Ar Ages Research Articles

Combined geochemical and geochronological analyses of stone artefacts provide unambiguous evidence of intra- and inter-island interactions in Polynesia

Stone tool geochemistry provides hard evidence for post-settlement voyaging and inter-island contacts in East Polynesia. This offers promising information on inter-community exchange networks that are not documented by ethnographic accounts. Drawing on “geochemical fingerprints”, provenance studies have slowly integrated geological data in order to better constrain the origin of archaeological material. However, the occurrence of overlapping geochemical signatures within Polynesian islands can cause difficulties when attempting to differentiate sources. Identifying clear geochemical characteristics for each island is therefore critical for a comprehensive understanding of the interaction patterns in East Polynesian societies. In this paper, we combine major and trace element data with isotopes as well as 40Ar-39Ar ages to constrain the local procurement networks that were occurring on the island of Tupua'i (Austral-Cook chain). We also demonstrate that some Tupua'i archaeological material originated from Eiao Island (Marquesas chain) circa 2000km north. This represents the first hard evidence for inter-island connections between the Austral and the Marquesas Archipelagos.

A relic slice of archean–early Paleoproterozoic basement of Jiaobei Terrane identified within the Sulu UHP belt: Evidence from protolith and metamorphic ages from meta-mafic rocks, TTG–granitic gneisses, and metasedimentary rocks in the Haiyangsuo region

A great quantity of metamorphic rocks (Haiyangsuo Complex), including high-pressure (HP) mafic granulites with ‘red-eye-circle’ textures, retrogressive products of Grt-, Qtz-, Ep-, and Cpx-bearing amphibolites, TTG–granitic gneisses, and metasedimentary rocks, is distributed extensively within the Haiyangsuo region, northeastern Sulu ultrahigh-pressure (UHP) belt. The Haiyangsuo Complex crops out over an area of at least 70km2, rather than the 15-km2 area mentioned in previous studies. Based on integrated studies of petrography, mineral phase transformations, mineral inclusions, cathodoluminescence (CL) images of zircons, and zircon U–Pb LA–ICP–MS and amphibole Ar–Ar age dates, together with previous P–T condition estimates, abundant Neoarchean to Early Paleoproterozoic igneous protoliths, including ca. 2.6–2.5Ga meta-mafic rocks and ca. 2.7Ga, ca. 2.55–2.5Ga, and ca. 2.4Ga TTG–granitic gneisses, were identified in the Haiyangsuo region, whereas the Late Paleoproterozoic (∼1.7Ga) and Neoproterozoic (ca. 800–650 Ma) protolith ages common in the Sulu HP–UHP rocks have not been observed. The metasedimentary rocks in the region studied have a deposition age of ca. 2.1–1.9Ga, showing protolith ages similar to those of the voluminous Khondalite series of the Jingshan and Fenzishan groups from the Jiaobei Terrane, southwestern Paleoproterozoic Jiao–Liao–Ji orogenic belt.Two discrete episodic HP granulite–facies metamorphic events were identified for the first time in the meta-mafic rocks of the region and their country rocks. The first episodic HP granulite-facies metamorphism (M1) is characterized by the relict coarse-grained mineral assemblage of garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)±quartz (Qtz), which formed under P–T conditions of 740–850°C and 1.0–1.26GPa. In contrast, the second HP granulite-facies metamorphism (M21) is characterized by ‘red-eye-circle’ textures, with a typical fine-grained mineral assemblage of Grt+Cpx+Pl±Qtz surrounding the relict coarse-grained HP granulite-facies minerals of M1. It formed under P–T conditions of 765–845°C and 1.14–1.75GPa. Both of the HP granulite-facies metamorphic events were overprinted and replaced by a late amphibolite-facies retrogression (M22), which is typified by a fine-grained mineral assemblage of Amp+Pl+Grt and formed under P–T conditions of 580–660°C and 0.6–0.8GPa. Abundant metamorphic zircon U–Pb dating revealed that the two discrete episodic HP granulite-facies metamorphic events in the Haiyangsuo Complex occurred and recorded in metamorphic zircon domains with HP granulite-facies mineral inclusions at Late Paleoproterozoic (ca. 1.87–1.86Ga) and Middle Triassic (ca. 245–225Ma), respectively, and that each was overprinted by a subsequent amphibolite-facies retrogression at∼1.84Ga and 220–214Ma, respectively. Various metamorphic rocks (Haiyangsuo Complex) in the Haiyangsuo region have rock association, petrographical, geochronological, and P–T evolution affinities to the Paleoproterozoic Jiaobei Terrane, SW Jiao–Liao–Ji Belt, but are distinguished from the Sulu UHP rocks. Hence, the Haiyangsuo Complex is exotic to the Sulu UHP belt and can be regarded as an inseparable part of the Jiaobei Terrane during the Paleoproterozoic. The unique Middle Triassic HP granulite-facies metamorphic event first identified within the Haiyangsuo Complex coincides with that (245–225Ma) of the adjacent Jiaodong Terrane, NE Sulu UHP belt, indicating that the Haiyangsuo Complex was also involved in Triassic collision–subduction and experienced HP granulite-facies metamorphism at a lower crustal level. The ultimate formation of the Haiyangsuo Complex along the southeastern margin of the Jiaobei Terrane, SW Jiao−Liao−Ji orogenic belt, was genetically related to the collision between the North China and South China cratons. Thus, the juxtaposition of the Haiyangsuo Complex to the Sulu UHP metamorphic belt took place in the Middle–Late Triassic rather than in the Jurassic.

Using palaeomagnetic and isotopic data to investigate late to post-Caledonian tectonothermal processes within the Western Terrane of Svalbard

The analytical results of a total of 205 metabasic specimens from 10 palaeomagnetic sites collected from Oscar II Land in Western Spitsbergen are presented. Petrographic, structural and palaeomagnetic data all demonstrate that the pre-Caledonian ferromagnetic fabric of the metabasic rocks has been extensively reoriented and intensively remineralized. New in situ laser ablation inductively coupled plasma mass spectrometry 40 Ar/ 39 Ar age determinations suggest that the host rocks have been subject to three resetting events during the 426 – 380 Ma (Caledonian sensu lato ), 377 – 326 Ma and c. 300 Ma intervals. The latter two resetting events coincide in time with the Barents Shelf-wide rift-controlled subsidence events. The derived palaeomagnetic data do not fall on the expected apparent polar wander path of Laurussia for syn- to post-Caledonian time. Consequently, four models invoking palaeogeographical great and small circle rotations, regional tectonism involving thrusting and normal listric faulting have been investigated to account for this lack of correspondence. The palaeomagnetic data do not lend support to reconstructions linking Western Svalbard with Pearya but point instead to the importance of listric faulting related to the opening of the North Atlantic Ocean that modified the geometry of the West Spitsbergen Fold and Thrust Belt. Supplementary material: (1) Field characteristics of metabasic sites, (2) detailed description of applied rock magnetic and palaeomagnetic procedures, (3) microscopic images of investigated geochronological samples, (4) in situ LA-ICP-MS 40 Ar/ 39 Ar isotopic age determination results, (5) microscopic, SEM and BSE images of investigated metabasites, and (6) anisotropy of magnetic susceptibility are available at https://doi.org/10.6084/m9.figshare.c.3673924 .

Synthesis of the 780–740 Ma Chuar, Uinta Mountain, and Pahrump (ChUMP) groups, western USA: Implications for Laurentia-wide cratonic marine basins

The upper Tonian Chuar, Uinta Mountain, and middle Pahrump (ChUMP) groups of present-day western Laurentia collectively record the early breakup of Rodinia, large-scale perturbations in the carbon cycle, and eukaryotic evolution, all of which preceded the onset of global glaciation by tens of millions of years. The spectacularly preserved and shale-rich Chuar Group of the Grand Canyon Supergroup stands out as one of the best global records of this time period, particularly for paleobiology. A new U-Pb age of 782 Ma on detrital zircons ( n = 14 young grains) from the underlying Nankoweap Formation refines the Chuar Group’s maximum depositional age to younger than 782 Ma. A new 40 Ar/ 39 Ar age of 764 ± 16 Ma (2σ) from K-feldspar within early diagenetic marcasite nodules from the upper Chuar Group (Awatubi Member) helps calibrate the rich Chuar microfossil record and constrain the large-magnitude shift in δ 13 C org (up to 18‰; referred to here as the Awatubi positive carbon-isotope excursion or APCIE) to between ca. 764 and ca. 742 Ma, the date of an ash near the top of the Chuar Group. In addition to the maximum depositional age of ca. 782 Ma, U-Pb detrital zircon analyses ( n = 826 grains) on sandstone beds from the underlying Nankoweap Formation indicate the presence of multiple older Laurentian age peaks. The similarity of detrital zircon populations and sedimentary character to that of the overlying Chuar Group ( n = 764 grains) suggests that the Nankoweap Formation should be included as the lowermost unit in the Chuar Group. This revised geochronological framework indicates a 300 Ma unconformity between the Chuar Group (including the Nankoweap Formation) and the underlying 1.1 Ga Cardenas Basalt of the Unkar Group. Chuar Group detrital zircon populations share similarities with those of the Uinta Mountain Group and especially the middle Pahrump Group, including ca. 780 Ma grains. Biostratigraphic correlation using microfossils enhances the ChUMP connection and shows a trend of higher acritarch diversity in the lower Chuar and Uinta Mountain groups, and the presence of vase-shaped microfossils in the upper intervals of all three ChUMP units. Comparisons of δ 13 C org and δ 13 C carb among ChUMP successions suggest a combination of local and regional controls. Thus, ChUMP successions are coeval within the 780−740 Ma range, show similar fossil and C-isotope trends, and derived sediments from similar Laurentian sources or source types. In light of recent age constraints and compiled paleontology in other Neoproterozoic basins, our high-resolution correlation of ChUMP successions can be extended to the Callison Lake dolostone of NW Canada and the Akademikerbreen-Polarisbreen groups of Svalbard. Biostratigraphic correlation with poorly age-constrained strata such as the Akademikerbreen-Polarisbreen groups and, farther afield, the Visingso Group of Baltica suggests that ChUMP units record continentwide—and perhaps global—evolutionary patterns. The δ 13 C org and δ 13 C carb values in the Chuar Group and its equivalents in Canada and Svalbard show broadly similar trends, including the APCIE, suggesting that δ 13 C org values from organic-rich shale record variations in the C-isotope composition of late Tonian oceans. Intracratonic basins and contiguous rift margins of ChUMP age are inferred to have been important locations for microbial productivity and significant organic carbon burial that induced large positive shifts in δ 13 C and changes in global carbon balance prior to the onset of snowball Earth.

Orogen transplant: Taconic–Caledonian arc magmatism in the central Brooks Range of Alaska

The Doonerak fenster of the central Brooks Range, Alaska, exposes a unique package of Cambrian−Devonian(?) volcanic and sedimentary basement rocks (Apoon assemblage) within the Mesozoic−Cenozoic Brookian fold-and-thrust belt. Recognition of a major pre-Mississippian unconformity within the fenster led to previous correlations between the Apoon assemblage and age-equivalent strata in the North Slope subterrane of the composite Arctic Alaska−Chukotka microplate. Previous age constraints on the Apoon assemblage are limited to a handful of Cambrian−Devonian paleontological collections and hornblende K-Ar and 40 Ar/ 39 Ar ages from mafic dikes ranging from ca. 520 to 380 Ma. We conducted U-Pb and Hf isotopic analyses on igneous and sedimentary units in the Apoon assemblage to test links with the North Slope subterrane and assess the tectonic and paleogeographic setting of this early Paleozoic arc complex. Igneous zircon from a leucogabbro in the Apoon assemblage analyzed using a sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) provided a 207 Pb-corrected 206 Pb/ 238 U age of 462 ± 8 Ma (2σ). Detrital zircon analyzed using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) from volcaniclastic and tuffaceous strata of the Apoon assemblage yielded a spectrum of unimodal and polymodal age populations, including prominent age groups at ca. 490−420, 540−520 Ma, 1250−960, 1500−1380, 1945−1750, and 2830−2650 Ma. Lu-Hf isotopic data from the ca. 490−420 Ma age population, including the Middle Ordovician leucogabbro, are highly juvenile (e Hf ∼+7−10), implying a distinct lack of crustal assimilation during Ordovician−Silurian arc magmatism. In context, these data suggest that a juvenile arc complex, herein referred to as the Doonerak arc, marks a prominent tectonic boundary between pre-Mississippian crustal fragments of Laurentian- and non-Laurentian affinity in the Arctic Alaska−Chukotka microplate. The U-Pb geochronologic and Hf isotopic data also provide a connection to Taconic−Caledonian arc magmatism along the edge of the Iapetus Ocean and linkages between the Apoon assemblage and the Descon Formation and plutonic equivalents of the Alexander terrane (Prince of Wales Island, Alaska).

Early Mesozoic intracontinental orogeny and stress transmission in South China: evidence from Triassic peraluminous granites

The Early Mesozoic tectonic and magmatic events in South China are considered to represent intracontinental orogeny, although the mechanism and geodynamic setting remain debated. Here we report zircon U–Pb geochronology, Hf isotopes and whole-rock geochemistry from four granitic plutons in the Nanling and Yunkai domains. The zircon data define seven groups of weighted mean 206 Pb/ 238 U ages as 218 ± 2, 222 ± 4, 225 ± 3, 227 ± 3, 221 ± 2, 224 ± 3 and 231 ± 3 Ma, representing the magmatic crystallization ages. These intrusions are characterized by a peraluminous nature, with high A/CNK values (>1.1), negative Eu, Ba, Sr, Nb and Ti anomalies, and enrichment in LREE, Rb, Th, U and Pb. Zircon grains from all the samples show variably negative ε Hf ( t ) values (−3.8 to −18.6) with two-stage Hf model ages clustered in the range 1.4–2.5 Ga. Our data indicate that the Early Mesozoic granitic magmas were generated by the partial melting of crustal rocks with insignificant input of mantle materials. In conjunction with other geological evidence, we suggest that Triassic intracontinental deformation was triggered by far-field stress propagation from the plate boundary of South China. Supplementary materials: U–Pb geochronological data, zircon Lu–Hf isotopic compositions, major and trace element data, and 40 Ar/ 39 Ar ages are available at https://doi.org/10.6084/m9.figshare.c.3672838 .

Late Pliocene High-Sulfidation Epithermal Gold Mineralization at the La Bodega and La Mascota Deposits, Northeastern Cordillera of Colombia

La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies. Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitic alteration with epidote, chlorite, calcite, specularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40 Ar/ 39 Ar geochronology on hydrothermal muscovite (i.e., sericite) to ~3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40 Ar/ 39 Ar ages constrain epithermal mineralization to between ~2.6 and ~1.6 Ma. Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor-rich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from ~143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite δ 34 S ranges from −16.9 to −11.3‰ at La Mascota and −8.3 to −6.1‰ at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on δ 18 O and δ D data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water.

Extension rates across the northern Shanxi Grabens, China, from Quaternary geology, seismicity and geodesy

Discrepancies between geological, seismic and geodetic rates of strain can indicate that rates of crustal deformation, and hence seismic hazard, are varying through time. Previous studies in the northern Shanxi Grabens, at the northeastern corner of the Ordos Plateau in northern China, have found extension rates of anywhere between 0 and 6 mm/a at an azimuth of between 95 and 180 °. In this paper we determine extension rates across the northern Shanxi Grabens from: offset geomorphological features and a variety of Quaternary dating techniques (including new IRSL and Ar-Ar ages); a Kostrov summation using a 700 year catalogue of historical earthquakes; and recent campaign GPS measurements. We observe good agreement between Quaternary, seismic and geodetic rates of strain, and we find that the northern Shanxi Grabens are extending at around 1 to 2 mm/a at an azimuth of ≈ 151 °. The azimuth of extension is particularly well constrained and can be reliably inferred from catalogues of small earthquakes. We do not find evidence for any substantial variations in extension rate through time, though there is a notable seismic moment rate deficit since 1750. This deficit could indicate complex fault interactions across large regions, aseismic accommodation of deformation, or that we are quite late in the earthquake cycle with the potential for larger earthquakes in the relatively near future.

A tale of clusters: No resolvable periodicity in the terrestrial impact cratering record

Rampino & Caldeira (2015) carry out a circular spectral analysis (CSA) of the terrestrial impact cratering record over the past 260 million years (Ma), and suggest a ~26 Ma periodicity of impact events. For some of the impacts in that analysis, new accurate and high-precision ("robust"; 2SE<2%) 40Ar-39Ar ages have recently been published, resulting in significant age shifts. In a CSA of the updated impact age list, the periodicity is strongly reduced. In a CSA of a list containing only impacts with robust ages, we find no significant periodicity for the last 500 Ma. We show that if we relax the assumption of a fully periodic impact record, assuming it to be a mix of a periodic and a random component instead, we should have found a periodic component if it contributes more than ~80% of the impacts in the last 260 Ma. The difference between our CSA and the one by Rampino & Caldeira (2015) originates in a subset of "clustered" impacts (i.e., with overlapping ages). The ~26 Ma periodicity seemingly carried by these clusters alone is strongly significant if tested against a random distribution of ages, but this significance disappears if it is tested against a distribution containing (randomly-spaced) clusters. The presence of a few impact age clusters (e.g., from asteroid break-up events) in an otherwise random impact record can thus give rise to false periodicity peaks in a CSA. There is currently no evidence for periodicity in the impact record.

Geochemical signature of paleofluids in microstructures from Main Fault in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

The present study reports on elemental and Sr isotopic analyses of calcite and associated celestite infillings of various microtectonic features collected mostly in the Main Fault of the Opalinus Clay from Mont Terri rock laboratory. Based on a detailed microstructural description of veins, slickensides, scaly clay aggregates and gouges, the geochemical signatures of the infillings were compared to those of the leachates from undeformed Opalinus Clay, and to the calcite from veins crosscutting Hauptrogenstein, Passwang and Staffelegg Formations above and below the Opalinus Clay. Vein calcite and celestite from Main Fault yield identical 87Sr/86Sr ratios that are also close to those recorded in the Opalinus Clay matrix inside the Main Fault, but different from those of the diffuse Opalinus Clay calcite outside the fault. These varied 87Sr/86Sr ratios of the diffuse calcite evidence a lack of interaction among the associated connate waters and the flowing fluids characterized by a homogeneous Sr signature. The 87Sr/86Sr homogeneity at 0.70774 ± 0.00001 (2σ) for the infillings of most microstructures in the Main Fault, as well as of veins from nearby limestone layer and sediments around the Opalinus Clay, claims for an “infinite” homogeneous marine supply, whereas the gouge infillings apparently interacted with a fluid chemically more complex. According to the known regional paleogeographic evolution, two seawater supplies were inferred and documented in the Delemont Basin: either during the Priabonian (38–34 Ma ago) from western Bresse graben, and/or during the Rupelian (34–28 Ma ago) from northern Rhine Graben. The Rupelian seawater that yields a mean 87Sr/86Sr signature significantly higher than those of the microstructural infillings seems not to be the appropriate source. Alternatively, Priabonian seawater yields a mean 87Sr/86Sr ratio precisely matching that of the leachates from diffuse calcite of the Opalinus Clay inside the Main Fault, as well as that of its microstructures and the same features of the sediments above and below. To envision a Priabonian seawater supply, there is a need for its storage without a significant evolution in its Sr isotopic composition until the final deformation of the area. The paleo-hydrogeological context calls for a possible infiltration of the seawater into a limestone karst located above the Opalinus Clay that could have acted as the storage reservoir. The karstic nature of this reservoir also explains why the 87Sr/86Sr of the fluids was not modified significantly until expulsion. An alternative storage could have been provided by the regional faulting system that developed during the contemporary regional rifting of the Rhine Graben. The fluid expulsion started along these extensional faults during the further Upper Eocene–Lower Oligocene rifting phase. Later, the thin-skinned deformation of the Jura Belt affected the Mont Terri region in the form of the Main Fault, probably between approximately 9 and 4 Ma on the basis of preliminary K–Ar ages of nanometer-sized authigenic illite crystals recovered from gouge samples.

High-precision 40Ar/39Ar dating of pleistocene tuffs and temporal anchoring of the Matuyama-Brunhes boundary

Abstract High-precision 40 Ar/ 39 Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) ( 40 Ar/ 39 Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision 40 Ar/ 39 Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale.

Geology, geochemistry and genesis of the Eocene Lailishan Sn deposit in the Sanjiang region, SW China

The Lailishan deposit is an important tin deposit that is genetically associated with an Early Eocene biotite granite in the western Yunnan metallogenic belt in the Sanjiang region, SW China. This study reports new zircon U–Pb ages and Hf isotopic data, whole–rock elements, mica Ar–Ar age and C–H–O–S–Pb isotope for the Lailishan Sn deposit. The mineralization–related biotite granite crystallized during the Early Eocene (50.5Ma), with its zircon εHf(t) values ranging from −11.5 to −7.6 and two–stage Hf model ages (TDM2) ranging from 1.60 to 1.85Ga. The rocks are peraluminous with A/CNK values of 0.99–1.08. The granites display high Si, Al and K contents but low Mg, Fe and Ca contents. The rocks show flat chondrite-normalized REE patterns with strong Eu negative anomalies. These characteristics indicate that the magma originated from a continental crustal source. The hydrothermal muscovite exhibits an Ar–Ar plateau age of 50.4±0.2Ma. The δ18O and δD values of hydrothermal quartz from the deposit range from −7.32‰ to 4.01‰ and from −124.9‰ to −87.1‰, respectively. The δ13CPDB and δ18OSMOW values of calcite range from −11.3‰ to −3.7‰ and from +2.2‰ to +12.7‰, respectively. The sulfur isotopic compositions (δ34SV–CDT) range from +3.3‰ to +8.6‰ for sulfide separates, and the lead isotopic ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb range from 18.668 to 18.746, from 15.710 to 15.743 and from 39.202 to 39.295, respectively. These isotopic compositions are similar to those of magma–derived fluids, indicating that the ore–forming fluids and materials mainly originated from magmatic rocks with some input from meteoric water. This evidence suggests that the tin mineralization is closely linked to the Lailishan I–type granites. In combination with previous data, it is proposed in this study that widespread early Eocene magmatism resulted from the slab breakoff of the subducting Neo-Tethyan slab at ca. 55Ma.

Geology, geochronology, geochemistry and ore genesis of the Wangu gold deposit in northeastern Hunan Province, Jiangnan Orogen, South China

The Wangu gold deposit in northeastern Hunan, South China, is one of many structurally controlled gold deposits in the Jiangnan Orogen. The host rocks (slates of the Lengjiaxi Group) are of Neoproterozoic age, but the area is characterized by a number of Late Jurassic–Cretaceous granites and NE-trending faults. The timing of mineralization, tectonic setting and ore genesis of this deposit and many similar deposits in the Jiangnan Orogen are not well understood. The orebodies in the Wangu deposit include quartz veins and altered slates and breccias, and are controlled by WNW-trending faults. The principal ore minerals are arsenopyrite and pyrite, and the major gangue minerals are quartz and calcite. Alteration is developed around the auriferous veins, including silicification, pyritic, arsenopyritic and carbonate alterations. Field work and thin section observations indicate that the hydrothermal processes related to the Wangu gold mineralization can be divided into five stages: 1) quartz, 2) scheelite–quartz, 3) arsenopyrite–pyrite–quartz, 4) poly-sulfides–quartz, and, 5) quartz–calcite. The Lianyunshan S-type granite, which is in an emplacement contact with the NE-trending Changsha-Pingjiang fracture zone, has a zircon LA-ICPMS U–Pb age of 142±2Ma. The Dayan gold occurrence in the Changsha-Pingjiang fracture zone, which shares similar mineral assemblages with the Wangu deposit, is crosscut by a silicified rock that contains muscovite with a ca. 130Ma 40Ar–39Ar age. The gold mineralization age of the Wangu deposit is thus confined between 142Ma and 130Ma. This age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of NE-trending extensional faults, the emplacement of Late Jurassic–Cretaceous granites and the formation of Cretaceous basins filled with red-bed clastic rocks in northeastern Hunan, which forms part of the Basin and Range-like province in South China. EMPA analysis shows that the average As content in arsenopyrite is 28.7 atom %, and the mineralization temperature of the arsenopyrite–pyrite–quartz stage is estimated to be 245±20°C from arsenopyrite thermometry. The high but variable Au/As molar ratios (>0.02) of pyrite suggest that there are nanoparticles of native Au in the sulfides. An integration of S–Pb–H–O–He–Ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks, possibly driven by hydraulic potential gradient created by reactivation of the WNW-trending faults initially formed in Paleozoic, with possible involvement of magmatic and mantle components channeled through regional fault networks. The Wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic, epithermal and Carlin-type gold deposits, and may be better classified as an “intracontinental reactivation” type as proposed for many other gold deposits in the Jiangnan Orogen.

Appendix Geochronology in Myanmar (1964–2017)

This Appendix summarizes the published isotopic age data for Myanmar up to early 2017. It is intended as a resource for geoscientists to stimulate future investigations. A variety of age dating methods have been applied in Myanmar since the mid-1960s, when Searle & Ba Than Haq (1964) published ages of rocks from the Mogok area data based on their uranium contents. During the 1970s the K–Ar and Rb–Sr methods were applied to date biotite, muscovite and amphibole and aggregates of these minerals (‘whole-rock method’), mainly of granitic rocks. In the K–Ar dating method the original sample was split into two halves. One half of the sample was analysed for K and the second half of the sample for Ar. Ideally the K–Ar isotope data document the closure temperatures when the magmatic minerals stabilized, such as biotite (250–350°C), muscovite (300–400°C) and hornblende (450–500°C), the results being used to date the solidification of an igneous rock. In practice K–Ar age data were often ambiguous due to resetting of the magmatic Ar and K isotope system by later tectonic and metasomatic events. However, it was found that the isotopes of Rb and Sr in magmatic minerals were more stable and not so affected by later events. Dating of intrusion of magma could be derived from the linear plot of the ratios of 87Sr/86Sr v. 87Rb/86Sr from a collection of related igneous rock types from a locality to plot an ‘isochron age’. In addition, the initial 87Sr/86Sr ratios gave an indication of the source of an igneous rock. 87Sr/86Sr ratios less than 0.705 suggested a mantle origin, while higher 87Sr/86Sr ratios indicated crustal contaminations of some nature. The analysis of Rb and Sr isotopes, major oxides and trace elements …

Age constraints on the hydrothermal history of the Prominent Hill iron oxide copper-gold deposit, South Australia

The Mesoproterozoic Prominent Hill iron-oxide copper–gold deposit lies on the fault-bound southern edge of the Mt Woods Domain, Gawler Craton, South Australia. Chalcocite–bornite–chalcopyrite ores occur in a hematitic breccia complex that has similarities to the Olympic Dam deposit, but were emplaced in a shallow water clastic–carbonate package overlying a thick andesite–dacite pile. The sequence has been overturned against the major, steep, east–west, Hangingwall Fault, beyond which lies the clastic to potentially evaporitic Blue Duck Metasediments. Immediately north of the deposit, these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite–calc–silicate skarn ± pyrite–chalcopyrite. The hematitic breccia complex is strongly sericitised and silicified, has a large sericite ± chlorite halo, and was intruded by dykes during and after sericitisation. This paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation. The breccia complex contains fragments of granitoid and porphyry that are found here to be part of the Gawler Range Volcanics/Hiltaba Suite magmatic event at 1600–1570 Ma. This indicates that some breccia formation post-dated granitoid intrusion. Monazite and apatite in Fe-P-REE-albite metasomatised granitoid, paragenetically linked with magnetite skarn formation north of the Hangingwall Fault, grew soon after granitoid intrusion, although the apatite experienced U–Pb–LREE loss during later fluid–mineral interaction; this accounts for its calculated age of 1544 ± 39 Ma. To the south of the fault, within the breccia, 40Ar–39Ar ages yield a minimum age of sericitisation (+Cu+Fe+REE) of dykes and volcanics of ∼1575 Ma, firmly placing Prominent Hill ore formation as part of the Gawler Range Volcanics/Hiltaba Suite magmatic event within the Olympic Cu–Au province of the Gawler Craton.

Post-emplacement kinematics and exhumation history of the Almora klippe of the Kumaun–Garhwal Himalaya, NW India: revealed by fission track thermochronology

Tectonically transported crystalline thrust sheet over the Lesser Himalayan meta-sedimentary zone along the Main Central Thrust (MCT) is represented by Almora, Baijnath, Askot and Chiplakot crystalline klippen. The Almora–Dadeldhura klippe in the Kumaun–Garhwal and western Nepal Himalaya is the witness and largest representative of these crystalline klippen, south of MCT. Here, we investigate the post-emplacement kinematics and exhumation history of the Almora klippe. The newly derived zircon fission track (ZFT) ages combined with published apatite fission track (AFT), 40Ar–39Ar ages from the Almora–Dadeldhura klippe and Ramgarh thrust sheet to quantify the temporal variation in cooling ages and exhumation rates. Using 1-D numerical modelling approach, we calculate the transient exhumation rates with respect to different time intervals. New ZFT cooling ages along ~50-km-long orogeny perpendicular transect across the Almora klippe range between 13.4 ± 0.6 and 21.4 ± 0.9 Ma. Published AFT ages and 40Ar–39Ar ages from the Almora–Dadeldhura klippe range 3.7 ± 0.8–13.2 ± 2.7 and 18.20–25.69 Ma, respectively. AFT ages reported from Ramgarh thrust sheet range 6.3 ± 0.8–7.2 ± 1.0 Ma in Kumaun region and 10.3 ± 0.5–14.4 ± 2.2 Ma in western Nepal. The linear age trend along with youngest ZFT age (~14 Ma) close to the North Almora Thrust (NAT) in its hanging wall suggests rapid uplift close to the NAT due to its reactivation as back thrust. The transient exhumation rates of Almora klippe agree that the erosion rate was rapid (0.58 mma−1) close to the NAT in its hanging wall and relatively slow (0.31 mma−1) close to the SAT in its hanging wall during 15–11 Ma. We interpret that the fission track ages and transient exhumation rate pattern of crystalline klippen show a dynamic coupling between tectonic and erosion processes in the Kumaun–Garhwal Himalaya. However, the tectonic processes play the major role in controlling the exhumation pattern.

Compilation and appraisal of geochronological data from the North Atlantic Igneous Province (NAIP)

Abstract The North Atlantic Igneous Province (NAIP), composed of volcanic sequences and intrusive rocks, occurs onshore in Greenland, the Faeroe Islands, the UK and Ireland, and offshore surrounding these areas as well as the west coast of Norway. Geochronological data have been published for Cenozoic igneous and volcanic rocks for much of the province, and provide valuable information to analyse the evolution of the province and magmatic processes more broadly. As part of the NE Atlantic Geosciences (NAG) cooperation, we examined approximately 700 dates from over 70 published studies and created a comprehensive database to facilitate ready access to this important information. This includes U–Pb, Rb–Sr, Re–Os, 40 Ar/ 39 Ar and K–Ar ages presented relative to the Geological Time Scale 2012. 40 Ar/ 39 Ar and K–Ar ages have been recalculated to a common reference. The complete database includes data that range from approximately 177 to 0.19 Ma. Our evaluation shows that variable sample quality, ambiguous data-handling methods, inadequate data reporting and data interpretation should preclude the use of data for purposes of rigorous geochronological analysis. Through a series of filtering techniques described here, we suggest excluding &gt;500 dates as being of too poor a quality to use in age determinations. Our analysis highlights the need for published geochronological studies to include sufficient information to allow critical assessment of ages and interpretations. We present an ‘optimized’ dataset containing 130 ages that range from approximately 64 to 13 Ma. The filtered dataset emphasizes the need for firm chronological benchmarks and suggests that some sub-provinces in the NAIP would greatly benefit from renewed research attention. Supplementary material: The full NAG-TEC Geochronological Database 001 and Data Evaluation 002 are available at https://doi.org/10.6084/m9.figshare.c.3554472

The Vorontsovskoe Au-Hg-As ore deposit (Northern Urals, Russia): Geological setting, ore mineralogy, geochemistry, geochronology and genetic model

The large Vorontsovskoe Au-Hg-As deposit in the Urals is located in the exocontact of the Early Devonian Auerbah gabbro-diorite-granodiorite massif, which intrudes volcano-sedimentary rocks. The orebodies are confined to a tectonic contact of calcareous and tuffaceous rocks. They are composed of 6 types of disseminated ores, but the main reserves of gold are associated with the following ore types: gold-pyrite-arsenopyrite in altered tuffaceous rocks, pyrite-realgar ores in limestone breccia with a carbonate-volcanogenic cement, and gold-oxide-clay from regolith with residual gold. Early ore associations have been formed at 450–300°C, whereas the late ores have been formed at lower temperature of 260–110°C. We propose a model for the genesis of the Vorontsovskoe deposit based on synchronicity of mineralization with the formation of the Auerbah volcano-plutonic complex. The Ar-Ar age of hydromica from the gold-arsenopyrite association is 391.1±4.9Ma, which coincides with the age of igneous rocks of the Auerbah complex. The main sources of water and carbon dioxide were composed of the fluid derived from the magma chamber and the metamorphic water equilibrated with carbonate sedimentary rocks. Magmatic fluid dominated during the development of skarns, jasperoids and quartz veins, whereas metamorphic water was dominant during quartz-sericite alteration of volcano-sedimentary rocks and calcareous breccias. The bulk of the sulfur was supplied by a deep magma reservoir, however this source prevailed only during skarn ore formation. The mixing between deep-sourced sulfur and sedimentary or biogenic sulfur was established for other ore types. Gold and other ore components were possibly introduced during the volcanic and intrusive activity and also extracted from host sedimentary rocks.

Detrital History of the Lower Pennsylvanian Pottsville Formation In the Cahaba Synclinorium of Alabama, U.S.A.

Abstract: The Greater Black Warrior Basin (GBWB) is an important late Paleozoic depocenter of the southern Appalachian foreland basin containing a thick (> 2.5 km) synorogenic clastic wedge belonging to the Lower Pennsylvanian Pottsville Formation. Pottsville clastics are cyclothemic, consisting of interbedded sandstone, siltstone, claystone, shale, and numerous coal beds, with orthoquartzite sandstone and conglomerate at the base. Basal Pottsville is dominated by quartz-rich sediments; feldspars increase upsection. Presence of heavy minerals, including garnets and rutile, suggest exhumation of deeper sections of orogenic belts for Pottsville sandstones. Compositions of these sandstones (Qt 60 F 7 L 33 ) reflect a “recycled orogenic” provenance. 40 Ar/ 39 Ar analysis of detrital muscovite from the Pottsville Formation yields ages as young as Early Pennsylvanian, that appear to reflect a very brief lag time for the deposition of some of the Pottsville sediment. Prominent detrital-muscovite age modes of ca. 460–450 Ma, 380–370 Ma, and 330–320 Ma are evident among these samples, and are interpreted to represent sediment from catchments containing Taconian, Acadian, and Alleghanian lithologies. We interpret the GBWB orogenic repository to have received detritus via Lower Pennsylvanian drainages of the southern Appalachians, analogous to the modern Ganges–Brahmaputra drainages of the Himalayas. There are dramatic differences in single-crystal 40 Ar/ 39 Ar age modes for muscovite among these samples that likely reflect combinations of changes in transverse and longitudinal sediment supply, Alleghanian tectonics, and progressive development of Appalachian catchments.

Zircon U–Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U–Pb dating of zircon gave an age of 232–237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu–Hf mineral isochron and phengite Ar–Ar ages. The zircon U–Pb and mineral Lu–Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U–Pb and phengite Ar–Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP–UHP metamorphic rocks of oceanic derivation.