Search

Garnet Lu-Hf and Sm-Nd ages from the Shetland Caledonides provide evidence of a polyorogenic history as follows: 1) c. 1050 Ma Grenvillian reworking of Neoarchaean basement; 2) c. 910 Ma Renlandian metamorphism of the Westing Group; 3) c. 622-606 Ma metamorphism of the Walls Metamorphic Series but of uncertain significance because the eastern margin of Laurentia is thought to have been in extension at that time; 4) Grampian I ophiolite obduction at c. 491 Ma followed by crustal thickening and metamorphism between c. 485 and c. 466 Ma; 5) Grampian II metamorphism between c. 458 and c. 442 Ma that appears to have been focused in areas where pre-existing foliations were gently-inclined and thus may have been relatively easily reworked; 6) Scandian metamorphism at c. 430 Ma, although the paucity of these ages suggests that much of Shetland did not attain temperatures for garnet growth. There is no significant difference in the timing of Caledonian orogenic events either side of the Walls Boundary Fault, although this need not preclude linkage with the Great Glen Fault. However, the incompatibility of Ediacaran events either side of the Walls Boundary Fault may indicate significant lateral displacement and requires further investigation.

At Peniche, Portugal, the GSSP for the base of the Toarcian and in a Submediterranean province, a value of 0.707 073 ± 0.000 002 (2s.e., n = 13) is determined for 87Sr/86Sr at the base of the Toarcian. In Yorkshire, UK, a Subboreal province, the base of the Toarcian has an 87Sr/86Sr value of0.707 072 ± 0.000 002 (2s.e., n = 23). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. At Peniche, the base of the Serpentinum (Falciferum/Levisoni) Chronozone has an 87Sr/86Sr value of 0.707 096 ± 0.000 003 (2.s.e). The value for this boundary in Yorkshire is 0.707 094 ± 0.000 002 (2s.e., n = 27). The similarity of the values shows that, within analytical uncertainty, these levels are exactly equivalent in time. The same ammonite successions occur through the lowermost Toarcian successions at Peniche and in Yorkshire. Between Peniche and Yorkshire, there is therefore an exact coincidence, within measurement uncertainty, between the Lower Toarcian, Tethyan Realm, Tenuicostatum (Polymorphum) [ammonite] Chronozone of Peniche and the Lower Toarcian, Boreal Realm, Tenuicostatum [ammonite] Chronozone of Yorkshire. The 87Sr/86Sr profile through the Tenuicostatum Subchronozone at Peniche reveals large variations in sedimentation rate, with lowest rates in a condensed basal 3 m of the zone, highest rates in mid-zone, with rates decreasing upward through the upper half of the zone. Apportioning time in proportion to 87Sr/86Sr shows that around half of Tenuicostatum time is condensed into the basal 3 m of the section.

Dental enamel represents an important mineralized archive of an individual’s early life. Previously, isotopic (Ca) or trace element ratios (Sr/Ca, Ba/Ca) have been used to reveal dietary and weaning histories, although few studies have utilized both proxies to evaluate the respective results. Here we report histologically-defined, spatially-resolved Ca-isotope (laser-cut & TIMS) and trace element ratio (Sr/Ca, Ba/Ca; LA-ICPMS) profiles along the enamel-dentine-junction in three deciduous dental crowns of three early twentieth century Italian infants (Modern-22, 27, 29). Modern-27 and Modern-29 display overall similar patterns of Ca-isotope variation and reflect an overall increase of >1.0‰ in δ44/40Ca across and after birth. Whilst the Sr/Ca and Ba/Ca signals of Modern-27 suggest a mixed breast-formula feeding, the two elemental proxies in Modern-29 hint at nearly exclusive breastfeeding until ∼4 months, followed by introduction of formula. A ∼0.5‰ δ44/40Ca decrease across and after birth together with Sr/Ca ratios in Modern-22 suggest a dominant breastfeeding history for the first ∼5–8 months. Enamel Ca-isotope data alone are not sufficient to distinguish between breastfed or formula-fed infants. In addition, Ca-isotope profiles in deciduous enamel suggest a connection between prominent physiological stress like birth and negative Ca-isotope excursions, underlining the physiological overprint of Ca-isotope signatures.

The Miri Zone in Sarawak contains thick Paleogene to Neogene sedimentary successions that extend offshore into the Sarawak Basin (Balingian and Central Luconia Provinces) and Sabah Basin. Exploration offshore has shown the Sarawak Basin in the South China Sea contains major hydrocarbon reservoirs. The sediments on land are age equivalents of the offshore successions and can be used to provide insights into their sedimentological and stratigraphic relations. However, because the rocks are found in mountainous regions covered by dense rainforest much of the stratigraphy in the Miri Zone is poorly known, as are timings and causes of major unconformities in the region that are essential for understanding the tectonic history, basin development, and sedimentary pathways. In this study we integrate fieldwork, U Pb zircon dating, biostratigraphy, and light and heavy mineral analyses to present a revised stratigraphy for the region as well as paleogeographic maps, including major paleo-river systems for the main sedimentary basins. Rocks studied include parts of the Cretaceous to Eocene deep marine Rajang Group, fluvial to marginal marine sediments of the Oligocene to Early Miocene Tatau, Buan, Nyalau and Setap Shale Formations, and the Miocene sediments which are assigned to the Balingian, Begrih and Liang Formations in the Mukah-Balingian province, and the Belait Formation on Labuan. There is still much debate about the timings or even existence of some important unconformities offshore, such as the Middle Miocene Unconformity (MMU) and Deep Regional Unconformity (DRU). We propose to avoid the ambiguous time-based terminology that has been used for different events by different authors. Instead, our results from the on-land stratigraphy show two main unconformities in northern Sarawak; one at c. 37 Ma (Rajang Unconformity), marking the change from deep marine to fluvial – marginal marine sedimentation, and another one at c. 17 Ma (Nyalau Unconformity) which is related to widespread uplift in Borneo and changing river systems.

A Sr-isotope chronostratigraphy is presented for Miocene sediments deposited by the “Paranense” Sea along 1200 km of the southwestern Atlantic coast of Argentina and Uruguay. Numerical ages were obtained for shells of Aequipecten paranensis from the Argentinean Puerto Madryn Formation, Facies Balneario La Lobería, “Entrerriense Beds” of the Salado Basin, and Paraná Formation, and from the Camacho Formation (Uruguay). The 87Sr/86Sr ages fall into five age-groups that encompass the “Paranense” flooding in the latest Serravalian-Messinian interval. For the Puerto Madryn Formation, the ages span the latest Serravalian to the Tortonian and are stratigraphically coherent with the transgressive phase (11.9–10.4 Ma) and the regressive phase (10.2–9.82 Ma and 9.40–9.05 Ma) of that unit. Ages of 8.85–7.95 Ma for the “Entrerriense Beds” show them to be Tortonian while the Facies Balneario La Lobería, and the Paraná and Camacho formations span the age-range 7.50–6.00 Ma, comprising the Tortonian-Messinian interval. The regressive phase of the Puerto Madryn Formation, and the Facies Balneario La Lobería are respectively correlated with the basal and middle beds of the cliffs at Barranca Final, where the uppermost horizons of the Barranca Final Formation are exposed. The “Entrerriense Beds” are correlated with the “Cape Fairwheater Beds”. Dating the “Paranense” marine incursion permits a reappraisal of its paleogeography and differentiation of its deposits from those of the “Patagoniense” Sea. The flooding area was smaller than previously thought, with its northwestern-most boundary in the surroundings of the Santa Fe Province and its southernmost boundary in southern Santa Cruz Province. Moreover, the Paranaian Molluscan Bioprovince was coeval with the Valdesian Molluscan Bioprovince for 2.35 Ma and species included in the Aequipecten paranensis Zone lived for at least 5.90 Ma. Fil: del Río, Claudia Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentina Fil: Martinez, Sergio. Universidad de la República; Uruguay Fil: Mc Arthur, John. University College London; Estados Unidos Fil: Thirwall M,. University College London; Estados Unidos Fil: Pérez, Leandro Martín. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; Argentina

We present new geochronological and geochemical data for Neogene magmatism from West Sarawak. Zircon U-Pb geochronology divides Neogene magmatic rocks of West Sarawak into a Lower Miocene West Sarawak Sintang Suite with ages of c. 19 to 21 Ma, and a Middle Miocene Bau Suite with ages of c. 12 to 14 Ma. Magmatism occurred in multiple short-lived pulses from approximately 24 Ma and was coeval with magmatic activity in NW Kalimantan and East Kalimantan. The majority of, but not all, Bau Suite samples display adakitic chemistry, while the West Sarawak Sintang Suite is predominantly non-adakitic. There was no active subduction zone or subducted slab associated with this adakitic magmatism. Instead, the geochemical diversity is consistent with the Bau and West Sarawak Sintang suites representing mixtures of mafic, mantle-derived magma with felsic magma derived from remelting of hydrous, mafic rock that had been emplaced into the lithosphere of Borneo as arc basalt tens or hundreds of millions of years previously. This origin is most evident in the main Sintang Suite of central Borneo (Kalimantan) which has preserved less contaminated examples of the mafic endmember. This endmember resembles basaltic rocks from several locations across Borneo suggesting that intraplate, mantle-derived magmatism was responsible for remelting older, hydrated basaltic rocks in the crust.

In the north-east Jordan steppe, gazelle were of considerable economic importance to human groups during the Epipalaeolithic and Neolithic. An influential model argues that gazelle herds mi...

The island of Seram, eastern Indonesia, incorporates Miocene ultrahigh-temperature (UHT; greater than 900°C) garnet–sillimanite granulites that formed by extensional exhumation of hot mantle rocks behind the rolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry results and the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundances between garnet and c. 16 Ma zircon. Monazites also record identical U–Pb ages, within uncertainty. However, these geochronometers do not date peak UHT metamorphism; instead, they date retrograde, garnet-consuming (Zr- and rare earth element (REE)-liberating) reactions that produced the granulites’ post-peak cordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rims and so were unaffected by the entire UHT event. Miocene UHT metamorphism overprinted a Late Triassic–Early Jurassic upper-amphibolite facies event that grew garnet cores and 216–173 Ma zircon. In the Miocene, these garnet cores were overgrown by peritectic garnet rims during UHT metamorphism, with some rutiles recording c. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstrate that a component of Hf4+ produced since c. 200 Ma was retained through the c. 16 Ma UHT event. Accordingly, UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.

Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U–Pb apatite ages, U–Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U–Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at https://doi.org/10.6084/m9.figshare.c.3990639

The Da Lat Zone in SE Vietnam consists of a Mesozoic orogenic belt that is characterised by abundant Cretaceous igneous rocks that were intruded into or deposited on Permo-Triassic rocks (represented by the Chau Thoi Formation) and Jurassic metasedimentary rocks. The three Cretaceous magmatic phases of the Dinh Quan, Deo Ca, and Ankroet suites of the Da Lat Zone were previously subdivided on the basis of geochemical and isotopic differences and related to within-plate and continental arc magmatism during Paleo-Pacific subduction. Limited dating gave varied estimates for crystallisation ages of the different suites. Here we present new whole-rock chemical data and LA-ICP-MS U-Pb zircon analyses for Cretaceous plutonic and volcanic rocks. The results indicate volcanic arc magmatism started in the Early Cretaceous, represented by the Dinh Quan suite at c. 113–135 Ma, and continued into the mid to Late Cretaceous (Deo Ca, Nha Trang, Dak Rium and Don Duong formations) with magmatism from c. 90 to 109 Ma. Granitoid basement rocks of the Cuu Long Basin yielded similar ages of c. 90 to 92 Ma, representing offshore equivalents. The Ankroet suite represents the youngest magmatic phase and is interpreted as within-plate magmatism from c. 75 to 87 Ma. The Cretaceous rocks generally have very few older zircons and their rarity is interpreted to indicate magmas were generated predominantly by melting of juvenile arc crust.

During the Early Toarcian, volcanic gases released by the Karoo-Ferrar large igneous province are widely believed to have caused severe environmental disturbances, including ocean acidification. Here we show records of δ44/40Ca and δ88/86Sr through the Early Toarcian, as recorded in three groups of biogenic calcite: Megateuthididae belemnites, Passaloteuthididae belemnites, and brachiopods of the species Soaresirhynchia bouchardi. We evaluate the data to eliminate the influence on isotopic composition of varying temperature, calcification rate, and salinity, through the section that may mask the environmental signals. Neither δ44/40Ca nor δ88/86Sr show negative isotope excursions across the suggested acidification interval as would be expected had acidification occurred. A profile of δ11B, re-interpreted from a published study, shows no variation through the interval. Taken together, these data provide little support for ocean acidification at this time. In our belemnites, values of δ88/86Sr are independent of temperature or Sr/Ca. For brachiopods, too few data are available to determine whether such dependences exist. Values of δ44/40Ca show a weak temperature control of magnitude +0.020 ± 0.004 ‰/°C (2 s.d.). In belemnites, δ44/40Ca also correlates positively with Mg/Ca and Sr/Ca.

Besides impact melt rock, several large terrestrial impact structures, notably the Sudbury (Canada) and Vredefort (South Africa) structures, exhibit considerable occurrences of a second type of impact-generated melt rock, so-called pseudotachylitic breccia (previously often termed “pseudotachylite” – the term today reserved in structural geology for friction melt in shear or fault zones). At the Vredefort Dome, the eroded central uplift of the largest and oldest known terrestrial impact structure, pseudotachylitic breccia is well-exposed, with many massive occurrences of tens of meters width and many hundreds of meters extent. Genesis of these breccias has been discussed variably in terms of melt formation due to friction melting, melting due to decompression after initial shock compression, decompression melting upon formation/collapse of a central uplift, or a combination of these processes. In addition, it was recently suggested that they could have formed by the infiltration of impact melt into the crater floor, coming off a coherent melt sheet and under assimilation of wall rock; even seismic shaking has been invoked. Field evidence for generation of such massive melt bodies by friction on large shear/fault zones is missing. Also, no evidence for the generation of massive pseudotachylitic breccias in rocks of low to moderate shock degree by melting upon pressure release after shock compression has been demonstrated. The efficacy of seismic shaking to achieve sufficient melting as a foundation for massive pseudotachylitic melt generation as typified by the breccias of the Sudbury and Vredefort structures has so far remained entirely speculative. The available petrographic and chemical evidence has, thus, been interpreted to favor either decompression melting (i.e., in situ generation of melt) upon central uplift collapse, or the impact melt infiltration hypothesis. Importantly, all the past clast population and chemical analyses have invariably supported an origin of these breccias from local lithologies only. Here, the first Rb-Sr, Sm-Nd, and U-Pb isotopic data for Vredefort pseudotachylitic breccias and their host rocks, in comparison to data for Vredefort Granophyre (impact melt rock), are presented. They strongly support that the pseudotachylitic breccias were exclusively formed from local precursor lithologies – in agreement with earlier isotopic results for Sudbury Breccia and chemical results for Vredefort pseudotachylitic breccias. A contribution from a Granophyre-like impact melt component to form Vredefort pseudotachylitic breccia is not indicated. The most likely process for the genesis of voluminous pseudotachylitic breccias in large impact structures remains decompression melting upon formation and collapse of the central uplift, during the modification stage of impact cratering.

The hunting of Gazella subgutturosa was a dominant practice for Epipalaeolithic and early Neolithic hunter–gatherers in the east Jordan steppe. The seasonal mobility of this taxon in the Levant is ...

The Early/Middle Epipalaeolithic aggregation site of Kharaneh IV in the east Jordan steppe is one of the largest of the period, showing repeated use, high degrees of site investment, and relatively prolonged periods of occupation. Hunters relied heavily on the single prey species, Gazella subgutturosa. This paper reports on isotopic analyses of gazelle teeth, drawn from the Kharaneh IV assemblage, to explore the seasonal and spatial distribution of gazelle in life and examine models of long-distance seasonal gazelle migrations. Dental microwear analysis is also employed to assess hunting seasons. Results from sequential isotope analyses of 11 Epipalaeolithic gazelle molars show that, with one exception, gazelle did not move seasonally from the limestone steppe onto other geologies, nor did they seasonally relocate to areas with different climate regimes. Rather, seasonal herd mobility seems to have been local, meaning animals would have been available in the vicinity of Kharaneh IV year-round. Results from 19 microwear analyses show some gazelle to have died in spring when grass graze was available, whilst others had been browsing around their time of death, indicating non-spring hunting. Microwear evidence thus suggests gazelle were hunted in more than one restricted season. The implications of these new datasets for hunter-gatherer use of the steppe, and potential hunting methods, is highlighted.

We report methodological innovations and an initial application to retrieve Ca isotopic (δ 44/40 Ca) compositions of (bio)apatite at high spatial resolution. We utilize laser-microsampling via laser-cut polygons followed by extraction of μg-sized bioapatite fragments; and apply a 10 10 Ω resistor for 40 Ca and parafilm-dam loading to minimize in-run Ca isotope fractionation for TIMS analysis. We obtain identical internally-normalized TIMS Ca isotopic ratios for natural calcite, apatite and bone meal SRM1486 without chemical separation, suggesting no need of chemical purification of these materials for such analysis. Our 42 Ca– 48 Ca double spike (DS) corrected δ 44/40 Ca values of standard solutions HPS new , Fisher07 and bone meal SRM1486 are 0.70 ± 0.18‰, 1.03 ± 0.20‰, and − 1.03 ± 0.19‰ (2SD), relative to SRM915a, all well in line with published data. Solutions prepared from both laser-cut and non-laser cut Durango apatite have similar δ 44/40 Ca values (0.70 ± 0.17‰, 2SD), suggesting no resolvable Ca isotope fractionation effect induced from laser-cutting. A δ 44/40 Ca profile of a third molar from a modern human female, laser-sampled in the growth direction of the enamel apatite, reveals a 0.5‰ increase from age ~ 11 to ~ 11.5, followed by a 0.7‰ drop at age ~ 12 and a final return to enriched Ca-isotope compositions until age ~ 14. We surmise that the initial increase is related to her known phase-out of dairy products, whereas the drop may coincide with hormonal changes at the onset of her menstruation. The latter hints at a relationship between physiological change and δ 44/40 Ca variation, and raises the need for proper evaluation of physiological effects on Ca isotopes before reliable environmental signals can be extracted from the large Ca-isotope variability previously observed in skeletal tissues. We also investigated the potential for Ca isotopic analysis using an IsoProbe MC-ICPMS in both solution and laser ablation (LA) modes. While the use of a collision cell almost completely removes 40 Ar + interference on 40 Ca + , partially-resolved hydrocarbon interferences result in only ~ 30% useable peak flats, that potentially cause problems with peak jumping needed for analysis of 46 Ca and 48 Ca. Furthermore, it is difficult to correct for Ti isobaric interferences so that analysis without chemical separation is challenging. In-situ Ca isotope analysis of natural calcite (SRM915b), aragonite and apatite by LA-MC-ICPMS shows substantial matrix sensitivity, and therefore requires close matrix matching.

This study reports a geochemical investigation of two thick basalt sequences, exposed in the Bracco–Levanto ophiolite (northern Apennine, Italy) and in the Balagne ophiolite (central-northern Corsica, France). These ophiolites are considered to represent an oceanward and a continent-near paleogeographic domain of the Jurassic Liguria–Piedmont basin. Trace elements and Nd isotopic compositions were examined to obtain information about: (1) mantle source and melting process and (2) melt–rock reactions during basalt ascent. Whole-rock analyses revealed that the Balagne basalts are slightly enriched in LREE, Nb, and Ta with respect to the Bracco–Levanto counterparts. These variations are paralleled by clinopyroxene chemistry. In particular, clinopyroxene from the Balagne basalts has higher CeN/SmN (0.4–0.3 vs. 0.2) and ZrN/YN (0.9–0.6 vs. 0.4–0.3) than that from the Bracco–Levanto basalts. The basalts from the two ophiolites have homogeneous initial Nd isotopic compositions (initial eNd from + 8.8 to + 8.6), within typical depleted mantle values, thereby excluding an origin from a lithospheric mantle source. These data also reject the involvement of contaminant crustal material, as associated continent-derived clastic sediments and radiolarian cherts have a highly radiogenic Nd isotopic fingerprint (eNd at the time of basalt formation = − 5.5 and − 5.2, respectively). We propose that the Bracco–Levanto and the Balagne basalts formed by partial melts of a depleted mantle source, most likely containing a garnet-bearing enriched component. The decoupling between incompatible elements and Nd isotopic signature can be explained either by different degrees of partial melting of a similar asthenospheric source or by reaction of the ascending melts with a lower crustal crystal mush. Both hypotheses are reconcilable with the formation of these two basalt sequences in different domains of a nascent oceanic basin.

Central problems in the interpretation of the Neoproterozoic geology of the North Atlantic region arise from uncertainties in the ages of, and tectonic drivers for, Tonian orogenic events recorded in eastern Laurentia and northern Baltica. The identification and interpretation of these events is often problematic because most rock units that record Tonian orogenesis were strongly reworked at amphibolite facies during the Ordovician-Silurian Caledonian orogeny. Lu-Hf and Sm-Nd geochronology and metamorphic modelling carried out on large (>1 cm) garnets from the Meadie Pelite in the Moine Nappe of the northern Scottish Caledonides indicate prograde metamorphism between 950 and 940 Ma at pressures of 6–7 kbar and temperatures of 600 °C. This represents the first evidence for c. 950 Ma Tonian (Renlandian) metamorphism in mainland Scotland and significantly extends its geographic extent along the palaeo-Laurentian margin. The Meadie Pelite is believed to be part of the Morar Group within the Moine Supergroup. If this is correct: 1) the Morar Group was deposited between 980 ± 4 Ma (age of the youngest detrital zircon; Peters, 2001 , youngest published zircon date is 947 ± 189 (Friend et al., 2003)) and c. 950 Ma (age of regional metamorphism reported here), 2) an orogenic unconformity must separate the Morar Group from the 883 ± 35 Ma ( Cawood et al., 2004 ) Glenfinnan and Loch Eil groups, and 3) the term ‘Moine Supergroup’ may no longer be appropriate. The Morar Group is broadly correlative with similar aged metasedimentary successions in Shetland, East Greenland, Svalbard, Ellesmere Island and northern Baltica. All these successions were deposited after c. 1030 Ma, contain detritus from the Grenville orogen, and were later deformed and metamorphosed at 950–910 Ma during accretionary Renlandian orogenesis along an active plate margin developed around this part of Rodinia.

The Ediacaran–Cambrian interval was an eventful transitional period, when dynamic interactions between the biosphere and its physical environment allowed the Earth System to cross into a new state, characterized by the presence of metazoans, more equable climates and more expansive oxygenation of the oceans. Due to the retreat of widespread sulphidic conditions, redox-sensitive trace-metals could accumulate to a greater extent in ‘black shales’ deposited in localised anoxic/euxinic environments, such as highly productive ocean margins. This study investigates the concentrations of the redox-sensitive trace-metals molybdenum and vanadium in organic-rich sedimentary rocks from seven sections of the Yangtze platform, slope and basin. Iron speciation analyses were carried out in order to distinguish oxic, anoxic-ferruginous and anoxic-sulphidic settings, while sulphur and nitrogen isotope ratios were measured to gain insight into sulphate and nitrate availability, respectively, in the context of changing redox conditions. The data herein demonstrate an overall increase in redox-sensitive trace-metal contents in black shales across the Ediacaran–Cambrian transition, but with marked temporal and spatial variability. Euxinia is evident in South China before 551 Ma in the Ediacaran, and again in the early Cambrian. However, some time-equivalent sections are not enriched in redox-sensitive trace metals, and also exhibit contrasting S-isotope and N-isotope systematics. A more complex configuration of the Yangtze Platform, for example with vast intra-shelf basins, together with changing (generally rising) eustatic sea-level may account for this variability. In this regard, it is proposed that a mid-depth sulphidic wedge, caused by nutrient upwelling over the south-east platform margins, migrated over time (but generally landward), leading to spatially variable redox conditions determined by sea-level, currents and bathymetric constraints. The changing extents of anoxia and euxinia appear to have limited the distribution of emerging Ediacaran and Cambrian ecosystems.

The presented Sr, Nd, Hf and double-spike Pb-isotopic analyses of Quaternary basalts from the Payenia volcanic province in southern Mendoza, Argentina, confirm the presence of two distinct mantle types feeding the Payenia volcanism. The southern Payenia mantle source feeding the intraplate-type Rio Colorado and Payun Matru volcanism is isotopically distinct from the northern Payenia and arc mantle source with less radiogenic Pb and lower Sr-87/Sr-86 and Hf-176/Hf-177 for a given Nd-143/Nd-144 than the northern Payenia and arc rocks. The basalts from the northern Payenia province (Nevado volcanic field) and the retroarc zone have isotopic compositions overlapping the Andean southern volcanic zone (SVZ) arc rocks and they are probably derived from the same metasomatized South Atlantic N-MORB-like mantle source. A high field strength element melting model suggests addition of similar to 1-2% upper continental crustal material to the mantle source of the Nevado basalts and the transitional SVZ arc and retroarc rocks and similar degrees of melting throughout this arc segment. A gradual depletion of the pre-metasomatic mantle source going from the backarc over the retroarc to the arc is indicated. The depletion is suggested to be caused by step-wise melt extraction due to repeated injections of subduction zone fluids and melts. The lower crustal contamination trends found in the trace element variations of basalt groups from all parts of the Payenia province are also recognized in isotopic space and the dominant isotope assimilation trends indicate lower crustal contaminants with more unradiogenic Pb, lower Nd-143/Nd-144 and Hf-176/Hf-177 and higher or similar 87Sr/86Sr as the mantle melts. These characteristics are similar to those found in lower crustal xenoliths from the northern Proterozoic Cuyania terrane, and the Payenia basalts may have been contaminated by this type of crust during passage through the lithosphere or by more recently underplated or intruded material.

The faunas of the highly fossiliferous Pirabas Formation belong to the southern part of the biogeographical unit known as "Neogene Tropical America". This unit developed prior to the closure of the Central American Seaway by the Isthmus of Panama. Until now, the age of the Pirabas Formation was inferred only from biostratigraphy. The Sr-isotope (87Sr/86Sr) values of pectinid shells from the Pirabas Formation show that most parts of this unit were deposited during the Late Burdigalian (about 16-17 Ma ). This result does not contradict biostratigraphic data and it constrains the age of the Pirabas Formation more tightly than do previous estimates of age, it for future, more precise biogeographical comparisons. Fil: Martinez, Sergio. Universidad de la República; Uruguay Fil: Feijó Ramos, María Inês. Museo Paraense Emilio Goeldi; Brasil Fil: McArthur, John M.. University College London; Estados Unidos Fil: del Río, Claudia Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentina Fil: Thirlwall, Matthew F.. Royal Holloway University Of London; Reino Unido

[1] This paper presents major, trace-elements, and Sr-Nd isotopes for two prominent sills formed during the opening of the North Atlantic, sampled by the Utgard borehole (6607/5-2) in the Voring Plateau. The Utgard sills are compared to opening-related lavas recovered from ODP Leg 104 Hole 642E farther west on the Voring Plateau and on the NE Greenland conjugate margin. The Utgard sills (3.6–5.9 wt % MgO) are enriched in strongly relative to moderately incompatible trace elements and have 87Sr/86Sr and 143Nd/144Nd ratios of 0.70380–70387 and 0.51292–0.51293, respectively, in the Upper Utgard Sill, and 0.70303–0.70306 and 0.51297–0.51299 in the Lower Sill. Alteration is minor. The Utgard melts originated by partial melting of an asthenospheric, depleted mantle source (DMM or Iceland Rift Zone, IRZ, type) with chemical characteristics similar to the source that gave rise to NE Greenland lavas. The Utgard magmas underwent extensive fractional crystallization in the lower crust (Upper Sill: >70%; Lower Sill: >55%) with removal mainly of olivine and pyroxenes, accompanied by ≤1% assimilation of crustal melts. This crystallization formed significant masses of dense cumulates (∼3.25 g/cm3) (underplating). Assuming an areal extent similar to that of the two sills, we estimate a composite layer of ultramafic cumulates mixed with less dense country rocks to be >320 m thick beneath the two Utgard sills and >8.8 km beneath the thickest part of the Voring Plateau lavas. Opening-related cumulates may thus account for a significant part of the lower crustal high-velocity, high-density bodies (average density 3.1 g/cm3) along the Norwegian margin.

Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc–continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). Lu–Hf and Sm–Nd garnet ages of c . 475–460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. Lu–Hf and Sm–Nd ages of c . 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event. The existing two-stage Grampian–Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution. The restriction of the Late Ordovician and Silurian events to the Northern Highland terrane reinforces the suggestion that it was far removed from the Grampian terrane until juxtaposition following major end-Caledonian (Devonian) sinistral displacement along the Great Glen Fault. A similar record of Mid- and Late Ordovician metamorphic events within the Laurentian-derived Uppermost Allochthons of Norway has been attributed to episodic accretion significantly prior to Silurian continent–continent collision and closure of the Iapetus Ocean. Supplementary materials: Results of trace element analysis of the garnets by laser ablation inductively coupled plasma mass spectrometry are available at www.geolsoc.org.uk/SUP18583.

The Xiaotan section, situated on the southwestern Yangtze Platform, South China, contains an unusually expanded sedimentary record, spanning the upper Neoproterozoic to the Cambrian and representing more than 40 My of marine sedimentation. Three distinct black shale units of the Shiyantou and Yuanshan formations were analysed for redox-sensitive trace elements and iron speciation, in order to constrain the evolution of the marine environment during the Early Cambrian metazoan biodiversification on the southwestern Yangtze Platform. For comparison we also include black shale samples of the Shiyantou Fm. from the Meishucun section, samples from a previous study covering the Shiyantou and Yuanshan Fm. at Dapotuo near Chengjiang, and samples from the strongly condensed Early Cambrian Niutitang Fm. from the Zhongnan section situated on the platform margin. Th/U and Fe HR /Fe T ratios reveal that bottom waters on the southwestern Yangtze Platform were commonly anoxic and ferruginous into the Cambrian, but became oxic after deposition of the black shales of the lower Yuanshan Formation, which elsewhere contains the Chengjiang Biota. Average Mo/TOC ratios, used as an indicator of the size of the Mo reservoir, increase up-section, and together with generally increasing S/C ratios suggest a trend towards normal (Phanerozoic) marine conditions. A distinctive Ni peak within the upper Shiyantou Fm. at Xiaotan and maximum Mo concentrations at the base of the Yuanshan Fm. can be correlated to the Ni–Mo sulphide layer within the lower Niutitang Fm. and thus indicate two separate pulses of sedimentary-exhalative nickel enrichment and subsequent, redox controlled molybdenum accumulation. A strong decoupling of organic carbon and pyrite sulphur within both sections further suggests upwelling of H 2 S-rich waters onto the platform. We argue here for an opening of the depositional environment on the northwestern Yangtze Platform which led to the upwelling of sulphidic waters, ventilation of the seafloor and ultimately its colonisation by newly emergent animal groups during the ‘Cambrian Explosion’.

This study reports a high-resolution carbon and strontium isotope profile for the Ediacaran–Cambrian Xiaotan section, situated in northeastern Yunnan, South China. Xiaotan section represents a more distal setting than the more condensed Meishucun section in eastern Yunnan, and is, biostratigraphically and chemostratigraphically, the best constrained section on the Yangtze platform covering the Ediacaran–Cambrian interval. The carbonate carbon isotopic ratios of the late Ediacaran, upper Dengying Formation to the early Cambrian, Zhujiaqing Formation, exhibit a large negative excursion (N1, −12.2‰) in the Daibu Member, just below the first appearance of small shelly fossils (SSF) at the base of the Zhongyicun Member, and a sustained pre-‘Tommotian’ positive excursion (P4, δ 13 C up to +7.3‰) which coincides with the occurrence of the Heraultipegma yunnanensis (= Watsonella crosbyi ) Assemblage SSF in the Dahai Member. Least altered strontium isotope ratios, based on sequential acid leaching of limestones as well as dolostones and phosphorites following rigid petrographic and geochemical selection, reveal a systematic decreasing trend from 0.7085–6 in the latest Ediacaran (Daibu Member) to about 0.7082–3 by the FAD of Watsonella crosbyi in China (Dahai Member). The carbon and strontium isotope features of the Xiaotan section can be correlated with those from sections in Morocco, Mongolia and Siberia, and confirm a decrease in seawater 87 Sr/ 86 Sr during Cambrian Stage 1 accompanied by C-isotope oscillations, which together may assist global stratigraphic correlation of the early Cambrian. As previously intimated by C-isotopes and biostratigraphy, this new Sr-isotope evidence confirms a depositional gap during the pre-Tommotian in the Aldan River area of SE Siberia. The decreasing trend of 87 Sr/ 86 Sr in the Cambrian Stage 1 marks a temporary (∼16 Myrs) reversal of the overall increasing trend during the Ediacaran and Cambrian periods, and potentially records a short-term decrease in continental weathering within a prolonged interval of generally increasing weathering rates. Other factors, such as increased submarine hydrothermal alteration, ocean spreading rates and/or chemical weathering of volcanic provinces and young carbonate dissolution might also have affected this decrease in seawater 87 Sr/ 86 Sr.

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean-continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66-71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (Ce-N/Sm-N=0.1-0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet + clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis. Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm-Nd and Lu-Hf isotope correlations. The Nd-Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (epsilon(Nd) = +4.7 to +7.6, epsilon(Hf) = +4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (delta O-18 = +4.9%0 to +5.2 parts per thousand) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts. (C) 2012 Elsevier B.V. All rights reserved

Lu–Hf and Sm–Nd garnet geochronology combined with geothermobarometric calculations allowed us to gain new insight into the metamorphic evolution of the Northern Nagssugtoqidian orogen (NNO), in West Greenland. Three samples were collected from the southern (SM 310 and SM 316) and northern (SM 339) parts of the NNO. They revealed amphibolite-grade Palaeoproterozoic metamorphism under peak temperature conditions decreasing from c. 810 to 840 °C at 7.5 kbar (SM 310, 316) to c. 690 °C at 10 kbar (SM 339) in the southern and northern part of the NNO, respectively. Our garnet dating defines two episodes: (1) at c. 1780 Ma, determined by Lu–Hf in sample SM 316 and Sm–Nd in sample SM 339, most likely approximating the time of garnet growth, and (2) at c. 1750 Ma defined by the remaining Sm–Nd dates, reflecting time of cooling below isotopic closure. Original garnet growth patterns preserved by HREE (SM 316) and clearly seen zonation of Sm and Nd (SM 339) suggest that the garnets may not have been completely reset and thus, the obtained age of 1780 Ma could be fairly close to the time of garnet crystallisation. The growth of peak metamorphic mineral assemblages in the southern NNO took place after the end of deformation, as demonstrated by sample SM 316 that was collected from an undeformed mafic dyke cutting across the regional fabric. In contrast, sample SM 339 revealed synkinematic growth of peak metamorphic paragenesis that crystallised in a low angle top-to-SW shear zone overprinted on the pre-existing fabric. Our results are consistent with the interpretation of the NNO as an Archaean crustal block, the Aasiaat domain, that was affected by Palaeoproterozoic metamorphism and localised deformation at 1780 Ma, i.e., 70 Ma after the Nagssugtoqidian collision. This event can be compared to the formation of steep belts (major left-lateral strike-slip shear zones) in the central part of the Nagssugtoqidian orogen. The Aasiaat domain is considered to be an independent microplate separating the Nagssugtoqidian and Rinkian orogens which define respectively its southern and northern margins. The interpretation presented implies the existence of two discrete tectonic sutures to the north and south of the Aasiaat domain.

Diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy (SIS). This is especially important for Precambrian and Cambrian studies for which no suitable skeletal material is available. One proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre-leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution. We applied a similar technique to dissolve a range of lithologies (limestone, dolostone and phosphorite) and compared the strontium isotopic composition of various leaches to contemporaneous seawater. Our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near-primary seawater 87Sr/86Sr composition once contaminant strontium from secondary calcite and other phases has been removed. Commonly applied trace element (e.g. Mn/Sr, Mg/Ca) and isotopic (C, O) screening cut-offs were also examined in the light of these results, and proved unreliable particularly in the case of dolostones and partially dolomitized limestones. We recommend that rigid sample selection and sequential leaching procedures be applied to all SIS studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future.

Eight samples of brachiopod shell material have been analyzed for their strontium isotope composition in order to more accurately date Lower to Upper Permian siliceous biogenic strata of Spitsbergen (Kapp Starostin Formation) and the southern Barents Sea (Roye Formation). The results are interpreted as showing a mid-Artinskian age for the basal Voringen Member of the Kapp Starostin Formation and a range of late Artinskian to Roadian for the overlying part of this unit. The upper part of the Roye Formation yields ages in the range Roadian to Wuchiapingian. These results are consistent with available biostratigraphic data and confirm the potential of strontium isotope stratigraphy for developing a more accurate chronology of the widespread spiculite deposits that characterize the northern margin of Pangea in late-Early Permian to Late Permian time and which constitute a potential target for petroleum exploration.

The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite–tonalite–granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age ( c . 1.1 Ga) eclogite and metasediments. U–Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U–Pb zircon, Sm–Nd and Lu–Hf garnet–clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean ( c . 2.6–2.8 Ga) and Palaeoproterozoic ( c . 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times. Supplementary material: U–Pb analytical data and LA-ICP-MS trace element data are available at http://www.geolsoc.org.uk/SUP18389.

Cratonic eclogites and garnet pyroxenites from the Kaapvaal craton have heterogeneous Hf^Nd^Sr^(O) isotope ratios that define a positive Hf^Nd isotope array and a negative Nd^Sr isotope array. Isotopic variability encompasses depleted (mid-ocean ridge basalt and ocean-island basalt) to enriched mantle compositions (Group I and II kimberlites) and overlaps with that of the Kaapvaal craton garnet peridotite xenoliths. Isotopic heterogeneity at Roberts Victor is less extreme than previously reported and ranges from eclogites with a highly depleted MORB-like signature to enriched eclogites similar to Group II and transitional kimberlites and Group II megacrysts (eHf1⁄4 ^32·8). Much of this similarity may well be due to partial or complete resetting during entrainment. For the majority of eclogites and garnet pyroxenites the Lu^Hf system records ‘older’ mantle events than the Sm^Nd system, but neither necessarily records the protolith age. Both the Lu^Hf and the Sm^Nd systems are prone to being reset by entrainment in high-temperature kimberlite and/or basaltic magmas (e.g. Kaapvaal) and emplacement in orogenic belts (e.g. Beni Bousera). In the case of one eclogite from Roberts Victor the Sm^Nd cpx^gt mineral isochron age (963·1 42·3Ma) differs from the Lu^Hf cpx^gt mineral isochron age (1953 13Ma) by 1Ga and the Rb^Sr clinopyroxene model age (3·15 Ga) is 1Ga older than the Lu^Hf age and the reconstructed whole-rock isochron age. Ironically, it may be that, in this instance, the Rb^Sr system gives a better indication of protolith age than Sm^Nd or Lu^Hf. Overall variable resetting of isotope systems between protolith formation in the Archaean (42·5 Ga) and kimberlite and/or basalt entrainment ( 0·2 Ga) masks our understanding of the exact protolith age of eclogites.

Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c. 120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c. 88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c. 116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap (c. 27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.

Prograde P-T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction-type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85-83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge-approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu-Hf dating of eclogite in the Sanbagawa belt gives ages of 89-88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P-T-t results implies vertical exhumation rates of greater than 2.5 cm yr )1 . This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.

In late Oligocene or early Miocene, a series of basalts and basaltic andesites erupted on the island of Canouan and formed the Old Volcanic Massif (OVM). In a second eruptive phase a series of basalts, basaltic andesites and andesites formed the upper part of the Canouan Formation (CF; lower Middle Miocene; post-16 Ma) and the Upper Volcanic System (UVS; end-Miocene; 6.8 Ma). These two eruptive phases are characterized by systematic differences in isotope ratios and chemical composition. The OVM rocks have 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.70512 to 0.70657 and 0.512415 to 0.512414, whereas in the CF/UVS rocks the ratios range from 0.70454 to 0.70493 and 0.512924 to 0.512843. In addition, the rocks of OVM also have lower ΣFe2O3 and Co contents, and higher La/Yb ratios than those of the CF/UVS. The isotopic and chemical data suggest that both suites were formed from similar partial melts of a subduction-modified, depleted spinel-lherzolite source. These data also show that the OVM melts assimilated relatively large amounts of highly radiogenic crustal material, whereas the CF/UVS melts assimilated relatively small amounts of highly radiogenic crustal material. The chemical and isotopic variations of the OVM rocks are consistent with the removal of mixtures of clinopyroxene, plagioclase and magnetite during assimilation-fractional-crystallization. Within the CF/UVS, the chemical and isotopic variations can be explained in terms of minor crustal assimilation; the removal of olivine, clinopyroxene, plagioclase and magnetite to form the basalts and basaltic andesites; and the removal of clinopyroxene, plagioclase, magnetite and amphibole to form the andesites. Copyright © 2008 John Wiley & Sons, Ltd.

We present new major and trace element and O–Sr–Nd-isotope data for igneous rocks from the western Mediterranean Alboran Sea, collected during the METEOR 51/1 cruise, and for high-grade schists and gneisses from the continental Alboran basement, drilled during the Ocean Drilling Programme (ODP Leg 161, Site 976). The geochemical data allow a detailed examination of crustal and mantle processes involved in the petrogenesis of the lavas and for the first time reveal a zonation of the Miocene Alboran Sea volcanism: (1) a keel-shaped area of LREE-depleted (mainly tholeiitic series) lavas in the central Alboran Sea, generated by high degrees of partial melting of a depleted mantle source and involving hydrous fluids from subducted marine sediments, that is surrounded by (2) a horseshoe-shaped zone with LREE-enriched (mainly calc-alkaline series) lavas subparallel to the arcuate Betic-Gibraltar-Rif mountain belt. We propose that the geochemical zonation of the Miocene Alboran Basin volcanism results from eastward subduction of Tethys oceanic lithosphere coupled with increasing lithospheric thickness between the central Alboran Sea and the continental margins of Iberia and Africa.

The Zuni–Bandera Volcanic Field (ZBVF) is a late-Neogene volcanic field on the boundary of the stable Colorado Plateau and the active Rio Grande Rift. Alkalic and tholeiitic magmas have erupted through Proterozoic continental crust with the tholeiitic magmas having undergone shallow-level fractional crystallization of olivine ± clinopyroxene ± spinel. The alkaline–tholeiitic lava flows lack elemental and isotopic correlations usually indicative of concomitant crust assimilation and fractional crystallization (AFC) and appear to have inherited their geochemistry from sub-Moho depths. Consideration of isotopic data, and modelling of REE and U–Th data constrains the mantle-melting history. The tholeiite basalts are primarily spinel-facies mantle melts (95–100% for spinel-facies), whereas the alkali basalts have a much higher proportion of garnet-facies mantle melts (15–25% for all samples except QBO 607 assumed to be 100% garnet-facies). The increased contribution from garnet-facies mantle in the alkali basalts is supported by U–Th isotopic data, where a shift towards 230 Th excess is observed. Since the lithospheric thickness increases from 45–55 km beneath the Rio Grande Rift, to 120–150 km beneath the Colorado Plateau, the ZBVF volcanic rocks are most likely a mixture of asthenosphere-derived (garnet-facies) alkali basalts and lithosphere-derived (spinel-facies) tholeiitic basalts. This is further supported by Sr–Nd isotopic data with alkali basalts having isotopic compositions similar to depleted-mantle values ( 87 Sr/ 86 Sr = 0.702986 to 0.70378, 143 Nd/ 144 Nd = 0.512712 to 0.512978), while the tholeiite basalts have higher 87 Sr/ 86 Sr (0.704725 to 0.706003) and lower 143 Nd/ 144 Nd (0.512379 to 0.512913) typical of basaltic magmas derived from ancient, LREE-enriched lithospheric mantle. Overall, melting is observed to be polybaric in nature, with mixing of melts over an extended depth range with the development of melting columns that span (a) the garnet-to spinel-facies phase boundaries in the mantle, and, (b) the asthenosphere–lithosphere boundary.

We report major and trace element abundances and Sr, Nd and Pb isotopic data for Miocene (16� 5^11 Ma) calc-alkaline volcanic rocks from the western segment of the Carpathian arc. This volcanic suite consists mostly of andesites and dacites; basalts and basaltic andesites as well as rhyolites are rare and occur only at a late stage. Amphibole fractionation both at high and low pressure played a significant role in magmatic differentiation, accompanied by highpressure garnet fractionation during the early stages. Sr^Nd^Pb isotopic data indicate a major role for crustal materials in the petrogenesis of the magmas.The parental mafic magmas could have been generated from an enriched mid-ocean ridge basalt (E-MORB)type mantle source, previously metasomatized by fluids derived from subducted sediment. Initially, the mafic magmas ponded beneath the thick continental crust and initiated melting in the lower crust. Mixing of mafic magmas with silicic melts from metasedimentary lower crust resulted in relatively Al-rich hybrid dacitic magmas, from which almandine could crystallize at high pressure. The amount of crustal involvement in the petrogenesis of the magmas decreased with time as the continental crust thinned. A striking change of mantle source occurred at about 13 Ma. The basaltic magmas generated during the later stages of the calc-alkaline magmatism were derived from a more enriched mantle source, akin to FOZO. An upwelling mantle plume is unlikely to be present in this area; therefore this mantle component probably resides in the heterogeneous upper mantle. Following the calc-alkaline magmatism, alkaline mafic magmas erupted that were also generated from an enriched asthenospheric source. We propose that both types of magmatism were related in some way to lithospheric extension of the Pannonian Basin and that subduction played only an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase of extension by melting of metasomatized, enriched lithospheric mantle and were contaminated by various crustal materials, whereas the alkaline mafic magmas were generated during the post-extensional stage by low-degree melting of the shallow asthenosphere. The western Carpathian volcanic areas provide an example of long-lasting magmatism in which magma compositions changed continuously in response to changing geodynamic setting.

Fault-associated dolomitized carbonates are proven hydrocarbon reservoirs in the subsurface; yet their origins and spatial variability in reservoir quality are poorly understood. Fault-associated dolomitization has affected a 4–8-km (2.5–5-mi)-wide strip of inner-platform carbonates of the Oligocene–Miocene Taballar Limestone, exposed onshore northeastern Borneo, where they are juxtaposed against deep-marine shales of the Eocene Maliu Mudstone. On textural grounds (planar dolomites postdating compaction) and from the predominance of monophase aqueous inclusions, replacive dolomite rhombs and later dolomite cements appear to have formed at about 50–60C and at shallow burial depths up to 0.5–1 km (0.3–0.6 mi). Isotopic and inclusion data indicate that the most likely dolomitizing fluid was dominated by Neogene seawater (for dolomites, 18OV-PDB = 5.9 to 10.3; [consistent with the precipitation from southeast Asian Oligocene–Miocene seawater {18O = 1.5 to 4.2} at temperatures of 40C up to a maximum of about 50–75C], 87Sr/86Sr = 0.708566–0.708697; from Tm = 2.1–3.4 wt.% NaCl). However, replacement and remobilization of the precursor limestone also contributed to the isotopic signature of the dolomites (87Sr/86Sr = 0.708115–0.708197), whereas an evolved formation water component cannot be ruled out. Dolomitizing fluids used faults and fractures as conduits to move into and alter the limestone, migrating farthest into the most permeable strata. The likely driver for fluid movement was convective flow caused by nearby Neogene igneous activity, perhaps in combination with tectonically induced hydrologic drive related to fault reactivation.Close to the main fault, late-stage dolomite cements occlude pore throats, reducing porosity (5%) and permeability (5 md). The best reservoir quality occurs in medium- to coarsely crystalline idiotopic mosaics of dolomite that have completely replaced the limestone 0.5–2 km (0.3–1.2 mi) away from the main fault where the late-stage cements did not form. These dolomites have 12–20% intercrystalline porosity and moderate to good permeability (tens of millidarcys). Fracturing has had variable impact on reservoir quality. Where late fractures remained open, permeability is enhanced (tens to hundreds of millidarcys), whereas brecciation and fault gouges result in sealing and reduced permeability. The textures, reservoir quality characteristics, and mechanisms of dolomitization show similarities to the few other detailed studies in the literature. This study refines the types of predictive reservoir models used in hydrocarbon exploration for subsurface faulted and dolomitized carbonates and increases the understanding of dolomitizing mechanisms of equatorial carbonates.

Following the end of the Kosovo conflict, in June 1999, a study was instigated to evaluate whether there was a cause for concern of health risk from depleted uranium (DU) to German peacekeeping personnel serving in the Balkans. In addition, the investigations were extended to residents of Kosovo and southern Serbia, who lived in areas where DU ammunitions were deployed. In order to assess a possible DU intake, both the urinary uranium excretion of volunteer residents and water samples were collected and analysed using inductively coupled plasma-mass spectrometry (ICP-MS). More than 1300 urine samples from peacekeeping personnel and unexposed controls of different genders and age were analysed to determine uranium excretion parameters. The urine measurements for 113 unexposed subjects revealed a daily uranium excretion rate with a geometric mean of 13.9 ng/d (geometric standard deviation (GSD)=2.17). The analysis of 1228 urine samples from the peacekeeping personnel resulted in a geometric mean of 12.8 ng/d (GSD=2.60). It follows that both unexposed controls and peacekeeping personnel excreted similar amounts of uranium. Inter-subject variation in uranium excretion was high and no significant age-specific differences were found. The second part of the study monitored 24 h urine samples provided by selected residents of Kosovo and adjacent regions of Serbia compared to controls from Munich, Germany. Total uranium and isotope ratios were measured in order to determine DU content. (235)U/(238)U ratios were within +/-0.3% of the natural value, and (236)U/(238)U was less than 2 x 10(-7), indicating no significant DU in any of the urine samples provided, despite total uranium excretion being relatively high in some cases. Measurements of ground and tap water samples from regions where DU munitions were deployed did not show any contamination with DU, except in one sample. It is concluded that both peacekeeping personnel and residents serving or living in the Balkans, respectively, were not exposed to significant amounts of DU.

The riebeckite-bearing rhyolite of Winkston Hill, Peebles, is shown to be peralkaline and may be named a pantellerite. Trace element comparison with other more altered members of the Tweeddale lavas from Wrae Hill suggests that the whole suite is peralkaline. It is thought most unlikely that the suite was developed in association with an Upper Ordovician subduction zone, and it was perhaps erupted on an oceanic island.

The Asmari Formation has been studied in the subsurface at the Bibi Hakimeh, Marun and Ahwaz oilfields and in an outcrop section from the Khaviz anticline. It consists of approximately 400 m of cyclic platform limestones and dolostones with subordinate intervals of sandstone and shale. The method of Sr-isotope stratigraphy is well suited for dating these strata because of the rapid rate of change of marine Sr-87/Sr-86 during Asmari deposition (roughly 32-18 Ma) and the common presence of well-preserved macrofossils.Profiles of age against depth in the four areas show a decrease from higher stratigraphic accumulation rates in the lower Asmari to lower rates in the middle to upper part of the formation. There is also a trend towards less open-marine depositional conditions and increasing early dolomitization and anhydrite abundance above the lower part of the formation. These changes reflect the dynamics of platform progradation across the areas studied, from early deposition along relatively high accommodation margin to slope settings to later conditions of lower accommodation on the shelf top.Ages of sequence boundaries are estimated from the age-depth profiles at each locality, providing a framework for stratigraphic correlation. Asmari deposition began in early Rupelian time (34-33 Ma) in the Bibi Hakimeh area, when the studied areas to the NW were accumulating basinal marl facies. Progradation of the platform across the Marun and Ahwaz areas took place in mid-Chattian time (27-26 Ma) and somewhat later (26-25 Ma) in the more basinward Khaviz area. Depositional sequences have durations of 1-3 Ma, whereas component cycles represent average time intervals of 100-300 Ky.Sr analyses of most dolomite, anhydrite and celestite samples plot close to or below the macrofossil age-depth trend for each locality, indicating formation from waters preserving seawater Sr-87/Sr-86 approximately contemporaneous with or slightly younger than the time of sediment deposition. Local deviations from this trend are interpreted as indicating episodes of seepage-reflux and also a contribution of Sr from non-marine sources during formation of the Gachsaran cap rock anhydrite.

We present new major and trace element, high-precision Sr-Nd-Pb (double spike), and Oisotope data for the whole range of rocks from the Holocene Tolbachik volcanic field in the Central Kamchatka Depression (CKD). The Tolbachik rocks range from high-Mg basalts to low-Mg basaltic trachyandesites. The rocks considered in this paper represent mostly Late Holocene eruptions (using tephrochronological dating), including historic ones in 1941, 1975-1976 and 2012-2013. Major compositional features of the Tolbachik volcanic rocks include the prolonged predominance of one erupted magma type, close association of middle-K primitive and high-K evolved rocks, large variations in incompatible element abundances and ratios but narrow range in isotopic composition. We quantify the conditions of the Tolbachik magma origin and evolution and revise previously proposed models. We conclude that all Tolbachik rocks are genetically related by crystal fractionation of medium-K primary magmas with only a small range in trace element and isotope composition. The primary Tolbachik magmas contain ~14 wt% MgO and ~4% wt% H2O and originated by partial melting (~6%) of moderately depleted mantle peridotite with Indian-MORB-type isotopic composition at temperature of ~1250oC and pressure of ~2 GPa. The melting of the mantle wedge was triggered by slab-derived hydrous melts formed at ~2.8 GPa and ~725oC from a mixture of sediments and MORB- and Meiji- type altered oceanic crust. The primary magmas experienced a complex open-system evolution termed Recharge-Evacuation-Fractional Crystallization (REFC). First the original primary magmas underwent open-system crystal fractionation combined with periodic recharge of the magma chamber with more primitive magma, followed by mixing of both magma types, further fractionation and finally eruption. Evolved high-K basalts, which predominate in the Tolbachik field, and basaltic trachyandesites erupted in 2012-2013 approach steady-state REFC liquid compositions at different eruption or replenishment rates. Intermediate rocks, including high-K, high-Mg basalts, are formed by mixing of the evolved and primitive magmas. Evolution of Tolbachik magmas is associated with large fractionation between incompatible trace elements (e.g., Rb/Ba, La/Nb, Ba/Th) and is strongly controlled by the relative difference in partitioning between crystal and liquid phases. The Tolbachik volcanic field shows that open-system scenarios provide more plausible and precise descriptions of long-lived arc magmatic systems than simpler, but often geologically unrealistic, closed-system models.

The ability to make rapid measurements on small samples using laser fluorination enhances the potential of oxygen isotopes in the investigation of early inorganic materials and technologies. delta O-18 and Sr-87/Sr-86 values are presented for glass from two primary production sites, four secondary production sites and a consumer site in the Near East, dating from Late Antiquity to the medieval period. delta O-18 is in general slightly less effective than Sr-87/Sr-86 in discriminating between sources, as the spread of measured values from a single source is somewhat broader relative to the available range. However, while Sr-87/Sr-86 is derived predominantly from either the lime-bearing fraction of the glass-making sand or the plant ash used as a source of alkali, delta O-18 derives mainly from the silica. Thus the two measurements can provide complementary information. A comparison of delta O-18 for late Roman - Islamic glasses made on the coast of Syria-Palestine with those of previously analysed glasses from Roman Europe suggests that the European glasses are relatively enriched in O-18. This appears to contradict the view that most Roman glass was made using Levantine sand and possible interpretations are discussed.

New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr / Y and depletion of the heavy rare earth elements. 87Sr / 86Sr and 143Nd / 144Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth elements, the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge. The wide range of SiO2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature. (1) Adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing garnet melted to yield adakitic magma. In either case the basaltic precursor was generated from fluid-modified mantle then differentiated within the garnet stability field. In Surigao this requires differentiation within mantle. The Surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth. This has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks.

Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ18O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr–Nd–Pb isotope ratios that terminate at δ18O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ18O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ18O. The hyperbolic 143Nd/144Nd–δ18O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are 4.5× greater and source Nd contents 9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ18O and 143Nd/144Nd or 206Pb/204Pb, extending to δ18O of +4.3‰ at 143Nd/144Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ18O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ18O in eastern Iceland. It is very difficult to explain the low δ18O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ18O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ18O enriched sources by recycling into the mantle, and there is no evidence for changes in δ18O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ18O appears to be associated with high 3He/4He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.

Strontium-isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two Miocene carbonate platforms cored by Ocean Drilling Program (ODP) Leg 194, just seaward of the Great Barrier Reef. The results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off-platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform-top sites. Oyster-bearing beds at the base of both platform successions yield early Oligocene ages (29–31 Ma), thus dating initial transgression of the Marion Plateau's volcanic basement. There followed a period of slow accumulation of shallow-water grainstones rich in quartz and phosphate grains in late Oligocene time (29–23 Ma; seismic Megasequence A). The main growth of the carbonate platforms took place in early to late Miocene time (23–7 Ma), comprising five depositional sequences. The first four of these (seismic Megasequence B) are common to both platforms and terminated with a possible karst surface at 10.7 Ma. Different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied. The final growth stage (seismic Megasequence C) occurred only in the southern platform and terminated at 6.9 Ma. Both platform-demise events (10.7 and 6.9 Ma) approximately coincide with falls in global sea level combined with longer-term trends of decreasing water temperature. Sr-isotope ages of dolostones increase with depositional age, and older dolostones in the southern platform have more coarsely crystalline and fabric-destructive textures than overlying younger dolostones. These relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata.

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km3. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×107 km2 and estimated maximum total volumes of up to 5,000 km3, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (

The most widespread blocks within the Cretaceous ophiolitic melange (North Anatolian ophiolitic melange) in Central Anatolia (Turkey) are pillow basalts, radiolarites, other ophiolitic fragments and Jurassic-Cretaceous carbonate blocks. The pillow basalts crop out as discrete blocks in close relation to radiolarites and ophiolitic units in Cretaceous ophiolitic melange. The geochemical results suggest that analyzed pillow basalts are within-plate ocean island alkali basalts. The enrichment of incompatible elements (Nb, Ta, Light REE, Th, U, Cs, Rb, Ba, K) demonstrates the ocean island environment (both tholeiites and alkali basalts) and enriched MORB. Dated calcareous intrafills and biodetrital carbonates reveal an age span of Callovian—Early Aptian. The thin-shelled protoglobigerinids, belonging to the genus Globuligerina, in the calcareous intrafills between pillow basalt lobes indicates a Callovian—Barremian age interval, most probably, Valanginian to Late Barremian. The volcanic and radiolarite detrit...

Lu–Hf analyses of garnet from metabasic amphibolite, glaucophane schist and eclogite facies blocks from the Franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the Franciscan subduction zone. Garnets yield 176 Lu/ 177 Hf ratios ranging from 1.5 to 28 with the highest ratios from garnets with high spessartine/pyrope ratio. Sulphuric acid leaching (SAL) of garnets revealed the presence of inclusions with significantly higher Lu/Hf ratios than those of garnet itself (most likely apatite). Their removal by SAL brings the 176 Lu/ 177 Hf ratios in garnets down by as much as 40%. This suggests that 176 Lu/ 177 Hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions. Sm–Nd garnet analyses were dominated by inclusions (mainly sphene), and failed to provide precise and accurate age information. The oldest Lu–Hf ages are 168.7F0.8 and 162.5F0.5 Ma on plagioclase-bearing garnet amphibolite from Panoche Pass and the Berkeley Hills, respectively, which suggests initiation of the subduction zone at about 169 Ma, coeval with the formation of the tectonically overlying Coast Range Ophiolite. Relatively high temperature conditions persisted for about 14 Ma as indicated by 153.4F0.8 Ma garnet growth recorded in epidote amphibolite and 157.9F0.7 in eclogite from Ring Mountain and Jenner, respectively. A 146.7F0.7 Ma age was obtained from garnet glacuophane schist, metamorphosed at around 400 jC. The sequence of ages from central and northern California shows a younging trend with decreasing metamorphic grade, which supports previous suggestions that the high-grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system. Combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca. 15 jC/Ma. The thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km/Ma during the initial stages of Franciscan subduction. Such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre. The data also suggest slow exhumation rates and significant residence time at depth of the earliest Franciscan rocks.