Cryptotephrochronology of the Eemian and the last interglacial–glacial transition in the North East Atlantic

The Eemian interglacial and the onset of the subsequent glacial period serve as the most recent analogue for the natural operation of the climate system during the current interglacial. Pronounced climatic oscillations occurred during this period, but their nature and pattern are poorly understood due to dating limitations and unknown phase relationships between different regions and archives. Tephrochronology offers considerable potential for precise correlation of disparate palaeoclimatic archives preserving evidence of these rapid climatic transitions through the tracing of common isochronous tephra horizons. We outline the identification of three previously unknown cryptotephra horizons within a marine core from the Rockall Trough, North East Atlantic. This sequence preserves a high‐resolution record of this interval and shard size, geochemical heterogeneity and the co‐variance of shard concentrations with ice‐rafted debris data are utilized to demonstrate that primary airfall was the most likely transport and depositional pathway. The main geochemical populations of these horizons have similar transitional alkali major and trace element compositions, suggesting that they were derived from a common Icelandic source, potentially the Öræfajökull volcanic system. These tephra horizons represent additions to the North Atlantic event stratigraphy for this period and tentative correlations to Icelandic terrestrial deposits are proposed for two horizons.     

Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus

Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr-Nd-Pb-Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The ⁴⁰Ar/³⁹Ar age and geochemical data show that the Hikurangi basement lavas (118-96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99-87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (∼52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ∼1% of the Earth’s surface was covered with volcanism, resulted from a thermo-chemical superplume/dome that stalled at the transition zone, similar to but larger than the structure imaged presently beneath the South Pacific superswell. The later alkalic volcanism on the Hikurangi Plateau and the Zealandia micro-continent may have been part of a second large-scale volcanic event that may have also triggered the final breakup stage of Gondwana, which resulted in the separation of Zealandia fragments from West Antarctica.     

Petrogenesis of basalt–trachyte lavas from Olmoti Crater, Tanzania

Olmoti Crater is part of the Plio-Pleistocene Ngorongoro Volcanic Highland (NVH) in northern Tanzania to the south of Gregory Rift. The Gregory Rift is part of the eastern branch of the East African Rift System (EARS) that stretches some 4000 km from the Read Sea and Gulf of Aden in the north to the Zambezi River in Mozambique. Here, we (1) characterize the chemistry and mineral compositions of lavas from Olmoti Crater, (2) determine the age and duration of Olmoti volcanic activity through ⁴⁰Ar/³⁹Ar dating of Olmoti Crater wall lavas and (3) determine the genesis of Olmoti lavas and the relationship to other NVH and EARS volcanics and (4) their correlation with volcanics in the Olduvai and Laetoli stratigraphic sequences.Olmoti lavas collected from the lower part of the exposed crater wall section (OLS) range from basalt to trachyandesite whereas the upper part of the section (OUS) is trachytic. Petrography and major and trace element data reflect a very low degree partial melt origin for the Olmoti lavas, presumably of peridotite, followed by extensive fractionation. The ⁸⁷Sr/⁸⁶Sr data overlap whereas Nd and Pb isotope data are distinct between OLS and OUS samples. Interpretation of the isotope data suggests mixing of enriched mantle (EM I) with high-μ-like reservoirs, consistent with the model of Bell and Blenkinsop [Bell, K., Blenkinsop, J., 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology 5, 99–102] for East African carbonatite lavas. The isotope ratios are within the range of values defined by Oceanic Island Basalt (OIB) globally and moderate normalized Tb/Yb ratios (2.3–1.6) in these lavas suggest melting in the lithospheric mantle consistent with other studies in the region.⁴⁰Ar/³⁹Ar incremental-heating analyses of matrix and anorthoclase separates from Olmoti OLS and OUS lavas indicate that volcanic activity was short in duration, lasting 200 kyr from 2.01 ± 0.03 Ma to 1.80 ± 0.01 Ma. The age of Olmoti activity overlaps with ages reported for Ngorongoro Caldera, implying contemporaneous activity of multiple NVH volcanic centers during part of the eruption interval.Olmoti is considered the source for the bulk of interbedded volcanics and volcaniclastic deposits that comprise much of the upper Bed I section of nearby Olduvai Gorge, and part of the Laetoli sequence, both known for their well preserved fossils and archaeological remains. Age and chemical data reported here are compatible with those derived from tephra and lava interbedded in Bed I at Olduvai Gorge and from the Olpiro Beds at Laetoli.     

Holocene tephra horizons at Klocka Bog,west‐central Sweden: aspects of reproducibility in subarctic peat deposits

This paper presents one of the most extensive Holocene tephra records found to date in Scandinavia. Microtephra horizons originating from Icelandic eruptions were recorded in two ca. 2 m thick peat profiles at Klocka Bog in west‐central Sweden. Five of the microtephra horizons were geochemically correlated to the Askja‐1875, Hekla‐3, Kebister, Hekla‐4 and Lairg A tephras respectively. Radiocarbon‐based dating of these tephras broadly agree with previously published ages from Iceland, Sweden, Germany and the British Isles. The identification of the Lairg A tephra demonstrates a more widespread distribution than previously thought, extending the usefulness of Icelandic Holocene tephrochronology further north into west‐central Scandinavia. Long‐lasting snow cover and seasonal wind distribution in the lower stratosphere are suggested as factors that may be responsible for fragmentary tephra deposition patterns in peat deposits of subarctic Scandinavia. Copyright © 2004 John Wiley & Sons, Ltd.     

Tephrochronology of the Siple Dome ice core,West Antarctica: correlations and sources

A total of 24 tephra-bearing volcanic layers have been recognized between 550 and 987 m depth in the Siple Dome A (SDM-A) ice core, in addition to a number already recognized tephra in the upper 550 m (Dunbar et al., 2003, Kurbatov et al., 2006). The uniform composition and distinctive morphological of the particles composing these tephra layers suggest deposition as a result of explosive volcanic eruptions and that the layers therefore represent time-stratigraphic markers in the ice core. Despite the very fine grain size of these tephra (mostly less than 20 microns), robust geochemical compositions were determined by electron microprobe analysis. The source volcanoes for these tephra layers are largely found within the Antarctic plate. Statistical geochemical correlations tie nine of the tephra layers to known eruptions from Mt. Berlin, a West Antarctic volcano that has been very active for the past 100,000 years. Previous correlations were made to an eruption of Mt. Takahe, another West Antarctic volcano, and one to Mt. Hudson, located in South America (Kurbatov et al., 2006). The lowest tephra layer in the ice core, located at 986.21 m depth, is correlated to a source eruption with an age of 118.1 ± 1.3 ka, suggesting a chronological pinning point for the lower ice. An episode of anomalously high volcanic activity in the ice in the SDM-A core between 18 and 35 ka (Gow and Meese, 2007) appears to be related to eruptive activity of Mt. Berlin volcano. At least some of the tephra layers found in the SDM-A core appear to be the result of very explosive eruptions that spread ash across large parts of West Antarctica, off the West Antarctic coast, as well as also being recognized in East Antarctica (Basile et al., 2001, Narcisi et al., 2005, Narcisi et al., 2006). Some of these layers would be expected to should be found in other deep Antarctic ice cores, particularly ones drilled in West Antarctica, providing correlative markers between different cores. The analysis of the tephra layers in the Siple Dome core, along with other Antarctic cores, provides a timing framework for the relatively proximal Antarctic and South American volcanic eruptive events, allowing these to be distinguished from the tropical eruptions that may play a greater role in climate forcing.     

Two distinct Precambrian terranes in the Southern Prince Charles Mountains, East Antarctica: SHRIMP dating and geochemical constraints

The Southern Prince Charles Mountains (SPCM) are mostly occupied by the Archaean Ruker Terrane. The Lambert Terrane crops out in the northeastern part of the SPCM. New geochemical and zircon U–Pb SHRIMP ages for felsic orthogneisses and granitoids from both terranes are presented. Orthogneisses from the Ruker and Lambert terranes differ significantly in their major and trace-element compositions. Those from the Ruker Terrane comprise two distinct groups: rare Y-depleted and abundant Y-undepleted. U–Pb isotopic data provide evidence for tonalite−trondhjemite emplacement at 3392 ± 9 and 3377 ± 9 Ma, pre-tectonic granite emplacement at 3182 ± 9 Ma, metamorphism(?) at c. 3145 Ma, and thermal events at c. 1300(?) and 626 ± 51 Ma. The Lambert Terrane orthogneisses probably originated in a continental magmatic arc. Zircon dating shows a very different geological history: pre-tectonic granitoid emplacement at 2423 ± 18 Ma, metamorphism at 2065 ± 23 Ma, and syn-tectonic granitoid emplacement at 528 ± 6 Ma, syn-tectonic pegmatite emplacement at 495 ± 18 Ma. The Lambert Terrane can be correlated with neither the Meso- to Neoproterozoic Beaver Terrane in the Northern PCM, which differs in isotopic composition, nor with the Archaean Ruker Terrane, which differs in both granitoid chemical composition and the timing of major geological events. It represents a Palaeoproterozoic orogen which experienced strong tectonic re-activation in Pan-African times. The Lambert Terrane has some geochronological features in common with the Mawson Block, which comprises south Australia and some areas in East Antarctica.     

Geochemical prospecting for stratabound mineralization in late Precambrian sediments of East Greenland (72–74°N)

A geochemical investigation was carried out mainly in a 14-km thick sedimentary sequence of late Precambrian age. The project included analysis of rock samples and drainage samples for Cu, Co, and Zn. The drainage samples were heavy-mineral concentrates and stream sediment samples.The results for the drainage sediments distinguish three well defined cupriferous horizons in the sedimentary column. Lithogeochemical studies of quartzites, pelites, and carbonates confirm that the anomalous values obtained in the drainage sediments arise from several Cu-mineralized horizons.The study has shown that geochemical prospecting appears to be well suited to Cu prospecting in Central East Greenland under arctic climate conditions even with a low sampling density of one sample per 5–10 km² for the drainage samples, and one sample per 10–100 m of the sedimentary stratigraphic column for the rock samples.     

Characteristics of tephra in Holocene lake sediments on King George Island, West Antarctica: implications for deglaciation and paleoenvironment

Several reworked tephra layers in gravity-flow deposits are present in lacustrine core sediments collected from Hotel and Rudy Lakes on King George Island, South Shetland Islands, maritime sub-Antarctica. This study tests the values of tephra for establishing regional tephrochronologies for lakes in ice-covered landscapes in the vicinity of volcanoes. The tephra record is more abundant in a long Hotel Lake core (515 cm long). This study uses volcanic glass samples from five tephra layers of Hotel Lake and from one tephra layer of Rudy Lake. Morphologically, tephras are mixtures of basaltic and pumice shards, both having various degrees of vesicularity. Major element analyses of glass shards reveal that the majority of the glass fragments belong to basic glass (<60 wt% SiO₂), compositionally ranging from basalt to low-silica andesite and subalkaline series medium-K tholeiites, probably sourced from Deception Island located 130 km southwest of King George Island. Less than 20% of tephra belongs to silicic glass and occurs in three tephra horizons E of Hotel Lake. However, source volcano(es) for about 10% of basic tephra and silicic tephra are not readily identified from nearby volcanic centers. Except for the studied tephra in Rudy Lake, all tephra samples in Hotel Lake are not ashfall deposits but reworked and redeposited pyroclasts derived from retreating ice sheet, resulting in the occurrence of geochemically equivalent tephra samples in different tephra horizons. The dating of the studied tephra horizons represents the timing of deglaciation rather than that of volcanic eruptions. The result of this study implies that combined with sedimentological information more chemical criterion is necessary to study tephrochronology and regional correlation and to understand paleoenvironmental changes using tephra.     

Geochemistry of Rare Earth Elements in Ferromanganese Micro- and Macronodules from the Pacific Nonproductive Zone

Ferromanganese micro- and macronodules in eupelagic clays at Site 35 of the South Basin were examined in order to check the REE distribution during the ferromanganese ore formation in nonproductive zones of the Pacific Ocean. We studied host sediments and their labile fraction, ferromanganese micronodules (fractions 50–100, 100–250, 250–500, and >500 m) from eupelagic clays (horizons 37–40, 105–110, 165–175, and 189–190 cm), and buried ferromanganese micronodules (horizons 64–68, 158–159, and 165–166 cm). Based on phase analysis data, the anomalous REE enrichment of eupelagic clays from Site 35 is related to the accumulation of rare earth elements in iron hydroxophosphates. The Ce concentration, generally linked to manganese oxyhydroxides, is governed by the oxidation of Mn and Ce in oceanic surficial waters. Micronodules (Mn/Fe = 0.7–1.6) inherit compositional features of the labile fraction of sediments. The Ce, Co, and Th concentrations depend on the micronodule dimension. The enrichment of micronodules in hydrogenic or hydrothermal substance is governed by their dimension and the dominant source of suspended oxyhydroxide material. The study of buried ferromanganese micronodules revealed general regularities in the compositional evolution of oxyhydroxide matrices of ferromanganese micro- and macronodules. The compositional variation of micro- and macronodules, relative to the labile fraction of sediments, in the Pacific nonproductive zone dramatically differs from the pattern in bioproductive zones, where micronodule compositions in larger fractions are similar to those in associated macronodules and labile fractions of the host sediment as a result of the more intense suboxidative diagenesis.     

Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between 800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, 2650, 2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.     

Ordovician igneous and metamorphic units in southeastern Puna: New U–Pb and Sm–Nd data and implications for the evolution of northwestern Argentina

New field, petrological, geochemical, and geochronological data (U–Pb and Sm–Nd) for Ordovician rock units in the southeastern Puna, NW Argentina, indicate two lithostratigraphic units at the eastern–northeastern border of salar Centenario: (1) a bimodal volcanosedimentary sequence affected by low- to medium-grade metamorphism, comprising metasediments associated with basic and felsic metavolcanic rocks, dated 485 ± 5 Ma, and (2) a plutonic unit composed of syenogranites to quartz-rich leucogranites with U–Pb zircon ages between 462 ± 7 and 475 ± 5 Ma. Felsic metavolcanic and plutonic rocks are peraluminous and show similar geochemical differentiation trends. They also have similar Sm–Nd isotopic compositions (T_DM model ages of 1.54–1.78 Ga; ε_Nd(T) values ranging from −3.2 to −7.5) that suggest a common origin and derivation of the original magmas from older (Meso-Paleoproterozoic?) continental crust. Mafic rocks show ε_Nd(T) ranging from +2.3 to +2.5, indicating a depleted mantle source. The data presented here, combined with those in the literature, suggest Ordovician magmatism mainly recycles preexisting crust with minor additions of juvenile mantle-derived material.     

Variable involvements of mantle plumes in the genesis of mid-Neoproterozoic basaltic rocks in South China: A review

Ca. 825–720 Ma global continental intraplate magmatism is generally linked to mantle plumes or a mantle superplume that caused rifting and fragmentation of the supercontinent Rodinia. Widespread Neoproterozoic igneous rocks in South China are dated at ca. 825–760 Ma. There is a hot debate on their petrogenesis and tectonic affiliations, i.e., mantle plume/rift settings or collision/arc settings. Such competing interpretations have contrasting implications to the position of South China in the supercontinent Rodinia and in Rodinia reconstruction models.Variations in the bulk-rock compositions of primary basaltic melts can provide first order constraints on the mantle thermal–chemical structure, and thus distinguish between the plume/rift and arc/collision models. Whole-rock geochemical data of 14 mid-Neoproterozoic (825–760 Ma) basaltic successions are reviewed here in order to (1) estimate the primary melts compositions; (2) calculate the melting conditions and mantle potential temperature; and (3) identify the contributions of subcontinental lithosphere mantle (SCLM) and asenthospheric mantles to the generation of these basaltic rocks.In order to quantify the mantle potential temperatures and percentages of decompression melting, the primary MgO, FeO, and SiO₂ contents of basalts are calculated through carefully selecting less-evolved samples using a melting model based on the partitioning of FeO and MgO in olivine. The mid-Neoproterozoic (825–760 Ma) potential temperatures predicted from the primary melts range from 1390 °C to 1630 °C (mostly > 1480 °C), suggesting that most 825–760 Ma basaltic rocks in South China were generated by melting of anomalously hot mantle sources with potential temperatures 80–200 °C higher than the ambient Middle Ocean Ridge Basalt (MORB)-source mantle.The mantle source regions of these Neoproterozoic basaltic rocks have complex histories and heterogeneous compositions. Enriched mantle sources (e.g., pyroxenite and eclogite) are recognized as an important source for the Bikou and Suxiong basalts, suggesting that their generations may have involved recycled components. Trace elements variations show that interactions between asthenospheric mantle (OIB-type mantle) and SCLM played a very important role in generation of the 825–760 Ma basalts. Our results indicate that the SCLM metasomatized by subduction-induced melts/fluids during the 1.0–0.9 Ga orogenesis as a distinct geochemical reservoir that contributed significantly to the trace-elements and isotope inventory of these basalts.The continental intraplate geochemical signatures (e.g., OIB-type), high mantle potential temperatures and recycled components suggest the presence of a mantle plume beneath the Neoproterozoic South China block. We use the available data to develop an integrated plume-lithosphere interaction model for the ca. 825–760 Ma basalts. The early phases of basaltic rocks (825–810 Ma) were most likely formed by melting within the metasomatized SCLM heated by the rising mantle plume. The subsequent continental rift allowed adiabatic decompression partial melting of an upwelling mantle plumes at relatively shallow depth to form the widespread syn-rifting basaltic rocks at ca. 810–800 Ma and 790–760 Ma.     

Combining trace-element compositions, U–Pb geochronology and Hf isotopes in zircons to unravel complex calcalkaline magma chambers in the Upper Cretaceous Srednogorie zone (Bulgaria)

Precise U–Pb geochronology and Hf isotope tracing of zircon is combined with whole-rock geochemical and Sr and Nd isotope data in order to unravel processes affecting mafic to felsic calcalkaline magmas prior to and during their crystallization in crustal magma chambers along the southern border of Central Srednogorie tectonic zone in Bulgaria (SE Europe). ID-TIMS U–Pb dating of single zircons from felsic and mixed/mingled dioritic to gabbroic horizons of single plutons define crystallization ages of around 86.5–86.0, 85.0–84.5 and 82 Ma. Concordia age uncertainties are generally less than 0.3 Ma (0.35%–2σ), and as good as 0.08 Ma (0.1%), when the weighted mean ²⁰⁶Pb/²³⁸U value is used. Such precision allows the distinction of magma replenishment processes if separated by more than 0.6–1.0 Ma and when they are marked by newly saturated zircons. We interpret zircon dates from a single sample that do not overlap to reflect new zircon growth during magma recharge in a long-lived crustal chamber. Mingling/mixing of the basaltic magma with colder granitoid mush at mid- to upper-crustal levels is proposed to explain zircon saturation and fast crystallization of U- and REE-rich zircons in the hybrid gabbro.Major and trace-element distribution and Sr and Nd whole-rock isotope chemistry define island arc affinities for the studied plutons. Slab derived fluids and a sediment component are constrained as enrichment sources for the mantle wedge-derived magma, though Hf isotopes in zircon suggest crustal assimilation was also important. Inherited zircons, and their corresponding ε-Hf, from the hybrid gabbroic rocks trace the lower crust as possible source for enrichment of the mantle magma. These inherited zircons are about 440 Ma old with ε-Hf of − 7 at 82 Ma, whereas newly saturated concordant Upper Cretaceous zircons reveal mantle ε-Hf values of + 7.2 to + 10.1. The upper and middle crusts contribute in the generation of the granitoid rocks. Their zircon inheritance is Lower Palaeozoic or significantly older and crustal dominated with 82–85 Ma corrected ε-Hf values of − 28. The Cretaceous concordant zircons in the granitoids are mantle dominated with a ε-Hf values spreading from + 3.9 to + 7.     

Geochronology of the khondalite belt of Trivandrum Block, Southern India: Electron probe ages and implications for Gondwana tectonics

In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma.The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities.The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.     

Age and origin of middle Neoproterozoic mafic magmatism in southern Yangtze Block and relevance to the break-up of Rodinia

SHRIMP U–Pb zircon age, geochemical and Sm–Nd isotopic data are reported for mid-Neoproterozoic volcanic rocks and mafic intrusions in northern Guangxi (Guibei) and western Hunan (Xiangxi) Provinces along the southern margin of the Yangtze Block. The mafic igneous rocks studied are generally synchronous, dated at  765 Ma. The least-contaminated dolerite samples from Xiangxi are characterized by high εNd(T) value of 3.3 to 5.3 and OIB-type geochemical features, indicating that they were derived from an OIB-like mantle source in a continental rift setting. The spilites and gabbros in Guibei show basaltic compositions transitional between the tholeiitic and calc-alkaline series. Despite depletion in Nb and Ta relative to La and Th, they have Zr/Sm = 27–35 and Ti/V = 30–40, affinitive to intraplate basalts. Their εNd(T) values are variable, ranging from − 1.2 to 3.2 for the spilites and from − 1.7 to 2.9 for the gabbros, suggesting that these spilites and gabbros crystallized from crustal-contaminated mafic magmas derived from a metasomatised subcontinental lithospheric mantle source. We conclude that the  765 Ma mafic magmatic rocks in Guibei and Xiangxi were formed in a single continental rift setting as part of the broadly concurrent  780–750 Ma rift magmatism over much of South China, which may be related to the plume activities during the breakup of Rodinia.     

Some numerical considerations in the geochemical analysis of distal microtephra

The use of Late Quaternary tephra horizons as isochronous markers for high resolution correlation between records is of growing importance in a number of scientific fields. Particular advancements have been made through the identification of microtephra deposits (very distal deposits of volcanic ash invisible to the naked eye). The successful correlation of ash layers and volcanic eruptions depends on the acquisition of the major element chemistries of a tephra layer, derived from microprobe analyses of individual glass shards. This is particularly important for microtephras where aeolian fractionation has removed much of the mineral phase of the ash deposit, which often aids tephra discrimination. There are, however, difficulties in distinguishing precisely between different eruptions of the same volcano using major element composition and this may not always be resolved by the use of trace elements. Given the potential of tephrochronology as a correlative tool in many disciplines it is important to resolve this difficulty. One way forward is to perform more robust statistical analyses on the geochemical data, as in general, analyses are confined to a series of bi-plots of major elements. Here the authors explore some of the problems associated with dealing with microprobe data generated for individual tephras and pay particular attention to the ‘unit sum problem’. Using a subset of data generated as part of a EURODELTA project on tephra distribution in the Adriatic, the authors demonstrate that this problem is detectable in major element data from tephras and have consequently applied the logratio method before further analyses of the data. It is demonstrated that the use of logratios combined with discriminant functions analysis provides a more robust assessment of likely chemical correlations between tephras, and are superior to the use of bi-plots alone; and obviate the need for any data normalization, a particular point of contention between tephrochronologists.     

Two Tephra Layers Bracketing Late Holocene Paleoecological Changes in Northern Germany

Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry ¹⁴C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog.     

Timing of arrival of the Danube to the Black Sea: Provenance of sediments from DSDP site 380/380A

Estimates for the timing of the arrival of Danube sediment to the Black Sea range from Messinian to Pleistocene; the river is currently the largest sediment contributor, supplying 88 MT/yr. We identify two changes in siltstone provenance‐sensitive heavy mineral abundances at DSDP site 380/380A in the southwest Black Sea. Comparison with modern river sediment compositions indicates that siltstones above 571.5 mbsf (metres below sea floor) were supplied by the Danube, while sediments below 651.0 m were sourced by other supply systems. Palaeo‐magnetic, ⁴⁰Ar/³⁹Ar and biostratigraphic data reveal that the influx of Danube‐supplied sediment to the southwest Black Sea began between 4.36 ± 0.19 Ma and 1 Ma ago (Zanclean–Calabrian). Our results provide an independent time constraint on palaeogeographic reconstructions of the Pannonian and Dacian basins, which acted as upstream sediment sinks, and suggest that significant volumes of Danube‐supplied sediment only started to reach the Black Sea at least 1 Ma after the Messinian Salinity Crisis (5.971–5.33 Ma) had ended.     

Petrology and geodynamic significance of the post-collisional Pan-African magmatism in the Eastern Saghro area (Anti-Atlas, Morocco)

The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T_DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption.Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd T_DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts.Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.