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Bolt's Farm is the name given to a series of non-hominin bearing fossil sites that have often been suggested to be some of the oldest Pliocene sites in the Cradle of Humankind, South Africa. This article reports the results of the first combined Uranium-Series and Electron Spin Resonance (US-ESR) dating of bovid teeth at Milo's Cave and Aves Cave at Bolt's Farm. Both tooth enamel fragments and tooth enamel powder ages were presented for comparison. US-ESR, EU and LU models are calculated. Overall, the powder ages are consistent with previous uranium-lead and palaeomagnetic age estimates for the Aves Cave deposit, which suggest an age between ~3.15 and 2.61 Ma and provide the first ages for Milo's Cave dates to between ~3.1 and 2.7 Ma. The final ages were not overly dependent on the models used (US-ESR, LU or EU), which all overlap within error. These ages are all consistent with the biochronological age estimate (<3.4–>2.6 Ma) based on the occurrence of Stage I Metridiochoerus andrewsi. Preliminary palaeomagnetic analysis from Milo's Cave indicates a reversal takes place at the site with predominantly intermediate directions, suggesting the deposit may date to the period between ~3.03 and 3.11 Ma within error of the ESR ages. This further suggests that there are no definitive examples of palaeocave deposits at Bolt's Farm older than 3.2 Ma. This research indicates that US-ESR dating has the potential to date fossil sites in the Cradle of Humankind to over 3 Ma. However, bulk sample analysis for US-ESR dating is recommended for sites over 3 Ma. [ABSTRACT FROM AUTHOR]

The Oaxacan Complex Carboniferous-Permian sedimentary cover in southern México records provenance shifts through time, reflecting the collision between Gondwana and Laurentia to amalgamate Pangea. The integration of petrological analysis and LA-ICP-MS U-Pb zircon geochronology from Santiago, Ixtaltepec and Yododeñe formations compared with adjacent terranes suggests that: (1) during the Early Mississippian, the Santiago Formation received sediments mainly from local sources such as the Oaxacan Complex and Tiñu Formation, with minor contributions from adjacent peri-Gondwana sources. The magmatic activity may have started during this time (~359–346 Ma) (2) during the Late Mississippian (Ixtaltepec Formation), detrital zircon grains of Ediacaran-Cambrian age are dominant, derived from sediments either related to the Pan-African/Brasiliano orogeny or the opening of the Iapetus Ocean; (3) during the Late Mississippian-Middle Pennsylvanian, intercalated marine volcaniclastic sandstone (Ixtaltepec Formation) provides the first record of Carboniferous arc-related volcanism reported in southern Mexico, dated between 330 and 308 Ma; (4) the early Permian Yododeñe Formation records the exhumation and erosion of the sedimentary cover during the final stage of Pangea assembly. Rhyodacitic subvolcanic sills and lavas dated at ~282–270 Ma are present throughout the succession. Volcanism and ca. 360–308 Ma detrital zircon grains could be associated with a Carboniferous magmatic arc formed by subduction of the Rheic oceanic plate beneath Gondwana. Slightly younger detrital zircon and subvolcanic rocks dated at ~300–270 Ma are linked to a western Pangea arc developed in response to the subduction of the Paleo-Pacific Ocean following Pangea assembly. Our results suggest that the Carboniferous-Permian units were deposited in a peri-arc basin, sharing sediment provenance with the Maya and Coahuila blocks, the Sierra de Juárez Complex, and northwestern South America. [ABSTRACT FROM AUTHOR]

In a reconnaissance study, we investigated the potential of γ‐ray induced production of 38ArK from 39K for geochronological applications. For this purpose, various age monitors commonly in use for the established 40Ar/39Ar‐method were co‐irradiated for 60 h at 17.6 MeV maximum energy in the ELBE facility, Dresden‐Rossendorf, Germany. Because the available energy was low, total production of 38ArK was depressed, leading to low J38‐values of (2.1–4.1) × 10‐6 and hence resulted in only minor 38Ar excess when compared with atmospheric 38Ar/36Ar ratios. In spite of these restrictions, ages of younger monitors could be reproduced within error, whereas older age reference materials showed discrepancies due to the low production rate. We observed Ca‐derived contributions on 36Ar in analysed CaF2 reference materials, and calculated a limit for Ca‐interference on 38ArCa of (38Ar/36Ar)Ca = 0.07 ± 0.03 (1s). In addition, we investigated a potential recoil redistribution of 38Ar by stepwise heating experiments, but could not quantify this further because of concurring processes. More work at higher photon energies is necessary to resolve other open issues, in particular the potential of utilising 40Ar/37Ar ratios for age determination and the possibility of 42Ar production from 44Ca, which would allow correction for Ca‐interference reactions on other Ar isotopes. This would be a pre‐requisite for dating extra‐terrestrial rocks. [ABSTRACT FROM AUTHOR]

• Drill core samples constrain the age and evolution of the buried Wyoming Province. • Zircon U-Pb geochronology records Mesoarchean magmatism and metamorphism. • Reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and isotopic signature. • Geochemical, geochronologic, and geophysical data are most easily explained by a contiguous eastern Wyoming Province. • Integration of geologic, geochemical, geochemical, and geophysical datasets is critical to evaluate Archean and Proterozoic terrane boundaries. The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution. [ABSTRACT FROM AUTHOR]

New geochemical and geochronological data lead to a fuller understanding of different aspects of the Rajmahal flood basalt province. Earlier work was on surface Rajmahal flows as well as from the nearby Bengal Traps. Samples from a ~300 m borehole near the northwestern corner of the Rajmahal lavas, analyzed herein, reveal the presence of carbonatitic (aillikitic) material. This small province, including the Bengal Traps, thus exhibits quartz and olivine tholeiites, andesitic basalts, alkali basalts as well as carbonatitic material. The Sylhet Traps, lying ~500 km to the east of the surface Rajmahal flows, show geochemical similarities, but were probably formed ~10 Myr after Rajmahal volcanism. Sr–Nd isotopic plots suggest that (a) not all lavas in the Rajmahal Traps were homogenized isotopically prior to eruption and (b) the Sylhet Traps show a much wider variety of isotopic values at eruption, possibly caused by alteration. Published and new 40Ar/39Ar ages for both the Rajmahal and other mafic rocks postulated to have been formed by the Kerguelen Hotspot in the Southern Indian Ocean, are critically examined; emphasis is placed on the freshness of the rocks dated. It is shown that a number of 'ages' for both the Rajmahal Province, and for rocks recovered from the Kerguelen Plateau area, should be rejected as invalid measures of the time of crystallization, as altered material was dated. For the surface Rajmahal material, valid ages span ~120 (quartz tholeiites) to ~116 Ma (andesitic basalts); alkali basalts and olivine tholeiites (Bengal Traps) as well as the aillikite were also formed within this time interval. The postulated genetic link to the Kerguelen Hotspot and basalts in the Southern Indian Ocean appears valid; in particular, a section of the southern Kerguelen Plateau (~58°S; 75°E) has strong geochemical and temporal links to the Rajmahal Traps. [ABSTRACT FROM AUTHOR]

• First in-situ apatite U-Pb ages from pseudotachylytes. • The lower crust fails seismically during the early stages of continental collision. • Earthquakes create long-lived fluid pathways in the lower crust. Determining the precise age of fossil earthquakes is essential for understanding the tectonic setting in which they occurred. Pseudotachylytes are quenched frictional melts that occur along coseismic fractures and represent evidence of fossil earthquakes in the rock record. Here, we present the first in-situ apatite U-Pb ages for three samples from Lofoten in the northern Norwegian Caledonides, that occur within and adjacent to lower-crustal pseudotachylytes: a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite that contains pseudotachylyte transposed into the shear plane. The apatite U-Pb ages are combined with cathodoluminescence and electron backscatter diffraction analysis to provide microstructural constraints on the interpretation of ages. Apatite shows evidence of either fragmentation and annealing, or dislocation creep in response to coseismic rupture. The results suggest that the short-lived thermal pulse generated by the earthquake is sufficient to reset the apatite U-Pb system. The age of 426 ± 19 Ma is interpreted to date lower-crustal seismicity during the early Caledonian orogeny. Later fluid-rock interaction along the same structures led to partial resetting of the apatite U-Pb systematics, indicating that seismic brittle failure of the lower continental crust occurs at the incipient stages of continental collision, and that the resulting structures are preferential pathways for later fluid flow in the otherwise impermeable anhydrous lower crust. [ABSTRACT FROM AUTHOR]

The abandoned deposit Dobšiná in eastern Slovakia used to be exploited for Ni-Fe-Co sulfarsenides in siderite-ankerite hydrothermal veins. These veins are locally accompanied by alteration zones of fuchsite-quartz-carbonate rocks. The Ni-Fe-Co sulfarsenides are postdated by chalcopyrite and tetrahedrite. This work aims to determine the age of these mineralizations utilizing U/Pb, Re/Os, and K/Ar dating of selected minerals. U/Pb dating of hydrothermal siderite and ankerite gave ages from 145 ± 5 Ma to 114 ± 24 Ma. They broadly coincide with the metamorphic peak in the Gemeric Unit and the formation of the Gemeric cleavage fan (CGF) during partial exhumation of this unit. Hydrothermal gersdorffite was dated (Re/Os) to 93.6 ± 0.9 Ma, the time of exhumation of the Gemeric unit. No data were extracted for the younger Cu sulfides. By correlation with other units in the Western Carpathians, they could be assigned Late Cretaceous age, corresponding to welding of the Gemeric and Veporic units and the formation of the Trans-Gemeric shear zone. The fuchsite rocks were dated (K/Ar) to 110 Ma, but this datum conflicts with textural evidence, which shows that these rocks are older than the hydrothermal carbonates. The age of 110 Ma is therefore considered to be cooling age after the reset of the geochronometer by the Alpine metamorphism, in agreement with earlier data. The geochronological results are supported by structural measurements and field observations. The main mineralized zone, investigated in one of the accessible adits, conforms with the Variscan - ?Alpine metamorphic foliation. There are also younger, less significant structures whose relevance to the ore mineralization is not yet clear. [ABSTRACT FROM AUTHOR]

The Hegenshan–Heihe Suture (HHS) is an important suture zone between the Xing'an and Songliao–Xilinhot blocks in the eastern Central Asian Orogenic Belt (CAOB). Many studies have examined the northern and southern parts of this suture zone, but the middle segment remains under-researched and controversial. This paper presents new research into Permian felsic rocks along the middle segment of the HHS, including alkaline granites, rhyolites, and granodiorites. Zircon U–Pb dating of these rocks yielded precise Permian ages of 300.4 ± 2.5, 292.8 ± 2.7, and 293.1 ± 2.2 Ma. The alkaline granites and the rhyolites are A-type rocks that were derived from lower crustal protoliths, with the melting of these protoliths caused by asthenospheric upwelling associated with lithospheric delamination. In comparison, the granodiorites are I-type granites were generated from a mantle-derived magmatic source. The petrogenesis of these granites also involved the assimilation of sedimentary rocks and fractional crystallization. This Permian magmatism occurred during post-collisional extension following the amalgamation of the Xing'an and Songliao–Xilinhot blocks. [ABSTRACT FROM AUTHOR]

The Sierra Nevada de Santa Marta in NW Colombia is an isolated massif at the northernmost end of the Andes chain near the boundary with the Caribbean plate. Previous geologic mapping and K-Ar dating have shown that Jurassic plutonic and volcanic units make up a large part of the Santa Marta Massif (SMM). These rocks have been considered to be part of a Jurassic magmatic arc extending from NW Colombia to northern Chile, but without any geochemical basis for comparison. This paper reports on a geochemical and Sr-Nd-Pb isotope study of the Jurassic rocks in the SMM and provides 12 new U-Pb zircon ages from in-situ laser ICP-MS dating. The plutonic and volcanic units span a range from 45 to 78 wt.% SiO 2 , with a dominance of intermediate to felsic compositions with SiO 2 > 57 wt.%. They classify as calc-alkaline, medium to high-K, metaluminous rocks with trace-element features typical for arc-derived magma series. In terms of their major and trace-element compositions, the SMM Jurassic units overlap with contemporary plutonic and volcanic rocks from other regions of the Central and Eastern Cordilleras of Colombia, and confirm an arc affinity. The new U-Pb ages range from 176 ± 1 Ma to 192 ± 2 Ma (n = 12), with most between 180 and 188 Ma (n = 7). The initial Sr isotope ratios (at 180 Ma) are between 0.7012 and 0.7071 (n = 29), with 3 outliers attributed to mobilization of Rb and/or Sr. Nearly all samples have negative εNd (180) values between −10.3 and 0.0 (n = 30), the two exceptions being only slightly positive (1.1 and 1.9). Measured Pb isotope ratios fall in a narrow range, with 206 Pb/ 204 Pb from 18.02 to 19.95, 207 Pb/ 204 Pb from 15.56 to 15.67 and 208 Pb/ 204 Pb from 37.76 to 39.04 (n = 28). In the regional context of previous studies, these results confirm early Jurassic ages and an arc affinity for the widespread magmatism exposed in the eastern and northeastern Colombian Andes. We also note patterns in the distribution and composition of magmas. The magmatic activity in the Central Cordillera tends to be younger than in the Eastern Cordillera and is spatially more restricted to the vicinity of regional fault systems. In terms of composition, Jurassic igneous rocks in the Eastern Cordillera have systematically lower εNd (180) values than those from the Central Cordillera, whereas the Pb isotope ratios overlap. We ascribe the Nd isotope variations to heterogeneity in the mantle source and/or degree of crustal contamination, whereas the Pb isotope ratios are crust-dominated and similar throughout the region. The spatio-temporal and compositional evolution of Jurassic magmatic rocks in the Northern Andes reflect the major plate kinematic readjustment between the Triassic and the Early Jurassic in the proto-Andean margin. [ABSTRACT FROM AUTHOR]

Abstract Apatite is an accessory mineral that is frequently found in both igneous and clastic sedimentary rocks. It is conventionally considered to be characterized by a closure temperature range between 375 and 600 °C and hence has been employed to address mid-temperature thermochronology questions relevant to the reconstruction of thermal events in the middle to lower crust. However, questions remain as to whether apatite faithfully records thermally-activated volume diffusion profiles, or rather is influenced by recrystallization and new growth processes. We present a case study of two apatite samples from the Akia Terrane in Greenland that help chart some of the post magmatic history of this region. Apatite in a tonalitic gneiss has distinct U-enriched rims and its U-Pb apparent ages correlate with Mn chemistry, with a high Mn group yielding an age of c. 2813 Ma. The U-Pb and trace element chemistry and morphology support an interpretation in which these apatite crystals are originally igneous and record cooling after metamorphism, with subsequent generation of discrete new rims. Epidote observed in the sample implies a <600 °C fluid infiltration event associated with apatite rims. The second sample, from a granitic leucosome, contains apparently homogeneous apatite, however U-Pb analyses define two distinct discordia arrays with different common Pb components. An older, c. 2490 Ma, component is associated with elevated Sr, whereas a younger, c. 1800 Ma, component has lower Sr concentration. A depth profile reveals an older core with progressively younger ages towards a compositionally discrete late Paleoproterozoic rim. The chemical and age profiles do not directly correspond, implying different diffusion rates between trace elements and U and Pb. The variation in core ages is interpreted to reflect radiogenic-Pb loss from a metamorphic population during new rim growth. The younger, c. 1800 Ma U-Pb age is interpreted to date new apatite growth from a compositionally distinct reservoir driven by tectonothermal and fluid activity, consistent with regional mica Ar-Ar ages. Results from these two samples show that recrystallization, dissolution and regrowth processes likely formed the younger rim overgrowths, and at temperatures below those often considered to be closure temperatures for Pb diffusion in apatite. The results from these samples imply many apatite grains may not record simple thermally activated Pb diffusion profiles and cautions against inversion of apatite U-Pb data to thermal histories without detailed knowledge of the grain growth/alteration processes. Highlights • Present U-Pb apatite geochronology from the Akia Terrane Greenland • Apatites from the same sample may incorporate different common Pb. • Chemical and age zoning in apatite need not correlate • Dissolution and regrowth forming high U rims is prevalent. • Recrystallization may dominate over Pb diffusion in apatite. [ABSTRACT FROM AUTHOR]

Abstract A next-generation, high-flux DD neutron generator has been designed, commissioned, and characterized, and is now operational in a new facility at the University of California Berkeley. The generator, originally designed for 40Ar/39Ar dating of geological materials, has since served numerous additional applications, including medical isotope production studies, with others planned for the near future. In this work, we present an overview of the High Flux Neutron Generator (HFNG) which includes a variety of simulations, analytical models, and experimental validation of results. Extensive analysis was performed in order to characterize the neutron yield, flux, and energy distribution at specific locations where samples may be loaded for irradiation. A notable design feature of the HFNG is the possibility for sample irradiation internal to the cathode, just 8 mm away from the neutron production site, thus maximizing the neutron flux (n/cm 2 /s). The generator’s maximum neutron flux at this irradiation position is 2.58 × 10 7 n/cm 2 /s ± 5% (approximately 3 × 10 8 n/s total yield) as measured via activation of small natural indium foils. However, future development is aimed at achieving an order of magnitude increase in flux. Additionally, the deuterium ion beam optics were optimized by simulations for various extraction configurations in order to achieve a uniform neutron flux distribution and an acceptable heat load. Finally, experiments were performed in order to benchmark the modeling and characterization of the HFNG. [ABSTRACT FROM AUTHOR]

The geochemistry, geochronology, and isotope geochemical systematics (Nd, Sr, Hf, and Pb) of the granitoids of the Pozdnestanovoy complex of the Dzhugdzhur-Stanovoy superterrane of the Central Asia fold belt were investigated. It was shown that their age is Mesozoic (142-138 Ma) rather than Early Precambrian, as was previously supposed. The main sources of parental melts for these granitoids were the Neoarchean and Paleoproterozoic rocks of the lower continental crust of the Dzhugdzhur-Stanovoy superterrane and the rocks of the Late Paleozoic-Early Mesozoic continental crust of the Amur microplate. They were formed at depths of >40 km and temperatures of 700-800°C, most likely through the melting of mafic feldspar granulites under the conditions of aqueous fluid infiltration without any significant contribution from a juvenile heat source. The granitoids of the Pozdnestanovoy complex were emplaced during the closure of the eastern segment of the Mongolia-Okhotsk Ocean owing to the collision of the Siberian and Sino-Korean continents. [ABSTRACT FROM AUTHOR]

This paper reports zircon U–Pb geochronology, whole-rock chemistry and Sr–Nd–Hf isotopic data of intermediate-acid intrusive rocks from the Gerlache Strait area, Antarctic Peninsula. These intrusive rocks can be divided into three groups. Group #1 has an emplacement age of ca. 117 Ma, and is further subdivided according to K 2 O contents into High-K (HKS) and Low-K (LKS) Subgroups. The HKS monzogranites are weakly peraluminous, and yield ε Hf ( t ) and ε Nd ( t ) values of +0.9 to +2.3 and +0.1 to +2.3, respectively, with corresponding model Nd ages (T DM Nd ) of 766–471 Ma, indicating that they were derived from the partial melting of dominantly early Neoproterozoic to early Paleozoic juvenile basaltic materials with minor involvement of sediments. In contrast, the LKS granodioritic magma was derived from partial melting of early Paleozoic juvenile basaltic materials with ε Hf ( t ) and ε Nd ( t ) values of +10.3 to +12.5 and +5.4 to +5.5, respectively, and corresponding T DM Nd ages of 548–500 Ma. Group #2 consists of granite porphyry that formed at ca. 82 Ma and is characterized by positive values of ε Hf ( t ) (+5.1 to +8.1) and ε Nd ( t ) (+1.9 to +3.1) with weakly peraluminous chemistry and corresponding T DM Nd ages of 712–548 Ma, suggesting that it was generated from partial melting of dominantly Neoproterozoic juvenile basaltic materials with minor involvement of sediments. Group #3 comprises dioritic–granodioritic rocks that were emplaced at ca. 62–54 Ma. These rocks have Mg# values of 39–46, and ε Hf ( t ) and ε Nd ( t ) values of +6.7 to +12.2 and +3.6 to +7.2, respectively, with corresponding T DM Nd ages of 529–307 Ma, indicating that they were derived from partial melting of Paleozoic juvenile basaltic materials with minor injection of mantle materials. Spatially, the lithology of these plutons roughly changes from acid to intermediate from east to west, concomitant with increasing ε Hf ( t ) and ε Nd ( t ) values and decreasing initial 87 Sr/ 86 Sr ratios and T DM Nd ages. This trend reflects changes in source characteristics from a Gondwana affinity to a juvenile crust affinity. Overall, the southeast-to-northwest migration of magmatism in the northern Antarctic Peninsula since the Mesozoic may have been related to retreating subduction of the paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

The late Carboniferous to Triassic tectonic history of eastern Australia includes important periods of regional-scale crustal extension and contraction. Evidence for these periods of tectonism is recorded by the extensive Pennsylvanian (late Carboniferous) to Triassic basin system of eastern Australia. In this study, we investigate the use of U-Pb dating of detrital zircons in reconstructing the tectonic development of one of these basins, the eastern Galilee Basin of Queensland. U-Pb detrital zircon ages were obtained from samples of stratigraphically well-constrained Cisuralian and Lopingian (early and late Permian, respectively) sandstone in the Galilee Basin. Detrital zircons in these sandstones are dominated by a population with ages in the range of 300-250 Ma, and ages from the youngest detrital zircons closely approximate depositional ages. We attribute these two fundamental findings to (1) appreciable derivation of detrital zircons in the Galilee Basin from the New England Orogen of easternmost Australia and (2) syndepositional magmatism. Furthermore, Cisuralian sandstone of the Galilee Basin contains significantly more >300 Ma detrital zircons than Lopingian sandstone. The transition in detrital zircon population, which is bracketed between 296 and 252 Ma based on previous high-precision U-Pb zircon ages from Permian ash beds in the Galilee Basin, corresponds with the Hunter-Bowen Orogeny and reflects a change in the Galilee Basin from an earlier extensional setting to a later foreland basin environment. During the Lopingian foreland basin phase, the individual depocentres of the Galilee and Bowen basins were linked to form a single and enormous foreland basin that covered >300 000 km2 in central and eastern Queensland. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Gawler Silicic Large Igneous Province (SLIP) in the Gawler Craton and Curnamona Province, southern Australia, comprises extensive felsic and lesser mafic volcanic sequences, with only limited sedimentary successions. The Roopena Basin is a rare example of a synvolcanic sedimentary basin that formed within the Gawler SLIP in the eastern Gawler Craton. It is a north-south-trending basin with a preserved area of 75 km2, bound by the Roopena and Wizzo Well faults, and contains three units of the lower Gawler Range Volcanics; the Angle Dam Dacite, Fresh Well Formation and Roopena Basalt. The Angle Dam Dacite is a porphyritic lava and the oldest part of the volcanic succession, directly overlying basement. The Fresh Well Formation overlies the Angle Dam Dacite or basement, comprises three coarsening-upwards volcaniclastic packages of claystone, siltstone, fine-grained to coarse-grained lithic sandstone and conglomerate deposited in a fluvio-lacustrine setting, and contains three tuff members. The Roopena Basalt is interlayered with the Fresh Well Formation, and comprises auto-brecciated lavas that exhibit only local interaction with water or wet sediment. Sharp basal contacts of the prograding packages within the Fresh Well Formation provide evidence of rapid flooding events within the basin. New detrital zircon geochronology of a sandstone within the Fresh Well Formation yielded a maximum depositional age of ca 1580 Ma, with provenance dominated by felsic volcanic units of the 1635-1605 Ma St Peter Suite. Sedimentation in the Gawler SLIP appears to have occurred in isolated basins with limited areal extent. It was largely restricted to the eastern Gawler Craton, and as well as the Roopena Basin, and includes similar basins at the Olympic Dam and Prominent Hill iron oxide-copper-gold ± uranium (IOCG ± U) deposits. The coincidence of sedimentation and mafic volcanism in the eastern Gawler Craton suggests that this region underwent extension at this time, although high-temperature metamorphism and compressional deformation occurred in some parts of the Gawler Craton and Curnamona Province synchronous with the Gawler SLIP. The Roopena Basin sedimentary rocks and underlying basement contain hematite-chlorite-sericite-white mica assemblages, permissive of hematite-style IOCG mineral deposits; however, no significant ore deposit has yet been discovered in the Roopena Basin. [ABSTRACT FROM AUTHOR]

Observations of discordant ages, meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock, began in the early days of geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages. Advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions, then the analysis of individual grains, and even pieces of grains, with higher degrees of concordancy. Further advances allowed a higher analytical precision, a clearer perception of accuracy, and a better statistical resolution of age discordance. As for understanding the cause(s) of discordance, belief revision followed the coupling of imaging, cathodoluminescence (CL), and backscattered electrons (BSE), to in situ dating by secondary ion mass spectrometry (SIMS) or by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Discordant zircon and other accessory minerals (e.g., monazite, apatite, etc.) often consist of young rims accreted onto/into older cores. Age gradients are sharp, and no Pb diffusion gradients are observed. As U-Pb discordance in crystalline, non-radiation damaged grains is caused by diachronous, heterochemical mineral generations, interpretations of mineral ages, based on the exclusive role of diffusion, are superseded, and closure temperatures of zircon and monazite are irrelevant in geological reality. Other isotopic systems (Rb-Sr, K-Ar) were believed, since the 1960s, to be similarly controlled by the diffusivity of radiogenic daughters. When zircon and monazite discordance were recognized as zone accretion/reaction with sharp boundaries that showed little or no high-temperature diffusive re-equilibration, the other chronometric systems were left behind, and interpretations of mineral ages based on the exclusive role of diffusion survived. The evidence from textural-petrologic imaging (CL, BSE) and element mapping by electron probe microanalyzer (EPMA) or high spatial resolution SIMS or LA-ICP-MS provides the decisive constraints. All microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions. It is important not to confuse causes and effects; heterochemical microstructures are not the cause of Ar and Sr loss; rather, they follow it. Ar and Sr loss by dissolution of the older mineral generation occurs first, heterochemical textures form later, when the replacive assemblage recrystallizes. Heterochemical mineral generations are identified and dated by their Ca/Cl/K systematics in 39Ar-40Ar. Replacive reactions adding or removing Cl, such as, e.g., sericite overgrowths on K-feldspar, retrograde muscovite intergrowths with phengite, etc. are detected by Cl/K vs. Ar/K isotope correlation diagrams. Ca-poor reaction products, such as, e.g., young biotite intergrown with older amphibole, adularia replacing microcline, etc., can be easily identified by Ca/K vs. Ar/K diagrams supported by EPMA analyses. Mixed mineral generations are observed to be the cause of discordant, staircase-shaped age spectra, while step-heating of crystals with age gradients produces concordant plateaus. Age gradients are therefore unrelated to staircase age spectra. There is a profound analogy between the U-Pb, Rb-Sr, and K-Ar systems. Pb and Ar diffusion rates are both much slower than mineral replacement rates for all T < 750 °C. Patchy retrogression textures are always associated with heterochemical signatures (U/Th ratios, REE patterns, Ca/Cl/K ratios). As a rule, single-generation minerals with low amounts of radiation damage give concordant ages, whereas discordance is caused by mixtures of heterochemical, resolvably diachronous, mineral generations in petrologic disequilibrium. This can also include (sub-)grains that have accumulated significant amounts of radiation damage. For accurate geochronology the petrologic characterization with the appropriate technique(s) of the minerals to be dated, and the petrologic context at large, are as essential as the mass spectrometric analyses. [ABSTRACT FROM AUTHOR]

The Galería complex is located in the intermediate level of the Atapuerca multilevel karst in Burgos (Spain). The Galería complex is formed by three subsections filled with sediments and connected to each other: Covacha de los Zarpazos, Galería site, and Tres simas. These cavities are filled with extensively dated entrance facies, rich in fossil and archaeological materials, and on the other hand, with interior facies, barely dated and without fossil and archaeological remains. Studying in greater depth the interior facies deposits is of interest as they record the entrenchment of the regional fluvial system and a transition period between the formation of the intermediate and lower levels of the Atapuerca endokarst. Previous geological works suggest that these interior facies in the Galería-Tres Simas Complex should have been deposited during the Lower Pleistocene-Middle Pleistocene; however, geochronological studies have provided disparate ages by ESR and OSL on the top of the interior facies. Therefore, the objective of this study is to date the lowermost part of the interior facies to complete the chronological framework of the Galería complex and, to provide minimum dates for the formation of this keyhole cross section. With this objective, 5 and 22 samples were taken to be dated by ESR and Paleomagnetism, respectively. Our first results suggest Middle Pleistocene ESR ages that are in agreement with previous results obtained by OSL. In contrast, paleomagnetic data reveal reversal polarity. These results may suggest postsedimentary processes or geological events that underestimate the ages obtained by trapped charge dating techniques. Our results are important to better understand this interval of the fluvio-karstic system of the Sierra de Atapuerca, which preceded the opening of the Gran Dolina and Gallery cavities and their infill by entrance deposits. [ABSTRACT FROM AUTHOR]

New Ar-Ar muscovite and Rb-Sr biotite age data in combination with structural analyses from the Apuseni Mountains provide new constraints on the timing and kinematics of deformation during the Cretaceous. Time-temperature paths from the structurally highest basement nappe of the Apuseni Mountains in combination with sedimentary data indicate exhumation and a position close to the surface after the Late Jurassic emplacement of the South Apuseni Ophiolites. Early Cretaceous Ar-Ar muscovite ages from structurally lower parts in the Biharia Nappe System (Dacia Mega-Unit) show cooling from medium-grade conditions. NE-SW-trending stretching lineation and associated kinematic indicators of this deformation phase (D1) are overprinted by top-NW-directed thrusting during D2. An Albian to Turonian age (110-90 Ma) is proposed for the main deformation (D2) that formed the present-day geometry of the nappe stack and led to a pervasive retrograde greenschist-facies overprint. Thermochronological and structural data from the Bihor Unit (Tisza Mega-Unit) allowed to establish E-directed differential exhumation during Early-Late Cretaceous times (D3.1). Brittle detachment faulting (D3.2) and the deposition of syn-extensional sediments indicate general uplift and partial surface exposure during the Late Cretaceous. Brittle conditions persist during the latest Cretaceous compressional overprint (D4). [ABSTRACT FROM AUTHOR]

Liaoning Province in China is an area known for the occurrence of numerous copper and/or molybdenum deposits of variable size. However, the age of mineralization and tectonic setting in this region are still a subject of debate. In this study we describe the geology of these deposits and apply zircon U–Pb and molybdenite Re–Os isotopic dating to constrain their ages and define the metallogenic epochs of this province. The Huatong Cu–Mo deposit yields molybdenite Re–Os model ages of 127.6–126.3 Ma and an isochron age of 127.4 ± 0.7 Ma. The Dongbeigou Mo deposit yields molybdenite Re–Os model ages of 132.6–127.1 Ma, an isochron age of 128.1 ± 5.1 Ma, and a zircon U–Pb age of 129.4 ± 0.3 Ma for the associated monzogranite. The granodiorite associated with the Wanbaoyuan Cu–Mo deposit yields a zircon U–Pb age of 128.4 ± 1.1 Ma; the plagiogranite associated with the Yaojiagou Mo deposit yields an age of 167.5 ± 0.9 Ma; and the biotite–plagioclase gneiss from the Shujigou Cu deposit yields an age of 2549.4 ± 5.6 Ma. These results, together with previous geochronology data, show that intense Cu–Mo porphyry and skarn mineralization were coeval with Early–Middle Jurassic and Early Cretaceous granitic magmatism. The former was associated with the orogeny that followed the collision of the Siberian and North China plates and the resulting closure of the palaeo-Asian Ocean, and the latter with rifting that followed the subduction of the palaeo-Pacific Plate and associated lithospheric thinning. Volcanogenic massive sulfide Cu deposit. mineralization took place much earlier, in the late Archaean, and was related to continent–continent collision, palaeo-ocean closure, the formation of a united continental landmass, bimodal volcanism, magma emplacement, and subsequent metamorphism and deformation of syn-collisional granites. [ABSTRACT FROM PUBLISHER]

This study reports zircon U-Pb and Hf isotopes and whole-rock elemental data for granodiorites from the East Kunlun orogen. The zircon U-Pb dating defines their crystallization age of 235 Ma. The rocks are characterized by high-K calc-alkaline, magnesian and metaluminous with (KO+NaO)=6.38 wt.%-7.01 wt.%, Mg#=42-50 [Mg#=100×molar Mg/(Mg+FeO)], A/CNK=0.92-0.98, coupled with high ε( t) values from -0.65 to -1.80. The rocks were derived from partial melting of a juvenile mafic crustal source within normal crust thickness. The juvenile lower crust was generated by mixing lithospheric mantle-derived melt (55%-60%) and supracrustal melt (40%-45%) during the seafloor subduction. Together with available data from the East Kunlun, it is proposed that the studied Middle Triassic granodiorites were formed in post-collisional extension setting, in which melting of the juvenile lower crust in response to the basaltic magma underplating resulted in the production of high-K granodioritic melts. [ABSTRACT FROM AUTHOR]

U-Pb geochronological studies reveal that the gabbro of the Mikitkin Massif and quartz diorite of the Ust'-Garin Massif of the Mamyn Terrane, which were provisionally assigned to the Early Proterozoic Garin Complex, have ages of 583 ± 6 Ma and 607 ± 8 Ma, i.e. Upper Riphean-Lower Vendian. The geochemical features of the magmatic rocks of the studied massifs indicate their suprasubduction origin. The parental melts of the magmatic rocks of the Garin Complex were formed in an active continental margin or ensialic island-arc setting due to melting of the subduction-modified depleted mantle source and the occurrence of contamination and fractional crystallization. These massifs were developed during the Ediacaran Stage of the geological evolution of the Mamyn Terrane and presumably reflect Neoproterozoic convergent geodynamic processes. These processes likely caused the ultimate formation of the Precambrian continental massifs in the eastern Central Asian fold belt, which were subsequently amalgamated into the structure of the epi-Paleozoic Amur microcontinent. [ABSTRACT FROM AUTHOR]

The ability to resolve the stratigraphic record of Earth history is critical to understanding past global change events and the range of natural variability within the global paleoclimate. The geosphere–hydrosphere–atmosphere–biosphere system operates on geologically very short time scales and we must be able to resolve the stratigraphic record of past events on similar time scales if we are to understand the cause-and-effect relationships operating within the system. Global resolution ≤ 100 kyr is commonly available when evaluating events from the Cenozoic Era, but such resolution is less common for older portions of Earth history. The introduction of High-Resolution Event Stratigraphy (HiRES) more than two decades ago revolutionized the ability to resolve the Mesozoic and Cenozoic stratigraphic records of global change, and a similar revolution is currently underway for the Paleozoic as well. Here, we review the application of the principles of HiRES to the Paleozoic stratigraphic record with examples from the Silurian System, discuss the quantification of chronostratigraphic uncertainty, and highlight the current deep-time Earth history revolution that has the potential to open the entire Phanerozoic record to the evaluation of short time scale global climate dynamics. [ABSTRACT FROM AUTHOR]

Atmospherically-derived cosmogenic nuclides such as meteoric 10 Be offer great potential in Quaternary science because they can be used as landform geochronometers and as sediment tracers. Use of meteoric 10 Be inventories for determining landform age, soil residence time, or erosion rates requires knowing the long-term rate of deposition for this cosmogenic nuclide to the Earth's surface. Here, we present meteoric 10 Be inventories measured at 6 dated sites near the Boulder Creek Critical Zone Observatory, Front Range, Colorado. Each site lies on a relatively stable landform (broad moraine or fluvial terrace) at slopes between <1 and 7° and supports well-developed soil horizons. Time-averaged rates of total meteoric 10 Be deposition (precipitation, dryfall, and dust deposition) calculated from measured inventories at these sites range from 0.4 to 5.0 × 10 6 atoms cm −2 yr −1 ; none of the inventories match expected primary meteoric 10 Be deposition rates that global, latitude-based models or precipitation-specific models predict for this region. Meteoric 10 Be inventories and calculated deposition rates at the three older (>90 ka) sites fall below (0.3–0.7×) model predictions; inventories and rates at the three Pinedale-age sites (LGM; 21–15 ka) are higher (1.5–4×) than model predictions. Previous analysis of soils in the Front Range over the same timeframe as these sites indicate that long-term dust accumulation rates are low and associated secondary meteoric 10 Be deposition is unlikely to account for the deviation from model predictions. At the Pinedale-age sites in particular, long-term dustfall would have to be 1–2 orders of magnitude higher to add the meteoric 10 Be required to account for measured inventories. Low inventories at older sites are consistent with loss of meteoric 10 Be through physical erosion or selective removal of near-surface soil enriched in meteoric 10 Be; time-averaged rates of total meteoric 10 Be deposition for these sites are minima. We speculate that higher inventories at Pinedale-age sites reflect the addition of meteoric 10 Be due to snowdrifts and/or interflow associated with landform location, increasing the effective precipitation-related primary meteoric 10 Be deposition at the sampled sites. Given these constraints, and the spatial variation observed around the Front Range, we suggest that 1.5 ± 0.2 × 10 6 atoms cm −2 yr −1 is an appropriate long-term meteoric 10 Be deposition rate to use across much of the Front Range region that receives 45–55 cm of precipitation a year. This long-term total meteoric 10 Be deposition rate is ∼30–50% higher than that predicted by global or annual precipitation-specific models, emphasizing that local calibration of deposition rates is essential for geomorphic studies around the globe. [ABSTRACT FROM AUTHOR]

Presented is an overview of the traditionally used absolute dating techniques for Quaternary geomorphological complexes. It is shown that they have a narrow range of coverage both as regards the time interval and with respect to the set of objects dated; therefore, they are unsuitable for age diagnostics of most geomorphological formations. The novel methods of dating are examined, and an assessment is made of the most promising of them. Technologies of age diagnostics, the merits and demerits of the existing and new methods, and the possibilities of using them in Siberia are considered. [ABSTRACT FROM AUTHOR]

Abstract: We evaluate the 190Pt–186Os decay system, measured by laser ablation MC-ICPMS, as a useful geochronometric tool for direct dating of platinum-group minerals (PGM) in large mafic/ultra-mafic intrusions by analysing mineralised rocks from the geochronologically well constrained Bushveld Complex, S. Africa. Mixed PGM (laurite, cooperite, laurite–platarsite, sperrylite and Pt–Fe alloy) from the Merensky Reef yield a Pt–Os isochron age of 1995±50Ma (2σ, n=48, MSWD=1.16, initial 186Os/188Os=0.119819±0.000006). While this is 59Myr younger than the U–Pb zircon age for the Merensky Reef (Scoates and Friedman, 2008) it is consistent with recent Re–Os ages for the Platreef. Considering the younger age recorded by both Re–Os and Pt–Os systems, we propose that the Pt–Os isochron age records a late, low temperature hydrothermal event affecting PGM and sulphides in the Platreef and Merensky Reef. Pt-rich phases may also yield single grain 190Pt–186Os model ages since initial 186Os/188Os can be well constrained. A Pt–Os model age of 2024Ma±101Ma was calculated for a Merensky cooperite; a Pt–Fe alloy from the Onverwacht pipe and a Tweefontein Hill sperrylite give Pt–Os model ages of 2113±106Ma and 2042±102Ma, respectively. A Pt–Os isochron derived from all Bushveld units gives an age of 2012±47Ma (2σ, 186Os/188Osi =0.119818±0.000006) with an MSWD of 1.19, hence PGE mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the Pt–Os chronometer. Nonetheless, the potential for the Pt–Os system in dating events in PGE mineralised systems is evident and the agreement of single grain model ages in high Pt/Os phases with the Pt–Os and Re–Os isochron ages indicates their usefulness in the geochronology of these systems. [Copyright &y& Elsevier]

Abstract: The Nihewan Basin sedimentary sequences in northern China are rich in mammalian fossil and Paleolithic sites, thus providing insights into our understanding of Quaternary land mammal biochronology and early human settlements in East Asia. Here we present high-resolution magnetostratigraphic results that place stringent age controls on the Xiashagou (XSG) Fauna (that is, the well-known Nihewan Fauna sensu stricto) in the Nihewan Basin, northern China. Results show that the XSG sequence recorded the Brunhes normal chron, the Matuyama reverse chron and the late Gauss normal chron. The XSG Fauna resides in the Matuyama reverse chron (between the pre-Reunion Matuyama chron and the post-Olduvai Matuyama chron), yielding an estimated age of ca. 2.2–1.7Ma. The Pliocene–Pleistocene boundary (Gauss/Matuyama boundary) is located in the lower part of the XSG section. The combination of our study and previously published magnetostratigraphic data suggest that the mammalian fossil sites in the eastern Nihewan Basin can be placed between the Gauss–Matuyama geomagnetic reversal and the Matuyama–Brunhes geomagnetic reversal (2.58–0.78Ma), leading to a time range of ca. 2.6–0.8Ma for the Nihewan mammalian faunas. [Copyright &y& Elsevier]

The Phanerozoic tectonic evolution of the Chuya-Kurai zone was studied by means of zircon U/Pb-dating, apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronology performed on basement rocks. Our results suggest that multiple magmatic episodes during the Paleozoic, related to the accretion-collision tectonics in Central Asia, affected our study area. Mesozoic and Cenozoic basement cooling events are interpreted as periods of tectonic reactivation. A new tectonic model for Late Cenozoic evolution of the Chuya-Kurai zone is proposed. [ABSTRACT FROM AUTHOR]

Abstract: In-situ cosmogenic 3He exposure ages of pyroxene phenocrysts from basalts from the Upper Neostromboli formation in southwest Stromboli date its eruption at 7.0±0.3ka (1σ, n =3, Ginostra site) and 6.8±0.2ka (1σ, n =10, Timpone del Fuoco site) respectively. Correlation of our new data to previous K/Ar and palaeomagnetic ages from the northwestern Neostromboli phase suggests that it erupted within a confined period between roughly 6 and 14ka. The low uncertainty on the 3Hecos ages as well as on individual exposure ages (4.4–8.7%) demonstrates that 3Hecos exposure dating is a viable tool for dating Holocene basalt lavas. The ages compare favourably to uncertainties obtained for radiocarbon dating of similar rocks. [Copyright &y& Elsevier]

Abstract: Magnetic sector mass spectrometers dominate the field of 40Ar/39Ar geochronology. Recent advances in quadrupole mass spectrometer technology, especially improvements in resolution, have increased the performance of these instruments to the extent that they can be used for isotopic determinations. We describe a triple filter quadrupole mass spectrometer (Hiden HAL 3F Series Pulse Ion Counting Triple Filter QMS) linked to an automated furnace extraction and cleaning system dedicated to 40Ar/39Ar incremental heating experiments. The instrument produces peaks with broad flat tops and a width of 0.9amu at 10cps height and 0.84amu at 500,000cps height on a 1 millioncps high peak (peak width at 0.01‰ and 50% peak height respectively). This allows measurement of ratios of the main Ar peaks in the 1‰ range. Measurements of 1.6×10−12 mole of air reference gas over two years yields 40Ar/36Ar=257.9±1.3 (1σ, n =34). The ability of the instrument to produce 40Ar/39Ar ages from rocks/minerals of a wide age range, reaching into the late Quaternary, are demonstrated by a series of tests and comparison with geochronological data from other studies and an in-house MAP 215-50 magnetic sector mass spectrometer. We demonstrate that high-end quadrupole systems can be used for routine 40Ar/39Ar dating purposes. [Copyright &y& Elsevier]

Abstract: Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this. Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ±2–3% for monazites with around 1500–2000ppm total Pb, or an additional absolute uncertainty of 20–50ppm Pb. [Copyright &y& Elsevier]

Abstract: We present a new, reliably dated Mesoproterozoic paleopole for Siberia, based on a combined geochronological and paleomagnetic study of mafic rocks within the Mesoproterozoic Sololi Group of the Olenek Uplift in northern Siberia. Ion microprobe (SHRIMP) U–Pb analysis yields crystallisation ages of 2036±11Ma for zircon from a basement granite and 1473±24Ma for baddeleyite from a large dolerite sill within the Kyutingde Formation. The baddeleyite result indicates that the lower Sololi Group is significantly older than was suggested by previous K–Ar results. Paleomagnetic analysis of the dolerite sill and related mafic intrusive rocks yields a paleopole at 33.6°N, 253.1°E, A 95 =10.4°. A positive baked-contact test between the Kyutingde sill and sedimentary country rocks shows that the magnetisation is primary. Comparison of this paleopole with coeval results for Laurentia provides a revised reconstruction between Siberia and Laurentia, and implies that these two continents were parts of a single Mesoproterozoic supercontinent since at least 1473Ma. We argue that Siberia, Laurentia, and Baltica belonged to the same supercontinent between 1473Ma and mid-Neoproterozoic time. [Copyright &y& Elsevier]

Abstract: U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement. Group 1 gneiss-migmatitic rocks (ca. 3200Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162±26Ma), Araci (3072±2Ma), Requeijão (2989±11Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton. Group 2 granitoid plutons were emplaced at 2.16–2.13Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement. [Copyright &y& Elsevier]

U Pb dating of hydrogenic and fossil material can be a significant challenge because samples often contain low U levels and variable amounts of non-radiogenic Pb. Line scans using LA-ICPMS provide the widest range of ratios available for defining the radiogenic to common Pb mixing line, hence the best age estimate, but extracting this information requires processing many low-count data. Conventional least-squares regression of ratios based on such data can give inconsistent results. A new approach is introduced where data are regressed as count rates in a 3D space rather than ratios on a 2D plot. This is a maximum likelihood approach in that mixing planes covering a range of ages and common 207Pb/206Pb ratios (PbC) are compared to the data set and used to determine a relative probability density surface whose maximum should fix the best-fit age and common 207Pb/206Pb ratio. 95% confidence errors are determined from T and PbC values that cut off 2.5% of the area to the left and right of 2D plots generated by integration of the 3D plot over PbC and age, respectively. Results from a rugose coral fossil show that this approach gives age results similar to regressions of concordia data and more accurate than Tera-Wasserburg regressions on data from natural samples. Tests on randomly generated synthetic data do not show significant discrepancy between Isoplot ages but they do between values of PbC. The 3D approach is the only one that gives accurate results for both age and PbC. [ABSTRACT FROM AUTHOR]

Geochronology studies in South Australia were developed in three Adelaide institutions. (i) University of Adelaide: (ia) Department of Geology and Mineralogy, where four single chemical measurements were reported in 1923-1955 and 14 lead-alpha measurements in 1961; MSID Rb-Sr work began in 1972 followed by zircon U-Pb about 1980 and Sm-Nd later; (ib) luminescence equipment was developed in the Department of Physics and is used to date recent sediments. (ii) Flinders University, where alpha spectrometry disequilibrium dating of recent sediments was active during 1972-2000; and (iii) AMDEL, which supplied a variable commercial service of K-Ar, Rb-Sr and zircon dating during period 1972-1999. [ABSTRACT FROM AUTHOR]

Zircon LA-ICP MS U-Pb dating of six metamorphic rocks and a metagranite (breccia) from southern basement of the Songliao Basin are reported In order to constrain the formation ages of basement. The basement metamorphic rocks in the Songliao Basin mainly consist of metagabbro (L45-1), amphibolite (SN117), metarhyolitical tuff (G190), sericite (Ser) schist (N103), chlorite (ChI) schist (T5-1), blotite (Bi)-actlnolite (Act)-quartz (Q) schist (Y205), and metagranite (L44-1). The cathodoluminesence (CL) Images of the zircons from metagabbro (L45-1) and metagranite (L44-1) Indicate that they have cores of magmatic origin and rims of metamorphic overgrowths. Their U-Pb isotopic ages are 1808±21 Ma and 1873±13 Ma, respectively. The zircons with oscillatory zoning from amphibolite (SN117) and Chi schist (T5-1), being similar to those of mafic igneous rocks, yield ages of 274 ± 3.4 Ma and 264 ± 3.2 Ma, respectively. The zircons from metarhyolitical tuff (G190) and Ser schist (N103) display typical magmatic growth zoning and yield ages of 424 ± 4.5 Ma and 287 ± 5.1 Ma, respectively. Most of zircons from Bi-Act-Q schist (Y205) are round in shape and different in absorption degree in the CL Images, Implying their sedimentary detritals. U-Pb dating yield concordant ages of 427 ± 3.1 Ma, 455 ± 12 Ma, 696 ± 13 Ma, 1384±62 Ma, 1649±36 Ma, 1778±18 Ma, 2450±9 Ma, 2579±10 Ma, 2793±4 Ma and 2953±14 Ma. The above-mentioned results indicate that the Precambrian crystalline basement (1808-1873 Ma) exists in the southern Songliao Basin and could be related to tectonic thrust, and that the Early Paleozoic (424-490 Ma) and Late Paleozoic magmatisms (264-292 Ma) also occur in the basin basement, which are consistent with the ages of the detrital zircons from Bi-Act-Q schist in the basement. [ABSTRACT FROM AUTHOR]

In the western part of the Mt Isa Inlier one of the most obvious lithostratigraphic correlations is the Torpedo Creek Quartzite (Lawn Hill Platform) and the Warrina Park Quartzite (Leichhardt River Fault Trough). These two sand bodies form the basal lithostratigraphic unit of their respective groups – McNamara and Mt Isa – and crop out as easily mappable, prominent ridges of white quartzite. Correlation of these sand bodies forms the linchpins for lithostratigraphic subdivision in the region. To test this correlation a 2000&ThickSpace;m-thick stratigraphic interval around these two quartzite units was analysed using two modern techniques. A sequence stratigraphic analysis, which included detailed surface gamma-ray logging, was undertaken at 20 measured sections over an area of about 18 000&ThickSpace;km 2 to identify and correlate important stratal surfaces. Complementary high precision U–Pb SHRIMP dating of tuffaceous intervals was also done to provide independent control for the resulting new subdivisions of the section. The sequence analysis and SHRIMP dating of these units and the underlying Surprise Creek Formation demonstrate major miscorrelations in the current lithostratigraphy. SHRIMP dating helps to define a major unconformity with up to 20 million years of missing rock record: rocks above the surface are around 1670–1660 Ma, whilst those below are around 1690 Ma. Although 1:100 000 lithostratigraphic mapping enabled the recognition of an unconformity at the base of the Mt Isa and McNamara Groups, in the majority of cases this surface does not correlate with the major lithostratigraphic boundary with which it was associated. In consequence this unconformity and supersequence boundary remains unrecognised in most areas. The consequences of such miscorrelations are inadequate and inaccurate reconstructions of basin geometry and stratigraphic architecture, and have important economic implications when searching for stratabound base metals. [ABSTRACT FROM AUTHOR]

The Keraf Shear Zone (KSZ) of NE Sudan formed in Neoproterozoic time when juvenile crust of the Arabian–Nubian Shield collided with reworked older crust of the Saharan Metacraton. Orthogneisses and deformed granitoids which intrude carbonate sediments are calc-alkaline, medium-K, metaluminous and I-type diorites and granodiorites, with subduction-related trace element signatures. 207Pb/206Pb zircon single grain ages are ∼730 Ma for the high-grade gneiss and ∼710 Ma for granitoids. K/Ar ages are 660 Ma for hornblende from the granitoids and 560 Ma for biotite from the high-grade gneiss and the plutons. The geochronologic data indicate that the crust formed ∼730–710 Ma and that orogenic activity ended by ∼565 Ma. Cooling occurred over much of NE Africa at about the same time, possibly caused by orogenic collapse or unroofing caused by extensive glaciation. Pb isotopic ratios indicate an oceanic source for both the magmatic and metamorphic rocks in the KSZ. Sm/Nd model ages are 620–770 Ma for the post-tectonic plutons and 830–900 Ma for the gneiss, indicating negligible involvement of a pre-Neoproterozoic material in the evolution of Keraf igneous rocks. The (87Sr/86Sr)i values range from 0.7026 to 0.7046, which are indistinguishable from initial ratios for juvenile crust of the Arabian Nubian Shield and are lower than initial Sr isotopic compositions for the Saharan Metacraton. The isotopic data indicate that Keraf igneous rocks are juvenile additions to the crust and that the boundary between the Saharan Metacraton and the Arabian–Nubian Shield lies to the west. [Copyright &y& Elsevier]

Different isotopic geochronometers have been used to study the nature of the mid-Paleozoic tectonothermal event in the North Qinling arc system at the southern margin of the North China Block. 40Ar/39Ar age of amphibole from sample QH-3 (426±2 Ma), Rb–Sr mineral isochron age (411±5 Ma) of sample QSP-10 of the Danfeng Group and Rb–Sr whole rock isochron age (406±22 Ma) of a greenschist facies pillow lava sample of the Erlangping Group indicate a widespread Late Silurian tectonothermal event in the North Qinling area. The data also suggest a limited uplift in the North Qinling during the Late Silurian tectonothermal event. This event is restricted to the North Qinling area and the southern margin of North China Block, and not observed in the South Qinling belt and the northern margin of the South China Block. Consequently, it is related to the collision between the North China Block and a North Qinling arc system, which resulted in the closure of the North Qinling back-arc basin(s). [Copyright &y& Elsevier]

Enveloped by orogenic belts during the assembly of Laurentia (∼2.0–1.7 Ga), the Rae and Hearne domains of the western Churchill Province experienced a complex sequence of tectonic events historically referred to as the “Hudsonian orogeny”. In Hearne domain, diverse thick-skinned intraplate structures developed in response to accretionary processes in Trans-Hudson orogen. These structures include infolds of Archean basement and Paleoproterozoic cover, and broken foreland thrusts. In central Hearne domain, an intracratonic basin succession of continental to shallow-marine strata (Hurwitz Group; <2.45–<1.91 Ga) is exposed in outliers that constitute the keels of doubly plunging basement-cover synclinoria, defining a fold train with wavelengths of 35–45 km. With the exception of local out-of-syncline shear zones, basement and Hurwitz Group are welded. Formed under greenschist facies conditions, the basement-cover infolds are open, concentric, and commonly display complex structures in their interiors. In the Poorfish–Windy outlier, southwestern Hearne domain, continental rift deposits of the Kiyuk Group unconformably overlie the Hurwitz Group. In contrast to basement-cover infolding to the north and east, Archean basement, the Hurwitz Group and the Kiyuk Group are imbricated by thick-skinned northwest-vergent thrusts and folds. Thrusting in the Poorfish–Windy belt was completed by 1815±10 Ma, as indicated by new U–Pb data from hydrothermal–metamorphic titanite recovered from recrystallized cements at the top of the Kiyuk Group. We interpret that shortening from shallow-level basement-cover infolding is balanced at depth by intrabasement faulting and distributed ductile strain, and that the geometric link between central Hearne basement-cover infolding and Poorfish–Windy thrusting is via intrabasement faults that cut to the surface. The main controls on thick-skinned deformation were: the high competency of the lower Hurwitz Group; previous thermal weakening; basement heterogeneities; and possibly, shallow subduction preceding terminal collision in Trans-Hudson orogen. [Copyright &y& Elsevier]

The paper documents a late Palaeoproterozoic episode of metamorphism and anatexis in the Suomuja¨rvi Complex, an Archaean enclave in the Palaeoproterozoic Central Lapland Granitoid Complex of northern Finland. U–Pb ages were obtained from monazite and titanite in four Archaean granitoid rocks, one quartzite and a massive granite. The results show that growth of monazite and titanite in these rocks spanned a period of about 15 million years between 1780 and 1765 Ma, presumably reflecting complex processes of metamorphic (re-) crystallisation, invasion by granitic dykes, veins and fluids and post-peak-metamorphic retrogression. The timing of this activity corresponds to post-orogenic granitoid magmatism throughout the Baltic Shield. In this region, between the Lapland-Kola and that of Svecofennian orogens, it may represent a retarded response to their coeval, ca. 1.9 Ga convergent collision. The results support a proposed affinity of the Suomuja¨rvi Complex with the Belomorian rather than the Archaean Karelian Province. [Copyright &y& Elsevier]

The amphibolite facies Moyar, Bhavani and Palghat shear zones separate the South Indian Dharwar Craton (DC) from the Nilgiri and Madurai high-grade crustal provinces. We present Sm–Nd and Rb–Sr isotopic whole rock, mineral and small slab data for remnant granulites, mylonitic gneisses and granitoids that reveal a Neoproterozoic tectonothermal imprint in these shear zones. Sm–Nd garnet crystallisation ages and Rb–Sr mica cooling ages from reworked rocks suggest that Pan-African structurally-controlled retrogression and successive cooling in the Moyar shear zone (MSZ) (garnet 624–591 Ma; muscovite 594 Ma; biotite 604–540 Ma) predates tectonometamorphism in the Bhavani shear zone (BSZ) (garnet 552 Ma; biotite 521–491 Ma) and Palghat shear zone (PSZ) (garnet 521 Ma; biotite 488–485 Ma). A Rb–Sr small slab isochron age of ∼624 Ma indicates that Pan-African retrogression in the MSZ also led to isotopic reequilibration of the Rb–Sr system on a cm-scale. A Sm–Nd garnet crystallisation age of 513±5 Ma reflects emplacement of a postdeformative pegmatite in the BSZ during the late Cambrian. A Sm–Nd garnet age of 2355±18 Ma obtained from a granulite remnant in the MSZ records early Palaeoproterozoic high-grade metamorphism, which may be related to the amalgamation of the Nilgiri Block and DC along the palaeo-Moyar Zone. A relict charnockite from the southern BSZ yielded a reproducible Sm–Nd garnet age of 1705±11 Ma, presumably recording late Palaeoproterozoic metamorphism. Neoarchaean and Palaeoproterozoic Nd model ages (3.0–2.2 Ga) suggest that the shear zone rocks are mainly reworked material from the adjacent Nilgiri and Madurai crustal domains. Nd model ages of granites and tonalites, which intruded the MSZ syntectonically, provide evidence for ≤1.7 Ga crust generation. Unsheared metasediments, exposed between the Bhavani and Palghat high-strain zones, were subjected to Pan-African high-grade metamorphism, as constrained by Sm–Nd age data of garnets grown during coaxial deformation at ∼610–560 Ma. Sm–Nd whole rock data from these supracrustal rocks indicate average crustal residence ages of ∼1.9 Ga, which implies contributions from an unknown Proterozoic crustal source and disproves their previously suggested correlation to the Archaean Sargur Group. These model ages provide an upper limit for the juxtaposition of DC and Madurai Block along the BSZ and PSZ. [Copyright &y& Elsevier]

The age of the Ashburton Province, comprising an older divergent-margin megasequence and a younger convergent-margin megasequence, is poorly constrained. The Boolaloo Granodiorite, which intruded the divergent-margin megasequence on the western margin of the Ashburton Province, has given a SHRIMP zircon U–Pb age of 1786 ± 5 Ma, and therefore post-dates convergent-margin, backarc basin sequences, with established conventional zircon U–Pb ages of ca 1843–1828 Ma. However, it pre-dated deformation of convergent-margin, remnant-ocean sequences. Similarly aged (ca 1797–1791 Ma) granitoids are present in the adjacent Gascoyne Province, thereby identifying a magmatic fold-thrust belt that abutted a remnant ocean in the Ashburton Province. [ABSTRACT FROM AUTHOR]

Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures < 850 °C should be interpreted as the time of titanite (re)crystallization, not cooling ages. The length scales of Zr diffusion are negligible (< 20 µm), even at T > 900 °C; thus, Zr-in-titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments. [ABSTRACT FROM AUTHOR]

We present coupled textural observations and trace element and geochronological data from metasomatic monazite and zircon, to constrain the timing of high-grade Na-metasomatism (albitization) of an Archean orthogneiss in southwest Montana, USA. Field, mineral textures, and geochemical evidence indicate albitization occurred as a rind along the margin of a ~3.2 Ga granodioritic orthogneiss (Pl + Hbl + Kfs + Qz + Bt + Zrn) exposed in the Northern Madison range. The metasomatic product is a weakly deformed albitite (Ab + Bt + OAm + Zrn + Mnz + Ap + Rt). Orthoamphibole and biotite grew synkinematically with the regional foliation fabric, which developed during metamorphism that locally peaked at upper amphibolite-facies during the 1800–1710 Ma Big Sky orogeny. Metasomatism resulted in an increase in Na, a decrease in Ca, K, Ba, Fe, and Sr, a complete transformation of plagioclase and K-feldspar into albite, and loss of quartz. In situ geochronology on zoned monazite and zircon indicate growth by dissolution–precipitation in both phases at ~1750–1735 Ma. Trace element geochemistry of rim domains in these phases are best explained by dissolution–reprecipitation in equilibrium with Na-rich fluid. Together, these data temporally and mechanistically link metasomatism with high-grade tectonism and prograde metamorphism during the Big Sky orogeny. [ABSTRACT FROM AUTHOR]

Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Kar glacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. [ABSTRACT FROM AUTHOR]