Ar Ages Research Articles

Magmatic and metamorphic evolution of the Late Cretaceous Muslim Bagh Ophiolite, western Pakistan: implications for ridge subduction after subduction initiation

ABSTRACT The magmatic and metamorphic evolutions of the Muslim Bagh Ophiolite (MBO) can help understand the relationship among supra-subduction ophiolite, subduction initiation, and the spreading centre in the forearc. Here, we investigated the whole-rock chemistry, mineral composition, P–T conditions, and K–Ar ages of the MBO and its associated units. The whole-rock and magmatic clinopyroxene compositions indicate that the MBO amphibolites and tholeiitic basalts of the underlying Bagh Complex originated in a supra-subduction setting and mid-ocean ridges, respectively. Late Cretaceous oceanic island basalts found in the Bagh Complex and the Bibai Formation occurred in oceans and near the passive continental margin of western India, respectively. Metamorphic Mg-hornblendes in the MBO amphibolites record low-P/T type upper amphibolite-facies metamorphism (2.4–2.9 kbar, 612–690 °C), indicating a > 50 ºC/km geothermal gradient. The K–Ar hornblende and plagioclase ages suggest contemporaneous low-P/T type metamorphism at ca. 78–71 Ma in the MBO formed in a forearc and tholeiitic ridge-related magmatism of ca. 78–77 Ma in the Bagh Complex, indicating a spreading ridge was active during the subduction. Our results, combined with plate kinematic reconstructions of the Neotethys Ocean in the previous studies, suggest that the abnormally high geothermal gradient in the MBO could be due to orthogonal ridge subduction near the triple junction after the ca. 80 Ma subduction initiation in the forearc. It is also likely that the geothermal gradient in the MBO dropped from >50 °C/km at 78–71 Ma, through 35–45 °C/km, to <25 °C/km at approximately 65 Ma due to gradual steepening of the subducting slab, followed by the obduction of the MBO during the Paleocene to Eocene period.

Tectono-stratigraphy, formation and emplacement ages of the Beyşehir-Hoyran Nappes in the south of the Sultan Dağları (Isparta, SW Türkiye)

The Beyşehir-Hoyran Nappes (BHN), in the south of Sultan Dağları, consist of ophiolite, mélange and nappe slices of different lithologies and ages belonging to the oceanic crust and related rocks, which originated from the northern Neotethys and were thrusted southwards over the Taurus Platform. These nappes, namely the Marmaris ophiolite nappe, the Gülbahar nappe and the Domuzdağ nappe, are represented by three tectonically related nappe-slices that extend as a narrow NW-SE trending belt to the south of the Sultan Dağları. The Marmaris ophiolite nappe (Upper Cretaceous) is formed by three subunits namely the Marmaris ophiolite, the Kızılcadağ mélange and the Yenicekale metamorphics. Hornblende minerals from amphibolites of the Yenicekale metamorphic rock unit yielded a 40Ar-39Ar age of 93.9±0.34 Ma (Cenomanian-Turonian boundary). Besides, late Triassic to early Jurassic ages were obtained from the deep marine sedimentary rocks of the Gülbahar nappe based on the radiolarian assemblages. Paleontological and radiometric ages obtained from this study show that the formation of the BHN should began in the Turonian and ended in the Late Maastrichtian. The nappes, on the other hand, were emplaced over southern Sultan Dağları in the early-middle Paleocene and reached their present position as a result of late Eocene movements.

The tectonostratigraphic evolution of a Miocene half-graben, Kerio Valley Basin, East African Rift of Kenya: analysis of a 3 km thick volcano-sedimentary succession

The Kerio Valley Basin is a classic half-graben within the Eastern Branch of the East African Rift System, though to date its tectonostratigraphic evolution has only been interpreted through an incomplete stratigraphic record exposed along structural highs. This study examines a 3 km thick, volcano-sedimentary succession recorded within a hydrocarbon exploration well, Cheptuket-1, located within the hanging wall of the Kerio Valley Basin. A suite of silicic extrusive volcanic and volcaniclastic rocks are also recorded, calibrated by a dense collection of thin-sections from 84 side-wall cores. These volcanic rocks include phonolite lava flows. Through documentation of the well stratigraphy and integrating newly acquired 40 Ar/ 39 Ar age dates from outcrop, we detail a substantially revised, simpler tectonostratigraphic evolution of this basin. Cheptuket-1 penetrated a Middle Miocene to recent stratigraphic succession, bottoming in metamorphic basement, likely through a fault plane. Several phases of lake development are elucidated, including both pre- and syn-mechanical rifting, with the latter phase containing beds exhibiting high organic matter content. This study ultimately represents a rare insight into a near complete rift-basin hanging-wall succession, as well as detailing a workflow for characterising mixed volcanic and sedimentary successions.

&lt;SUP&gt;40&lt;/SUP&gt;Ar/&lt;SUP&gt;39&lt;/SUP&gt;Ar age of thrusting in sedimentary complexes of Chukotka and Wrangel Island (North-East Russia)

&lt;SUP&gt;40&lt;/SUP&gt;Ar/&lt;SUP&gt;39&lt;/SUP&gt;Ar age of thrusting in sedimentary complexes of Chukotka and Wrangel Island (North-East Russia)

Lithology, U[sbnd]Pb zircon geochronology, and geochemistry of the Austroalpine Sieggraben Complex: Pre-Alpine vs. Alpine history

The Sieggraben Complex (SC) in Eastern Austria provides comprehensive information on the pre-Permian, Permian, and Alpine (Cretaceous) evolutional stages of the crustal and mantle sections included into the Austroalpine basement HP/UHP belt. The SC in the Sieggraben–Schwarzenbach area consists of three, top-to-bottom tectono-stratigraphic units: (1) the Paragneiss unit, (2) the Marble–Eclogite unit, which contains MORB and OIB types of mafic rocks, and (3) the Micaschist–Calcschist unit. The N-MORB type mafic rocks yield a UPb age of 369.7 ± 2.0 Ma, have εNd(370) = +8.5 to +7.4 and TDM(2st) of 0.42–0.50 Ga, while the E-MORB types (UPb ages around 400 Ma) show εNd(400) = +9.4 to +5.6 and TDM(2st) of 0.37–0.67 Ga, both indicating rather juvenile mantle sources during the formation of the inferred oceanic Devonian basin. The rocks of these units revealed Cambrian, Proterozoic, and Archean zircon sources. The ages of 285.8 ± 9.2 and 265.7 ± 2.1 Ma from eclogite and amphibolite, respectively, and 265.5 ± 1.4 Ma from paragneiss indicate an important Permian metamorphic overprint. Permian granitic dykes dated from ∼265 to 260 Ma, exceptionally at 273 and 253 Ma, crosscut the lithological units. A clinopyroxenite dyke, which crosscuts harzburgite of the underlying mantle fragment, yielded a magmatic age of 253.3 ± 2.9 Ma. Overall, these ages suggest a strong Permian extension, overheating, and melting of the crustal and mantle rocks. The eclogite ages from ∼100 Ma, but mainly between 92 and 90 Ma, constrain the Late Cretaceous metamorphic event terminated by an 88.4 ± 0.8 Ma pegmatite intrusion in the Paragneiss unit. The Permian and Cretaceous metamorphic events often caused resetting of the relatively older zircon ages, together with a decrease of zircon εHf(t) values, and an increase of whole-rock 87Sr/86Sr values. A Carboniferous metamorphic age of ∼340 Ma was rarely detected. The SC finally represents a part of an early Late Cretaceous intra-continental subduction zone, which formed within a strongly-thinned crystalline basement due to the Devonian and Permian extensions. A similar Cretaceous suture zone with HP amphibolite, dated at ∼130–100 Ma by white mica ArAr ages (100–90 Ma exhumation), has been described around the Lubeník line between the Gemeric and Veporic tectonic units of the Inner Western Carpathians in the framework of the Cenozoic AL–CA–PA (Alpine–Carpathian–Pannonian) microplate.

On the Age-Old Problem of Dating a Granite: Combined Zircon, Apatite, and Titanite Petrochronology in an I-Type Granite from Mt Stirling, Australia

Abstract To address the limitations of current dating methods, it is crucial to not only enhance existing techniques like U–Pb zircon dating but also explore alternative tools. This study focuses on three common mineral phases—zircon, apatite, and titanite—in an I-type granite. The goal is to assess their reliability as dating tools and propose improved methods for dating granitic rocks. In the case study of the Mt Stirling pluton within the Mt Buller igneous suite in Southeastern Australia, significant variability in laser ablation U–Pb zircon ages (around 100 million years) was observed. To improve the reliability of zircon age data and reduce non-magmatic-related variabilities, a data filtering protocol is applied. This protocol involves several steps such as trimming zircons with excessive K and Ca, excluding zircons with unusual core–rim age relationships, removing zircons with excessive non-formula elements (Al, Fe, and Mn), identifying hydrothermally altered zircons, and applying a 10% discordance threshold. The filtered Concordia Age (406 ± 1 Ma; mean square weighted deviation (MSWD) = 0.7, n = 80) of the host rock exhibits improved precision and reduced error compared to the unfiltered data (399 ± 2 Ma; MSWD = 9.3, n = 240). The filtered individual dates show less scatter and a mean that is different (i.e. outside statistical uncertainty), noting that their total still spans a considerable time range of ~50 million years, exceeding the individual zircon analytical reproducibility of 2 standard errors (~15 million years of 2 SE). Caution is advised when using the proposed error for the pooled analyses as a definitive precision. Similarly, trace element filtering approaches were applied to apatite and titanite samples from Mt Stirling, two phases that arguably cannot be inherited. For apatite, monitoring Ca and P as well as Zr/Y and Th/U ratios, along with identifying age groupings based on Sr concentrations, was effective in eliminating outliers and enhancing dating precision. In the case of titanite, monitoring Ca and Ti, Sr/Zr and Sr/Th ratios, and Sr/Ca and Zr/Ti ratios successfully enhanced dating precision. Notably, apatite and titanite grains were grouped in distinct Sr concentrations (high-, mid-, and low-Sr), with these groups corresponding to different date groups: high-Sr apatite and high- and low-Sr titanite returned c. 403 Ma, while low-Sr apatite and mid-Sr titanite returned c. 420 and 393 Ma, respectively. The spuriously younger or older dates may indicate an open system and influences from various common-Pb sources. The 403 Ma date coincides with the filtered zircon data, placing further confidence in the coupled approach, and is interpreted here as the igneous intrusion age. Notable is that this age is 25 Myr older than previously reported K–Ar age data, thus far considered to be the age of the intrusion. This study underscores the potential for erroneous zircon dates due to cryptic chemical influences. To enhance the reliability of age interpretation using laser ablation analyses, employing a petrochronological approach using split-stream combined age and trace element data is recommended in addition to the combination of multiple geochronometers. In the case of Mt Buller, it has proven crucial to carefully verify chemical closure of all applied geochronometers by monitoring concomitant trace element concentrations. Applied to other intrusions, petrochronology can play a critical role in obtaining reliable age information, even for igneous rocks that appear pristine. With this, we emphasise the importance of a careful approach towards individual age data interpretation, which can be produced fast and in abundance with modern analytical approaches.

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Middle Triassic Cu–Pb–Zn Skarn mineralization in the Wulonggou gold ore field, Eastern Kunlun Orogen, NW China: Insights from phlogopite Ar–Ar and zircon U–Pb dating and Sr–Nd–Pb–Hf isotopes

The skarn is one of the most significant Cu–Pb–Zn mineralization worldwide. The timing of the magmatic-hydrothermal activity is of importance for understanding the Cu–Pb–Zn mineralization process. The Cu–Pb–Zn skarn mineralization has been recognized in the northwestern section of the Wulonggou gold ore field, Eastern Kunlun Orogen (EKLO), NW China. However, no precise geochronological data has been measured for the Cu–Pb–Zn mineralization, and the source of ore-forming materials is also not well studied. In this study, we present new LA-ICPMS U–Pb ages and Sr–Nd–Hf isotopic compositions for granodiorite and diorite in the northern section of the Wulonggou gold ore field, as well as an Ar–Ar age for phlogopite and Pb isotopic compositions for sulfides and intrusions, further to constrain the ages of mineralization and magmatism, and the source of ore-forming materials. Two granodiorite samples yield mean ages of 245 ± 2 to 243 ± 2 Ma and a diorite has a mean age of 245 ± 2 Ma. A phlogopite sample, which is associated with Cu–Pb–Zn mineralization, has been chosen to precisely date the formation of Cu–Pb–Zn ores by Ar–Ar method and yields a well–defined plateau age of 240.4 ± 1.2 Ma. These results indicate that the Cu–Pb–Zn skarn mineralization formed in the Middle Triassic and is closely related to the coeval magmatism. The εHf(t) values of the ore-related granodioritic and dioritic rocks range from −2.8 to +2.5, whereas an un-mineralized granodiorite sample (WHFG–1) shows lower εHf(t) values of –9.3 to –3.4, indicating that these magmatic rocks are derived from the mixing of magma with different proportions of mantle- and crust-components. A mixing model with ∼40–70 % mantle contribution involved in the formation of ore-related granodiorite and diorite was revealed by Sr–Nd–Hf isotopic compositions. Combined with Pb isotopic compositions for sulfides, feldspar, and bulk rocks, we consider a common source for the ore-forming materials and magmatism. In addition, higher εHf(t) and εNd(t) values for ore-related magmatism implicate that mantle-derived melts show a vital role in the Cu–Pb–Zn skarn mineralization process.

An oblique transpressive soft-collision records the amalgamation of the Siderian-Orosirian Mineiro belt (Brazil) and the São Francisco paleo-craton (Kenorland Supercontinent): Insights from the Bom Sucesso metasedimentary sequence

The southernmost São Francisco craton carries a long and polycyclic tectonic evolution from the Archean to the Proterozoic. The worldwide characterizations of rocks formed between these Eons, such as those of the Bom Sucesso metasedimentary sequence, is key to understand significant changes on Earth's geodynamic and atmosphere. This sequence is located between two important geotectonic domains in Brazil: the Archean São Francisco paleo-craton (a piece of the Kenorland Supercontinent) and the Siderian-Orosirian Mineiro belt. Although the Bom Sucesso metasedimentary sequence presents a strategic position to investigate the docking between different tectonic domains, longtime tectono-metamorphic episodes challenge the full understanding of the broad tectonic scenario. This work was performed aiming to unravel the links between the tectonic evolution of the Bom Sucesso metasedimentary sequence and their adjoining domains, represented by the Archean São Francisco paleo-craton and the Paleoproterozoic Mineiro belt. The original sedimentation of the Lagoa da Prata unit at the base of the Bom Sucesso metasedimentary sequence, the Bom Sucesso Banded Iron Formation and the Tabuãozinho unit developed in a passive margin setting with different Archean sources associated with nearby orthogneisses and metagranites of the São Francisco paleo-craton. This sequence yields an MDA of 2615 ± 4 Ma and is chronocorrelated to the Moeda Fm. at the base of the Minas Supergroup. After the deposition of the Bom Sucesso metasedimentary sequence in a passive margin setting, the amalgamation of the Paleoproterozoic Mineiro belt to the São Francisco paleo-craton between ca. 2.07 and 2.00 Ga by a soft-collision process produced two distinct compressional events. The first with N–S direction (σ1 = 181/16) originated 2 km-scale thrust faults, associated with a transcurrent movement between the Bom Sucesso metasedimentary sequence and both the adjoining Archean and Paleoproterozoic domains. The same compressional event also developed tectonic foliations under amphibolite facies conditions (2.8–6.3 kbar; 550–632 °C) and imprints a sillimanite-andalusite isograd along the Ibituruna and Bom Sucesso ridges. The second event has a NW-SE direction (σ1 = 333/18) and produced crosscutting structures under a ductile-brittle regime at lower temperatures been considered as retrogressive. A thermal influence of the Brasiliano orogeny in the region is suggested by new LA-ICPMS zircon Concordia age of 623 ± 4 Ma obtained in a garnet schist of the Lagoa da Prata unit, which is in agreement with Neoproterozoic Concordia Lower intercepts (probably Pb-loss) reported in zircon grains, and Ar–Ar ages (biotite) obtained along most of the Mineiro Belt area. This age may imply in a superposition of temporally distinct metamorphic events (Paleoproterozoic and Neoproterozoic) on the rocks of the Bom Sucesso metasedimentary sequence. The structural, metamorphic, and geochronologic data presented here brings new insights on structural and P-T-X conditions of the Paleoproterozoic geotectonic evolution of the southernmost São Francisco craton.

The Kangâmiut dykes in West Greenland: markers of the tectono-metamorphic evolution of the southern Nagssugtoqidian orogen and its foreland

The general extent and structural evolution of the southern Nagssugtoqidian orogen of West Greenland were first described by Hans Ramberg who based much of his paper on the deformation of the regional Kangâmiut dyke swarm. The southern boundary is marked by a transition from undeformed, discordant dykes in the south to highly deformed dykes and host rocks to the north. Our analysis of the southern Nagssugtoqidian orogen and its southern foreland uses a comprehensive compilation of available data and covers the area from Sisimiut in the north to Alanngua, south of Maniitsoq. This represents almost the entire c. 200 km latitudinal extent of the Kangâmiut dyke swarm and encompasses the complete range of Nagssugtoqidian overprint on these dykes and their country rocks. South of Itillip Ilua (Itilleq), the structural and metamorphic overprints on the dykes exhibit a considerable range in both intensity and P–T conditions between and even within outcrops. In contrast, north of Itillip Ilua, the rocks show more systematic gradual increases in the degree of structural overprints and metamorphic grade, culminating in the Ikertooq thrust zone where granulite facies rocks are brought southwards over amphibolite facies rocks. Currently, available age data from the Nagssugtoqidian orogen permits the identification of two metamorphic episodes at c. 1850–1800 Ma and c. 1780–1720 Ma. These groups of metamorphic ages are supported by recent 40Ar–39Ar ages from dykes in the same area, which cluster at c. 1860 Ma and c. 1740 Ma, respectively. Albeit geographically sporadic, both age intervals support a subdivision of the Nagssugtoqidian structural and metamorphic overprints across the southern Nagssugtoqidian orogen and its foreland into two distinguishable temporal phases. Further geochronological investigations may well, however, find these two phases to be part of a tectonic continuum. For now, it is thought that the older event records south-directed thrusting over the foreland and concomitant loading of this crust, at least as far south as Maniitsoq. This c. 1860–1800 Ma crustal shortening and thrusting likely also closed a depositional basin located at the current latitude of Ikertooq, which could have formed during an early-orogenic extensional event that enabled and accompanied the c. 2035 Ma emplacement of Kangâmiut dykes. Up to 50–100 Ma later, a younger (c. 1780–1720 Ma) phase of shearing and thrusting mainly affected the Itillip Ilua – Ikertooq area and likely overprinted elements of the former event. This local younger overprint generated a separate trend of distinctly northward-increasing deformation and metamorphism.

Assessing the geometry of the Main Himalayan thrust in central Nepal: A thermokinematic approach

Abstract Since the 2015 Gorkha earthquake in Nepal, the relationship between the geometry of megathrusts and the control it exerts over the nucleation and propagation of major earthquakes has become an important topic of debate. In this study, we integrate new geologic mapping, a newly interpreted cross section from the Daraundi valley of central Nepal, two published cross sections from the neighboring Marsyangdi and Budhi Gandaki valleys, and a suite of 270 thermochronometric ages to create an integrated and validated three-dimensional kinematic model for the central Nepal Himalaya. We use this model to investigate the assertion that the westward propagation of the Gorkha rupture was restricted by deep-seated structures in the Main Himalayan thrust. The integrated kinematic model based on these cross sections indicates that the ~30 km southward step in the Main Central thrust system mapped in the Daraundi valley, along with the corresponding step in the distribution of reset muscovite (Ar-Ar) ages, is not the result of a lateral structure in the modern Main Himalayan thrust. Instead, the step in the surface geology is the result of a considerably shorter Trishuli thrust sheet in the Daraundi transect (~30 km compared to between 105 and 120 km in the other transects). The corresponding southward step in the distribution of reset muscovite Ar-Ar ages is the result of the Lesser Himalayan duplex being completely translated over the Main Himalayan thrust ramp, elevating and exposing rocks heated to &amp;gt;400 °C farther south in the Daraundi transect. Our integrated model also highlights the 10–15 km of out-of-sequence thrusting that occurs on the Main Central thrust system across central Nepal. Importantly, these out-of-sequence thrusts sole directly into the modern Main Himalayan thrust ramp, and, together with the distribution of reset zircon (U-Th)/He and apatite fission track ages, show that the modern ramp is distinctly linear from east to west, with no support for a lateral structure at the ramp or to the south.

Late-Pleistocene rejuvenated volcanism and flank collapse deposits on a Cretaceous seamount near El Hierro, Canary Archipelago

Henry Seamount is a Cretaceous submarine volcano located 40 km southeast of El Hierro, the youngest of the Canary Islands, at 3700 m water depth. On the seamount's summit region, a widespread layer of heterolithologic volcaniclastic ash and lapilli beneath centimeters to decimeters of pelagic sediment was discovered and sampled during R/V Meteor cruise 146. The dominant lithology is a glassy basaltic ash that is depleted in highly incompatible elements and enriched in sulfur (S/K2O ratios of 0.10–0.20) compared to El Hierro lavas, suggesting an origin by a deep-sea volcanic eruption on Henry Seamount. Uranium-series disequilibria constrain the age of this ash to <350 ka, which implies rejuvenated volcanic activity of the seamount after up to 126 Ma of dormancy. This rejuvenated activity is possibly related to the Canary hotspot, where heating of lithosphere that had become amphibole-metasomatized during the formation of Henry Seamount led to renewed melt production. In contrast to the dominant ash type, most other volcaniclastic samples are geochemically indistinguishable from El Hierro lavas. The variety of lithologies, the angular to edge-rounded shapes of many fragments, and intimate mixture with the predominant ash suggest that this group of volcaniclastics was transported from El Hierro to Henry Seamount by a submarine debris avalanche and associated turbidity current. This implies a runup of up to 700 m even for centimeter-sized basaltic clasts after up to 40 km of lateral transport. ArAr age constraints for two samples are ∼190–200 ka, which is consistent with the southeast-directed giant Las Playas II landslide from El Hierro as the most likely source. Henry Seamount thus provides a rare example where collapse-induced deposits from another volcanic edifice are found on top of a seamount and are mingled with ash to lapilli from previous rejuvenated volcanism. Mingling and reworking of the tephra may explain the lack of a discernible eruption center on top of the seamount.

Last 150 kyr volcanic activity on Mauritius island (Indian ocean) revealed by new Cassignol-Gillot unspiked K–Ar ages

We present new K–Ar ages extending the volcanic history of Mauritius Island towards the Holocene. Mauritius volcanism is associated with the activity of the Réunion hotspot, the magmatism of which produced the Deccan Traps across the Cretaceous-Tertiary (KT) boundary and continues up to present on Réunion Island. After shield building from before 8.9 Ma to 4.7 Ma, Mauritius Island underwent an unusual volcanic evolution involving two rejuvenation stages (3.5–1.9 Ma and <0.7 Ma) separated by a 1.2 Myr hiatus. The lower bound of the second rejuvenation stage being poorly constrained, it is not clear whether volcanism in this island has come to a rest. Given the occurrence of uneroded, and yet undated strombolian cones in Mauritius, it is important to better constrain the latest activity of this island in order to assess its volcanic hazard. The importance of dating the end of this stage is also enhanced by the fact that most of the surface of Mauritius is covered by the lavas of the rejuvenation stages, hence indicating a recent volcanic activity that needs to be better characterized for risk assessment.We focus here on the last 150 ka interval which lacked precise age controls with only a few whole-rock and groundmass 40Ar/39Ar plateau ages available (Moore et al., 2011), ranging from 138 ± 29 to 40 ± 48 ka (uncertainties are 1 σ). Our six new Cassignol-Gillot unspiked K–Ar ages obtained on plagioclase or groundmass range from 113 ± 7 to 14 ± 3 ka, with much lower uncertainties than previous ages available. Our major and trace element analyses of the dated samples highlight the compositional homogeneity of the lavas erupted in the last rejuvenation stage, showing no geochemical evolution.Our results, with ages as young as 14 ± 3 ka, demonstrate that near-Holocene volcanism is present in Mauritius Island, with no evidence of fading magmatism in the last 150 ka. Our second youngest age at 44 ± 5 ka implies that the latest activity occurred after a ∼30 kyr repose interval, with the consequence that Mauritius volcanism may resume at any time.

The complex continental margin of NE Oman: Fault-gouge dating of the Wadi Kabir Fault at the northern margin of the Saih Hatat window, Sultanate of Oman, and its wider implications

The complex structural geometry along the northern continental margin of Oman formed during polyphase deformation from the mid-Cretaceous to the Miocene. We focus on the kinematics and tectonics of the Wadi Kabir Fault, which represents an eastern segment of the large-scale, composite Frontal Range fault system that forms the northern boundary of the central Al Hajar Mountains. The most exhumed rocks outcrop in the Saih Hatat window of the Al Hajar Mountains, which occurs in the footwall of the Wadi Kabir Fault. To better understand the tectonic history of the Wadi Kabir Fault, we report three K-Ar illite fault-gouge ages. Sample FG16-1 from a shallowly north-dipping gouge zone of the eastern segment of the Wadi Kabir Fault yielded two consistent ages that overlap within 2σ uncertainties at ∼90 Ma. Sample FG16-2 from the central segment of the fault yielded a K–Ar age of 57.9 ± 2.0 Ma (2σ uncertainties), which is interpreted as a late Paleocene faulting event as it perfectly matches the cooling history of the high-pressure rocks of the Saih Hatat window in the footwall of the Wadi Kabir Fault. We present two tectonic interpretations for the ∼90 Ma fault-gouge age and prefer a synchronous kinematic relationship with the parallel and contractional mid-Cretaceous Yenkit Shear Zone that was active at the distal Arabian Platform since 114 Ma. Because the 57.9 ± 2.0 Ma age is associated with pronounced footwall cooling, we interpret this age to be related to the formation of the Bandar Jissah Basin in the hanging wall of the Wadi Kabir Fault. We conclude that the Frontal Range fault system likely formed during the Paleocene and that its young (≤∼43 Ma) history was accommodated along the western segment of the fault, as there is no evidence of this Eocene fault interval at the Wadi Kabir Fault.

Geochronology and geochemistry of the Banjiaoyuan tin deposit, southern Hunan Province and its geological implications

Situated in the central part of the Southern Hunan segment of the Nanling Metallogenic Belt, the Banjiaoyuan tin deposit represents a significant Indosinian quartz vein type tin deposit. Based on field geological surveys and petrographic observations, the mineralization in the area can be divided into three stages: (I) early greisen stage, (II) cassiterite-quartz vein stage, and (III) late barren quartz vein stage. The dating results show that the 40Ar-39Ar plateau age of muscovite in stage I is 215.51 ± 0.87 Ma, and the Rb-Sr isochronous age of quartz mineral in stage II is 214.2 ± 3.1 Ma (MSWD = 1.3), both of which are coincide with the cassiterite U-Pb lower intercept age (215.7 ± 1.3 Ma, MSWD = 1.2), confirming the existence of metallogenic events in this region during the late Indosinian. The δD and δ18O of quartz in stage II range from −66.2 to −80.2 ‰, and 3.71 to 6.17 ‰, respectively, indicating that the ore-forming fluid is mainly derived from a mixture of magmatic-hydrothermal fluids and a small amount of meteoric water. We applied LA-ICP-MS chemical mapping and trace elements analysis to cassiterites, found that they underwent a transition from oxidation to reduction during precipitation when the fluids tend to be more volatile-rich and highly fractionated. The formation of the Banjiaoyuan tin deposit is closely related to the late Indosinian magmatic activity in the Yangmingshan region. Besides, the deep and peripheral parts of the mining area also have the potential for large-scale vein-type and greisen-type W-Sn mineralization.

Field geology, petrology and geochronology of pluton–dyke systems: emplacement conditions of the Trapecio and Ushuaia peninsula dyke swarms (Fuegian Andes) and their relations with tectonics

We report the field geology, petrography, and geochemical and geochronological data for two dyke swarms in the Fuegian Andes, with the aim of correlating them with known suites and improving our knowledge of the magmatism of the Late Cretaceous rear arc and its relations with ductile deformation. We provide keys for correlation based on the K 2 O content of amphiboles. The Trapecio dyke swarm is mildly alkaline and ferriferous, with a zircon U–Pb age of 75 Ma, and therefore belongs to the Fuegian potassic magmatism (78–68 Ma). The Ushuaia peninsula dyke swarm, also known as the Ushuaia Peninsula Andesites, is shown to be high-K, calc-alkaline and magnesian, with new radiometric ages of 87–86 Ma. Although the Ushuaia peninsula dyke swarm exhibits foliation associated with ductile deformation, the Trapecio dyke swarm is post-kinematic and reveals emplacement controlled by the slaty cleavage in the metapelite host. Field and geophysical evidence suggest both swarms overlie small upper crustal plutons that are mostly buried. The variable composition of the dykes suggests a protracted history of dyke injection, mostly fed from deeper reservoirs. Contact metamorphism around both pluton–dyke systems is very weak. By comparison with adjacent plutons, we argue that the main variables controlling the development of the aureole are the small magma volumes and low rates of injection. Supplementary material: Full analytical procedures (Online Resource 1); detailed petrography of the investigated Trapecio dyke swarm samples (Table S1 and Fig. S1) with geochemical data (Table S3 and Fig. S2) (Online Resource 2); extended petrography and photomicrographs (Fig. S3) of the Ushuaia Peninsula Andesite dyke swarm (Online Resource 3); mineral chemistry data (Table S2) and the results of zircon U–Pb and hornblende Ar–Ar age measurements and ratios (Tables S4–S8) (Online Resource 4), and thermobarometric results (Table S9) (Online Resource 5) are available at https://doi.org/10.6084/m9.figshare.c.7097733

Is the Yermak Plateau a continental fragment from North America? Constraints from Cretaceous and early Eocene magmatic events

The Yermak Plateau (YP) north of Svalbard is a prominent bathymetric feature in the Eurasia Basin of the Arctic Ocean, forming the northwesternmost margin of the Eurasian plate. Seismic data indicate that the YP comprises continental basement; however, little is known about its geology. New petrographic, geochemical, Sr–Nd isotopic, and Ar–Ar geochronological data were obtained on rock fragments, which were previously recovered from basement highs of the northeastern and southwestern YP and are dominantly of magmatic origin. These new data combined with available literature data, and comparisons with volcanic and sedimentary rocks from onshore and offshore areas adjacent to the YP indicate that the northeastern YP and the southwestern YP are different regarding their geological evolution. The southwestern YP comprises an alkaline basaltic suite for which an Ar–Ar biotite age of 51 Ma was previously reported. The suite was formed in a continental extensional regime offshore northern Svalbard. Associated sedimentary rocks (sandstone, several limestones) show petrographic similarity with rocks of the Devonian Old Red Sandstone on Svalbard. From the northeastern YP, in contrast, we recovered mildly alkaline basaltic rocks with mid-Cretaceous Ar–Ar ages (102 ± 3 and 98 ± 3 Ma). The rocks show certain geochemical characteristics (partial enrichments of P, Ba, and Eu), which overlap with similar-aged Cretaceous basaltic rocks from northern Ellesmere Island of Canada and North Greenland. We suggest that the northeastern YP is a continental fragment derived from the North American plate, which was separated from the conjugate Morris Jesup Rise and juxtaposed to the geologically distinct southwestern YP by the propagation of the Gakkel Ridge spreading center since the early Oligocene.Graphical

Revisiting the depositional age and provenance characteristics of metasedimentary rocks from the basement units in the Central Pontides (N Turkey): New constraints for tectonic evolution of the southern Black Sea region

The pre-Jurassic tectonic evolution of the Central Pontides in northern Turkey is still poorly understood due to the lack of detailed geochemical and geochronological data from the basement units. Therefore, this study reports for the first time combined whole rock geochemistry, trace element data and U-Pb ages for detrital zircons, and Ar-Ar geochronological data from the metasedimentary rocks of the Devrekani Massif, Geme Complex, and the Serveçay Unit to better understand their provenance, depositional age characteristics, and tectonic evolutions. The detrital zircon U-Pb data of metasedimentary units from the Devrekani Massif, Geme Complex, and the Serveçay Unit yield ca. 191, 185 and 298 Ma maximum depositional ages (youngest age peaks), respectively. The Devrekani Massif is dominated by Permo-Carboniferous-aged detrital zircons (67 %), whereas Neoproterozoic and Devonian-aged detrital zircons are more common in the Geme Complex (47 %) and the Serveçay Unit (53 %). These distinct age distributions clearly indicate that the Devrekani Massif may has been deposited in a dissimilar location compared to the Geme Complex and the Serveçay Unit. Furthermore, in contrast to the Devrekani Massif, the source area of the metasedimentary rocks from the Geme Complex is comparable with that of the Serveçay Unit. Lastly, the obtained detrital zircon U-Pb and mica Ar-Ar ages suggest that the Devrekani Massif and the Geme Complex were deposited and metamorphosed during the Early and Middle Jurassic, respectively. The Serveçay Unit was deposited and metamorphosed in the Permian.

Exothermic reactions and 39Ar–40Ar thermochronology: Hydration leads to younger apparent ages

Abstract Retrogression and hydration commonly affect large swaths of Earth’s crust, causing variable degrees of chloritization, sericitization, and/or serpentinization. Hydration is a strongly exothermic process that partially opens isotopic systems, thereby distorting the recorded apparent ages and cooling histories of reworked terranes. Using a simple one-dimensional numerical model involving heat released at variable temperatures to simulate exothermic hydration, we track the cooling histories for rocks that exhume from depth. The calculated cooling paths are used to quantify apparent 39Ar–40Ar ages in muscovite, biotite, and feldspar considering 40Ar production and diffusive Ar loss. For fluid incorporation relative to chloritization of ~10%, ~50 kJ of latent heat are released per kilogram of rock. For this scenario and exhumation rates between 1 mm·yr–1 and 4 mm·yr–1, muscovite grains ≤100 µm in diameter yield apparent ages that are younger by up to 10%, but always exceeding the typical uncertainty of Ar dating. Biotite and feldspar display a similar distortion, even for large grains of ~1 mm in size. The relative younging effect increases to &amp;gt;30% with enthalpy released, exhumation rate, and decreasing grain size, with younging reaching a maximum for hydration at approximately the nominal closure temperature of the respective thermochronometers. Using published data sets (from Sifnos, Greece, and Tian Shan, China), we suggest that rejuvenation of apparent mica ages is consistent with diffusive Ar loss due to exothermic hydration during exhumation. Our method applies to any thermally activated process, like element exchange in mineral thermometers or fission-track annealing, provided heat is released close to the characteristic closure temperature. This extends to processes beyond hydration, such as shear heating or localized magma emplacement, making our results pertinent for diverse thermochronometers and temperature-sensitive methods across a broad range of conditions.

Structural, 40Ar/39Ar Geochronological and Rheological Feature Analysis of the Guoxuepu Shear Zone: Indications for the Jitang Metamorphic Complex in the Northern Lancangjiang Zone

AbstractThe Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone. Through structural‐lithological mapping, structural analysis and laboratory testing, the composition of the Jitang metamorphic complex was determined. The macro‐ and microstructural analyses of the ductile detachment shear zone (Guoxuepu ductile shear zone, 2–4 km wide) between the metamorphic complex and the overlying sedimentary cap show that the shear sense of the ductile shear zones is top‐to‐the‐southeast. The presence of various deformation features and quartz C‐axis electron backscatter diffraction (EBSD) fabric analysis suggests multiple deformation events occurring at different temperatures. The average stress is 25.68 MPa, with the strain rates (έ) ranging from 9.77×10–14 s–1 to 6.52×10–16 s‐1. The finite strain of the Guoxuepu ductile shear zone indicates an elongated strain pattern. The average kinematic vorticity of the Guoxuepu ductile shear zone is 0.88, implying that the shear zone is dominated by simple shear. The muscovite selected from the protomylonite samples in the Guoxuepu ductile shear zone yields a 40Ar‐39Ar age of 60.09 ± 0.38 Ma. It is suggested that, coeval with the initial Indo–Eurasian collision, the development of strike‐slip faults led to a weak and unstable crust, upwelling of lower crust magma, then induced the detachment of the Jitang metamorphic complex in the Eocene.