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Lithic and quartz arenites of the Central Appalachian Basin deposited by late Paleozoic Laurentian fluvial systems are widely interpreted to be sourced by recycling of late Precambrian and early Paleozoic clastic sequences in the Appalachian Orogen. U-Pb and (U-Th)/He age distributions for detrital-zircon and Th-Pb age distributions for detrital-monazite, detrital-zircon and monazite textures (including detrital diagenetic monazite, which prove recycling), sandstone petrology, heavy-mineral abundances, and other detrital proxies are all accounted for by the following: 1) lithic arenite is directly sourced from late Neoproterozoic metasediments in the proximal Appalachian fold and thrust belt via transverse drainages, 2) the late Neoproterozoic sediments were recycled from early Neoproterozoic, post-Grenvillian clastic sequences, 3) Cambrian quartz arenites along the Laurentian margin were recycled from Neoproterozoic sequences with local cratonic input, 4) although dominated by sediment of ultimate Grenvillian age, quartz arenites require ∼ 40% of Paleoproterozoic and Archean input, interpreted to be recycled from late Neoproterozoic to Devonian clastic sequences of the northern Appalachians and/or southern (Scottish–Irish) Caledonides in the distal reaches of a longitudinal drainage system. Ordovician to Mississippian clastic sequences and their metamorphosed equivalents in the Appalachian crystalline core were also likely sediment sources. Quartz arenite does not result from mixing of lithic arenite with other sources because of differences in textural and compositional maturity, and in heavy-mineral characteristics. Input from the Laurentian craton, commonly cited as the source for Paleoproterozoic and Archean detrital zircon in the eastern Laurentian clastic systems, is untenable here because of: 1) the presence of Paleozoic monazite derived from Appalachian Neoproterozoic and early Paleozoic metasediments, 2) abundant detrital chromite, and 3) abundant Paleozoic detrital muscovite. Multiple recycling explains all observed sedimentologic and mineralogic characteristics of the two lithic types. Incorporation of published detrital-zircon data for Paleozoic to modern clastic sequences in eastern Laurentia reveals that Grenville-age zircons experienced at least five and potentially ten recycling events since entering the clastic system in the Neoproterozoic. Recycling also explains the abundance of quartz pebbles in conglomerates of the quartz-arenite lithofacies, and the range of detrital-muscovite 40Ar/39Ar ages in quartz arenites of the Appalachian Basin.

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

Grenvillian ages dominate Neoproterozoic to Paleozoic detrital zircon (DZ) populations across eastern Laurentia and persist through the present. The persistence of this dominance is inferred to result from recycling of DZ grains ultimately sourced from exceptionally Zr-rich and zircon-fertile Grenvillian granitoids. Pennsylvanian arenites of the Appalachian Basin (eastern United States) exhibit DZ U-Pb age distributions that are nearly identical to those of Neoproterozoic to Cambrian strata, and contain detrital diagenetic monazite grains formed via metamorphism or diagenesis of sedimentary rocks in the source region. Detrital zircon (U-Th)/He ages are mostly 475–300 Ma, yielding lag times [Δt = U-Pb age − (U-Th)/He age] of 500–1000 m.y. and 1200–2400 m.y. for Grenvillian and Paleoproterozoic to Archean DZ grains, respectively. Detrital monazite Th-Pb ages are comparable to (U-Th)/He cooling ages, reflecting formation of monazite during Paleozoic regional metamorphism of Neoproterozoic to Cambrian strata that reset the (U-Th)/He systematics of Grenvillian DZ grains within those metasediments. These results are either consistent with or prove recycling. Incorporation of other geological constraints permits definition of at least three (and potentially five) recycling events and their timing following initial post-Grenvillian exhumation and erosion (the “great Grenvillian sedimentation episode”). Recycling events include dispersal of post-Grenvillian sediment during deposition of Neoproterozoic to Cambrian strata (formation of the “Great Unconformity”: cycle 1), subsequent erosion of metamorphosed Neoproterozoic to Cambrian strata generating detritus for the Pennsylvanian arenites sampled here (cycle 2), and modern erosion of those arenites (cycle 3). Pancontinental river systems facilitated dispersal of sediment of ultimate Grenvillian age during or after each cycle.

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.

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 igneo...

The lithologic diversity and protracted geologic history of the Minnesota River Valley (MRV) gneiss terrane at the southern margin of the Archean Superior Province provide key perspectives on the growth and modification of this craton. Innovations in geochronology, specifically the application of the chemical abrasion technique to remove Pb-loss domains in zircon, and the tandem use of in situ geochemistry and precise isotope dilution age analysis for establishing the petrologic context of radioisotopic ages, are yielding new insights into these old rocks. First, U-Pb geochronology on a variety of plutonic to mesoscale intrusive rocks from the Morton and Montevideo blocks of the MRV provide precise constraints on the Neoarchean (2603–2584 Ma) Sacred Heart orogeny, which sutured the Mesoarchean gneisses of the MRV to the southern margin of the Superior craton in an episode of contractional deformation, high-grade metamorphism, and voluminous plutonism. Second, we report the first radioisotopic age constraints on the St. Leo and Taunton belts of greenstone and supracrustal rocks within the Yellow Medicine shear zone, an inferred lithosphere-scale structure juxtaposed between the Morton and Montevideo gneisses. Titanite from a metadiorite intrusion into the St. Leo belt yields igneous (∼2539 Ma) and metamorphic (∼2510 Ma) ages, confirming the Archean provenance of the host greenstones and revealing a latest Neoarchean episode of magmatism and deformation focused along and within the shear zone. Third, a new ∼1781 Ma U-Pb zircon age for one of a number of small granitic plugs in the southern Montevideo block confirms its correlation to the more voluminous East-Central Minnesota Batholith and its origin in the Yavapai orogeny. Again later magmatism was focused along the Yellow Medicine shear zone suggesting the lithosphere-scale significance of that structure. This more resolved history for the Neoarchean to Proterozoic evolution of the southern Superior craton provides new opportunities to test Archean craton correlations and reconstructions, including links to the Wyoming and Slave cratons.

The majority of granitic magmas generated during the Grenville Orogeny (1.2–1.0 Ga) are extremely zirconium rich (Zr contents often >500 ppm) and thus typically contain very abundant zircon. The extreme zircon fertility of Grenville granitoids appears to be unique in the geological record – no major magmatic event before or after produced on average such zircon-rich granitoids. Five potential mechanisms could explain the cause of the very high concentration of Zr (and other high field strength elements) in Grenville age magmas: an unusually high percentage of xenocrystic zircon; production of magmas from extremely trace element enriched sources; enrichment of crustal sources by moderate-temperature melting prior to Grenville magmatism; unusually high magmatic temperatures allowing for considerable Zr incorporation from source to magma prior to zircon saturation; massive continental crustal recycling via assimilation of significant amounts of sediment in subduction zones. Current data are not sufficiently abundant to completely dismiss any of these potential mechanisms. However, the notion that most high-Zr Grenville granitoids are solely the result of a very significant proportion of xenocrystic zircon is viewed as unlikely due to the limited number of xenocrystic ages that have been determined in conventional U Pb dating studies as well as the large number of zircon crystals (N ≥ 100) analyzed in this study of three extremely high-Zr granitoids (Zr range = 504–1306 ppm) that failed to demonstrate evidence of significant xenocrystic components. Massive degrees of crustal recycling via sediment subduction is also viewed as an unlikely dominant mechanism as it does not appear that the oxygen isotope composition of Grenville-age zircon is unique. Enrichment mechanisms of potential source regions, such as metasomatizing of the mantle or low-temperature melting of lower crust prior to Grenville magmatism, are viewed as more plausible mechanisms to produce high-Zr magmas. The occurrence of very high magmatic temperatures of at least some high-Zr granitoids, based on modeling and Ti-in-zircon thermometry, also appears to be critical in helping to explain the exceptional zircon fertility of 1.2–1.0 Ga granitoids in North America. Because Grenville magmas had such high Zr contents they produced intermediate-felsic plutons with a much higher zircon content compared to typical granitoids. This resulted in a disproportionate amount of ~ 1 Ga detrital zircon in the sedimentary record. This superabundance of detrital grains can cause biases in provenance studies and crustal growth models that use the magnitude of detrital zircon age modes as a proxy for source contribution or amount of new crustal material generated.

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).

The utilization of detrital zircon ages as a successful provenance indicator necessitates that source rocks and derived sediment, particularly first-cycle sediment, closely match each other in terms of age and age distribution. In order to test how closely first-cycle sediment resembles its source, zircon ages were determined from a homogenous plutonic source and sediments deposited directly on it in the Stepladder Mountains, southeastern California. U-Pb zircon ages from the pluton and proximal downstream alluvium exhibit substantial differences in proportions of Cretaceous, Mesoproterozoic, and Paleoproterozoic ages. Although geographic relations and major-element geochemistry require that the sediment was primarily derived from the weathering of the adjacent pluton, sample likeness values for zircon ages are commonly low, and age distribution differences are sufficiently large to reject the null hypothesis that they were drawn from the same population. The Stepladder pluton has abundant Cretace...

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.

The clastic record is commonly interrogated by analysis of detrital heavy mineral assemblages, with the bulk of modern detrital geochronological studies employing U-Pb dating of detrital zircon. However, the bias of zircon toward felsic igneous sources, and the limited ability of the U-Pb system in zircon to record low- to medium-grade metamorphic events, means that the U-Pb detrital zircon record is largely insensitive to magma-poor orogens. In this study, U-Pb ages were obtained by laser ablation–inductively coupled plasma–mass spectrometry for apatite and rutile extracted from alluvium of the French Broad River (FBR) in the southern Appalachians (southeastern United States). In contrast to previously published FBR U-Pb zircon data sets, which yield essentially no record of the most recent Appalachian metamorphic events (ca. 320 Ma) associated with assembly of Pangea, the U-Pb detrital rutile and especially the U-Pb apatite systems together provide a complete record of complex polyphase Appalachian orogenesis. It is unexpected that the apatite and rutile U-Pb Appalachian age populations differ significantly, with probable low-temperature breakdown of rutile biasing the rutile data set toward the most recent (Alleghanian) metamorphic event. These data make the FBR one of the most intensely studied river systems globally for multiproxy single-grain U-Pb analysis, clearly demonstrate dependence of provenance information on mineral proxy choice, and emphasize the resolving power of multiproxy provenance studies.

Paleoarchean to Mesoarchean gneisses are exposed in the Minnesota River Valley (MRV) and in northern Michigan, USA. The MRV rocks include a complex of granitic migmatites, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton gneiss and the Montevideo gneiss. U–Pb geochronology for the MRV rocks shows that they have a long and complex history, with initial crystallization at c.3500 Ma and other igneous events at c.3420 Ma, c.3385 Ma, and c.3140 Ma. Many gneisses are clearly now mixtures of material formed, probably by both tectonic and magmatic processes, during at least the first three of these events. Hf and Nd isotopic systematics suggest derivation from precursor material that had resided in the crust for at least 100 m.y. prior to 3500 Ma and further demonstrate closed-system melting and remelting for about 1000 m.y. The ancient gneisses were intruded by synkinematic granite at 2603 Ma and by syenogranite and aplite at 2590 Ma, igneous events likely related to granulite facies metamorphism recorded in paragneisses and events related to suturing of the MRV terrane to the Superior craton. Rocks exposed in northern Michigan include tonalitic gneiss and granite. Although they are less well-studied, the tonalitic gneiss is c.3600 Ma and a late-granitic intrusion is c.2650 Ma. Thus, the two terranes are similar and were probably once continuous, constituting a major feature on the southern margin of the Superior craton. Similarly ancient rocks are known on the northern margin of, and within, the Superior craton. Thus, the MRV rocks were likely a crustal component that was sutured to the Superior Province during the 2.6 Ga Sacred Heart Event.

We present an integrated study combining detailed field, geochronological and geochemical data of a Neoproterozoic intra-oceanic arc systems exposed in the Pan-African belt of the Moroccan Anti-Atlas. The arc rock units exposed in Bou Azzer and Sirwa inliers consist of a tectonic patchwork of back-arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south. Arc complexes are composed of amphibolite, granodioritic and granitic gneisses intruded by various undeformed hydrous ultramafic (hornblendite), mafic (hornblende-gabbro, diorite) and felsic (granodiorite, tonalite, granite) arc lithologies. We show that these complexes are remnants of a long-lived (120 Myr) Neoproterozoic oceanic arc, punctuated by three successive magmatic episodes (760–730 Ma, 710–690 Ma, 660–640 Ma respectively) interspersed with periods of magmatic quiescence. The typical geochemical arc signature and positive ƐNdt values for the igneous rocks emplaced during each magmatic episode (medians at +7.1, +5.4 and +5.7, from older to younger) attest that their parental magmas derived from a depleted mantle source without substantial assimilation by the WAC older crustal basement. Trace-element geochemistry, i.e. Sr/Y, La/Yb, of intermediate to felsic arc rocks produced during each magmatic pulse suggests that the arc crust was thickened (>30–35 km) over a short time period between the first and second magmatic episodes (730–710 Ma) which coincides with an important regional shortening event. Soft-docking of the oceanic arc on a buoyant transitional margin is invoked to explain tectonic inversion in overriding plate, leading to shortening and related thickening of the arc crust. Concomitant magmatic shutdown resulting from a reorganization of subduction dynamics (i.e. change in slab geometry, flip in subduction polarity). A non-tectonic critical thickening of the arc crust is invoked to explain the second magmatic shutdown (680–660 Ma), by freezing the subarc mantle influx. This lull period is followed by a third magmatic episode which is likely triggered by delamination of the dense lower crust and reactivation of subarc mantle flow. This is supported by the bimodal chemical signature of evolved magmatic products, suggesting two distinct sources partial melts from the foundered lower crust and new magmatic products which differentiated from a post-delamination thinned crust., SCOPUS: ar.j, info:eu-repo/semantics/published

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).

Chemical analysis of detrital garnet is a widely utilized quantitative tool for determining the provenance of sediments. The goal of this study is to investigate the ability of the compositions of detrital garnet to provide a direct linkage to the specific source rocks from which the garnet was derived. To achieve this goal, we analyzed detrital garnet isolated from alluvial samples collected from the French Broad River and several of its tributaries, a watershed that drains portions of the multiply deformed southern Appalachian Orogen, USA. To test whether potential linkages between detrital and potential source-rock garnet could be achieved, garnet crystals were chemically characterized from a wide variety of lithologies that crop out in the French Broad watershed, and thus are the most likely local sources of detrital garnet. Mahalanobis distances measured on canonical discriminant functions successfully differentiated garnet compositions among the 18 analyzed source rocks. These metrics were then applied to ∼ 2,300 detrital-garnet compositions to link each grain to its potential source rock. Approximately 94% of the detrital garnet crystals were linked to their potential source regions. Thus only 6% of the detrital crystals were treated as “unclassified.” We interpret the “unclassified” high Ca garnets to have been derived from minor sub-map-scale lenses and boudins of calc-silicate or skarns from exposed Ocoee or Ashe metaclastic units within the French Broad River watershed. The “unclassified” high-Mn detrital garnet grains are interpreted to have been derived from the various felsic granitiods and pegmatites that intrude the region of study.

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 source-to-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.

In the Variscan fold belt of Morocco, the Jebilet massif is characterized by Palaeozoic metasedimentary rocks intruded by syntectonic magmatism that includes an ultramafic–granitoid bimodal association and peraluminous granodiorites emplacedc. 330 Ma, intruded by younger leucogranitesc. 300 Ma. The mafic–ultramafic rocks belong to a tholeiitic series, and display chemical and isotopic signatures consistent with mixing between mantle-derived and crust-derived magmas or assimilation and fractional crystallization. The granites within the bimodal association are mainly metaluminous to weakly peraluminous microgranites that show characteristics of A2-type granites. The peraluminous, calc-alkaline series consists mainly of cordierite-bearing granodiorites enclosing magmatic microgranular enclaves and pelitic xenoliths. Detailed element and isotope data suggest that the alkaline and the peraluminous granitoids were formed in the shallow crust (c. 750°C), respectively. Mixing between the coeval mantle-derived and crust-derived magmas contributed to the large variation of initial εNdvalues and initial Sr isotopic ratios observed in the granitoids. Further contamination occurred by wall-rock assimilation during ascent of the granodioritic plutons to the upper crust. The ultramafic–granitoid association has been intruded by leucogranites that have high initial Sr isotopic ratios and low initial εNdvalues, indicating a purely crustal origin. The heating events that caused emplacement of the Jebilet magmatism are related to cessation of continental subduction and convective erosion/thinning of the lithospheric mantle during plate convergence.

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the suture of West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctive high-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to the east of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formed during subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses in Ghana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatitic gneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblende as the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zone inferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for ∼2500 km from the Sahara to the Gulf of Guinea. The granitoids are characterized by I-type signatures, display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a subduction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss) that forms a sliver just east of the suture zone, yielded U–Pb zircon ages ranging from 615 to 589 Ma. This age range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzed display concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zircon separates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U–Pb zircon crystallization ages ranging from 2.19 to 2.14 Ga. These are the first U–Pb zircon ages to document Paleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages of rocks in the suture zone are also consistent with the existence of widespread involvement of older crust in the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain of the northwest Borborema province in Brazil.

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 eNd = −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.

The 3.5–2.6 Ga rocks of the Minnesota River Valley (MRV) provide a unique opportunity to examine rocks that span the now recognized crucial time (3.2–3.0 Ga) on the Earth when Wilson style plate tectonic processes likely started. To elucidate the crustal evolutionary history of these ancient rocks we obtained Hf isotopic compositions by LA-MC-ICPMS from zircon, previously dated by SHRIMP U–Pb methods; whole-rock Nd isotopic compositions by TIMS; and whole-rock major and trace element concentrations. The oldest, ca. 3500 Ma rocks have major and trace element compositions similar to other Archean tonalite–trondhjemite–granodiorite (TTG) suites, which are consistent with derivation from a mafic source. The ca. 3500 Ma Morton and Montevideo gneisses have similar initial ɛ Hf values that range from 1.9 to −2.6 at 3500 Ma, with model ages ranging from 3.83 to 3.64 Ga. The model ages are interpreted as resulting from melting of a source that is >100 Ma older than the ca. 3500 Ma gneisses. Elevated Nb/Ta ratios and large negative Nb, Ta and Ti anomalies suggest that the source may have been eclogite, consistent with melt formation in an overthickened basaltic lithosphere. Younger (3380 Ma), more chemically evolved, intrusions into both the Morton and Montevideo gneisses plot on a closed-system isotopic evolution path (Nd, Hf), indicating that they were in situ melts of the older ca. 3500 Ma rocks. These intrusions contrast with a more mafic 3140 Ma intrusion, which has zircon with more positive ɛ Hf than the closed-system isotopic evolution defined by the older gneisses. The last major event was the emplacement of the 2604 Ma Sacred Heart granite and formation of a garnet-biotite paragneiss. Isotopic values for the Sacred Heart granite show a large juvenile component relative to all other rocks, suggesting it is a mix of juvenile material and recycled Morton or Montevideo gneiss. The transition from closed to open system evolution of rocks in the MRV at 3140 Ma, and continuing at 2600 Ma, is consistent with a proposed global change in continental crust formation and evolution that began at 3.2–3.0 Ga.

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.

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 (~ 55 Ma) 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 400 km, 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.

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.

Avalonia in the northern Appalachian orogen is generally assumed to have originated near the margin of Gond- wana, although its position along that extensive margin and the timing of its separation remain disputed. U-Pb ages deter- mined by laser ablation - inductively coupled plasma - mass spectrometry are reported here for detrital zircon from Avalonian clastic sedimentary units, three in southern New Brunswick, and one in southeastern Cape Breton Island, with depositional ages ranging from ca. 630 to ca. 505 Ma. Considered in combination with previously published detrital data sets from two other samples from southern New Brunswick deposited in a similar age range, the samples show a wide spec- trum of ages from Ediacaran to Neoarchean (as old as 3.2 Ga). The dominant zircon populations in all six samples are Edia- caran-Cryogenian with ages between 770 and 540 Ma and can be matched with known magmatic events in Avalonia. Pre- Avalonian cycle detrital zircon dates in the samples are relatively sparse and do not reveal a consistent signature that might be used to place Avalonia in a unique paleogeographic position, although they are compatible with Amazonian provenance. An increase in ca. 2.2-1.9 Ga zircon dates in Early Cambrian through Early Ordovician samples suggests that sources for zircon of those ages became available in Avalonia by the end of the Ediacaran, possibly by motion of Avalonia to a different part of the Amazonian margin or to the West African craton. Resume : Il est presume que l'origine d'Avalonie dans l'orogene du nord des Appalaches se situait a proximite de la bor- dure de Gondwana, bien que sa position le long de cette longue bordure et le moment de sa separation demeurent des points de controverse. Des âges U-Pb obtenus par ablation laser - spectrometrie de masse a plasma inductif (LA-ICP-MS) sont donnes dans cet article pour des zircons detritiques provenant d'unites sedimentaires clastiques d'Avalonie, soit trois unites du sud du Nouveau-Brunswick et une du sud-est de l'ile du Cap-Breton; les âges des depositions varient entre ∼630 Ma a ∼505 Ma. Lorsque ces echantillons sont etudies a la lumiere d'ensembles de donnees publies anterieurement provenant de deux autres echantillons du sud du Nouveau-Brunswick, deposes dans une plage de temps similaire, ils montrent une vaste plage d'âge, de l'Ediacarien au Neoarcheen (atteignant 3,2 Ga). Les populations dominantes de zircons dans tous les six echantillons datent de l'Ediacarien-Cryogenien avec des âges entre 770 et 540 Ma; elles peuvent etre reliees a des evene- ments magmatiques connus en Avalonie. Dans les echantillons, les dates pour le zircon detritique d'avant l'Avalonie sont peu nombreuses et ne revelent pas une signature constante qui pourrait servir a placer l'Avalonie dans une position paleo- geographique unique, bien qu'elles soient compatibles avec une provenance amazonienne. Une augmentation, ∼2,2 a 1,9 Ga, des dates du zircon dans les echantillons du Cambrien precoce a l'Ordovicien precoce suggere que les sources pour les zircons de ces âges soient devenues disponibles en Avalonie a la fin de l'Ediacarien, possiblement par le deplacement d'A- valonie a une autre partie de la bordure d'Amazonie ou vers le craton ouest-africain. (Traduit par la Redaction)

The provenance potential of detrital monazite was investigated by in situ measurement of 232 Th– 208 Pb dates of grains isolated from six Middle Carboniferous–Permian sandstones from the Appalachian foreland basin. Provenance assessment of these units was previously investigated by measuring U–Pb crystallization ages of detrital zircon (Thomas et al., 2004; Becker et al., 2005, 2006). Approximately 90% of the detrital zircon ages record Mesoproterozoic or older ages, with only 10% recording the three major pulses of tectonism (Taconian, Acadian and Alleghanian) that are the hallmark of the Appalachian Orogen. 232 Th– 208 Pb ages of detrital monazite, however, strongly record the complex phases of Paleozoic orogenesis. Nearly 65% of the ages record Paleozoic events, while 35% record Neoproterozoic or older ages. In several of the analyzed sandstones, detrital monazite ages record Paleozoic orogenic events that are completely missed by detrital zircon ages, demonstrating that monazite ages more accurately reflect the character of the sediment source rocks. The inferred maximum age of sediment deposition, as determined by the youngest monazite grains, is ~ 550 Ma younger for two of the analyzed sandstones compared to depositional constraints based on the youngest detrital zircon. The different physical properties and petrogenesis of zircon and monazite are interpreted to be factors for the dramatic differences in sediment provenance information provided by each mineral. The results from this study have important implications for determining sediment provenance, constraining maximum age of sediment deposition, and developing robust regional tectonic models.

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 confl icting 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 signifi cantly cooler than contemporaneous estimates from high southern latitudes.

Geochemical correlation of ash-fall beds with conventional tephrochronologic methods is not feasible when original glass composition is altered. Thus, alternative correlation methods may be required. Initial studies of heavily altered Paleozoic tephra (K-bentonites) have suggested the potential for employing trace-element concentrations in apatite as ash-fall bed discriminators. To further test the utility of apatite trace-element tephrochronology, we analyzed apatite phenocrysts from unaltered volcanic rocks with an electron microprobe: nine samples from rocks erupted during the Quaternary and one sample from a rock erupted during the Paleogene. The resulting apatite trace-element data provide unique bed discriminators despite within-crystal variability. Each of the volcanic rocks studied possesses unique trends in Mg, Cl, Mn, Fe, Ce and Y concentrations in apatite. The results from this study establish an important tephrochronologic method that can be applied to nearly all portions of the Phanerozoic stratigraphic record and greatly assist development of an advanced timescale. In addition to establishing a fingerprint for a particular eruption, apatite chemistry provides useful information about the source magma.

Ages of detrital monazite and zircon from alluvium collected from the French Broad River drainage basin, an orogen-crossing main trunk river, and alluvium in first-order tributary streams, provide an unconventional perspective for examining the regional tectonic and metamorphic history of the southern Appalachian orogen (eastern United States). The French Broad River system samples migmatitic Ashe–Tallulah Falls suite paragneisses (with inferred Neoproterozoic clastic protoliths) of the Eastern Blue Ridge and western Inner Piedmont, Mesoproterozoic basement orthogneisses, numerous Paleozoic metaplutonic gneisses, and tectonite equivalents of these lithologies in the Brevard fault zone. Middle Ordovician ages dominate the monazite age spectrum. Monazite from tributaries has a dominant 208 Pb- 232 Th age peak ca. 463 Ma. Monazite from the French Broad River alluvium suite yields a dominant 208 Pb- 232 Th age mode ca. 450 Ma, but differs from the tributaries in having scattered Mesoproterozoic, Silurian–Devonian, and Carboniferous ages. Electron microprobe total Th-U-Pb chemical ages for selected tributary monazite grains also analyzed by ion microprobe reveal additional monazite growth events (i.e., metamorphic reaction) at 480–475 Ma and 445–440 Ma. Tributary and French Broad River zircon age spectra are dominated by Mesoproterozoic and Ordovician grains. Most Ordovician zircon from the French Broad River has Th/U > 0.1 and is most likely derived from the Henderson orthogneiss (447.6 ± 5.4 Ma), the largest pluton in the French Broad River headwaters region. A minor zircon age population at 450 Ma, represented primarily by metamorphic zircon rims with Th/U Ordovician monazite ages and zircon rim ages correspond to Taconian metamorphism in the Eastern Blue Ridge province. The dominant monazite age mode (463 Ma) from tributaries is slightly older than the Ordovician metamorphic zircon age mode (450 Ma), which is accounted for by monazite growth primarily via prograde metamorphic reactions, and zircon growth by melt-forming reactions in migmatites at the thermal peak. The scattered middle to late Paleozoic zircon and monazite ages attest to the lack of significant thermotectonic and magmatic events of that age in the Southern Blue Ridge providing sediment to the French Broad River drainage system. This dearth of ages is consistent with the pattern of nonpenetrative late Paleozoic deformation, retrograde metamorphism, and scattered plutonism northwest of the Brevard fault zone. A reasonable source of Neoproterozoic zircon in alluvium or in Ashe–Tallulah Falls paragneisses is Neoproterozoic rift-related magmatic rocks. The rarity of Mesoproterozoic monazite compared to zircon contrasts with the marked abundance of monazite in Ashe–Tallulah Falls paragneisses, and emphasizes the responsiveness of monazite compared to zircon in regional metamorphism. The abundant Mesoproterozoic zircon was inherited from Mesoproterozoic basement lithologies by Neoproterozoic sediments, preserved through regional metamorphism and three phases of orogenesis, and persists in modern alluvium being shed by the orogen.

The Ordovician Millbrig K-bentonite of North America and the Kinnekulle "Big Bed" of Scandinavia have been considered correlative, thus representing one of the largest volcanic eruptions of the Phanerozoic. Whether these two K-bentonites correlate has implications for the paleogeographic reconstructions of Laurentia and Baltica, transoceanic biostratigraphic correlations of graptolites and conodonts, and the possibly global occurrence of the Guttenberg carbon isotope excursion. Trace element analyses of apatite phenocrysts have proven to be good discriminators for other Ordovician K-bentonites and it is shown here, on the basis of significantly different Mg, Cl, Mn, Fe, Ce, and Y concentrations in apatite phenocrysts from the Kinnekulle and Millbrig K-bentonites, that the beds must be viewed as representing two or more different eruptions. Vertical subsampling of several layers from each bed suggests that both beds are likely composed of multiple components, but none of the components coincide with one another. These results indicate that the Millbrig and Kinnekulle beds cannot both be derived from a single ultra-Plinian eruption, as previously proposed.

Determining detrital zircon U–Pb ages has become the method of choice for single- mineral-based provenance studies focused on the identification of potential source regions of siliciclastic sediments. Advances in microanalytical methods have significantly accelerated the acquisition rate of U–Pb ages, thus allowing for more statistically significant zircon age datasets to be acquired than previously. However, several studies have demonstrated limitations of relying solely on detrital zircon as a provenance proxy. To further assess the utility of this provenance indicator we measured U–Pb ages of detrital zircon derived from modern sediment collected from the French Broad River and its tributaries that drain portions of the Appalachian Orogen in southeastern USA. The results demonstrate that significant detrital zircon age variations occur along the length of the river. The age variations suggest that characterization of entire sedimentary formations by analysis of single samples may be misleading and that a multiple-sample approach is required. In addition, by incorporating high-magnification cathodoluminescence images with Th/U for each detrital grain, a more robust interpretation can be made regarding zircon source. Supplementary material: Global positioning system coordinates for each sampling location and complete zircon data are available at http:/www.geolsoc.org.uk/SUP18445.

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the E-W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellar feldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutile and zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that the dyke underwent hydrothermal alteration; however, immobile Zr vs Ti02 for the Ben Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic crystallization temperatures were calculated as 1067 + 14°C using Ti-in-zircon thermo metry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of < 574CC based on Ti-in-quartz thermometry. An age of 373.9±4.0Ma was determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis of zircon. The Ben Levy porphyritic felsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway Batholith to the south, and may represent the northernmost extension of late-stage dyke activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between deposition of the Lough Kilbride Formation (436 -428.2Ma) and intrusion of the Ben Levy Dyke at 373.9 ±4.0Ma. Introduction Coolin Fault. The Coolin Fault lies within a thr ust belt associated with the Caledonian Orogeny A felsic porphyritic dyke outcrops along the foot(Phillips 2001). Based upon field relationships in this wall scarp of a NNE-trending normal fault, location, motion along the Coolin Fault must have which transects Ben Levy near Ballard, Co. Galway occurred after deposition of the Kilbride Formation. (Fig. 1). The dyke has a strike of ~040°, dips steeply The Kilbride is constrained to the Telychian Stage to the W, and is about 2-3 metres wide in locations (436^428.2Ma) of the Llandovery Series (Holland where contacts can be clearly seen on both sides 1985; Cocks 1989) by the presence of the brachiopod of the porphyritic felsite (Locations A and C). The Eocoelia curtisi curtisi within the Annelid Grits porphyritic felsite extends through the Dalradian member in the lower Kilbride (Harper et al. 1995). Ben Levy Formation and into the Lower Silurian In this study, the timing of motion along the Lough Mask and Kilbride Formations on the north Coolin Fault is constrained using U-Pb age dat side of Ben Levy, cross-cutting the E-W-trending ing. Cathodoluminescence (CL) imaging, major and Irish Journal of Earth Sciences 29 (2011), 1-13 © 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.1 This content downloaded from 157.55.39.217 on Mon, 18 Apr 2016 08:03:01 UTC All use subject to http://about.jstor.org/terms 2 Irish Journal of Earth Sciences (2011 ) Ordovician-Silurian Sediments and Volcanics Carboniferous Oughterard Granite p Connemara Metamorphic Complex Galway Batholith

Neoproterozoic though Cambrian clastic sedimentary and/or metasedimentary rocks occur in the Hammondvale Metamorphic Suite (HMS; ca. 680–620 Ma) and the Broad River (ca. 620 Ma), Coldbrook (ca. 560–550 Ma), and Saint John (ca. 540–490 Ma) groups in the Avalonian Caledonia terrane of southern New Brunswick. Evolved Nd isotopic values show that the HMS and the metasedimentary rocks of the Broad River Group were in part derived from older cratonic material. The presence of Mesoproterozoic and the absence of Paleoproterozoic zircon in the HMS suggest that the source of this older cratonic material was the Amazonian craton. In contrast, sedimentary rocks of the Coldbrook Group show generally positive eNd values and likely were derived from local Avalonian sources. More evolved isotopic signatures in the lower Saint John Group indicate a return to older cratonic source areas. Detrital zircon from the lowest part of the Saint John Group has Mesoproterozoic ages like those of the HMS but also has 2000‐ a...