals, combined with the high abundance of grains between 2.20 and 2.00 Ga, corresponds closely to the ages of exposed rocks in the West Africa Craton, and thus it is suggested that Cadornia was in close proximity to West Africa by c. 580 Ma. In contrast, the main age groups of detrital zircon from the Neoproterozoic Avalon terrane are Mesoproterozoic and there is a distinct gap of ages between 2.40 and 2.05 Ga. These significant differences suggest that the two terranes were in different locations relative to major Gondwanan cratons in latest Neoproterozoic time. Keywords: Cadomian Orogeny, U-Pb, detrital zircon, palaeogeography.

Gabbro from the Caledonia Mountain Pluton in the northeastern Caledonian Highlands of southern New Brunswick yielded a U-Pb (zircon) crystallization age of 615 ± 1 Ma. The age shows that [...]

A precise U-Pb zircon age of [Mathematical Expression Omitted] Ma has been determined for the foliated quartz diorite at Perelle Bay, Guernsey, UK Channel Islands. The age is based on the analysis of single zircons, half of a single grain, and groups of two and three zircons. The age is nearly 100 Ma younger than that suggested by other workers, based on 207Pb/206Pb dates of large, multi-grain fractions of zircon, and thus rules out this intrusion as the earliest marker of the onset of Cadomian magmatism in the North Armorican Massif. A 611 Ma crystallization age for the Perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid-state deformation fabrics resulting from syntectonic emplacement during Cadomian orogenesis. The U-Pb age is approximately 8-15 Ma older than 40Ar/39Ar hornblende dates for the pluton, consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient crustal temperatures. These new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events, but only mark events within a period of regional tectonism. The ability to analyse single zircons. and selected portions of single zircons. using ultra-low blank U-Pb analytical techniques documents the feasibility of determining precise emplacement ages of other Neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon. Keywords: Cadomian Orogeny, English Channel Islands, U-Pb, syntectonic processes, quartz diorites.

Gneissic units of the Channel Islands, United Kingdom, have traditionally been considered to be Palaeoproterozoic basement to the late Neoproterozoic Cadomia terrane, based on lithological correlations and imprecise or ambiguous isotopic data. A new precise U-Pb date of 2061 [+ or -] 2 Ma, based on analyses of single zircons, for the Icart granitic orthogneiss of Guernsey confirms a Palaeoproterozoic age of its igneous protolith. The Nd depleted mantle model age ([T.sub.DM]) of this gneiss is 2220 Ma, only slightly older than its crystallization age, indicating that it represents juvenile crust. Other gneisses from Guernsey have similar [T.sub.DM] ages ranging from 2210 to 2370 Ma, suggesting that they are also exposures of juvenile Palaeoproterozoic crust. A component from a penetratively deformed orthogneiss from Sark, previously correlated with Icartian gneisses on Guernsey, yields a U-Pb zircon date of [Mathematical Expression Omitted] Ma. This crystallization age demonstrates that the protolith was a Cadomian intrusion and that penetrative deformation and amphibolite-facies metamorphism in the northern Channel Islands were Cadomian in age. The use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable. The presence of 2170 [+ or -] 7 Ma zircon xenocrysts within the Sark orthogneiss, combined with its initial [[Epsilon].sub.Nd](615) = - 9.5, are strong evidence that pre-Cadomian basement was a significant source component, and that documenting the extent and age of pre-Cadomian basement may be possible by indirect geochemical methods. Keywords: Cadomian Orogeny, English Channel Islands, U/Pb, Sm/Nd, absolute age.

Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Sheiburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

Synkinematic Yanceyville granite gneiss, syn-to late synkinematic Kilgore granite crystallization and the crystallization of the Farmers Lake suggest that the Hyco shear zone was active during mid-Mississippian time and that its deformation and metamorphisms happened in lte Mississippian times. This finding compared to that of southerly segments of the central Piedmont shear zone suggests that Alleghanian deformation happened at the same time along the entire zone.

The arcuate surface of the newly recognized Hyco shear zone supports the theory that the Hyco Lake and Virginia segments of the zone are lateral and frontal ramps of the Alleghanian thrust regime. The earlier belief that the zone was a contact area between Milton and Carolina terrain has been challenged by the new findings. The nature, timing and kinematics of the zone boundary should be re-examined.

The Yadong cross structure (YCS) found in the east central Himalaya is the largest across-strike discontinuity in the geologic structure and topography of the High Himalaya between the Himalayan syntaxes. It is a surface manifestation of a lateral ramp in the Main Boundary Thrust, which is measured at about 10 km in height facing west-northwest. Furthermore, the South Tibetan Detachment System offsets the YCS at 70 km across.

The chemical, isotopic and mineralogical characteristics of mid-Ordovician North American K-bentonites were analyzed to determine whether Laurentia and Gondwana were contiguous during the early Paleozoic era. A link between Laurentia and Gondwana may be established if an appropriate source region for the North American K-bentonites is found in South America. A discovery of K-bentonites in the North American Caradocian strata with chemical traits of North American beds also demonstrates such a link.

The Middle Ordovician rocks of the Taconic foredeep in the Mohawk Valley, New York, contain numerous altered volcanic ash beds (K-bentonites). These synchronous beds have the potential to link disparate facies in a way that is not feasible with biostratigraphy or sequence stratigraphy alone. Geochemical fingerprinting of glass inclusions within volcanic quartz phenocrysts permits unambiguous matching of the compositionally unique ash layers. The resulting correlations demonstrate that time lines based on the graptolite biozones and transgressive-regressive facies patterns parallel the K-bentonite isochrons. They also demonstrate that much of the upper Trenton Group is older than previously believed and facies relations are markedly different from previous interpretations. Although some facies patterns appear to be synchronous across the region, the likely existence of substantial structural control on facies development in the basin suggests that it is unwise to attribute these patterns to eustatic sea-level changes.

The stratigraphy of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet, Southern Alaska is comprised of Jura-Cretaceous rocks of the Gravina belt, pre-Permian to Triassic rocks of theTaku terrane, and mid-Proterozoic to upper paleozoic continental margin assemblages. These assemblages may be part of the Yukon-Tanana terrane, which lies underneath most of Yukon and eastern Alaksa. Gravina belt, Taku and Yukon-Tanana terrane strata lie side-by-side along a mid-Cretaceous thrust fault system comprising the Alexander-Wrangellia terrane inboard margin.

The Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane. [ABSTRACT FROM AUTHOR]

A U-Pb zircon age of 530.7 + or - 0.9 Ma for a volcanic ash in the lower Placentian Series of the Avalonian terrane in southern New Brunswick provides a precise point for calibration of Early Cambrian biologic evolution. The ash is from the middle Rusophycus avalonensis Zone in Saint John, New Brunswick. This date is on subtrilobitic, Lower Cambrian strata that may be approximately correlative with the sub-Tommotian Manykaian Stage of Siberia and the lowest Meishucunian Stage of the South China platform. This age, when combined with published geochronologic data from the Cambrian, suggests that the Manykaian Stage may have lasted approximately 13 m.y. and that the Tommotian may have been considerably shorter.

Uranium-lead dates for single and small multiple grain zircon separates are presented for the main geological units from Jersey, British Channel Islands. A turbiditic sandstone from the Jersey Shale Formation (Brioverian Supergroup) contains detrital zircons of a variety of ages, ranging from c. 2.6 to 0.6 Ga. The youngest zircons yielded nearly concordant U-Pb dates of 586.7 [+ or -] 3.0 Ma and 587.1 [+ or -] 2.7 Ma providing a maximum depositional age for the unit. Zircons from overlying volcanic rocks (Anne Port Rhyolite) give an upper intercept date of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Ma, considered to be the age of eruption. The undeformed SW Jersey granite, which intrudes and locally thermally metamorphoses the folded Jersey Shale Formation, is dated at [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Ma. These dates constrain tightly the timing of deposition of the Jersey Shale Formation and the age of deformation on Jersey, and indicate a dynamic arc environment during the late Neoproterozoic. Zircons from the NW Jersey granite define an upper intercept date of 482.7 [+ or -] 1.6 Ma, similar to the previously reported [sup.40]Ar/[sup.39]Ar hornblende date of c. 473 Ma, confirming a renewed pulse of magmatism during the Ordovician in this sector of Cadomia. Keywords: Cadomian Orogeny, U-Pb, absolute age, Channel Islands, Jersey.