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The early Late Ordovician sedimentary rocks of eastern North America contain a relatively large number (>100) of widespread heavily altered tephra layers (K-bentonites). These beds represent an intense period of subaerial volcanism that occurred from ca. 455 to 449 Ma. The sedimentary rocks that contain these K-bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies. Accurate correlation of these ancient ash-fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono-sedimentary provinces. Despite the relatively thorough study of a few of these K-bentonites over the past several decades, the full stratigraphic potential of these beds has yet to be realized. To test the utility of the apatite trace-element K-bentonite correlation method on a larger scale, we studied over 200 K-bentonite samples from the Mohawkian Stage of eastern North America and statistically compared our results with previous studies on the same suites of K-bentonites. Electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) results show that apatite trace-element data provide unique bed discriminators. Each of the K-bentonite layers exhibits unique and reproducible trends in Mg, Cl, Mn, Fe, Ce, Y, and other trace-element concentrations in apatite. Statistical evaluation of results from our apatite analyses suggests correlations for 12 K-bentonite beds, providing a significant improvement in stratigraphic resolution. The stratigraphic relations indicated by these new K-bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic, chemostratigraphic, and sequence stratigraphic studies in eastern North America. [ABSTRACT FROM AUTHOR]

The metasedimentary Arvonia Formation in the Piedmont Province of central Virginia ranges primarily from slates to garnet schists, which unconformably overly Middle Ordovician metavolcanic rocks of the Chopawamsic arc terrane. The formation has typically been interpreted as Late Ordovician based on poorly preserved fossils, perhaps having formed as a successor basin following accretion of the Chopawamsic arc to Laurentia. However, alternative interpretations have been proposed, including deposition outboard of Laurentia on Chopawamsic arc crust (that is, prior to accretion). Furthermore, the origin of the Chopawamsic arc as peri-Laurentian or peri-Gondwanan is an unresolved issue. We report whole-rock major and trace element compositions as well as Nd-isotopic compositions of the Arvonia Formation, and use these results to evaluate further its origin. Samples collected along the length of the outcrop belt show similar major element compositions, which are typical of shales. Chemical Index of Alteration values (68-89), corrected for K-metasomatism, indicate intermediate to extreme levels of source weathering. A majority of the samples show uniform rare-earth element (REE) patterns that are light REE-enriched with negative Eu-anomalies. A few samples display anomalous REE patterns, indicative of some disturbance of the light REE at approximately the time of deposition. A variety of trace element values or ratios indicate that Arvonia sediments are similar to shale composites (thus upper continental crust). Nd-isotopic compositions for eight samples are similar, with εNd = -5.6 to -8.9 (400 Ma). Collectively, these results are consistent with derivation of Arvonia sediments from a single source or well-mixed sources of typical upper crustal composition. The isotopic data are compatible with a Laurentian provenance, however, they are equally compatible with a strictly Chopawamsic crustal source based on available Nd isotopic data. [ABSTRACT FROM AUTHOR]

Abstract: Granitic intrusions emplaced within Laurentia during the Grenville Orogeny (1.15–1.05 Ga) are exceptionally Zr-rich, primarily a function of high modal zircon, relative to Paleozoic granitoids emplaced within eastern Laurentia during phases of Appalachian orogenesis. Erosion of Grenville source rocks would generate disproportionately large volumes of detrital zircon compared to less zircon-fertile source regions. The latter sources are difficult to detect by standard in situ U–Pb dating methods of detrital zircon (SHRIMP or LA-ICP-MS U–Pb analysis of centers of > 100 μm grains), or could be overlooked during sampling. Grenvillian zircon fertility biased the Neoproterozoic to Recent sedimentary record as a result of two factors: (1) zircon durability and insolubility led to recycling during repeated orogenesis; (2) inertness of zircon below the onset of anatexis means dominantly metamorphosed sedimentary terranes failed to generate significant new zircon corresponding in age to the time of accretion of those terranes to Laurentia. Zircon growth under anatectic conditions generates new zircon, commonly as overgrowths on preexisting zircon, which may be too narrow to easily analyze by laser or ion beam techniques. Accretion of metasedimentary terranes therefore could be rendered all but undetectable via standard U–Pb detrital zircon dating. Grenville age modes dominate detrital zircon age distributions for Laurentian Neoproterozoic rift basins, Appalachian Paleozoic synorogenic clastic sequences, Appalachian metasedimentary terranes, and modern rivers draining these terranes. This is an artifact of the Grenville zircon signal that echoed throughout all Paleozoic orogenies on the eastern Laurentian margin. The natural Grenville bias in the detrital zircon record is further amplified by standard sampling biases (such that large (e.g. 100–300 μm) crystals are preferentially chosen) and analytical biases (e.g. cores are much more frequently analyzed). [Copyright &y& Elsevier]

During the Neoproterozoic Cadomian orogeny in northern France, supracrustal rocks of the Brioverian Supergroup were deposited in marginal and back arc basins, and were subsequently variably deformed and metamorphosed. New U–Pb analyses of single, and small multigrain fractions of zircon from selected plutons from the Baie de St. Brieuc region provide robust geochronological constraints on the timing of these events. The Jospinet granodiorite forms part of the local basement directly overlain by Brioverian metasediments and basic volcanics, and yields a U–Pb zircon date of 625.9+3.6/−1.9 (2σ) Ma. The pre-tectonic Port Moguer tonalite, which has been strongly sheared along with its amphibolite facies country rocks, has a crystallization age of 600.4±0.9 Ma. Emplacement ages of 576.3+1.5/−1.2 Ma for the syn-tectonic Fort La Latte quartz diorite and 574.6+1.8/−1.5 Ma for the late-tectonic St. Quay quartz diorite place limits on termination of deposition and timing of subsequent regional deformation of the Brioverian sequence in the Baie de St. Brieuc region. The new dates constrain the age of Brioverian sedimentation to the interval 626–575 Ma, a range consistent with a previously published Pb–Pb zircon evaporation age of ca. 588±22 Ma for Brioverian volcanic rocks (Lanvollen Formation). Deformation within this sector of the Cadomian belt is believed to have occurred shortly before 575 Ma, revising previously published estimates for the age of this major tectonothermal Cadomian event by 10–20 My. [Copyright &y& Elsevier]

• Mesoproterozoic basement rocks were dated by zircon U-Pb geochronology. • Shawinigan and Ottawan igneous and metamorphic ages were determined. • Tectonomagmatic events consistent between southern and northern French Broad massif. • Large- n zircon U-Pb data sets required to adequately characterize metaigneous rocks. Mesoproterozoic metaplutonic rocks from the southern French Broad massif in southwestern North Carolina preserve Grenville magmatic and metamorphic ages (1.28–1.0 Ga). New in-situ zircon U-Pb measurements from eight basement lithologies, obtained by laser ablation and ion-probe techniques, delineate Elzevirian (1283–1250 Ma), Shawinigan (1191–1126 Ma), and Ottawan (1034–1008 Ma) magmatism and metamorphism. Magmatism produced dominantly calc-alkaline plutons with normative compositions of quartz diorite, quartz monzodiorite, granodiorite, and monzogranite. The majority of magmatism in the field area occurred between ca. 1190 and 1125 Ma, a period correlative to the Shawinigan phase of the Grenville orogeny. Syn- to post-emplacement metamorphism of these units is evidenced by complex zircon U-Pb systematics in all Shawinigan age plutons. Ottawan magmatism, expressed by emplacement of the Max Patch granite and protoliths of the Earlies Gap granitoid gneiss, is less voluminous than Shawinigan magmatism; a difference that mirrors their relative proportions elsewhere in the orogen. Ottawan metamorphism is recorded by growth of new zircon rims around magmatic core domains in several samples and spans a period of ca. 25 m.y. from ca. 1035 to 1010 Ma. The timing of the older and younger magmatic suites from the southern French Broad massif correlate to well-established times of Shawinigan and Ottawan magmatism and metamorphism in other Grenville-age terranes within the U.S. (e.g., Appalachian inliers, Adirondacks). [ABSTRACT FROM AUTHOR]

States that tectonic models of the Ordovician Taconian orogeny in western New England, usually cause a collision between the Laurentian margin and a magmatic arc known as the Bronson Hill arc. Details on study conducted in this area; Presentation of a tectonic model; Indications of findings

Presents neodymium (Nd) isotopic data from clastic sedimentary rocks within the Taconic and Sevier foreland basins to examine changes in provenance of sedimentary rocks within a basin. Geologic constraints on provenance; Determination of time of deposition of Nd based on isotopic ratios.

Examines the precise U-Pb dates for an orthogneiss, a granitic gneiss, and synkinematic granites within the Hyco shear zone, with focus on geological sciences. Methodology used to gather the information; Indications of the findings; Importance of the discovery of the Hyco shear zone.

Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrital garnet ages were compared to published detrital zircon and monazite ages from the same river tributary system. Results show that 87% of the zircons have inherited Proterozoic ages; only zircon rims give Paleozoic ages. Monazites are exclusively Ordovician (weighted average: 460.9 ± 3.0 Ma). Our new detrital garnet ages (and the detrital zircon rims) record younger ages spanning the Late Ordovician to Early Devonian, likely reflecting prolonged metamorphic heating. The weighted average age of the detrital garnet population is Silurian (430.1 ± 7.2 Ma). Statistical tests confirm that the garnet population is younger than the monazite. The new detrital garnet ages illuminate a previously uninterpreted Silurian tectonometamorphic signal in this region. [ABSTRACT FROM AUTHOR]

Sm-Nd garnet-whole rock geochronology and phase equilibria modeling have been used to determine the age and conditions of regional metamorphism within the Tasriwine ophiolite complex, Sirwa, Morocco. Pressure and temperature estimates obtained using a NaCaKFMASHT phase diagram (pseudosection) and garnet core and rim compositions predict that garnet growth began at ∼0.72 GPa and ∼615 °C and ended at ∼0.8 GPa and ∼640 °C. A bulk garnet Sm-Nd age of 647.2 ± 1.7 Ma, calculated from a four point isochron that combines whole rock, garnet full dissolution and two successively more aggressive partial dissolutions, provides a precise date for garnet formation and regional metamorphism. The age is over 15 million years younger than a previous age estimate of regional metamorphism of 663 ± 13 Ma based upon a SHRIMP U-Pb date from rims on zircon from the Iriri migmatite. The new data provide further constraints on the age and nature of regional metamorphism in the Anti-Atlas mountains and emphasizes that garnet growth during regional metamorphism may not necessarily coincide with magmatism/anatexis which predominate the signature witnessed by previous U-Pb studies. The ability to couple PT estimates for garnet formation with high precision Sm-Nd geochronology highlights the utility of garnet studies for uncovering the detailed metamorphic history of the Anti-Atlas mountain belt. [ABSTRACT FROM AUTHOR]

For over 50 years the Pan African orogeny has been recognized as a Neoproterozoic tectonothermal episode affecting West Africa 800–550 Ma. As such, the Pan African events are similar to the Appalachian orogenic cycle extending from ∼1100 to 250 Ma and the Cordilleran orogenic cycle of 350 Ma to the present. A significant difference is that the Appalachian orogenic cycle has long been recognized as consisting of separate Grenville, Taconic, Acadian and Alleghenian orogenies. Similarly, the Cordilleran orogenic cycle consists of distinct Antler, Sonoma, Nevadan, Sevier, Laramide and ongoing Cascadian-Andean orogenies. Failure to distinguish individual tectonic events in the Anti-Atlas Mountains has been attributable to the dearth of radiometric dates in this region. Since 2000, precision geochronologic dating in the Anti-Atlas Mountains, Morocco, has provided a means by which it is now appropriate to designate a Pan African orogenic cycle consisting of separate distinct orogenic events. We herein propose the following distinct orogenic events in the Anti-Atlas Mountains of Morocco: Iriri-Tichibanine orogeny (760–700 Ma), Bou Azzer orogeny (680–640 Ma) and the WACadomian orogeny (620–555 Ma). [ABSTRACT FROM AUTHOR]

Provenance and tectonic setting of clastic rocks of the Neoproterozoic Ocoee Supergroup were assessed using modes of detrital framework minerals, detrital feldspar chemistry, whole-rock geochemistry, heavy mineral assemblages and chemistry, and detrital zircon U-Pb geochronology. Putative source rocks comprising locally exposed Grenville basement were analyzed for comparison. Samples studied are primarily arkosic to subarkosic siltstones, sandstones, and minor conglomerate. Ternary tectonic discrimination diagrams indicate a continental basement uplift setting, although the mineralogically most mature Longarm quartzite indicates a continental interior setting. The range of detrital feldspar compositions among Ocoee units is comparable to feldspar in basement rocks. Orthoclase-rich detrital alkali feldspar predominates over detrital albitic plagioclase. Whole-rock normative Qz-Or-Pl values and trace-element abundances closely match those of local basement. Snowbird Group siltstones and sandstones contain high abundances of zircon, apatite, titanite, ilmenite, and epidote, which locally occur as concentrations of heavy minerals in discrete laminae. Zircon-tourmaline-rutile indices and titanite compositions are most consistent with a granitic source for these sediments. The majority of detrital zircon U-Pb ages are late Mesoproterozoic, with scattered late Neoproterozoic and Paleoproterozoic grains. All but one of the ∼1200 detrital zircons (at ca. 2000 Ma) can be accounted for by a local Blue Ridge basement magmatic source terrane. Archean detrital zircon ages are absent from the Ocoee detrital zircon age population. This absence is consistent with the general paucity of Archean grains in other Neoproterozoic Southern Appalachian clastic sequences. All data are consistent with a lithologically and geochronologically restricted, proximal, flank source for sediment filling a marginal continental rift. The Ocoee represents the first pulse of sediment dominated by Grenville-age zircon on the eastern Laurentian margin and serves as a major source of recycled zircon for Paleozoic clastic sequences in eastern Laurentia. The youngest detrital zircon age obtained from the basal Ocoee Wading Branch Formation (639 ± 8 Ma) provides a maximum depositional age for the Ocoee Supergroup and onset of rift-filling sedimentation in the intracontinental Ocoee basin. [ABSTRACT FROM AUTHOR]

Abstract: A series of flux-grown synthetic zircon crystals doped with Hf and selected rare earth elements (REE) were produced for use as a potential reference material for Lu–Hf isotopic analysis of zircon by in situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS). The synthetic zircon crystals were doped such as to produce a large range of REE/Hf, allowing for a robust means of monitoring and correcting the sometimes large 176Yb and 176Lu isobaric interferences on the low-abundance 176Hf in natural zircon, as well as potential oxide interferences from Gd, Tb, and Dy. The synthetic zircon crystals have a homogeneous Hf isotopic composition, both within and between grains, as documented by solution MC-ICPMS analyses of Hf chemically separated from multiple fragments of zircon from three different syntheses, and 321 laser ablation MC-ICPMS analyses done in two different laboratories. An additional 110 in situ analyses of natural zircon crystals reveal identical behavior of natural and synthetic zircon during Hf isotope analysis by laser ablation. Hence, the synthetic zircon crystals provide a means of monitoring a wide range of Yb and Lu interferences (up to 50% of the total signal intensity at mass 176) during routine isotopic analysis for geological studies. They also may be helpful in developing improved laboratory protocols for accurate in situ Lu–Hf isotopic measurement of natural zircon. The present data illustrate the importance of the 176Yb interference correction on 176Hf, which can result in large biases if not properly applied to natural samples with their variable and often high 176Yb/177Hf. This study definitively shows that use of high-REE/Hf synthetic zircon standards for calibrating the Yb mass bias correction is more accurate than use of Yb-doped natural Hf solutions. Finally, the results of the present work suggest that synthetic minerals may prove useful as both in situ method development tools and isotopic reference materials. [Copyright &y& Elsevier]

Abstract: The late Paleozoic Alleghanian orogeny culminated with the continent–continent collision between Laurentia and Gondwana in the construction of the supercontinent Pangea. Within the Appalachian basin, the sedimentological record of the early stages of the continental collision is preserved as late Mississippian-age siltstones and mudstones that mark the end of carbonate production. An upward transition to coarse, quartz-rich, cratonward-prograding, fluvial deposits of Pennsylvanian age is cited to mark the initiation of continental collision and cratonward advancement of the tectonic load. Seven samples of early Pennsylvanian (early-middle Morrowan) sandstones and conglomerates from the Appalachian basin were collected along the orogen from eastern Pennsylvania to central Alabama for U–Pb dating of detrital zircons. All of the samples reveal a dominance of zircons with ages that can be attributed to crust-forming events in Laurentia. A small percentage of the detrital zircons have ages (530–680 Ma and 2000–2200 Ma) that correspond to the Pan-African/Brasiliano orogenic belts and Trans-Amazonian/Eburnian cratons typical of Gondwanan crust. Although recycling of pre-orogenic strata may disproportionately bias the detrital-zircon population toward ages of Laurentian affinity, the dominance of the Laurentian “signature” in the Appalachian basin along strike suggests that the early components of the Alleghanian orogen were composed mainly of recycled Laurentian crust. Detrital-zircon populations are not indicative of substantial incorporation of Gondwanan crust in the orogenic highlands. On the basis of the timing of dextral displacement along shear zones in the Appalachian hinterland, the early Pennsylvanian clastic deposits preserved in the Appalachian basin likely correspond to transpressional exhumation of the Laurentian margin, consistent with hypotheses of oblique collision between Gondwana and Laurentia in the late Paleozoic. [Copyright &y& Elsevier]

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]

Although northern peatlands represent a globally significant reservoir for carbon, considerable uncertainty exists concerning solute transport systems within large (>1000 km 2 ) peat deposits. We therefore delineated geochemical gradients linked to groundwater recharge and discharge along a 6 km transect within the 1200 km 2 Red Lake Peatland of northwestern Minnesota. We used ratios of Ca/Mg and 87 Sr/ 86 Sr to distinguish discharge of calcareous groundwater (∼1.4 and 0.7155, respectively) to the peatland from the mineral substratum along a topographic gradient from a bog crest downslope to an internal fen water track and bog islands. In contrast, the stable isotopes of the porewaters (δ 18 O from −12.8 ‰ to −7.8 ‰) show that the active pore-spaces in these peat profiles have been flushed by recharge from the near-surface peat. We hypothesize that back-diffusion of groundwater-derived solutes from the peat matrix to active pore-spaces has allowed the geochemical signal from paleo-hydrogeologic discharge to persist into the current regime of dilute recharge. This effect has not been observed previously on the landform-scale and has important implications for carbon cycling in peatlands. [ABSTRACT FROM AUTHOR]

The advent of new procedures for the rapid collection of large amounts of detrital zircon age data has created significant challenges to the representation, quantitative interpretation, and analysis of these results. Principal among these is the development of the most efficient and informative methods to accurately present and quantitatively compare numerous age data from large numbers of samples in a statistically valid, geologically meaningful, and easily assessable manner. Many detrital zircon studies have adopted a subjective-graphic approach to these tasks, primarily through the examination of vertically stacked probability density plots (PDPs). Although this methodology allows for quick visual assessment of changes in zircon age frequencies in space and/or time, it is fundamentally qualitative, and becomes rather impractical as numbers of analyses become large. More quantitative approaches to the comparison of PDPs (e.g., Kolmogorov-Smirnov test and multi-dimensional scaling) have also become more popular, but are rather computationally intense. In order to examine degrees of sameness among detrital zircon age populations in geographic and/or stratigraphic contexts, we develop a "likeness" metric that quantifies the degree of overlap between pairs of PDPs. We compare this metric to several others that have been proposed previously, and then evaluate its usefulness in describing sourceto- sink changes in sample age populations by analyzing data from four published studies of detrital zircon age frequencies. Likeness among 26 Paleozoic samples from the Grand Canyon (southwestern United States) decreases with increasing stratigraphic separation, but stratigraphically contiguous sample likeness exhibits statistically insignificant upsection change. The degree to which these patterns are representative of other stratigraphic successions awaits further evaluation. Likenesses among 15 samples of modern fluvial sand from the Amazon River (South America), 11 stream samples from the French Broad River in the southern Appalachians (eastern United States), and 10 Quaternary coastal sand samples from eastern Australia decrease with increasing distance between samples; as might be expected, greater geographic disjunction results in greater dissimilarity. In contrast, likenesses among spatially contiguous fluvial and littoral sample ages all exhibit trends reflecting a greater down-system increase in likeness. These results suggest that sediment grain ages may exhibit increasing homogenization with transport in both fluvial and littoral systems. The extent to which these source-to-sink changes may characterize lateral variations in other sedimentary systems also awaits additional assessment. Nonetheless, each of these four examples demonstrates that the determination of sample-pair likenesses, and evaluation of their geographic and/or stratigraphic variation, serve to effectively highlight those aspects of differences in detrital zircon age frequencies that ultimately record the histories of geologic and geomorphic processes in both space and time. [ABSTRACT FROM AUTHOR]

Neoproterozoic though Cambrian clastic sedimentary and/or metasedimentary rocks occur in the Hammondvale Metamorphic Suite (> ca. 620 to <680 Ma) and Broad River (ca. 620 Ma), Coldbrook (ca. 560-542 Ma) and Saint John (ca. 540-490 Ma) groups in the Avalonian Caledonia terrane of southern New Brunswick. The petrographic, major and trace element chemical, and Nd isotopic compositions of the sedimentary rocks provide constraints on the provenance and tectonic setting of these units and hence on the tectonic evolution of this typical part of Avalonia. Nd isotopic and whole-rock chemical data show that the Hammondvale Metamorphic Suite and metasedimentary rocks of the Broad River Group have negative εNd values, were derived from recycled sedimentary and mafic igneous sources, and were deposited in intra-arc basins as part of the ca. 620 volcanic-arc complex. In contrast, sedimentary rocks of the Coldbrook Group show generally positive εNd values and likely were derived from Avalonian felsic to intermediate igneous sources and deposited in rift basins associated with 560-550 Ma arc extension. Samples from the overlying Saint John Group have felsic to mafic igneous sources, but are characterized by negative εNd values and likely were deposited as part of a newly forming passive margin sequence. Many metasedimentary and sedimentary samples from the Hammondvale Metamorphic Suite, Broad River Group, and Saint John Group fall outside the normal Nd isotopic range for Avalonian igneous rocks, whereas Coldbrook Group samples fall mainly in the typical Avalonian igneous Nd isotopic range, suggesting a substantially larger Avalonian crustal component in their evolution. Based on their mostly positive εNd values, the sedimentary units in the Coldbrook Group were most likely derived from associated volcanic units as well as the older Broad River Group igneous units. The more negative εNd values for samples from the Hammondvale Metamorphic Suite and Broad River Group indicate a large, isotopically mature source from more interior locations in Gondwana. Based on overlapping detrital muscovite ages and similar Nd isotopic values, the lower Saint John Group units were derived from the Hammondvale Metamorphic Suite, whereas the middle to upper units, which have more evolved Nd isotopic values, appear to require a source outside of the Caledonia terrane. [ABSTRACT FROM AUTHOR]

Abstract: The oxygen and strontium isotopic ratios of marine carbonates are widely employed to derive paleotemperature and age, respectively. While open ocean samples are the standard source for these measures, shelf settings can also provide detailed records. However, the implementation of shelf data for these purposes has been hindered by the perception that local environmental conditions (such as freshwater influx) confound regional and global signals. Here, we directly evaluate this concern by assessing the spatial variability in the isotopic composition of biogenic carbonate precipitated along the depositional strike of an inner shelf environment. Data are derived from mollusc shells collected from a stratigraphically restricted, transgressive unit in the early Eocene (~55Ma) of the US Gulf Coastal Plain (GCP). Lithofacies and fossil assemblages suggest that an estuary was present in the westernmost study region, whereas more typically marine facies predominated elsewhere. Isotopic composition varies predictably with paleoenvironmental setting; shells from the estuary have δ18O values and 87Sr/86Sr ratios that reflect high freshwater input. Specimens in the eastern GCP, however, from within and between outcrops separated by up to 400km, all yield similar results. Serial microsampling reveals congruence not only in mean δ18O values, but also in winter and summer extremes. Estimated paleotemperatures (mean annual temperature of 26°C) agree well with proxy and model evidence for the early Eocene and the GCP, and 87Sr/86Sr ratios are consistent with the established early Eocene seawater ratio. The fidelity of isotopic values relative to environment indicates that shallow shelf records have much to contribute to studies of ancient climate and ocean chemistry. [Copyright &y& Elsevier]

The early Eocene (ca. 55-48 Ma) encompasses one of the warmest intervals of the past 65 m.y. and is characterized by an unusually low equator-to-pole thermal gradient. Recent proxy studies suggest temperatures well in excess of 30 °C even at high latitudes, but conflicting interpretations derived from different types of data leave considerable uncertainty about actual early Eocene temperatures. A robust comparison among new paleotemperature proxies may provide insight into possible biases in their temperature estimates, and additional detail on the spatial distribution of temperatures will further resolve the early Eocene meridional temperature gradient. We use a suite of paleotemperature proxies based on the chemistry of bivalve shell carbonate and associated sedimentary organic matter from the United States Gulf Coastal Plain to constrain climate at a subtropical site during this key interval of Earth history. Oxygen isotope and clumped isotope analyses of shell carbonate and two tetraether lipid analyses of sedimentary organic carbon all yield temperatures of ~27 °C. High-resolution, intraannual oxygen isotope data reveal a consistent, large range of seasonal variation, but clumped isotope data suggest that seasonality is due primarily to precipitation, not to temperature. These paleotemperature estimates are 2-3 °C warmer than the northern Gulf of Mexico today, and generally consistent with early Eocene temperature estimates from other low and mid-latitude locations, but are significantly cooler than contemporaneous estimates from high southern latitudes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Blueschists from the Bou Azzer inlier provide compelling evidence for Late Proterozoic subduction in the Anti-Atlas Mountains of Morocco. High-pressure/low-temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar. Together with regional relationships, the geologic setting of the blueschists constrains the polarity of Pan African subduction in this region, which occurred from ∼750 to 600 Ma. Blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix. The me´lange belt containing the blueschist facies rocks is intruded by a number of diorite plutons, one of which has yielded a U/Pb radiometric age of 650 Ma. Together with Transaharan Belt to the southeast, the Anti-Atlas suture zone exposed within the Bou Azzer inlier contains among the oldest known blueschist-bearing, ophiolitic me´langes in the world. [Copyright &y& Elsevier]

Focuses on the Hyco shear zone in North Carolina and southern Virginia, with reference to Appalachians and science. Details on the geochronilogical study that constrains the absolute timing of the HUG; Importance of the Hyco shear zone in the southerN Appalachians; Conclusions reached.