10Be ages from central east Greenland constrain the extent of the Greenland ice sheet during the Last Glacial Maximum

Traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the East Greenland continental margin during the Last Glacial Maximum (LGM): ice to the shelf break south of Scoresby Sund and ice extending no further than to the inner shelf at and north of Scoresby Sund. We report new ¹⁰Be ages from erratic boulders perched at 250 m a.s.l. on the Kap Brewster peninsula at the mouth of Scoresby Sund. The average ¹⁰Be ages, calculated with an assumed maximum erosion rate of 1 cm/ka and no erosion (respectively, 17.3±2.3 ka and 15.1±1.7 ka) overlap with a period of increased sediment input to the Scoresby Sund fan (19–15 ka). The results presented here suggest that ice reached at least 250 m a.s.l. at the mouth of Scoresby Sund during the LGM and add to a growing body of evidence indicating that LGM ice extended onto the outer shelf in northeast Greenland.     

Revised 14C dating of ice wedge growth in interior Alaska (USA) to MIS 2 reveals cold paleoclimate and carbon recycling in ancient permafrost terrain

Establishing firm radiocarbon chronologies for Quaternary permafrost sequences remains a challenge because of the persistence of old carbon in younger deposits. To investigate carbon dynamics and establish ice wedge formation ages in Interior Alaska, we dated a late Pleistocene ice wedge, formerly assigned to Marine Isotope Stage (MIS) 3, and host sediments near Fairbanks, Alaska, with 24 radiocarbon analyses on wood, particulate organic carbon (POC), air-bubble CO₂, and dissolved organic carbon (DOC). Our new CO₂ and DOC ages are up to 11,170 yr younger than ice wedge POC ages, indicating that POC is detrital in origin. We conclude an ice wedge formation age between 28 and 22 cal ka BP during cold stadial conditions of MIS 2 and solar insolation minimum, possibly associated with Heinrich event 2 or the last glacial maximum. A DOC age for an ice lens in a thaw unconformity above the ice wedge returned a maximum age of 21,470 ± 200 cal yr BP. Our variable ¹⁴C data indicate recycling of older carbon in ancient permafrost terrain, resulting in radiocarbon ages significantly older than the period of ice-wedge activity. Release of ancient carbon with climatic warming will therefore affect the global ¹⁴C budget.     

The deglacial history of NW Alexander Island,Antarctica, from surface exposure dating

Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic ¹⁰Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the ¹⁰Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic ³He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes.     

Last Glacial Maximum ice sheet dynamics in Arctic Canada inferred from young erratics perched on ancient tors

A long-standing debate regarding the reconstruction of former ice sheets revolves around the use of relative weathering of landscapes, i.e., the assumption that highly weathered landscapes have not been recently glaciated. New cosmogenic isotope measurements from upland bedrock surfaces and erratics along the northeastern margin of the Laurentide Ice Sheet (LIS) shed light on this debate. ¹⁰Be and ²⁶Al concentrations from three perched erratics, yielding cosmogenic exposure ages of 17–11 ka, are much lower than those measured in two unmodified, highly weathered tors upon which they lie, which yield cosmogenic exposure ages of >60 ka. These findings suggest that non-erosive ice covered weathered upland surfaces along the northeastern margin of the LIS during the last glacial maximum. These data challenge the use of relative weathering to define the margins of Pleistocene ice sheets. The juxtaposition of non-erosive ice over upland plateaus and erosive ice in adjacent fiords requires strong gradients in basal thermal regimes, suggestive of an ice-stream mode of glaciation.     

SHRIMP zircon U–Pb ages of eclogite and orthogneiss from Sulu ultrahigh-pressure zone in Yangkou area,eastern China

We report SHRIMP U–Pb age of zircons in four samples of eclogite and one sample of orthogneiss from Sulu ultrahigh-pressure (UHP) zone in Yangkou area, eastern China. UHP rocks are distributed along the Sulu orogenic belt suturing North China Block with South China Block. In Yangkou area, UHP unit is well exposed for about 200 m along Yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para- and orthogneiss which are highly sheared partly. Zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in CL images, and most of the grains have high Th/U ratio ranging from 0.8 to 2.1 indicating an igneous origin. The weighted mean ²⁰⁶Pb/²³⁸U ages of zircons from the four samples range from 690 to 734 Ma. These ages can be correlated to the magmatic stage of the protoliths. In rare cases, zircon grains possess a narrow rim with very low Th/U ratio (< 0.02). EPMA U–Th-total Pb dating of such rim yields younger ages that range from 240 to 405 Ma marking the metamorphic stage. On the other hand, zircons from the orthogneiss show irregular shape and zoning with inclusion-rich core and inclusion-free rim. These grains of zircon yield U–Pb discordia intercept ages of 226 ± 63 Ma and 714 ± 110 Ma (MSWD 0.78). Bulk of the areas of the rims rim of the zircons demonstrate younger ²⁰⁶Pb/²³⁸U ages close to the upper intercept, with low Th/U ratio (< 0.20) indicating their metamorphic origin. In contrast, the cores show older ²⁰⁶Pb/²³⁸U ages close to lower intercept and high Th/U ratio of (0.14–5.25) indicating their igneous origin. The upper intercept age is also commonly noted in zircons from eclogite. Our results suggest a bimodal igneous activity along this zone during the Neoproterozoic, probably related to the rifting of the Rodinia supercontinent.     

Cosmogenic 10Be and 36Cl ages from Late Pleistocene terminal moraine complexes in the Taylor River drainage basin,central Colorado,USA

Cosmogenic surface-exposure ages from boulders on a terminal moraine complex establish the timing of the local last glacial maximum (LGM) in the Taylor River drainage basin, central Colorado. Five zero-erosion ¹⁰Be ages have a mean of 19.5±1.8 ka while that for three ³⁶Cl ages is 20.7±2.3 ka. Corrections for modest rates (∼1 mm ka⁻¹) of boulder surface erosion result in individual and mean ages that are generally within 2% of their zero-erosion values. Both the means and the range in ages of individual boulders are consistent with those reported for late Pleistocene moraines elsewhere in the southern and middle Rocky Mountains, and thus suggest local LGM glacier activity was regionally synchronous. Two anomalously young (?) zero-erosion ¹⁰Be ages (mean 14.4±0.8 ka) from a second terminal moraine are tentatively attributed to the boulders having been melted out during a late phase of ice stagnation.     

In situ U–Pb isotopic dating of columbite–tantalite by LA–ICP–MS

Columbite–tantalite (Coltan) is the most important niobium (Nb)- and tantalum (Ta)-bearing economic mineral, commonly occurring in rare metal granite and pegmatite, alkaline granite, syenite and carbonatite. Its high U but low common Pb contents make it an ideal mineral for U–Pb isotopic dating of Nb–Ta mineralization. In order to establish a feasible coltan dating method by in situ laser-ablation (LA) ICP–MS, we determined the U–Pb ages of five coltan samples from different pegmatites and rare-metal granites in China. In order to evaluate the potential matrix effect between different minerals, a 91500 zircon was used as external standard during analyses. The results show that, compared to the recommended ages, approximately 7–15% younger ages were yielded for the analyzed coltan samples in both single spot and line raster scan analytical methods, indicating a significant matrix effect between coltan and zircon. However, by using a coltan standard from Namibia (Coltan139), the coltan sample from Dahe pegmatite (SNNT) has a weighted mean ²⁰⁶Pb/²³⁸U age of 363 ± 4 Ma (2σ, n = 25) and 357 ± 5 Ma (2σ, n = 20) in single spot and line raster scan analytical methods, respectively; the coltan samples from Altai No.3 pegmatite (713-79), Yichun topaz-lepidolite granite (Yi-1) and Huangshan albite granite (LS-15) have weighted mean ²⁰⁶Pb/²³⁸U ages of 218 ± 2 Ma (2σ, n = 20), 160 ± 1 Ma (2σ, n = 20) and 130 ± 1 Ma (2σ, n = 20), respectively, in single spot mode. These ages agree well with the previously published data, and hence support the reliability of our analytical method. Although the analyzed coltan minerals show a large variation of chemical compositions, no significant matrix effect was observed, which suggests that a coltan material should be used as an external standard for U–Pb dating of coltan by LA–ICP–MS. Using the established analytical protocol, we date the Nanping pegmatite (NP155), a main Nb–Ta deposit in China without known age, and obtain a weighted mean ²⁰⁶Pb/²³⁸U age of 391 ± 4 Ma (2σ, n = 20), which is considered as the best estimation of Nb–Ta mineralization time in the area.     

Late Quaternary erosion events in lowland and mid-altitude Tasmania in relation to climate change and first human arrival

The establishment of a chronology of landscape-forming events in lowland and mid-altitude Tasmania, essential for assessing the relative importance of climatic and human influences on erosion, and for assessing present erosion risk, has been limited by the small number of ages obtained and limitations of dating methods. In this paper we critically assess previous Tasmanian studies, list published radiocarbon ages considered to be dependable, present new radiocarbon and thermoluminescence (TL) ages for 25 sites around Tasmania, and consider the evidence for the hypotheses that erosion processes at low and mid altitudes have been: (1) purely climatically controlled; and (2) influenced both by climatic and anthropogenic (increased fire frequency) effects. A total of 94 dependable finite ages (calibrated for radiocarbon and ‘as measured’ for TL and optically stimulated luminescence (OSL) determinations) are listed for deposits comprising dunes, colluvium, alluvium and loess-like aeolian deposits. Two fall in the >100 ka period, 15 fall in the period 65–35 ka, and 77 fall in the period 35–0.3 ka. There was a sustained increase in erosion recorded in the period 35–15 ka, as reflected by a greater number of dated aeolian deposits during this period.We considered three possible biases that may have affected the age distribution obtained: the limitations of radiocarbon dating, sampling bias, and preservation bias. Sampling bias may have favoured more recent dune strata, but radiocarbon dating and preservation biases are unlikely to have significantly distorted the age distribution obtained.Long but intermittent aeolian deposition is recorded at two sites (Southwood B; c. 59–28 ka and Dunlin Dune; c. 29–14 ka) but there is no evidence of regional loess deposits such as found in New Zealand. The timing of increased erosion in Tasmania between 35 and 30 ka approximately coincides with the intermittent ten-fold increase of dust accumulation between 33 and 30 ka in the Antarctic Dome C ice core. The absence of widespread erosion before 35 ka, the abrupt increase of erosion around this time, the frequent association of erosion products with charcoal, the arrival of people in Tasmania at c. 40 cal ka, and the known use of fires by Aborigines to maintain areas of non-climax vegetation suggest that ecosystem disturbance by anthropogenic fires, in a drier climate than that presently prevailing, may have contributed to erosion in lowland and mid-altitude Tasmania after 35 ka. Thus the Tasmanian erosion record provides circumstantial support for the proposition that human dispersal in southeast Australia was accompanied by significant ecological change.     

Surface-exposure ages of Front Range moraines that may have formed during the Younger Dryas, 8.2 cal ka,and Little Ice Age events

Surface-exposure (¹⁰Be) ages have been obtained on boulders from three post-Pinedale end-moraine complexes in the Front Range, Colorado. Boulder rounding appears related to the cirque-to-moraine transport distance at each site with subrounded boulders being typical of the 2-km-long Chicago Lakes Glacier, subangular boulders being typical of the 1-km-long Butler Gulch Glacier, and angular boulders being typical of the few-hundred-m-long Isabelle Glacier. Surface-exposure ages of angular boulders from the Isabelle Glacier moraine, which formed during the Little Ice Age (LIA) according to previous lichenometric dating, indicate cosmogenic inheritance values ranging from 0 to ∼3.0 ¹⁰Be ka.¹ Subangular boulders from the Butler Gulch end moraine yielded surface-exposure ages ranging from 5 to 10.2 ¹⁰Be ka. We suggest that this moraine was deposited during the 8.2 cal ka event, which has been associated with outburst floods from Lake Agassiz and Lake Ojibway, and that the large age range associated with the Butler Gulch end moraine is caused by cosmogenic shielding of and(or) spalling from boulders that have ages in the younger part of the range and by cosmogenic inheritance in boulders that have ages in the older part of the range. The surface-exposure ages of eight of nine subrounded boulders from the Chicago Lakes area fall within the 13.0–11.7 ¹⁰Be ka age range, and appear to have been deposited during the Younger Dryas interval. The general lack of inheritance in the eight samples probably stems from the fact that only a few thousand years intervened between the retreat of the Pinedale glacier and the advance of the Chicago Lakes glacier; in addition, bedrock in the Chicago Lakes cirque area may have remained covered with snow and ice during that interval, thus partially shielding the bedrock from cosmogenic radiation.     

Glaciation and deglaciation of the SW Lake District,England: implications of cosmogenic 36Cl exposure dating

Exposure dating using cosmogenic ³⁶Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26–21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750–765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840–870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750–765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9–11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion.     

40Ar/39Ar and U–Pb dating of the Fish Canyon magmatic system,San Juan Volcanic field,Colorado: Evidence for an extended crystallization history

The ∼ 5000 km³ Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the ⁴⁰Ar/³⁹Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution ²³⁸U–²⁰⁶Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age “concordance” by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U–Pb zircon ages and ⁴⁰Ar/³⁹Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U–Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 ⁴⁰Ar/³⁹Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system.Disequilibrium-corrected ²⁰⁶Pb/²³⁸U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67–28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the “chemical abrasion” (CA) technique [Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1–2): 47–66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma ⁴⁰Ar/³⁹Ar eruption age of FCT – even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the ⁴⁰K decay constants – and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the ⁴⁰Ar/³⁹Ar method.Even though data statistics preclude unambiguous conclusions, ⁴⁰Ar/³⁹Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in ⁴⁰Ar/³⁹Ar dating. Chemical Geology, 145: 117–152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2–0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a “staircase” argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 10⁵ years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 10⁵ years, even at T ∼ 700 °C.     

In situ U–Pb dating of micro-baddeleyite by secondary ion mass spectrometry

We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS) in situ analysis of ng-mass crystals that cannot be efficiently extracted by conventional mineral separation techniques. Average ²⁰⁷Pb/²⁰⁶Pb ages for Precambrian baddeleyite crystals are within < 0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. ²⁰⁶Pb/²³⁸U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO₂⁺/U⁺ calibration plot. Calibration is performed on separated baddeleyite crystals (～ 100–200 μm in maximum dimension) mounted in random crystallographic orientation. ²⁰⁶Pb/²³⁸U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of ²⁰⁶Pb/²³⁸U between large crystals and micro-baddeleyite from FC4b is within ～ 1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO₂⁺/U⁺-based calibration to significantly reduce orientation dependent bias.     

Readvance of the last British–Irish Ice Sheet during Greenland Interstade 1 (GI-1): the Wester Ross Readvance,NW Scotland

Fourteen samples obtained from Torridon sandstone boulders on four moraines marking the limit of the Wester Ross Readvance (WRR) in NW Scotland yielded tightly clustered ¹⁰Be exposure ages confirming contemporaneous or penecontemporaneous moraine deposition. Collectively, the 14 samples yield mean ages of 13.5 ± 1.2 ka to 14.0 ± 1.7 ka, depending on choice of geomagnetic scaling and sampling surface erosion rates. All fourteen moraine ages are significantly younger than an age of ca 16.3 ka previously proposed for the WRR, and also younger than most samples obtained from rock outcrops within the WRR limits. The ages obtained for the WRR moraines appear to confirm that a substantial cover of glacier ice persisted over low ground in NW Scotland during at least the early part of the Lateglacial Interstade (≈Greenland Interstade 1). We infer that the WRR probably occurred in response to rapid short-lived cooling during the Older Dryas climatic reversal (≈Greenland Interstade 1d), though the possibilities that the WRR represents ice-margin response to a later climatic reversal during the Lateglacial Interstade or stabilization and readvance of the ice margin following rapid offshore calving cannot be discounted.     

Calibrating a glaciological model of the Greenland ice sheet from the Last Glacial Maximum to present-day using field observations of relative sea level and ice extent

We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.     

Late Glacial ice advances in southeast Tibet

The sensitivity of Tibetan glacial systems to North Atlantic climate forcing is a major issue in palaeoclimatology. In this study, we present surface exposure ages of erratic boulders from a valley system in the Hengduan Mountains, southeastern Tibet, showing evidence of an ice advance during Heinrich event 1. Cosmogenic nuclide analyses (¹⁰Be and ²¹Ne) revealed consistent exposure ages, indicating no major periods of burial or pre-exposure. Erosion-corrected (3 mm/ka) ¹⁰Be exposure ages range from 13.4 to 16.3 ka. This is in agreement with recalculated exposure ages from the same valley system by [Tschudi, S., Schäfer, J.M., Zhao, Z., Wu, X., Ivy-Ochs, S., Kubik, P.W., Schlüchter, C., 2003. Glacial advances in Tibet during the Younger Dryas? Evidence from cosmogenic ¹⁰Be, ²⁶Al, and ²¹Ne. Journal of Asian Earth Sciences 22, 301–306.]. Thus this indicates that local glaciers advanced in the investigated area as a response to Heinrich event 1 cooling and that periglacial surface adjustments during the Younger Dryas overprinted the glacial morphology, leading to deceptively young exposure ages of certain erratic boulders.     

First evidence of the Cambrian basement in Upper Peninsula of Thailand and its implication for crustal and tectonic evolution of the Sibumasu terrane

Although the Sibumasu terrane in Asia was previously considered to be composed of Phanerozoic rocks with Cambrian crystalline basement, no reliable or direct radiometric dating evidences of such crystalline basement was ever reported. Our new in-situ zircon U/Pb dating of the Khao Tao orthogneiss yields a concordant age of 501.5 ± 7.5 Ma (2σ), which provides the first robust evidence for the Cambrian crust in Upper Peninsula of Thailand. The zircon εHf_(T) values range from + 3.7 to − 6.1 with model ages (T^C_DM) of 1244–1827 Ma, suggests a mixed crust-mantle source. The chemical similarity and spatial continuity of the Khao Tao orthogneiss with other pre-Neotethys marginal Eurasian and Sibumasu granitoids indicate the linear paleogeographic association under a similar magmatic arc-related regime along the Gondwana India–Australia margin as part of the Pan-African Orogeny system.     

Phanerozoic reactivation of the Archean North China Craton through episodic magmatism: Evidence from zircon U–Pb geochronology and Hf isotopes from the Liaodong Peninsula

The Archean lithospheric root of the North China Craton (NCC) has been considerably eroded and modified by Phanerozoic magmatic processes. Here we investigate the decratonization of the NCC through U–Pb and Hf isotopic analyses of zircons from Cenozoic basalts in the Liaodong Peninsula using ion-probe and MC-ICPMS techniques. The U–Pb zircon geochronology identifies three zircon populations: Precambrian, Paleozoic and Mesozoic. The Precambrian zircons yield ²⁰⁷Pb/²⁰⁶Pb ages of 2275–2567 Ma with a peak at around 2.5 Ga. They define a U–Pb discordia with upper intercept ages of 2447 ± 50 Ma to 2556 ± 50 Ma and a wide range of Hf T_DM ages with a mode at 2.7–2.8 Ga. Our results clearly demonstrate the presence of an Archean lower crust in the Liaodong region. The Paleozoic zircons from the Liaodong region lack the clear internal zoning and are subhedral to rounded in shape, and yield a narrow ²⁰⁶Pb/²³⁸U concordant ages of 419–487 Ma with a weighted mean age of 462 ± 16 Ma. The Mesozoic zircons predominantly show crystallization in the early Cretaceous and yield a relatively large range in ²⁰⁶Pb/²³⁸U ages from 100 to 138 Ma (n = 53) with a peak around 120 Ma. Three samples give indistinguishable weighted mean ²⁰⁶Pb/²³⁸U ages of 120 ± 5 Ma, 120 ± 4 Ma and 121 ± 2 Ma. These early Cretaceous zircons have enriched Hf isotope compositions with ε_Hf(t) values from ?26 to ?16. Our results provide important constraints on episodic magmatism during the Phanerozoic in the Liaodong region, which led to substantial reactivation of the Archean basement of the North China Craton.     

Triassic volcanism in eastern Heilongjiang and Jilin provinces,NE China: Chronology,geochemistry, and tectonic implications

With the aim of constraining the Early Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt (CAOB), we undertook zircon U–Pb dating and geochemical analyses (major and trace elements, Sr–Nd isotopes) of volcanic rocks of the Luoquanzhan Formation and Daxinggou Group in eastern Heilongjiang and Jilin provinces, China. The analyzed rocks consist mainly of dacite and rhyolite, with SiO₂ contents of 68.52–76.65 wt%. Three samples from the Luoquanzhan Formation and one from the Daxinggou Group were analyzed using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon techniques. Three zircons with well-defined oscillatory zoning yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 217 ± 1, 214 ± 2, and 208 ± 1 Ma, and one zircon with oscillatory zoning yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 201 ± 1 Ma. These ages are interpreted to represent the timing of eruption of the volcanic rocks. The Triassic volcanic rocks are characterized by high SiO₂ and low MgO concentrations, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), (⁸⁷Sr/⁸⁶Sr)_i = 0.7040–0.7050 (Luoquanzhan Formation) and 0.7163–0.7381 (Daxinggou Group), and ε_Nd (t) = 1.89–3.94 (Luoquanzhan Formation) and 3.42–3.68 (Daxinggou Group). These geochemical features indicate an origin involving the partial melting of juvenile lower crust (Nd model ages (T_DM2) of 651–821 Ma) and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation. These volcanic rocks formed within an extensional environment following collision of the NCC and Jiamusi-Khanka Massif during the Late Paleozoic–Early Triassic.     

Was the Younger Dryas (Loch Lomond Stadial) icefield on Rannoch Moor,western Scotland,deglaciated as early as c.12.5 cal ka BP?

The concept that Rannoch Moor, the centre of the Younger Dryas (YD), West Highland Icefield, was deglaciated as early as 12.5 cal ka BP is discussed in the light of radiocarbon dates and varve sequences from outlet glaciers of this icefield, and climate change during the YD. The maximum positions of three YD glaciers were reached after 11.6–11.8 cal ka BP (Lomond), and after 11.8–11.9 cal ka BP (Spean and Treig) indicating that ice remained on Rannoch Moor until long after c.12.5 cal ka BP, and possibly until the YD/Holocene transition at c.11.7 cal ka BP. Further, the Spean glacier dammed a proglacial lake in Lochaber for at least 495 varve years over a period that included the deposition of the Vedde Ash (c.12.1 cal ka BP) and a late YD ash layer (c. 11.7–11.2 cal ka BP), a thesis at variance with supposed early YD deglaciation. Recent examination of this issue using ¹⁰Be exposure age determinations from Rannoch Moor is equivocal. In view of the presence of hard water algae at the sampling site on Rannoch Moor it is recommended that the ‘early’ ¹⁴C dates from Rannoch Moor need to be further reassessed using chronological constraints provided by dated microtephra, and a collaborative radiocarbon dating programme.     

Provenance and origins of a Late Paleozoic accretionary complex within the Khangai–Khentei belt in the Central Asian Orogenic Belt,central Mongolia

We have investigated the petrography, geochemistry, and detrital zircon U–Pb LA-ICPMS dating of sandstone from the Gorkhi Formation of the Khangai–Khentei belt in the Ulaanbaatar area, central Mongolia. These data are used to constrain the provenance and source rock composition of the accretionary complex, which is linked to subduction of the Paleo-Asian Ocean within the Central Asian Orogenic Belt during the Middle Devonian to Early Carboniferous. Field and microscopic observations of the modal composition of sandstone and constituent mineral chemistry indicate that the sandstone of the Gorkhi Formation is feldspathic arenite, enriched in saussuritized plagioclase. Geochemical data show that most of the sandstone and shale were derived from a continental margin to continental island arc setting, with plutonic rocks being the source rocks. Detrital zircon ²⁰⁶Pb/²³⁸U ages of two sandstones yields age peaks of 322 ± 3 and 346 ± 3 Ma. The zircon ²⁰⁶Pb/²³⁸U age of a quartz–pumpellyite vein that cuts sandstone has a weighted mean age of 339 ± 3 Ma. Based on these zircon ages, we infer that the depositional age of sandstone within the Gorkhi Formation ranges from 320 to 340 Ma (i.e., Early Carboniferous). The provenance and depositional age of the Gorkhi Formation suggest that the evolution of the accretionary complex was influenced by the intrusion and erosion of plutonic rocks during the Early Carboniferous. We also suggest that spatial and temporal changes in the provenance of the accretionary complex in the Khangai–Khentei belt, which developed aound the southern continental margin of the Siberian Craton in relation to island arc activity, were influenced by northward subduction of the Paleo-Asian Ocean plate.