Messinian events in the Black Sea

Past hydrological interactions between the Mediterranean Sea and Black Sea are poorly resolved due to complications in establishing a high‐resolution time frame for the Black Sea. We present a new greigite‐based magnetostratigraphic age model for the Mio‐Pliocene deposits of DSDP Hole 380/380A, drilled in the southwestern Black Sea. This age model is complemented by ⁴⁰Ar/³⁹Ar dating of a volcanic ash layer, allowing a direct correlation of Black Sea deposits to the Messinian salinity crisis (MSC) interval of the Mediterranean Sea. Proxy records divide these DSDP deposits into four intervals: (i) Pre‐MSC marine conditions (6.1–6.0 Ma); (ii) highstand, fresh to brackish water conditions (~6.0–5.6 Ma); (iii) lowstand, fresh‐water environment (5.6–5.4 Ma) and (iv) highstand, fresh‐water conditions (5.4–post 5.0 Ma). Our results indicate the Black Sea was a major fresh‐water source during gypsum precipitation in the Mediterranean Sea. The introduction of Lago Mare fauna during the final stage of the MSC coincides with a sea‐level rise in the Black Sea. Across the Mio‐Pliocene boundary, sea‐level and salinity in the Black Sea did not change significantly.     

The Danube River inception: Evidence for a 4 Ma continental‐scale river born from segmented ParaTethys basins

Before reaching the Black Sea, the Danube River passes through a string of Para‐Tethyan (Vienna, Pannonian and western and eastern Dacian) basins. The key question is, when and how did the Danube River become a continental‐scale river with a drainage similar to present? New data presented here show that the Late Miocene deepwater strata in the Black Sea have a significant sediment source and depositional style change at about 4 Ma. However, the presence of active Miocene basins within the Danube catchment raises questions about the timing of Danube River inception and whether the upstream palaeogeography would have allowed or disallowed delivery of large sediment volumes to the deepwater Black Sea. Stratigraphic analyses in the Pannonian and Dacian basins reveal a phase of coeval sedimentary fill of the basins along the Danube at about 4 Ma. This multi‐basin observation points to a concurrent basin‐fill model rather than the basin fill‐and‐spill or Messinian‐type lowstand models previously proposed for Danube inception.     

Formation of the Japan Sea basin: Reassessment from Ar–Ar ages and Nd–Sr isotopic data of basement basalts of the Japan Sea and adjacent regions

Many studies have examined the Japan Sea basalts recovered during Ocean Drilling Program (ODP) Leg127/128. Of these, the ⁴⁰Ar–³⁹Ar dating undertaken is important in constraining the timing of the formation of the Japan Sea; however, the implications of their results do not appear to be fully appreciated by the geological community. In this paper, I reassess the ⁴⁰Ar–³⁹Ar age data of the basalts with reference to Nd–Sr isotopic data. The ⁴⁰Ar–³⁹Ar dating was performed on basalts somewhat enriched in large-ion lithophile elements and recovered from ODP Sites 794, 795 and the lower part of 797, yielding the plateau ages of 21.2–17.7 Ma. These basalts show the Nd–Sr isotopic signature of a moderately depleted mantle source (ε_Nd: 0.6–6.9). In contrast, the basalts from the upper part of Site 797 have yet to be dated due to their low K content, although their Nd isotopic compositions are similar to that of MORB (ε_Nd: 8.4–10.4). By analogy to the secular Nd–Sr isotopic trends reported for Sikhote-Alin and northeast Japan, the age of the upper basalts at Site 797 may be inferred to be younger than the lower basalts, probably around 16 Ma. The Nd–Sr isotopic compositions of the Japan Sea basalts have been interpreted in terms of eastward asthenospheric flow, as have the lavas of the Sikhote-Alin and northeastern Japan. The timing of volcanic activity in the Japan Sea region (i.e., from 21.2 to 14.86 Ma) is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the Japanese Islands (i.e., 14.8–4.2 Ma for southwest Japan and 16.5–14.4 Ma for northeast Japan).     

Late Neogene sequence stratigraphic evolution of the Foz do Amazonas Basin,Brazil

The margin of the Foz do Amazonas Basin saw a shift from predominantly carbonate to siliciclastic sedimentation in the early late Miocene. By this time, the Amazon shelf had also been incised by a canyon that allowed direct influx of sediment to the basin floor, thus confirming that the palaeo‐Amazon fan had already initiated by that time (9.5–8.3 Ma). Above this interval, during a prolonged lowstand, Messinian third‐order sequences are preserved only in the incised‐valley fills of the canyon with no equivalent strata on the shelf. Third‐ and fourth‐order sequences younger than Messinian are preserved on the shelf after sea‐level rise above the shelf by the early Pliocene. Sequences younger than 3.8 Ma often show fourth‐order cyclicity with an average duration of 400 ka (larger scale eccentricity cycles) often preserved in high‐sedimentation‐rate areas of river deltas. Mass wasting and transportation of slope sediments to the basin began to play an important role in sediment dispersal at least as far back as the mid‐Pliocene, after rapid progradation had produced steeper slopes more prone to failure.     

Precise 40Ar/39Ar dating of volcanic tuffs within the upper Messinian sequences in the Melilla carbonate complex (NE Morocco): implications for the Messinian Salinity Crisis

The Melilla carbonate complex (NE Morocco) is the only area of the Paleo-Mediterranean Sea where volcanic activity was present throughout most of the Messinian. ⁴⁰Ar/³⁹Ar dating of volcanic tuffs interbedded within the upper Messinian sedimentary deposits, known as the Terminal Carbonate Complex (TCC), yields accurate ages of paleoenvironmental and sea-level changes related to the Messinian Salinity Crisis. The new chronologic data (1) provide an average of 5.95–5.99 Ma for the base of the TCC, thus being synchronous with the onset of the Messinian Salinity Crisis, (2) demonstrate for the first time that the basal unconformity of the TCC does not represent a hiatus of long duration, (3) define a precise time line at 5.87±0.02 Ma (2σ) corresponding to sedimentary rocks exhibiting a lateral transition between continental and marine deposits typical of the TCC and (4) yield evidence that emersion of the Melilla platform during deposition of the TCC is partly related to tectono-magmatic activity. An erosional surface, capping the TCC deposits in the Melilla basin, is related to the major Messinian Mediterranean drawdown. The duration of the hiatus, associated with this surface, is estimated to be at most 450 kyr, but is probably shorter.     

Local high relief at the southern margin of the Andean plateau by 9 Ma: evidence from ignimbritic valley fills and river incision

A valley‐filling ignimbrite re‐exposed through subsequent river incision at the southern margin of the Andean (Puna) plateau preserves pristine geological evidence of pre‐late Miocene palaeotopography in the north western Argentine Andes. Our new ⁴⁰Ar/³⁹Ar dating of the Las Papas Ignimbrites yields a plateau age of 9.24 ± 0.03 Ma, indicating valley‐relief and orographic‐barrier conditions comparable to the present‐day. A later infill of Plio–Pleistocene coarse conglomerates has been linked to wetter conditions, but resulted in no additional net incision of the Las Papas valley, considering that the base of the ignimbrite remains unexposed in the valley bottom. Our observations indicate that at least 550 m of local plateau margin relief (and likely >2 km) existed by 9 Ma at the southern Puna margin, which likely aided the efficiency of the orographic barrier to rainfall along the eastern and south eastern flanks of the Puna and causes aridity in the plateau interior.     

40Ar/39Ar Geochronology of Volcanic and Intrusive Rocks in the Papandayan Metallic Prospect Area,West Java,Indonesia

This study presents new ⁴⁰Ar/³⁹Ar ages on the volcanic and intrusive rocks from the Papandayan metallic district in West Java, Indonesia. The vein system in the Arinem area, one of the prospective areas in the district, has been considered as an epithermal gold–silver–base metal deposit, however, no published age results are available for the host volcanic rocks in the district. The ages of these rocks are critical in terms of their association with mineralization and are important to understand the evolution of volcanism in the region, which has implications for mineral exploration in the district. ⁴⁰Ar/³⁹Ar plateau ages of two typical basalt and one andesite samples of the Jampang Formation volcanic rocks yielded ages of 11.65 ± 0.52 Ma, 18.15 ± 0.46 Ma and 7.69 ± 0.05 Ma, respectively. ⁴⁰Ar/³⁹Ar ages of three intrusive rock samples from Gunung Halang diorite, Gunung Lingga diorite, and Gunung Buligir fine‐grained quartz diorite yielded ages of 12.98 ± 0.20 Ma, 10.81 ± 0.15 Ma, and 7.37 ± 0.05 Ma, respectively. The age of the youngest fine‐grained diorite (Gunung Wayang dike) is 3.95 ± 0.03 Ma. An ⁴⁰Ar/³⁹Ar age obtained from adularia in the Arinem mineralized vein (18.30 ± 0.20 Ma) is older than the age of altered basalt sample of this study (11.65 ± 0.52 Ma) and the K–Ar illite ages of the Arinem vein (9.4 ± 0.3 Ma and 8.8 ± 0.3 Ma) which resulted from a previous study. The age results suggest that the Papandayan district may have experienced multiple hydrothermal and mineralization events. This study, therefore, provides crucial age data to support future mineral exploration in the district.     

Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite facies mica 40Ar/39Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim ⁴⁰Ar/³⁹Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The ⁴⁰Ar/³⁹Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between ⁴⁰Ar/³⁹Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.     

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field,Saudi Arabia: Implications for duration and migration of volcanic activity

New ⁴⁰Ar/³⁹Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma–Recent. The new ⁴⁰Ar/³⁹Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the ⁴⁰Ar/³⁹Ar plateau ages show that volcanism in this area began in the Neogene (∼10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.     

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos,Greece: a record of continuous (re)crystallization during exhumation?

The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, ⁴⁰Ar/³⁹Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (⁴⁰Ar/³⁹Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The ⁴⁰Ar/³⁹Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new ⁴⁰Ar/³⁹Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of ⁴⁰Ar/³⁹Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.     

Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy,Lofoten islands,Norway

The determination of the thermal (temperature–time) histories of high‐P metamorphic terranes has been commonly based on the concepts of slow cooling and closure temperatures. In this paper, we find that this approach cannot reconcile a geochronological data set obtained from the amphibolite‐facies allochthonous Leknes Group of the Lofoten islands, Norway, which reveals an extremely complex thermal history. Using detailed results from several different geochronometers such as ⁴⁰Ar/³⁹Ar, Rb–Sr and U–Pb, we show that a model invoking multiple, short‐lived thermal pulses related to hot‐fluid infiltration channelized by shear zones can reconcile this complicated data set. This model suggests that hot fluids infiltrated throughout basement shear zones and affected the overlying cold allochthon, partially resetting U/Pb rutile and titanite ages, crystallizing new zircon and produced identical ⁴⁰Ar/³⁹Ar and Rb/Sr ages in muscovite, biotite and amphibole in various rocks throughout the region. This paper shows the enormous potential of coupling laser Ar‐spot data with thermal modelling to identify and constrain the duration of short‐lived events. An optimal P–T–t history has been derived by modelling the age data from a previously dated large muscovite crystal (Hames & Andresen, 1996, Geology, 24 :1005) and using Zr‐in‐rutile thermometry which is consistent with all geochronological data and geological constraints from the basement zones and allochthon cover. This tectonothermal model history suggests that there have been three episodic hot‐fluid and ⁴⁰Ar‐free infiltration events, resulting in the total resetting of Ar ages during the Scandian (425 Ma) for 1 Ma at 650°C and two reheating events at 415 Ma for 400 ka at 650°C and at 365 Ma for 50 ka at 600°C, which are modelled as thermal spikes above an ambient temperature of 300°C. Independent confirmation of these parameters was provided by Pb‐diffusion modelling in rutile and titanite. The model suggests that the amphibolite facies rocks of the Leknes Group probably remained cold before being exhumed for at least 60 Ma (425–365 Ma) and successfully explains the presence of different minerals that crystallized or were totally/partially reset in the allochthon and in the basement. The migration of hot fluids for short periods of times within conduits extending through the basement and allochthon rock units is likely associated with episodic seismic activity during the Caledonian orogeny.     

The onset of the Messinian salinity crisis in the deep Eastern Mediterranean basin

Astronomical tuning of the Messinian pre‐salt succession in the Levant Basin allows for the first time the reconstruction of a detailed chronology of the Messinian salinity crisis (MSC) events in deep setting and their correlation with marginal records that supports the CIESM ( 2008 ) 3‐stage model. Our main conclusions are (1) MSC events were synchronous across marginal and deep basins, (2) MSC onset in deep basins occurred at 5.97 Ma, (3) only foraminifera‐barren, evaporite‐free shales accumulated in deep settings between 5.97 and 5.60 Ma, (4) deep evaporites (anhydrite and halite) deposition started later, at 5.60 Ma and (5) new and published ⁸⁷Sr/⁸⁶Sr data indicate that during all stages, evaporites precipitated from the same water body in all the Mediterranean sub‐basins. The wide synchrony of events and ⁸⁷Sr/⁸⁶Sr homogeneity implies inter‐sub‐basin connection during the whole MSC and is not compatible with large sea‐level fall and desiccation of the Mediterranean.     

Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation

New eclogite localities and new ⁴⁰Ar/³⁹Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km²; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. ⁴⁰Ar/³⁹Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New ⁴⁰Ar/³⁹Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. ⁴⁰Ar/³⁹Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr^?1), perhaps because of continued tectonic unroofing.     

40Ar/39Ar geochronological constraints on the age and evolution of the Permo-Triassic Emeishan Volcanic Province,Southwest China

The biostratigraphically constrained Permo-Triassic Emeishan Volcanic Province (EVP), extending over wide areas in southwest China, has been recently considered as a Large Igneous Province contemporaneous with the Siberian Traps and the siliceous tuffs at the P–T boundary in South China. We report the first ⁴⁰Ar/³⁹Ar ages on this igneous province. Minimum ages have been obtained on phenocrystic plagioclase of the Emeishan basalt, which has undergone a pervasive metamorphism, most likely during subsequent tectonization as a consequence of terrane amalgamation. Comparison between the Ar–release spectra obtained on clear vs. cloudy plagioclase indicates a 40–30 Ma sericite resetting time. A minimum apparent age of 246 ± 4 Ma for plagioclase from a plagiogranite, a late-differentiate of the Panzhihua Layered Complex, and an age of 254 ± 5 Ma for phlogopite from a pyroxenite near Lake Erhai, provide the first absolute age constraint on this igneous province. Additional Ar–Ar age measurements on post-Emeishan alkaline and mafic magmatism yielded 104 ± 2 and 100 ± 2 Ma for an alkaline complex near Panzhihua, and 42 ± 1 Ma for a gabbro sill emplaced near the Ertan Dam. Further study is still needed to determine the age of the Emeishan volcanic emission accurately, and to test the validity of the assumed short duration of the eruption.     

Thermochronological record of Middle–Late Jurassic magmatic reheating to Eocene rift-related rapid cooling in the SE South China Block

We investigate the Mesozoic–Cenozoic thermal history of the Daxi region (central SE South China Block) to evaluate the influence of the subduction of the Paleo-Pacific oceanic plate beneath the SE South China Block along the block's southeast margin on the tectonothermal evolution of the upper plate. We apply a multi-chronological approach that includes U-Pb geochronology on zircon, ⁴⁰Ar/³⁹Ar dating on muscovite and biotite from granitic rocks as well as fission-track and (U-Th-Sm)/He analyses on zircon and apatite from granitic and sedimentary rocks. The Heping granite, located in the Daxi region, has a magmatic age of ca. 441 Ma. The biotite ⁴⁰Ar/³⁹Ar ages of ca. 193 Ma for the Early Jurassic Shibei granite and ca. 160 Ma for the Late Jurassic Fogang granite, respectively, reflect magmatic cooling. The Triassic Longyuanba granite yielded a muscovite ⁴⁰Ar/³⁹Ar age of ca. 167 Ma, recording heating to ≥ 350 °C induced by nearby intrusion of Middle Jurassic granites. Zircon fission-track and (U-Th-Sm)/He ages from Lower Carboniferous–Lower Jurassic sandstones (140–70 Ma) record continuous cooling during the Cretaceous that followed extensive Middle–Late Jurassic magmatism in the Daxi region. Cretaceous cooling is related to exhumation in an extensional tectonic setting, consistent with lithospheric rebound due to foundering and rollback of the subducted Paleo-Pacific oceanic plate. Apatite fission-track (53–42 Ma) and (U-Th-Sm)/He ages (43–36 Ma), and thermal modelling document rapid cooling in the Paleocene–Eocene, which temporally coincides with continental rifting in the SE South China Block in the leadup to the opening of the South China Sea.     

Closed system behaviour of argon in osumilite records protracted high‐T metamorphism within the Rogaland–Vest Agder Sector,Norway

Here, we present results of the first ⁴⁰Ar/³⁹Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 10⁸ cm²/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old ⁴⁰Ar/³⁹Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the ⁴⁰Ar/³⁹Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events.     

Diachronic metamorphic and structural evolution of the Connecticut Valley–Gaspé trough,Northern Appalachians

The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite ⁴⁰Ar/³⁹Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D₁ crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D₂ event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D₃ compressional exhumation event also evolved from south to north from 345 to 335 Ma. D₁ to D₃ deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D₁, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.     

Sediment and water discharge rates of Turkish Black Sea rivers before and after hydropower dam construction

Presently, the water discharge rate to the Black Sea by Turkish rivers is approximately 41 km³/yr. The sediment discharge rate of Turkish rivers to the Black Sea is 28×10⁶ t/yr. Before construction of the hydroelectric dams, the sediment discharge rate was approximately 70×10⁶ t/yr. The sharp reduction in sediment load is largely a result of the dams near the mouths of the Yesil Irmak and Kizil Irmak rivers. Before the construction of dams, Turkish rivers contributed approximately one third of the total amount of sediment received by the Black Sea from all surrounding rivers. The life-span of the major reservoirs varies from approximately only one century (Yesil Irmak river reservoirs) to several thousand years (Sakarya river reservoirs). Life-span for the large Altinkaya Dam reservoir is estimated with approximately 500 yr.     

Thermochronology of the PoSen complex,northern Vietnam: Implications for tectonic evolution in SE Asia

The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However, its thermochronological history has not been fully examined yet. Here we applied U–Pb and ⁴⁰Ar/³⁹Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of ⁴⁰Ar/³⁹Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.