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During the Late Miocene, Africa experienced a number of ecological transitions including the spread of C 4 grasslands, the expansion of the Sahara Desert, the Messinian Salinity Crisis and a number of mammalian migrations and expansions, including the origin of the hominin clade. A detailed understanding of the relationship between environmental change and hominin evolution is hampered by the paucity of data available from terrestrial localities, especially in southern Africa. Here, we present a stable isotope and trace element record from a speleothem from the South African cave site of Hoogland. Uranium-lead dating and magnetostratigraphy places the speleothem within the Messinian Age (7.25–5.33 Ma) of the Late Miocene, making it the oldest known cave deposit from the region near the UNESCO Fossil Hominids of South Africa World Heritage Site (locally known as the "Cradle of Humankind"). Low carbon isotope values indicate a predominantly C 3 vegetation in the vicinity of the cave throughout the period of speleothem growth. It is not possible to determine if this represents a C 3 grassland or a C 3 woodland, but it is clear that an equivalent C 3 -rich environment has yet to be found during the Messinian of east Africa. We conclude that the C 4 grass expansion occurred millions of years later in South Africa than it did in eastern Africa, and that this vegetation shift should be considered when comparing African vegetation change with the late Miocene hominin fossil record. • Speleothem from South Africa dated to late Messinian age, using U–Pb methods. • Carbon isotopes indicate a predominantly C 3 vegetation at this time. • First savannah grasslands in southern Africa occur later than in eastern Africa. • Implications for early hominin environments [ABSTRACT FROM AUTHOR]

The Shillong Plateau is the only raised topography (up to 2000 m elevation) in the Himalayan foreland. It is proposed to have had a major influence on strain partitioning and thus tectonics in the Eastern Himalaya. Additionally, its position on the trajectory of the summer monsoon means it has influenced the regional climate, with reduced erosion rates proposed over geological timescales in its lee. The timing of surface uplift of the plateau has been difficult to determine. Exhumation rates have been calculated over geological timescales, but these seem at variance with estimates based upon extrapolating the present day velocity field measured with GPS, and it has thus been suggested that exhumation and surface uplift are decoupled. We determine the timing of surface uplift using the sedimentary record in the adjacent Surma Basin to the south, which records the transition from a passive margin with southward thickening sedimentary packages to a flexural basin with north-thickening strata, due to loading by the uplifting plateau. Our method involves using all available 2D seismic data for the basin, coupled to well tie information, to produce isochore maps and construct a simple model of the subsidence of the Surma basin in order to assess the timing and magnitude of flexural loading by the Shillong Plateau. We conclude that the major period of flexural loading occurred from the deposition of the Tipam Formation ( 3.5 – ∼ 2 Ma ) onwards, which is likely to represent the timing of significant topographic growth of the Shillong Plateau. Our isochore maps and seismic sections also allow us to constrain the timing of thinning over the north–south trending anticlines of the adjacent basin-bounding Indo–Burman Ranges, as occurring over this same time interval. The combined effect of the uplift of the Shillong Plateau and the westward encroachment of the Indo–Burman Ranges to this region served to sever the palaeo-Brahmaputra drainage connection between Himalayan source and Surma Basin sink, at the end of Tipam Formation times ( ∼ 2 Ma ). [ABSTRACT FROM AUTHOR]

The Himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion. Within the broader orogen, fluvial drainages partly reflect surface uplift, different climatic zones and a response to crustal deformation. In the eastern Himalaya, the unusual drainage configuration of the Yarlung Tsangpo–Brahmaputra River has been interpreted either as antecedent drainage distorted by the India–Asia collision (and as such applied as a passive strain marker of lateral extrusion), latest Neogene tectonically-induced river capture, or glacial damming-induced river diversion events. Here we apply a multi-technique approach to the Neogene paleo-Brahmaputra deposits of the Surma Basin (Bengal Basin, Bangladesh) to test the long-debated occurrence and timing of river capture of the Yarlung Tsangpo by the Brahmaputra River. We provide U–Pb detrital zircon and rutile, isotopic (Sr–Nd and Hf) and petrographic evidence consistent with river capture of the Yarlung Tsangpo by the Brahmaputra River in the Early Miocene. We document influx of Cretaceous–Paleogene zircons in Early Miocene sediments of the paleo-Brahmaputra River that we interpret as first influx of material from the Asian plate (Transhimalayan arc) indicative of Yarlung Tsangpo contribution. Prior to capture, the predominantly Precambrian–Paleozoic zircons indicate that only the Indian plate was drained. Contemporaneous with Transhimalayan influx reflecting the river capture, we record arrival of detrital material affected by Cenozoic metamorphism, as indicated by rutiles and zircons with Cenozoic U–Pb ages and an increase in metamorphic grade of detritus as recorded by petrography. We interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen. Whole rock Sr–Nd isotopic data from the same samples provide further support to this interpretation. River capture may have been caused by a change in relative base level due to uplift of the Tibetan plateau. Assuming such river capture occurred via the Siang River in the Early Miocene, we refute the “tectonic aneurysm” model of tectonic–erosion coupling between river capture and rapid exhumation of the eastern syntaxis, since a time interval of at least 10 Ma between these two events is now demonstrated. This work is also the first to highlight U–Pb dating on detrital rutile as a powerful approach in provenance studies in the Himalaya in combination with zircon U–Pb chronology. [ABSTRACT FROM AUTHOR]

Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the prograde metamorphic history of Eohimalayan crustal thickening, which has been erased by Oligo-/Miocene migmatization elsewhere in the Himalaya. Our zircon and monazite U/Th–Pb data unravel a multi-stage prograde metamorphic evolution. The earliest evidence of prograde Barrovian metamorphic monazite growth is ~ 41 Ma. Peak metamorphic conditions (~ 8–8.5 kbar, ~ 600–700 °C) were attained at 37–36 Ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth, which may be related to the formation of the northern Himalayan nappes (e.g., Shikar Beh nappe, Nyimaling nappe). Rapid exhumation of the crystalline started at ~ 26 Ma and resulted in cooling through the muscovite 40 Ar/ 39 Ar closure temperature by 21.8 Ma. Although a local continuation of the South Tibetan detachment is not unambiguously identified in central Himachal Pradesh extrusion was likely facilitated by a system of several minor late Oligocene/early Miocene top-to-the-N to NE shear zones. In contrast to the crystalline of Zanskar and eastern Himachal Pradesh, extrusion was not accompanied by widespread decompression melting. [ABSTRACT FROM AUTHOR]

The Hardangervidda-Rogaland Block within southwest Norway is host to ~1.52 to 1.48 Ga continental building and variable reworking during the ~1.1 to 0.9 Ga Sveconorwegian orogeny. Due to the lack of geochronological and geochemical data, the timing and tectonic setting of early Mesoproterozoic magmatism has long been ambiguous. This paper presents zircon U-Pb-Hf-O isotope data combined with whole-rock geochemistry to address the age and petrogenesis of basement units within the Suldal region, located in the centre of the Hardangervidda-Rogaland Block. The basement comprises variably deformed grey gneisses and granitoids that petrologically and geochemically resemble mature volcanic arc lithologies. U-Pb ages confirm that magmatism occurred from ~1,521 to 1,485 Ma, and conspicuously lack any xenocrystic inheritance of distinctly older crust. Hafnium isotope data range from εHf +1 to +11, suggesting a rather juvenile magmatic source, but with possible involvement of late Palaeoproterozoic crust. Oxygen isotope data range from mantle-like (δO ~5 ‰) to elevated (~10 ‰) suggesting involvement of low-temperature altered material (e.g., supracrustal rocks) in the magma source. The Hf-O isotope array is compatible with mixing between mantle-derived material with young low-temperature altered material (oceanic crust/sediments) and older low-temperature altered material (continent-derived sediments). This, combined with a lack of xenoliths and xenocrysts, exposed older crust, AFC trends and S-type geochemistry, all point to mixing within a deep-crustal magma-generation zone. A proposed model comprises accretion of altered oceanic crust and the overlying sediments to a pre-existing continental margin, underthrusting to the magma-generation zone and remobilisation during arc magmatism. The geodynamic setting for this arc magmatism is comparable with that seen in the Phanerozoic (e.g., the Sierra Nevada and Coast Range batholiths), with compositions in the Suldal Sector reaching those of average upper continental crust. As within these younger examples, factors that drive magmatism towards the composition of the average continental crust include the addition of sedimentary material to magma source regions, and delamination of cumulate material. Underthrusting of sedimentary materials and their subsequent involvement in arc magmatism is perhaps a more widespread mechanism involved in continental growth than is currently recognised. Finally, the Suldal Arc magmatism represents a significant juvenile crustal addition to SW Fennoscandia. [ABSTRACT FROM AUTHOR]

Abstract: Deposited within the Indus–Tsangpo suture zone, the Cenozoic Indus Basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of India–Eurasia collision, a conclusion challenged by data presented here. The Eurasian derived 50.8–51Ma Chogdo Formation was previously considered to overlie Indian Plate marine sedimentary rocks in sedimentary contact, thus constraining the timing of collision as having occurred by this time. Using isotopic analysis (U–Pb dating on detrital zircons, Ar–Ar dating on detrital white mica, Sm–Nd analyses on detrital apatite), sandstone and conglomerate petrography, mudstone geochemistry, facies analysis and geological mapping to characterize and correlate the formations of the Indus Basin Sedimentary rocks, we review the nature of these contacts and the identification and correlation of the formations. Our results reveal that previously interpreted stratigraphic contacts identifying Chogdo Formation unconformably overlying Indian plate sedimentary rocks are incorrect. Rather, we suggest that the inaccuracy of previous interpretations is most likely a result of Formation misidentification and thus cannot be used to constrain the timing of India–Asia collision. [Copyright &y& Elsevier]

Abstract: We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite (P =14–18kbar, T ≈800°C) yield U–Pb ages of 24.0±0.3Ma and 18.8±0.3Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490±3Ma and 24.2±0.4Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0±0.4Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼24Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼0.6 to ∼2.0Ga with peak at 0.8–1.2Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼600Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼24Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene. [Copyright &y& Elsevier]

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lie above the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii) an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposed thickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the main kinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main Central Thrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22 Ma. The out-of-sequence Kakhtang thrust was active at 10–14 Ma and was concurrent with reactivation of the South Tibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that the Greater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetan plateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetan plateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater Himalayan Sequence. This process also led to occurrences of present-day surface rocks that were derived from variable distances down dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for the Greater Himalayan Sequence along the length of the Himalayas. [Copyright &y& Elsevier]

Examines the chronology of deformation, metamorphism, and magmatism of the Karakoram Mountains in Northern Pakistan. Evolution of plate crust after collision of the Kohistan arc and the Indian plate; .

The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene--Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected εHf ratios decrease from between -10 and -15 down to values as low as -23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in εNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust. [ABSTRACT FROM AUTHOR]

Numerous studies have documented rare-earth element (REE) mobility in hydrothermal and metamorphic fluids, but the processes and timing of REE mobility are rarely well constrained. The Round Top laccolith in the Trans-Pecos magmatic province of west Texas, a REE ore prospect, has crosscutting fractures filled with fluorite and calcite along with a variety of unusual minerals. Most notably among these is an yttrium and heavy rare-earth element (YHREE) carbonate mineral, which is hypothesized to be lokkaite based on elemental analyses. While the Round Top laccolith is dated to 36.2 ± 0.6 Ma based on K/Ar in biotite, U-Pb fluorite and nacrite ages presented here clearly show the mineralization in these veins is younger than 6.2 ± 0.4 Ma (the age of the oldest fluorite). This discrepancy in dates suggests that fluids interacted with the laccolith to mobilize REE more than 30 m.y. after igneous emplacement. The timing of observed REE mobilization overlaps with Rio Grande rift extension, and we suggest that F-bearing fluids associated with extension may be responsible for initial mobilization. A later generation of fluids was able to dissolve fluorite, and we hypothesize this later history involved sulfuric acid. Synchrotron spectroscopy and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of minerals that record these fluids offer tremendous potential for a more fundamental understanding of processes that are important not only for REE but other ore deposits as well. [ABSTRACT FROM AUTHOR]

Interannual variability of African rainfall impacts local and global communities, but its past behavior and response in future climate projections are poorly understood. This is primarily due to short instrumental records and a lack of long high-resolution palaeoclimate proxy records. Here we present an annually resolved 91,000 year Early Pleistocene record of hydroclimate from the early homininbearing Makapansgat Valley, South Africa. Changes in speleothem annual band thickness are dominated by precession over four consecutive orbital cycles with strong millennial-scale periodicity. The frequency of interannual variability (2.0-6.5 yr oscillations) does not change systematically, yet its amplitude is modulated by the orbital forcing. These long-term characteristics of interannual variability are reproduced with transient climate model simulations of water balance for South Africa from the Late Pleistocene to Recent. Based on these results, we suggest that the frequency of interannual variations in southern African rainfall is likely to be stable under anthropogenic warming, but that the size of year-to-year variations may increase. We see an orbitally forced increase in the amplitude of interannual climate variability between 1.8 Ma and 1.7 Ma coincident with the first evidence for the Acheulean stone tool technology. [ABSTRACT FROM AUTHOR]

The characterization of the geochemical reservoirs of the Earth's continental crust, including the determination of representative upper and lower crustal compositions, underpins our understanding of crustal evolution. The classic I- and S-type granite classification has often been invoked to distinguish between melts derived from igneous protoliths and those derived from the melting of a sedimentary source. Recent geochemical studies suggest that most granites, even those cited as typical examples of ‘S-type’, show evidence for a mixture of mantle and upper crustal sources, thereby implying that granite formation is evidence for overall crustal growth. We have examined the source of leucogranite bodies in one of the world's youngest collisional orogens using novel zircon techniques that can resolve the presence of even minor mantle contributions. 232 zircons from 12 granites from the Bhutan Himalaya were analysed by in-situ techniques for O, Hf and U–Pb isotopic signatures. In combination with data from the granite host rocks, our data show that the Himalayan leucogranites were derived solely from metamorphosed crustal sediments, and do not record any mantle contribution. This finding is consistent with the time-lag between crustal thickening and widespread crustal melting, and the heat-producing capacities of the pelitic source rocks. We conclude that Himalayan leucogranites provide a more suitable type locality for ‘S-type’ granites than the Lachlan area in South-East Australia where the term was first defined. The Himalayan leucogranites therefore provide evidence that syn-orogenic melting during collisional events does not necessarily result in crustal growth. Importantly, crustal growth models should not always assume that crustal growth is achieved during collisional orogenesis. [ABSTRACT FROM AUTHOR]

Multiproxy analysis of detrital minerals is a powerful tool for unravelling the source-to-sink evolution of syn-rift sedimentary basins that represent important depositional settings during major rift events. The Cenozoic Resende half-graben in SE Brazil is a classic example of a syn-rift basin developed under oblique extension, which preserves evidence of tectonic episodes and palaeoclimatic conditions during the opening of the Atlantic. Sediment in the basin was sourced from the surrounding basement rocks, which to the faulted margin consists of older Proterozoic orthogranulites and orthogneisses, whilst to the flexural margin is characterised by Neoproterozoic passive margin paragneisses. Cretaceous intra-plate alkaline stocks crop out at the western edge and in the central-south basement of the basin and represent a distinct provenance fingerprint. This study uses new detrital zircons U–Pb ages combined with previously published elemental and Nd and Sr isotopic data to characterise the sedimentary provenance of Eocene alluvial deposits and to understand how sediment sourcing has changed throughout the evolution of the basin. U–Pb detrital zircon analyses from samples collected across the Resende Basin are presented herein. Samples yield a spread of detrital age spectra from the Archaean to the Cretaceous. Precambrian zircons range from ca. 3228 Ma to ca. 542 Ma. Cretaceous zircons yield ages between ca. 138 Ma and ca. 67 Ma. The sediment source location is interpreted by comparing the yielded detrital zircon spectra with the age spectra of zircons from the basement rocks. The dominance of detrital zircons yielding ages of ca. 1500 Ma and ca. 1000 Ma is interpreted to be sourced from the Neoproterozoic basement rocks to the flexural border of the half-graben. Cretaceous zircons from samples with whole rock εNd 40Ma > −10 are found in both the western and eastern portions of the half-graben but not in the central portions. It is interpreted that an along-axis fluvial system was responsible for a small increase in the compositional maturity of the Resende sandstones from west to east, resulting in the progressive dilution of Cretaceous alkaline igneous detritus along the basin axis. Preserved metamorphic rims in zircon grains indicate low transport abrasion, suggesting that Cretaceous zircons in the eastern depocentre could be associated with other small proximal sources. U–Pb zircon analyses and Nd and Sr isotopes provide reliable insights regarding sediment provenance and transport processes, whilst element geochemistry supplies evidence for hydraulic sorting effects. Consequently, the data represents a more robust estimation of the provenance patterns of the Resende Basin, showing chemical and geochronological differences that support the division of the basin into three depocentres (western, central, and eastern) connected by a longitudinal fluvial system sourced mostly by sediments coming from the south-western flexural border. This study highlights the usefulness of using multiple provenance proxies to unravel the complex development of rift basins, assessing multiple source terrains and the variable degrees of mixing in sedimentary basins. [Display omitted] • Unravelling syn-rift sedimentary basins' source-to-sink behaviour • U–Pb detrital zircon dating reflects proximal source areas in a continental rift basin. • Palaeogeographic reconstruction and the role of a longitudinal fluvial system [ABSTRACT FROM AUTHOR]

We report progress in using the isotopic composition and concentration of Pb in the dentine and enamel of deciduous teeth to provide a high resolution time frame of exposure to Pb during fetal development and early childhood. Isotope measurements (total Pb and 208 Pb/ 206 Pb, 207 Pb/ 206 Pb ratios) were acquired by laser ablation inductively coupled mass spectrometry at contiguous 100 micron intervals across thin sections of the teeth; from the outer enamel surface to the pulp cavity. Teeth samples ( n =10) were selected from two cohorts of children, aged 5–8 years, living in NE England. By integrating the isotope data with histological analysis of the teeth, using the daily incremental lines in dentine, we were able to assign true estimated ages to each ablation point (first 2–3 years for molars, first 1–2 years for incisors+pre-natal growth). Significant differences were observed in the isotope composition and concentration of Pb between children, reflecting differences in the timing and sources of exposure during early childhood. Those born in 2000, after the withdrawal of leaded petrol in 1999, have the lowest dentine Pb levels (<0.2 µg Pb/g) with 208 Pb/ 206 Pb (mean ±2 σ : 2.126–2.079) 208 Pb/ 206 Pb (mean ±2 σ : 0.879–0.856) ratios that correlate very closely with modern day Western European industrial aerosols (PM 10 , PM 2.5 ) suggesting that diffuse airborne pollution was probably the primary source and exposure pathway. Legacy lead, if present, is insignificant. For those born in 1997, dentine lead levels are typically higher (>0.4 µgPb/g) with 208 Pb/ 206 Pb (mean ±2 σ : 2.145–2.117) 208 Pb/ 206 Pb (mean ±2 σ : 0.898–0.882) ratios that can be modelled as a binary mix between industrial aerosols and leaded petrol emissions. Short duration, high intensity exposure events (1–2 months) were readily identified, together with evidence that dentine provides a good proxy for childhood changes in the isotope composition of blood Pb. Our pilot study confirms that laser ablation Pb isotope analysis of deciduous teeth, when carried out in conjunction with histological analysis, permits a reconstruction of the timing, duration and source of exposure to Pb during early childhood. With further development, this approach has the potential to study larger cohorts and appraise environments where the levels of exposure to Pb are much higher. [ABSTRACT FROM AUTHOR]

Cape Kiber on the Arctic coast of Chukotka, northeast Russia, consists of a granite intruding Devonian (and older?) strata in the core of a large southeast trending anticline. These strata are structurally overlain by Carboniferous and younger strata. A U-Pb age of 351.4 ± 5.6 (2σ) Ma shows that the granite is Early Carboniferous in age. A large granite cobble extracted from a Carboniferous conglomerate produced a Late Devonian or Early Carboniferous U-Pb age of ∼355-361 Ma. Also, a deformed and altered granitic dyke yielded an age of 363.7 ± 5.7 (2σ) Ma. Major and trace elements suggest a syn-collisional (orogenic) setting. The granite's (biotite) Ar release spectrum is reset. The granitic dyke also shows a disturbed Ar-Ar whole-rock spectrum implying an Early Cretaceous age (∼122-130 Ma) for closure of the Ar system. We interpret this as due to widespread greenschist metamorphism accompanying regional deformation of the Anyuy-Chukotka Fold Belt that accompanied closure of the South Anyuy Ocean. Regionally, this event predates deposition of Aptian and Albian strata and the eruption of Okhotsk-Chukotsk magmatic rocks. An Ar-Ar (biotite) plateau age of 96.4 ± 1.0 (2σ) Ma from a mildly deformed, lamprophyre dyke reflects intrusion in a setting of regional extension. Its deformation reflects a younger tectonic event. The record of Devonian-Carboniferous magmatism and early Carboniferous unroofing is younger and less complex than that of Arctic Alaska. However, evidence for Early Devonian (Caledonian) or Late Devonian (Ellesmerian) deformation could have been masked by intense Mesozoic deformation. Outcrop data and geochronology support and refine regional interpretations of Early Cretaceous deformation and mineral growth accompanying accretion of Chukotka to north Asia, followed by regional extension and subsequent convergent deformation. [ABSTRACT FROM AUTHOR]

Abstract: Allanite has the potential to be a useful chronometer of crustal evolution, forming in response to a wide spectrum of metamorphic and magmatic conditions and incorporating weight-percent concentrations of LREE, Th and U. Despite its growing use in in situ U–Th–Pb geochronology, allanite reference materials lack sufficient U–Th–Pb isotopic characterisation and little is known concerning the response of U–Th–Pb systematics of allanite to hydrothermal alteration and self-irradiation. This contribution presents the results of a combined ID-TIMS and LA-ICPMS U–Th–Pb study on a suite of five allanite crystals, spanning ∼2.6Ga and including three commonly-used allanite reference materials: the Siss, Bona and Tara allanites. Siss and Bona allanites preserve an inherited ca. 1Ga Pb component, consistent with the presence of xenocrystic allanite cores or the presence of zircon micro-inclusions. Tara allanite yields concordant U–Pb ages (407–430Ma), but is affected by Th/U fractionation, likely caused by hydrothermal alteration. Additionally, the tendency for Th to become mobilised post-crystallisation is further evidenced by two Precambrian allanite megacrysts, LE40010 (ca. 2.8Ga) and LE2808 (ca. 1.1Ga), that both exhibit discordant Th/Pb analyses, linked to the formation of thorite micro-inclusions along hydration pathways. Self-irradiation dose versus discordance relationships show that a percolation threshold is present in allanite at cumulative dose values close to 3×1017 α-decayg−1, an order of magnitude smaller than zircon. Collectively, the presence of common-Pb and excess-206Pb, its susceptibility to incur Th/U fractionation and hydrothermal Pb-loss complicates the use of allanite as a geochronometer. These factors explain dispersion of ∼4% in the isotopic compositions of Siss and Tara allanites measured by LA-ICPMS, providing a fundamental limit on the accuracy of the allanite chronometer using these reference materials. [Copyright &y& Elsevier]

Abstract: The Early–Middle Miocene exhumation of the crystalline core of the Himalaya is a relatively well-understood process compared to the preceding phase of burial and prograde metamorphism in the Eocene–Oligocene. Highly deformed rocks of the Greater Himalayan Sequence (GHS) dominate the crystalline core, and feature a strong metamorphic and structural overprint related to the younger exhumation. The Tethyan Sedimentary Series was tectonically separated from the underlying GHS during the Miocene by the South Tibetan Detachment, and records a protracted and complex history of Cenozoic deformation. Unfortunately these typically low-grade or unmetamorphosed rocks generally yield little quantitative pressure–temperature–time information to accompany this deformation history. In parts of the western Himalaya, however, the basal unit of the Tethyan Sedimentary Series (the Haimanta Group) includes pelites metamorphosed to amphibolite facies. This presents a unique opportunity to explore the tectono-thermal evolution of crystalline rocks which record the early history of the orogen. Pressure–temperature–time–deformation (P–T–t–d) paths modelled for two Haimanta Group pelitic rocks reveal three distinct stages of metamorphism: (1) prograde Barrovian metamorphism to 610–620 °C at c. 7–8 kbars, with garnet growing over an early tectonic fabric (S1); (2) initial decompression during heating to 640–660 °C at c. 6–7 kbars, with development of a pervasive crenulation cleavage (S2) and staurolite and kyanite porphyroblast growth; (3) further exhumation during cooling, with minor retrograde metamorphism and modification of the pervasive S2 fabric. Monazite growth ages constrain the timing of initial garnet growth (>34 Ma), the start of D2 and maximum burial (c. 30 Ma), and the termination of garnet growth (c. 28 Ma). Muscovite Ar/Ar ages indicate cooling through c. 300 °C at c. 13 Ma, from which we derive an initial exhumation rate of c. 1.3 mm year−1 for the Haimanta Group. The underlying GHS was exhumed at a rate of 2.2 to 3 mm year−1 during this time. The difference in exhumation rate between these two units is considered to reflect Early Miocene displacement on the intervening South Tibetan Detachment. Slower exhumation (c. 0.6 mm year−1) of both units after c. 13 Ma followed the cessation of major displacement on this structure, after which time the Haimanta Group and the GHS were exhumed as one relatively coherent tectonic block. [Copyright &y& Elsevier]

A newly developed XRF-microprobe at the Institute of Mineralogy and Petrology, University of Bern, Switzerland has been applied for precise chemical Th–U–Pb dating of individual monazite grains separated from Pb-free polished petrographic thin sections. The nondestructive nature of the XRF-measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis (EMPA), chemical dating by XRF-microprobe analysis, and isotopic 208Pb/232Th dating by Laser Ablation Plasma Ionisation Multi-collector Mass Spectrometry (LA-PIMMS) analysis. As an example, the 2σ precision achieved with the XRF-microprobe for well characterised reference material, monazite FC-1 (TIMS age 54.3±1 Ma; μ-XRF age 55.3±2.6 Ma), doubly polished to 30 μm in thickness, is below 5% after 90 min integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. At 38-μm spatial resolution, the uncertainty is 35% for the same integration time. The sample characteristics are 200–300 ppm of Pb (μ-XRF), 3.8–5.1 wt.% of Th (EMPA), and 0.4–1.4 wt.% U (EMPA). Combined with an electron microprobe and conventional optical microscopy, the XRF-microprobe is thus a competitive low-cost and nondestructive alternative to more costly isotopic methods. The XRF-microprobe is easy to use and maintain. [Copyright &y& Elsevier]

Of the hypothesized causes of Gulf War Illness (GWI), a chronic multi-symptom illness afflicting approximately 25% of military personnel deployed to the 1991 Gulf War, exposure to depleted uranium (DU) munitions has attracted international concern. Past research has not tested the potential association of GWI with inhaled DU nor used isotope mass spectrometry of sufficient sensitivity to rigorously assess prior DU exposure. We applied a standard biokinetic model to predict the urinary concentration and uranium isotopic ratios for a range of inhalation exposures. We then applied sensitive mass spectrometry capable of detecting the predicted urinary DU to 154 individuals of a population-representative sample of U.S. veterans in whom GWI had been determined by standard case definitions and DU inhalation exposures obtained by medical history. We found no difference in the 238 U/ 235 U ratio in veterans meeting the standard case definitions of GWI versus control veterans, no differences by levels of DU inhalation exposure, and no 236 U associated with DU was detected. These findings show that even the highest likely levels of DU inhalation played no role in the development of GWI, leaving exposure to aerosolized organophosphate compounds (pesticides and sarin nerve agent) as the most likely cause(s) of GWI.