Discovery of an Eo-Meso-Neoarchean Terrane in the East Greenland Caledonides

tThis study investigates basement gneisses from the Niggli Spids thrust sheet of the East Greenland Caledonides in an attempt to place them into the broader context of the Archean–Paleoproterozoicarchitecture of the Greenland shield. Our combined whole-rock geochemical and U–Pb zircon geochronol-ogy results from Gåseland reveal an Archean terrane defined by TTG magmatic events at 3607 Ma and3070–2980 Ma followed by metamorphism and high-K granite intrusion at 2790–2677 Ma. These results identify a relatively pristine Archean terrane with previously unknown Eoarchean rocks that holds poten-tial for future investigations into the early evolution of continental crust, and adds to a growing body of data characterizing the Archean–Paleoproterozoic architecture of East Greenland

Campaign-style U-Pb titanite petrochronology: along-strike variations in timing of metamorphism in the Himalayan Metamorphic Core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core (HMC), in order to document and investigate the causes of along-strike variations in its Oligocene–Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya. This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ∼30 Ma to 12 Ma, recording (re-)crystallization between 625 °C and 815 °C. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen. Disparities in the duration and timing of metamorphism within the HMC are best explained by along-strike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes. These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger, thicker crystalline core in the eastern Himalaya. Keywords: Himalaya, Petrochronology, Titanite, Metamorphic petrolog

High-Temperature Deformation During Continental-Margin Subduction & Exhumation: The Ultrahigh-Pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction

Industrial-era lead and mercury contamination in southern Greenland implicates North American sources

We would like to thank Jesús R. Aboal (Universidade de Santiago de Compostela) and Kjell Billström (Naturhistoriska Riksmuseet) for access to the laboratory facilities; Antonio Rodríguez López helped with laboratory work. This research was done under the framework of the projects CGL2010-20672 (Plan Nacional I+D+i, Spanish Ministerio de Economía y Competitividad), R2014/001 and GPC2014-009 (Dirección Xeral I+D, Xunta de Galicia). The authors gratefully acknowledge the financial support of the UK Leverhulme Trust Footprints on the Edge of Thule programme award for core collection and associated environmental research.Peer reviewedPostprin

The temporal evolution of subduction initiation in the Samail ophiolite: high-precision U–Pb zircon petrochronology of the metamorphic sole

High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite

Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation

Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a postVariscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cmscale layered rocks showing compositional variation consisting of alternating ‘amphibole-rich’ (i.e., amphibolites) and ‘clinopyroxene/plagioclase-rich’ domains (i.e., calc-silicates). Titanite from amphibolerich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive coreto-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclaserich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs

Lead content and isotopic composition in submound and recent soils of the Volga upland

Literature data on the historical reconstructions of the atmospheric lead deposition in Europe and the isotopic composition of the ores that are potential sources of the anthropogenic lead in the atmospheric deposition in the lower Volga steppes during different time periods have been compiled. The effect of the increasing anthropogenic lead deposition recorded since the Bronze Age on the level of soil contamination has been investigated. For the first time paleosol buried under a burial mound of the Bronze Age has been used as a reference point to assess of the current contamination level. The contents and isotopic compositions of the mobile and total lead have been determined in submound paleosols of different ages and their recent remote and roadside analogues. An increase in the content and fraction of the mobile lead and a shift of its isotopic composition toward less radiogenic values (typical for lead from the recent anthropogenic sources) has been revealed when going from a Bronze-Age paleosol to a recent soil. In the Bronze-Age soil, the isotopic composition of the mobile lead is inherited from the parent rock to a greater extent than in the modern soils, where the lead is enriched with the less radiogenic component. The effect of the anthropogenic component is traced in the analysis of the mobile lead, but it is barely visible for the total lead. An exception is provided by the recent roadside soils characterized by increased contents and the significantly less radiogenic isotopic composition of the mobile and total lead