The Holocene climatic optimum and pollen records of sapropel 1 in the eastern Mediterranean, 9000–6000 BP

The most recent sapropel in the deep eastern Mediterranean Sea has been deposited between 9 and 6 ka bp. Climate conditions, as revealed by the pollen records of this sapropel in marine cores, were most favorable for temperate deciduous trees, which is in agreement with the inferences from records of peripheral land pollen sites. The abundance of deciduous oak pollen is much higher than that of Artemisia (sage-brush), indicating that annual precipitation in the mid-elevation borderlands was at least 550 mm without summer drought, but more probably in the range 800–1300 mm. The pollen of Pistacia, which formed a savanna at low elevations, is also at its highest abundance and signals the absence of frost in winter, while being capable of withstanding summer drought. The early Holocene therefore appears as the post-glacial climatic optimum with the highest moisture and mildest winters. In southwest Asia, this is also the time of the Neolithic population explosion with incipient domestication of cereals, possibly following natural selection of the ‘tough rachis’ mutation in wheat and barley by the extreme aridity of the preceding Younger Dryas.     

Decline of the mid‐ to late Holocene forests in China: climatic change or human impact?

Fossil pollen data from China indicate continued forest decline during the mid‐ to late Holocene in most regions north of the Yangtze River. The earliest forest decline can be detected ca. 5000 yr BP in the middle and lower Yellow River regions. North, northeast and northwest from this region, forest decline became progressively later, and almost no decline took place in the northernmost part of northeast China and in the remote areas of west China during the last 5000 yr. Climate changes could hardly account for the temporal and spatial patterns of the forest decline. Instead, anthropogenic disturbance may have been of overwhelming importance. Ancient agriculture and high‐density settlement expanded outward from the middle and lower Yellow River regions in similar patterns to those of forest change. This study also indicates that land‐use and land‐cover changes may have started in the early stage of Chinese civilization in an extensive area of the country. Copyright © 2000 John Wiley & Sons, Ltd.     

A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga?

Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga.Among the more important of these are the following:(1)increasing proportion of basalts with"arc-like"mantle sources;(2)an increasing abundance of basalts derived from enriched(EM)and depleted(DM)mantle sources;(3)onset of a Great Thermal Divergence in the mantle;(4)a decrease in degree of melting of the mantle;(5)beginning of large lateral plate motions;(6)appearance of eclogite inclusions in diamonds;(7)appearance and rapid increase in frequency of collisional orogens;(8)rapid increase in the production rate of continental crust as recorded by zircon age peaks;(9)appearance of ophiolites in the geologic record,and(10)appearance of global LIP(large igneous province)events some of which correlate with global zircon age peaks.All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere,and they may record the gradual onset and propagation of plate tectonics around the planet.To further understand the changes that occurred between 3 and 2 Ga,it is necessary to compare rocks,rock associations,tectonics and geochemistry during and between zircon age peaks.Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.     

U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.     

Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.     

U–Pb zircon ages,field geology and geochemistry of the Kermanshah ophiolite (Iran): From continental rifting at 79 Ma to oceanic core complex at ca. 36 Ma in the southern Neo-Tethys

The geodynamic evolution of the Zagros Mountains of Iran remains obscure. In particular, the time of formation of the Zagros ophiolites and the closure of the Neo-Tethys Ocean are highly controversial. Here we present new precise zircon U–Pb ages that show that the younger part (Sahneh–Kamyaran) of the Kermanshah ophiolite formed at 35.7 ± 0.5 Ma and the older part (Harsin) at 79.3 ± 0.9 Ma. Field relations and geochemical evidence show that the younger Sahneh–Kamyaran part is probably a fossil oceanic core complex, and the older Sahneh part is probably a continental-oceanic transition complex. Both the Sahneh–Kamyaran and Sahneh parts were later emplaced into an accretionary complex. We conclude and infer that the final closure time of the southern Neo-Tethys Ocean was after the Late Eocene. Our data and tectonic model have crucial implications for the geodynamic evolution of the Zagros region.     

Paleoclimate variability in central Taiwan during the past 30 Kyrs reflected by pollen, δ13CTOC,and n-alkane-δD records in a peat sequence from Toushe Basin

A total of 233 samples from the upper 16 m of the Toushe peat core retrieved in central Taiwan were measured for TOC and δ¹³C_TOC values. From these samples, 17 selected samples with large δ¹³C_TOC fluctuations were analyzed for n-alkane and δD of the C27 and C29 n-alkanes. Combining with the detailed high-resolution pollen and geochemical records, this study reveals more detailed climatic variations in terms of temperature and precipitation as well as abrupt climatic events during the past 30 Kyrs. Before the Last Glacial Maximum (LGM), climate was cold and damp with predominantly woodland vegetation in Toushe Basin, and turned to cold and dry after 25 Kyr BP. Climatic conditions there were the worst during LGM over the past 30 Kyrs, especially around 23 and 18 Kyr BP when the woodland was diminished and C4 grass was dominated. Although short durations of relatively wet conditions could be found at 17, 16 and 14 Kyr BP, cold and dry climates were prevailing during 29.5–28, 24–22, 17–15 and 13–11.5 Kyr BP, corresponding to Heinrich (H) Events 3, 2, 1, and Younger Dryas (YD), respectively. During the early Holocene, dry climate occurred at ∼11, ∼10, 9.7–9.2 and ∼8 Kyr BP; whereas wet condition appeared at 10.3, 9.8, 9–7.5 Kyr BP. In the middle Holocene, climate kept warm and moderate wet in the first half period, but many dry events existed in the second half following a cold and dry event at 6 Kyr BP. After a sharply warm peak at 5.2 Kyr BP, the climate in Toushe turned to cold quickly, and tree/shrub vegetation disappeared completely with the replacement of C3 grasses. In the late Holocene, climate was relatively wetter with predominant C3 grasses in the basin. Our climatic interpretations based on the peat records agree well with the Greenland ice core and Chinese speleothem records on millennium time scales during the last glacial period. Dry climates corresponding to weakening of the East Asian Summer Monsoon (EASM) during the Heinrich events and Younger Dryas in central Taiwan and eastern China demonstrate the climatic forcing on such long time scales in concert with regional monsoon climate. However, the discrepancies exist between our peat record and the Dongge/Hulu stalagmite record on: (1) the age of H2; (2) climate intensities of LGM and H1; and (3) wetness condition during Holocene. These observations call for further study on high-resolution climatic changes especially on moisture budget in the East Asian monsoonal region.     

Further paleomagnetic results from the ~ 155 Ma Tiaojishan Formation,Yanshan Belt,North China,and their implications for the tectonic evolution of the Mongol–Okhotsk suture

A new paleomagnetic study on well-dated (~ 155 Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A₉₅ = 5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~ 155 Ma. Our calculation shows a ~ 1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~ 155 Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~ 155 Ma.     

Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000 Ma Closure age for the Purana Basins?

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including ⁸⁷Sr/⁸⁶Sr and δ ¹³C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α₉₅ = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α₉₅ = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.     

Variability of Indian monsoonal rainfall over the past 100 ka and its implication for C3–C4 vegetational change

Oxygen and carbon isotope ratios of soil carbonate and carbon isotope ratios of soil organic matter (SOM) separated from three cores, Kalpi, IITK and Firozpur, of the Ganga Plain, India are used to reconstruct past rainfall variations and their effect on ambient vegetation. The δ¹⁸O values of soil carbonate (δ¹⁸O_SC) analyzed from the cores range from ? 8.2 to ? 4.1‰. Using these variations in δ¹⁸O_SC values we are able, for the first time, to show periodic change in rainfall amount between 100 and 18 ka with three peaks of higher monsoon at about 100, 40 and 25 ka. The estimation of rainfall variations using δ¹⁸O value of rainwater-amount effect suggests maximum decrease in rainfall intensity (~ 20%) during the last glacial maximum. The δ¹³C values of soil carbonate (δ¹³C_SC) and SOM (δ¹³C_SOM) range from ? 6.3 to + 1.6‰ and ? 28.9 to ? 19.4‰, respectively, implying varying proportions of C₃ and C₄ vegetations over the Ganga Plain during the last 100 ka. The comparison between monsoonal rainfall and atmospheric CO₂ with vegetation for the time period 84 to 18 ka indicate that relative abundances of C₃ and C₄ vegetations were mainly driven by variations in monsoonal rainfall.     

Petrogenesis and tectonic significance of the ∼ 850 Ma Gangbian alkaline complex in South China: Evidence from in situ zircon U–Pb dating,Hf–O isotopes and whole-rock geochemistry

The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848 ± 4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO₂ = 47.6–68.4%, K₂O + Na₂O = 4.5–10.5%, K₂O/Na₂O = 0.4–1.2, MgO = 1.2–8.5%, Cr = 4.5–239 ppm, and Ni = 4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd(T) (? 6.5 to ? 0.4) and εHf(T) (? 10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis. In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.     

The Afeição augen-gneiss Suite and the record of the Cariris Velhos Orogeny (1000–960 Ma) within the Riacho do Pontal fold belt,NE Brazil

Early Tonian (∼1000–920 Ma) rocks occur within the Transversal Zone of the Borborema Province in Northeast Brazil comprising the 700 km-long sigmoidal Cariris Velhos belt. The Afeição augen-gneiss Suite crops out in the internal zone of the Riacho do Pontal fold belt, about 100 km southwestward of the closest Cariris Velhos occurrence within the Transversal Zone, and has been proposed to represent a continuation of this belt within the Southern subprovince of the Borborema Province. Several plutons included within this unit intrude or are thrust upon metavolcanosedimentary sequences of three different units (Santa Filomena, Paulistana and Morro Branco complexes). The Afeição Suite is composed mostly of calc-alkaline, high-K, peraluminous, high-silica ferroan and magnesian granites. Chondrite-normalized REE patterns are moderate to highly fractionated, with a pronounced negative Eu anomaly. Incompatible element spidergrams show a negative Nb–Ta anomaly, akin to convergence setting (Cordilleran-type) granites. U–Pb zircon data constrain the age of crystallization between 1000 and 960 Ma, thus confirming chrono-correlation with the Cariris Velhos belt. Values of εNd(t) between −1.0 and +3.1 and T_DM of 1.2–1.5 Ga, similar to other Cariris Velhos occurrences, suggest variable mixing of Tonian juvenile sources with older crustal sources, the latter involving Archean/Paleoproterozoic basement. Although there are currently no reliable geochronological data for the supracrustal sequences of the internal zone, cross-cutting relationships indicate that the Santa Filomena and Morro Branco complexes are older or of similar age to the Afeição Suite, and thus could be related to the Cariris Velhos Orogeny. Based on petrographic, lithogeochemical, geochronological and isotope data, and according to previous models proposed for the Cariris Velhos belt, we interpret the Afeição Suite as the southwestern edge of a continental margin magmatic arc accreted to this portion of West Gondwana during the Early Tonian. Late-Brasiliano dextral displacement through the western branch of the Pernambuco shear zone separated these Cariris Velhos occurrences from its equivalents within the Transversal Zone of the Borborema Province (Recanto-type augen-gneiss of the Alto Pajeú Terrane).     

Biostratigraphy of the Çeşmeler and Elmalι Dere formations: First records of upper Cretaceous planktonic foraminifera and radiolarians from the Eastern Pontides (Maçka,Trabzon, Turkey)

Upper Cretaceous pelagic deposits outcropping in the Maçka (Trabzon) region include radiolarians and pelagic foraminifera. The Çatak Group represented by the volcano-sedimentary successions consists of three formations having different properties. Two sections, ÇTK1 and ÇTK2, are selected from the Çe meler and Elmalι Dere formations, respectively, establishing the biostratigraphy of outcropping sedimentary units. A total of 17 species of Whiteinella, Helvetoglobotruncana, Marginotruncana, Dicarinella, Praeglobotruncana, Archaeoglobigerina and Hedbergella demonstrating the early Turonian–Coniacian are established in the ÇTK1 stratigraphic section. The early Turonian radiolarian fauna consisting of Halesium sexangulum Pessagno, 1971, Crucella cachensis Pessagno, 1971, Stichomitra communis Squinabol, 1903 is also defined in the same section. A total of 30 species of Crucella, Halesium, Pessagnobrachia, Patulibracchium, Alievium, Archaeospongoprunum, Dicyomitra, Stichomitra, Diacanthocapsa, Dactiyliodiscus, Amphipydax, Pseudoaulophocus, Acaeniotyle, Archaeodictyomitra, Actinomma, Xitus, Neosciadocapsidae characterizing the early and late Turonian, as well as the Coniacian–early Santonian are recognized from red-coloured pelagic limestones of the ÇTK2 section. Also, planktonic foraminifera species of Marginotruncana, Hedbergella, Heterohelix, Globotruncana, Globotruncanita, Archaeoglobigerina, Dicarinella characterizing the Coniacian–Santonian are described in the thin sections of the same samples. The age of red-coloured limestones is identified as the Coniacian–Santonian benefit from radiolarians and pelagic foraminifera. Consequently, radiolarians and pelagic foraminifera within sedimentary successions of the investigation area are distributed in two intervals that coincide with the early Turonian–Coniacian and Coniacian–Santonian intervals.     

Late Barremian–early Aptian climate of the northern middle latitudes: Stable isotope evidence from bivalve and cephalopod molluscs of the Russian Platform

Palaeotemperatures during the late Barremian–early Aptian (Early Cretaceous) on the Russian Platform have been determined on the basis of oxygen isotope analysis of aragonitic bivalve molluscan and ammonoid shells and belemnite rostra with well-preserved microstructure from the Ulyanovsk area. Those obtained from the planispiral and heteromorph ammonoid shells from the lower Aptian Volgensis–Schilovkensis, Deshayesi–Tuberculatum, and Deshayesi–Renauxianum zones range from 26.7 to 33.2 °C, from 29.2 to 33.1 °C, and from 27.0 to 29.5 °C, respectively. A heteromorph Helicancylus? cf. philadelphius shell from the uppermost lower Aptian Bowerbanki Zone was secreted in highest temperature conditions (32.8–35.2 °C). In contrast, upper Barremian molluscs (bivalve Cyprina sp. and belemnite Oxyteuthis sp.) of the Ulyanovsk area show significantly lower palaeotemperatures: 16.9–18.5 °C and 7.9–17.8 °C, respectively, which is in accordance with known palaeogeographic and palaeobotanical evidences, showing that a distinct climatic optimum seems to have occurred during the late early Aptian, when warm Tethyan water penetrated into the basin. Marked changes in calculated growth temperatures for investigated molluscs from the Russian Platform were most likely connected with both the general warming trend during the late Barremian–early Aptian and local palaeonvironmental conditions. New data from the Bowerbanki Zone of the Russian Platform provide evidence on existence of the positive carbon isotope anomaly (2.4–6‰) at the end of the lower Aptian. There were apparently the three positive C-isotope anomalies during the late Barremian–early Aptian. The onset of mid early Aptian Oceanic Anoxic Event (OAE) 1a seems to coincide with both the beginning of significant warm conditions (followed by short-term cooling) and the abrupt decline in heavy carbon isotope concentrations in marine carbonates, which partly were the likely consequences of the intensive release of CO₂ (biased by volcanic activity) and/or dissociation of methane gas hydrate.     

Geochemistry and petrogenesis of late Ediacaran (605–580 Ma) post-collisional alkaline rocks from the Katherina ring complex,south Sinai,Egypt

The Katherina ring complex (KRC) in the central part of south Sinai, Egypt, is a typical ring complex of late Neoproterozoic age (605–580 Ma). It was developed during the final tectono-magmatic stage of the north Arabian–Nubian Shield (ANS) during evolution of the Pan-African crust. The KRC includes Katherina volcanics, subvolcanic bodies, ring dykes and Katherina granitic pluton. The Katherina volcanics represent the earliest stage of the KRC, which was subsequently followed by emplacement of the subvolcanic bodies and ring dykes. The Katherina granitic pluton depicts as the latest evolution stage of the KRC that intruded all the early formed rock units in the concerned area. The Katherina volcanics are essentially composed of rhyolites, ignimbrite, volcanic breccia and tuffs. Mineralogically, the peralkaline rhyolites contain sodic amphiboles and aegirine. The rhyolite whole rock chemistry has acmite-normative character. The subvolcanic bodies of the KRC are represented by peralkaline microgranite and porphyritic quartz syenite. The ring dykes are semicircular in shape and consist mainly of quartz syenite, quartz trachyte and trachybasalt rock types. The Katherina subvolcanic rocks, volcanic rocks as well as the ring dykes are alkaline or/and peralkaline in nature. The alkaline granitic pluton forms the inner core of the KRC, including the high mountainous areas of G. Abbas Pasha, G. Bab, G. Katherina and G. Musa. These mountains are made up of alkaline syenogranite and alkali feldspar granite. The mantle signature recorded in the KRC indicates a juvenile ANS crust partial melting process for the generation of this system. The evolution of the KRC rocks is mainly dominated by crystal fractionation and crustal contamination. Mineral geothermometry points to the high temperature character of the KRC, up to 700–1100 °C.     

High-P (P = 1.5–1.8 GPa) blueschist from Elba: Implications for underthrusting and exhumation of continental units in the Northern Apennines

The Acquadolce Subunit on the Island of Elba, Italy, records blueschist facies metamorphism related to the Oligocene–early Miocene stages of continental collision in the Northern Apennines. The blueschist facies metamorphism is represented by glaucophane- and lawsonite-bearing metabasite associated with marble and calcschist. These rock types occur as lenses in a schistose complex representing foredeep deposits of early Oligocene age. Detailed petrological analyses on metabasic and metapelitic protoliths, involving mineral and bulk-rock chemistry coupled with P–T and P–T–X(Fe₂O₃) pseudosection modelling using PERPLE_X, show that the Acquadolce Subunit recorded nearly isothermal exhumation from peak pressure–temperature conditions of 1.5–1.8 GPa and 320–370°C. During exhumation, peak lawsonite- and possibly carpholite- or stilpnomelane-bearing assemblages were overprinted and partially obliterated by epidote-blueschist and, subsequently, albite-greenschist facies metamorphic assemblages. This study sheds new light on the tectonic evolution of Adria-derived metamorphic units in the Northern Apennines, by showing (a) the deep underthrusting of continental crust during continental collision and (b) rapid exhumation along ‘cold’ and nearly isothermal paths, compatible with syn-orogenic extrusion.     

Early Cretaceous (ca. 100 Ma) magmatism in the southern Qiangtang subterrane,central Tibet: Product of slab break-off?

International Journal of Earth Sciences - The lack of Early Cretaceous magmatic records with high-quality geochemical data in the southern Qiangtang subterrane has inhibited a complete...     

Combined U–Pb and Lu–Hf isotope data on turbidites of the Paleozoic basement of NW Argentina and petrology of associated igneous rocks: Implications for the tectonic evolution of western Gondwana between 560 and 460 Ma

New combined U–Pb and Lu–Hf isotope analyses on zircon from three turbidite deposits, and petrologic data for associated igneous rocks were used to study the evolution of the Paleozoic basement of Eastern Cordillera, NW Argentina. Maximum and minimum ages for turbidite deposits, considered to be part of the Puncoviscana Fm., are reported. In the Tastil area, turbidites were deposited in a fore-arc setting after 560 Ma and intruded at 534 Ma by the Tastil batholith. In the El Niño Muerto Hill area turbidites with maximum depositional age of 496 ± 11 Ma were intruded by high-K dacites at 483 ± 3 Ma. In the Río Blanco Valley, the turbiditic/hemipelagitic sediments, with maximum depositional age of 463 ± 11 Ma were contemporaneous with E-MORB/OIB volcanism. The U–Pb and Lu–Hf data permitted to distinguish two major periods of magmatic activity during Late Mesoproterozoic–Early Neoproterozoic (0.95 to 1.2 Ga) and Late Neoproterozoic–Early Paleozoic (0.75 to 0.46 Ga) times, the former dominated by the input of juvenile crust and the latter by arc magmatism and recycling of Meso- to Paleoproterozoic crust. On the basis of new data we suggest that western margin of Gondwana was controlled by subduction processes and accretion of small terrains during Neoproterozoic–Early Paleozoic times.     

Petrography and geochemistry of intraclastic manganese–carbonates from the ∼2.2 Ga Nsuta deposit of Ghana: Significance for manganese sedimentation in the Palaeoproterozoic of West Africa

Intraclastic Mn carbonate rocks occur in the marginal areas of the manganese–carbonate orebody (manganesestone) of the Palaeoproterozoic Nsuta deposit in the Birimian of Ghana. Macroscopically the intraclastic rocks display graded bedding and are typified by a matrix-supported fabric with subangular to subrounded particles less than a millimetre to ∼1.5 × 0.5 cm. Both clasts and matrix consist mainly of varying proportions of microcrystalline and microconcretionary carbonates, quartz, muscovite and subordinate pyrite. Within individual intraclasts, carbonate minerals (including distinctly zoned microconcretions) are essentially Mg kutnahorite and Mg–Ca rhodochrosite, similar to the carbonate minerals in the manganesestone. Whole rock chemistry of the intraclastic carbonates shows significant variability in the amounts of SiO₂, Al₂O₃, MnO, MgO, CaO, Na₂O and, to a lesser extent, K₂O. Major element contents of the manganesestone similarly vary widely, except that these have, in particular, comparably higher MnO but less SiO₂ and Al₂O₃ than the intraclastic carbonates and host rock Mn phyllite. Rare earth element (REE) concentrations in the intraclastic carbonates are approximately an order of magnitude higher than in the manganesestone. Whereas both rocks exhibit positive Eu anomalies, only the manganesestone shows a discernibly negative Ce anomaly. Petrographic and geochemical features suggest that the intraclasts are fragments of reworked Mn carbonate sediments derived from intraformational erosion and subsequent (mass flow) deposition as carbonate “turbidite” mud. Processes such as submarine slumping, sliding and other sediment gravity flows may have likely interrupted Mn sedimentation and transported partially consolidated manganiferous sediments down slopes into the early Birimian ocean.     

Evidence for a 200 km thick diamond-bearing root beneath the Central Mackenzie Valley,Northwest Territories,Canada? Diamond indicator mineral geochemistry from the Horn Plateau and Trout Lake regions

The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.