Ar Ages Research Articles

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.

The 40Ar–39Ar Age of Folding in the Sedimentary Complexes of Chukotka and Wrangel Island (Northeastern Russia)

The 40Ar–39Ar Age of Folding in the Sedimentary Complexes of Chukotka and Wrangel Island (Northeastern Russia)

Late Oligocene to Early Pliocene Exhumation and Structural Development in the Western Himalaya, Northern Pakistan: Implications for the Cenozoic Metamorphic Overprint

Abstract New middle Miocene to Pliocene (~14–3 Ma) apatite fission track (AFT) cooling ages combined with published K–Ar/Ar–Ar and zircon fission track (ZFT) ages from the Hazara and Swat regions of Pakistan are used to explain the Oligocene to Pliocene structural evolution in the Western Himalaya. The structural model explains the distribution of K–Ar/Ar–Ar ages in three distinct age groups (Proterozoic, Paleozoic-Mesozoic, and Eocene to Oligocene). The Proterozoic to Mesozoic sequence of northern Hazara and Swat experienced elevated temperature and pressure conditions, evident by reset Eocene to Oligocene K–Ar/Ar–Ar hornblende and Eocene to Miocene muscovite ages, caused by Kohistan overthrusting the Indian margin during and after the India–Asia collision. Samples from the Indus syntaxis with Paleo to Mesoproterozoic K–Ar/Ar–Ar hornblende ages and Eocene to Oligocene Ar–Ar muscovite ages show no signs of Cenozoic metamorphism; these samples were thermally imprinted up to the Ar–Ar muscovite closure temperature. Neoproterozoic to Lower Paleozoic rocks from the southern parts of Hazara and Swat show Mesozoic to Oligocene partially reset Ar–Ar muscovite ages and preservation of Ordovician metamorphism. The combined analysis of published K–Ar/Ar–Ar (muscovite), ZFT, and new AFT ages (~14–12 Ma) suggests that the Main Central thrust/Panjal thrust was active from Oligocene to early Miocene (~30–18 Ma), and the Nathia-Gali and Main Boundary thrusts were active from the middle to late Miocene (~14–9 Ma) in the Hazara area. New and published AFT ages (~6–3 Ma) from the Indus syntaxis suggest that early Pliocene tectonic thickening in the hinterland formed the N–S trending Indus anticline, creating an erosional half window in the Main Mantle thrust, forming the Indus syntaxis, and dividing the Main Central thrust sheet into the Hazara and Swat segments.

Isotopic Geochronological Constraints on the Formation and Evolution of the Moon

One major task of studying the formation and evolution of the Moon is to construct a timeline of the important events with precise isotopic ages. Here, we review recent major isotopic geochronological progress in the past decade and the unsolved problems in isotopic geochronology. The Moon probably formed between 4.52 and 4.42 Ga. Recent high-precision whole-rock and mineral Sm-Nd isotopic dating results suggested that ferroan anorthosite and highlands magnesian suite rocks formed contemporarily around 4.37 to 4.33 Ga. Although the major mare basaltic volcanism took place from 3.85 to 2.93 Ga, new geochronological data from lunar meteorites and Chang’e-5 basalts suggested that lunar basaltic volcanism took place as old as up to 4.37 Ga and at least as young as 2.0 Ga, respectively. Impact events older than 3.9 Ga have also been revealed based on U-bearing minerals Pb/Pb ages and Ar-Ar ages and can provide important clues to understand the late heavy bombardment hypothesis. However, the reliable isotopic ages for the important events on the Moon are still far from conclusive, due to lack of pristine samples that directly crystallized from Lunar Magma Ocean and samples from impact melt sheets in large impact basins (e.g., the South Pole-Aitken basin). In the future, collection and return of pristine samples of ferroan anorthosite and highlands magnesian suite rocks from the farside, cryptomare basalts and late-stage basalts, quartz monzogabbros, granites/felsites, and rocks from impact melt sheets in large impact basins are required for better understanding the formation and evolution of the Moon.

Serravallian–Tortonian (Miocene) folding in the Amakusa region, northern Ryukyu arc: Possible subduction resumption of the Philippine Sea Plate

AbstractEocene to Lower Miocene coal‐bearing formations in northern and western Kyushu, northern Ryukyu arc, are folded, and the horizontal compression has been attributed to the opening of the Japan Sea or to the significant movement along the Median Tectonic Line and its southwestern extension. However, the timing and implication of the folding are not well understood. To deal with these issues, we studied the Amakusa region where the folded Eocene strata with a total thickness of a few kilometers. Paleomagnetic directions of Middle Miocene intrusions and of Late Miocene lavas were measured in this study to apply the fold test to judge the relative timing of the folding and magmatism. As a result, the concentration of the directions was improved by the tilt‐correction, indicating the folding younger than the magmatism. Our detailed geological mapping revealed that the folding is older than a horizontally‐lying basaltic lava which yielded a K–Ar age of 6.8 Ma, because folded Eocene formations were truncated and unconformably blanketed by the lava. In addition, we found that some of the normal faults trending perpendicular to the folds were reactivated as transfer faults after the normal faulting which also postdated the magmatism. This reactivation is concordant with the above‐mentioned relative timing. Synthesizing geological data from surrounding regions, we conclude that the folding was probably contemporaneous with the Taiwan‐Shinji fold belt which grew in the Ryukyu and southwest Japan backarcs. Since the simultaneous compression affected not only these regions but also northeast Japan, the compression possibly resulted from the resumed subduction of the Philippine Sea Plate in the Serravallian–Tortonian time.

Multiple geochronological evidences revealing oil charging and redistribution processes in the Tazhong-4 Oilfield, Tazhong Uplift (Tarim Basin): Implications for quantitative reconstruction of petroleum accumulation history in superimposed basins

Re-migration within a hydrocarbon reservoir is an important yet poorly constraint process in petroleum system analysis. Here we investigated a Paleozoic petroleum reservoir in the Tarim Basin and quantitatively reconstructed the oil re-migration process using fluid inclusions and multiple isotopic dating techniques including the calcite U–Pb dating, the oil Re–Os dating, and the illite K–Ar dating. The Re–Os dating for the Carboniferous oil samples demonstrated an oil generation process during the Middle Caledonian (479 ± 110Ma). Two episodes of oil charging were revealed by each represented oil inclusion fluorescing yellowish and blue color in the Cambrian and Ordovician intervals. Calcite U–Pb ages suggest the Cambrian and Ordovician paleo-reservoirs represented oil inclusion fluorescing yellowish color were charged by mature oils during the late Caledonian orogeny (420-384Ma), and illite K–Ar ages show the oils with similar maturity charged into the Carboniferous reservoir during the late Hercynian orogeny (278.4–246.0Ma). This contradiction in absolute ages reveals an oil re-migration process with oils that originally was generated during the Middle Caledonian and charged in the Cambrian and Ordovician reservoirs and then migrated vertically to the Carboniferous reservoir ∼200 Myrs later possibly due to fault reactivation. The successful application of fluid inclusion analysis and multiple absolute dating techniques in the Tarim Basin provides a paradigm for quantitative reconstruction of petroleum (re-) accumulation history in similar tectonic superimposed basins.

Submarine basalt xenoliths confirm the occurrence of post-rift volcanism in the Southern Santos Basin, SW Atlantic

We report here the finding and analysis of xenoliths of altered basalt clasts dredged from the top of two exhumed diapirs in the southern Santos Basin (SW Atlantic). The diapirs crossed the stratigraphic column from the Aptian and outcropped on the seafloor. Complemented by seismic data, these findings provide the first direct evidence of an Early Campanian post-rift magmatic event in the southern Santos Basin (SW Atlantic). One 82 Ma Ar–Ar age of these xenoliths indicates that the volcanism developed coeval with the intrusion of the Serra do Mar Alkaline Province rocks on land. The tholeiitic composition of the xenoliths excludes a genetic correlation with the alkaline province. Seismic reflection data indicate the possible source of the xenoliths from a 40-km-wide cluster of volcanic centers and sills located above the Aptian salt deposits (∼115 Ma), between the top-Turonian (90 Ma) and top-Campanian (72 Ma) interval. We suggest that salt diapirism peented the overlaying volcanic units and carried fragments upwards through the entire stratigraphic column to the ocean floor. Our findings stress that salt diapirism is an important mechanism for conveying unreachable subsurface fragments to the surface, particularly the ocean floor. With the expanding knowledge of the seascape worldwide, enhanced exploration of salt diapirism and its xenolith products has the potential of opening new windows to unknown parts of geologic history.

Volcanic debris avalanche deposits and their significance in the architecture and evolution of the Miocene-Quaternary Călimani-Gurghiu-Harghita volcanic range (Eastern Transylvania, Romania)

The Călimani-Gurghiu-Harghita volcanic range (CGH) represents the surface manifestation of the post-collisional magmatism that followed the docking of its basement, the Dacia Mega Unit, onto the East European margin. Between ca. 11 and 0.3 Ma, detachment of subducted slab, magma generation, and volcanic activity in CGH propagated south-southeastward, sub-parallel with the collisional Eastern Carpathian Orogen, resulting in the construction of a series of mostly composite volcanoes dominated by calc-alkaline lavas and pyroclastic products. The generally well-preserved axial row of the CGH volcanic edifices is accompanied by voluminous volcaniclastic deposits of which sources and emplacement history persist as a matter of debate. In the standard view reflected in regional scale geologic maps, these deposits represent products of complete erosion of hypothetical earlier volcanoes. Other interpretation, however, suggest that they are at least partially constituted by voluminous volcanic debris avalanche deposits (VDADs).By combining evidence from field observation and GIS analysis with published and new data from petrography, bulk-rock geochemistry, and K/Ar geochronology, here we demonstrate that VDADs vastly dominate these volcanoclastic deposits. In addition to two previously identified VDADs (Western Călimani and Vârghiş), we show that further six VDADs (Eastern Călimani, Fâncel-Lăpuşna, Ostoroş, Ivo-Cocoizaş, Luci, and Pilişca) were generated due to the sector failure of a series of CGH volcanoes, most of which still preserve well-distinguishable collapse scars. Based on our estimates, debris avalanches with volumes between <1 km3 and > 100 km3 displaced 0.4–39% of the source edifices, transferring nearly a third of their cumulated volcanic output to the observed volcaniclastic deposits.K/Ar age constraints suggest VDAD formation closely followed in space and time the migrating volcanic activity between ca. 7.8 and 1.5 Ma. Relying on this age pattern, collapse scar orientations, and VDAD runout directions, we propose that regional scale deformation of the Dacia lithosphere imposed by progressive detachment of the underlying subducted slab, manifested on the surface as a transient subsidence–uplift couple traveling along CGH as well as opening of intramountain basins at the eastern flank of the range, was responsible for basement tilting and faulting that destabilised the CGH volcanic edifices. Seismic activity associated with slab detachment and shallow crustal fault system as well as explosive volcanic eruptions may have triggered the collapse of these edifices, leading to the generation of the VDADs.

Tectonic influence of Cenozoic post-rift alkaline magmatic intrusions on the onshore basement of the Campos and Santos basins, SE Brazil

This study focuses on the tectonic control of post-rift Eocene alkaline intrusions that outcrop in the Atalaia Peninsula, SE Brazil, on the onshore limit of both the Campos and Santos basins, the most prolific Brazilian oil basins. To investigate these tectonic-magmatic processes, high-resolution unmanned aerial vehicle (UAV) images, structural field data, and geochronological analyses were performed. This study characterized several alkaline lithotypes (syenites, N–S and NW–SE trachytes, NW–SE lamprophyres, and E–W phonolites dikes), cutting a N–S oriented Paleoproterozoic orthogneiss basement and NE–SW tholeiitic dikes. A new Ar–Ar age of 53.82 ± 0.7 Ma was obtained on phlogopite from one lamprophyre dike. Geometric and dynamic analysis indicate that these alkaline intrusions are synchronous and their emplacements can be explained by a sub-horizontal NNE–SSW intraplate extension linked to a sub-vertical σ1. Magma rising occurred by forceful injection through transcrustal sets of discontinuities that reached the mantle. Strike to oblique-slip and late normal faults support an anticlockwise rotational extension direction after emplacement. These alkaline tectonic-magmatic dynamics may have caused an isostatic response, influencing the evolution of the abutting offshore basins while triggering a large influx of sediments during the Paleocene-Eocene.

Volcanism straddling the Miocene–Pliocene boundary on Patmos and Chiliomodi islands (southeastern Aegean Sea): insights from new 40Ar ∕ 39Ar ages

Abstract. The island of Patmos, in the eastern Aegean Sea, consists almost entirely of late Miocene to Pliocene volcanic rocks. The magmatism in the Aegean is governed by subduction of the African plate below the Eurasian plate, back-arc extension, slab rollback, slab edge processes and westward extrusion of central Anatolia to the west along the Northern Anatolian Fault into the Aegean domain. The evolution of the Aegean basin is that of a back-arc setting, with a southerly trend in the locus of both convergent tectonics and back-arc stretching, allowing intermittent upwelling of arc, lithospheric and asthenospheric magmas. Here, we present new 40Ar/39Ar age data for Patmos and the nearby small island of Chiliomodi to place this volcanism in a new high-resolution geochronological framework. High-resolution geochronology provides a key to understanding the mechanisms of both the tectonic and magmatic processes that cause the extrusion of magma locally and sheds light on the tectonic evolution of the larger region of the back-arc basin as a whole. The volcanic series on Patmos is alkalic, consistent with a back-arc extensional setting, and ranges from trachybasalt to phonolites, trachytes and rhyolites, with SiO2 ranging from 51.6 wt % to 80.5 wt %, K2O ranging from 2 wt % to 11.8 wt % and extrusion ages ranging from 6.59 ± 0.04 (0.14) Ma to 5.17 ± 0.02 (0.11) Ma. Volcanism on Patmos and adjacent Chiliomodi can be understood as a combination of mantle and crustal tectonic processes including the influence of transform faults and rotational crustal forces that also caused the widening of the southern Aegean basin due to two opposite rotational poles in the east and west and rollback of the subducting slab south of Crete.

Fitting Thermal Evolution Models to the Chronological Record of Erg Chech 002 and Modeling the Ejection Conditions of the Meteorite

The history of accretion and differentiation processes in the planetesimals is provided by various groups of meteorites. Sampling different parent body layers, they reveal the circumstances of the metal–silicate segregation and the internal structures of the protoplanets. The ungrouped achondrite Erg Chech 002 (EC 002) added to the suite of samples from primitive igneous crusts. Here we present models that utilize thermochronological data for EC 002 and fit the accretion time and size of its parent body to these data. The U-corrected Pb–Pb pyroxene, Pb–Pb phosphate, and Ar–Ar ages used imply a best-fit planetesimal with a radius of 20–30 km that formed at 0.1 Ma after calcium-aluminum-rich inclusions. Its interior melted early and differentiated by 0.5 Ma, allowing core and mantle formation with a transient lower mantle magma ocean and a melt fraction of <25% at the meteorite layering depth. EC 002 formed from this melt at a depth of 0.8 km in a partially differentiated region covered by an undifferentiated crust. By simulating collisions with impactors of different sizes and velocities, we analyzed the minimum ejection conditions of EC 002 from its original parent body and the surface composition of the impact site. The magma ocean region distinct from the layering depth of EC 002 implies that it was not involved in the EC 002 genesis. Our models estimate closure temperatures for the Al–Mg ages as 1030–1200 K. A fast parent body cooling attributes the late Ar–Ar age to a local reheating by another, late impact.

Ar-Ar ages and geochemistry of Late Cretaceous basalts in the Nanxiong Basin, SE China: Constraints on the subduction and rollback of the Paleo-Pacific Plate

The Cretaceous arc-related and intraplate mafic rocks are widely distributed across Southeast (SE) China, and provide important information for investigating the Cretaceous dynamical processes of the Paleo-Pacific Plate. This study presents new petrological, geochronological, and geochemical analyses for the Late Cretaceous basalts from the Nanxiong Basin in SE China. The Nanxiong basalts yield the 40Ar/39Ar plateaus ages of 76.2-69.9 Ma, they belong to alkaline-calc-alkaline basalts and have OIB-like geochemical patterns with enrichment on LILEs and LREEs. These basalts have initial 87Sr/86Sr ratios of 0.7055-0.7062 and εNd (t) values of +2.5-+3.4. Such signatures suggest that the Nanxiong basalts were the products of partial melting of asthenospheric mantle, which might be metasomatized by slab-derived melts with a proportion of sediments. Our new results, along with the regional reported ages and geological observations, suggest that the Nanxiong OIB-like basalts were formed in an intraplate extensional setting, in response to the rollback of the subducted Paleo-Pacific slab during the Late Cretaceous.

Pliocene-to-Holocene volcano-tectonic activity on Mohéli Island (Comoros archipelago) constrained by new K[sbnd]Ar ages

The volcanism of the Comoros archipelago (from west to east: Grande Comore, Mohéli, Anjouan, and Mayotte islands) has been under renewed scientific scrutiny since the eruption of a new submarine volcanic edifice (Fani Maoré) which begun in May 2018 east of Mayotte. West of Mayotte, Mohéli island has received little attention from the geoscience community, despite its largely uneroded volcanic landscapes suggesting a recent activity that has not been dated. Here we address this gap in knowledge by applying KAr geochronology on groundmass and major-trace element analyses to subaerial and submarine rocks, in an attempt to reconstruct the volcano-tectonic and geochemical evolution of Mohéli. Our results show that Mohéli's volcano-tectonic evolution proceeded in two constructional stages, including a primary edification along a N070°E axis from before ca. 3.8 Ma to ca. 3 Ma (Stage 1), and a second construction along a N110°E ridge shaping the present-day island since ca. 2 Ma (Stage 2). The two stages were separated by an increased subsidence (≥0.2 mm/yr) that drowned a large part of the N070°E primary edifice. The two identified volcano-tectonic stages are reflected also in the geochemical evolution of emitted magmas, which were moderately silica-undersaturated in Stage 1 and moderately to highly silica-undersaturated in Stage 2. Silica undersaturation increases with time up to the olivine melilitite field, together with enrichments in Ca, P, and incompatible trace elements, suggesting the increasing contribution of a metasomatized mantle in melts. The coeval changes of construction orientations and magmas compositions of Mohéli suggest a modification in the tectonics of the Comoros related to magmatism at about 2 Ma. The most recent volcanic morphology investigated is an olivine nephelinite lava flow on the north coast of the island dated at 8 ± 2 ka. This finding of Holocene volcanism on Mohéli implies that the entire Comoros archipelago should be considered as an active zone in any volcanic hazard assessment. The melilite-bearing rocks of Mohéli share geochemical similarities with those of Mayotte and of the Cenozoic volcanic provinces of Madagascar, further pointing to similar melting sources and magmatic processes over the Mozambique channel.

Effects of Metasomatism on Granite-Related Mineral Systems: A Boron-Rich Open Greisen System in the Highiş Granitoids (Apuseni Mountains, Romania)

Greisenization is typically linked with highly fractionated granites and is often associated with hydrothermal vein systems. Late to postmagmatic metasomatic processes involve the enrichment of volatile components such as boron and halogens as well as several metallic elements. The purpose of this study is to reveal the main metasomatic effects and paragenetic sequences of the related mineralizations in Highiş granitoids, Romania. In a natural outcrop, more than 30 samples were collected from granitoids, felsic veins, and country rocks. We carried out a detailed mineralogical and petrological characterization of carefully selected samples using X-ray powder diffractometry, electron microprobe analysis, and microscopic methods together with K–Ar ages of whole rocks and K-bearing minerals. Several characteristic features of albitization, sericitization, tourmalinization, epidotization, and hematitization were recognized in the studied samples. Crystallization of quartz, K-feldspar, and magnetite represents the first stage during the magmatic-hydrothermal transition. The mineral assemblage of albite, sericite, schorl, and quartz originates from the early and main stages of greisenization. While the subsequent mineral assemblages, which predominantly include dravite, specular hematite, and epidote, are closely related to the late vein-depositing stage. We propose that the study area could belong to a boron-rich open greisen system in the apical portion of Guadalupian A-type granite. Based on a new hypothesis, the previously published Permian crystallization ages (between ~272 Ma and ~259 Ma) could be homogenized and/or partially rejuvenated during the hydrothermal mineralization processes due to uraniferous vein minerals. Additionally, the Highiș granite-related system suffered a Cretaceous thermal overprint (between ~100 Ma and ~96 Ma). The results may help to understand the evolution of highly evolved granite intrusions worldwide and improve our knowledge of the effect of hydrothermal mineralization processes on the emplacement ages.

Recycling of subducted Indian continental crust constrained by late Cretaceous mafic dykes in Central Lhasa block of the Tibetan plateau

The question of whether subducted continental crust can be recycled into post-collisional magmatism in continental collisional zones remains controversial. Post-collisional mantle-derived ultrapotassic rocks are widespread in western part of central Lhasa block (WCL) of the Tibet-Himalaya orogen and show arc-type trace-element signatures and very enriched SrNd isotopes, which have been explained by the recycling of subducted Indian continental crust. However, due to the lack of late Cretaceous mafic magmatism in the WCL, the nature of the ancient subcontinental lithospheric mantle (SCLM) beneath the WCL has not been well constrained. It is therefore still unclear whether the enriched components of these ultrapotassic rocks were derived from subducted Indian continental crust or inherited from the ancient SCLM. Here we report the mafic dykes from the TangraYumco area in the WCL, which formed at ∼90 Ma (whole-rock 40Ar39Ar, and zircon and titanite UPb ages). The diabase-porphyrites and diorite-porphyrites are similar to intraplate Nb-enriched basalts and magnesium adakitic rocks, respectively. This rock association reveals the lithospheric foundering process. Furthermore, the diabase-porphyrites generated by the interaction between the ancient SCLM and asthenosphere provide important constraints on the nature of the ancient SCLM, which shows distinct Nd isotopes to the post-collisional ultrapotassic rocks. In addition, the NW Lhasa block was in a rear-arc environment since the late Cretaceous and thus this SCLM was not significantly influenced by the ongoing northward subduction of the Indus-YarlungZangbu Neo-Tethys ocean. Finally, we propose that the post-collisional ultrapotassic rocks cannot be sourced from the ancient SCLM, but instead were derived from a relatively depleted mantle (e.g., juvenile lithospheric mantle) metasomatized by subducted Indian continental crust. Thus, this study confirms the recycling of subducted continental materials in continental collisional orogens.

Age, duration and spatial distribution of ocean shields and rejuvenated volcanism: Fuerteventura and Lanzarote, Eastern Canaries

Fuerteventura and Lanzarote form the oldest emerged part of the Eastern Canary Islands archipelago. Geologically, they can be considered a single edifice, constituting a continuous volcanic ridge extending 250 km from SW to NE. This study completes the dating and the determination of the magnetic stratigraphy of the shields and the rejuvenated volcanism of Fuerteventura and Lanzarote, refining the volcanic stratigraphy and cartography. The new unspiked K–Ar ages and magnetostratigraphy of Fuerteventura and Lanzarote indicate that these islands developed patterns similar to those of the Central and Western Canary Islands, building adjacent and successively superimposed basaltic shield volcanoes during the Miocene, between 20.19 ± 0.30 and 6.30 ± 0.11 Ma. The overlay of post-Miocene rejuvenated volcanism hinders determination of the extent and interrelationship of the shields. These materials constitute only a small fraction by volume but cover a large part of the islands. Despite this, it is confirmed that the disposition of the shields is opposite to the insular progression induced by the hotspot, suggesting the presence of some SW–NE propagation volcanic front or fracture to explain its direction of development. Supplementary material: A description of the K–Ar method is available at https://doi.org/10.6084/m9.figshare.c.6641464

Volcanic evolution through geomorphological mapping: A case study of Cerro Bravo volcano (Colombia)

Geomorphological mapping is essential for identifying landforms, determining erosion, and reconstructing the morphological evolution over time. The relative chronology between features can be established, supporting the study of the potential hazard and associated risks in volcanic terrains. Geomorphological analysis, performed on a 12.5 m ALOS PALSAR DEM, along with field-work and new K–Ar geochronological data, help to clarify the overall evolution of the Cerro Bravo volcano. The observed north-south arrangement of the craters, the stratigraphic relationships, and the new K–Ar ages reveal that the volcanic activity migrated towards the north, with the younger activity concentrated north of the volcanic area. This evolution, which occurred from the Middle Pleistocene to the Holocene, is evidenced by four consecutive semicircular crater-like structures associated with four building stages. Effusive and explosive activity was observed in all recorded volcanic activity stages, indicating a consistent volcanological behavior of this volcano over time, characterized by a bimodal effusive-explosive eruption style. The effusive activity involved lava domes and lava flows, while the explosive activity included dome collapses and Plinian eruptions. This morphological study can help assess the geological risk and potential hazards of inaccessible areas.

40Ar-39Ar dating, fluid inclusion, and isotopic constraints on the timing, fluid evolution, and deposition of the polymetallic Nuocang Pb–Zn deposit in the western Nyainqêntanglha belt

The Nyainqêntanglha polymetallic belt (NPB) is western China's foremost skarn Pb–Zn metallogenic belt. Significant progress has been made in determining these ore deposits' age and origin. However, the precipitation mechanisms and mineralization controls for these deposits remain unresolved. 40Ar/39Ar dating, fluid inclusion (FI), and H-O-S-Pb isotopic data are presented for the Nuocang deposit, a newly discovered polymetallic Pb-Zn skarn deposit in the western NPB, with the goal to trace age and, especially, the major fluid processes associated with hydrothermal evolution leading to the precipitation of ore. The muscovite yields an 40Ar-39Ar plateau age of 59.68 ± 0.60 Ma, which closely resembles typical skarn deposits within the NPB, indicating that it formed during the main collisional stage of the India–Eurasia continent. In the Nuocang base-metal skarn deposit, three ore-related stages can be discerned and subdivided: 1) the pre-ore stage, containing the prograde and early retrograde metasomatic skarn minerals, 2) the syn-ore stage, including the late retrograde metasomatic minerals and sulfides, and 3) the post-ore stage of quartz, calcite, and chlorite alteration. Correspondingly, three types of FI (L, V, and S type) have been identified. The ore-forming fluids evolved from moderate to high temperature between 340 and 440 °C primarily, but up to 571 °C, and moderate to high salinity (concentrated ∼8–18 wt% NaCl-equiv., but up to 48.1 wt% NaCl equiv.) in the pre-ore stage. During the syn-ore stage, the fluids were at low to moderate temperature (between 200 and 320 °C) and low salinity (∼2 to 12 wt% NaCl-equiv.) and developed in low temperature (∼120–200 °C) and low salinity (∼2–8 wt% NaCl equiv.) in the post-ore stage. The calculated δDH2O values of the transparent minerals were −180 to −106 ‰ during the pre-ore stage, −141 to −59 ‰ during the syn-ore stage, and −152 to −105 ‰ during the post-ore stage. The δ18OH2O values were −1.2 to 3.8 ‰ during the pre-ore stage, −4.7 to −2.3 ‰ during the syn-ore stage, and −10.9 to −5.0 ‰ during the post-ore stage, respectively.Based on these results, the magmatic fluids appeared to have gradually evolved into meteoric. Meteoric water is believed to play a significant role in metal deposition within magmatic-hydrothermal systems. Locally, boiling is also considered another key mechanism for ore precipitation. The S-Pb isotope values of the sulfide metals indicate that the metallogenic materials were predominantly derived from an upper crustal source mixing with minor amounts of mantle–derived components. This study suggests that the fluid mixing is related to geostatic pressure variations and infiltration of meteoric waters into the hydrothermal system and are key mechanisms triggering Pb-Zn ore deposition in the NPB.

U-Pb and Ar-Ar geochronological constraints on timing of deformation and peak metamorphism in the Central Africa Orogenic Belt, Yaoundé Domain, Cameroon

ABSTRACT The Central Africa Orogenic Belt (CAOB) and the Transaharan and Sergipano belts form a major West Gondwana suturing orogen affected by D1–D4 compressional deformation. Microstructural observations, mineral chemistry, zircon, and titanite laser ablation ICP-MS U-Pb dating, and biotite, muscovite, and K-feldspar 40Ar-39Ar age data are used to constrain the timing of rocks formation and D1–D4 events in the Yaoundé Domain of the CAOB, as well as characterize the Neoproterozoic-Cambrian transition. The Yaoundé Domain rocks were deposited on a pre-Neoproterozoic structure that was overprinted by a c. 1000–700 Ma mega arc/back-arc system and related Tonian to Ediacaran magmatism. Older U-Pb dates (1000 up to 2700 Ma) are inheritance from a pre-existing crust. The D1–D4 compressional events relate to the inversion of the c. 1000–700 Ma mega-rift system. New dates at c. 700 Ma, c. 630 Ma, c. 580–555 Ma constrain the timing of D1, D2, D3+D4, respectively. The peak of accompanying metamorphism occurred at 614 ± 4 Ma during D2. This was followed by a c. 115 Ma period of deformation-driven crustal exhumation and cooling that extended into the Cambrian. New data document short episodes of rapid denudation and cooling (up to 0.9 km/Ma and 40°C/Ma at c. 578 Ma in gneiss and up to 5 km/Ma and >125°C/Ma at c. 555 Ma in micaschist). It is proposed that the CAOB and correlative belts are parts of the orogenic hinterland formed from accretion and collision between the West African Craton and the Sahara Metacraton precursor. The correlated belts appear to represent the result of a protracted tectono-metamorphic and magmatic process that started in late Mesoproterozoic (c. 1167 Ma) and continued through the Neoproterozoic to the Cambrian (c. 500 Ma).

40 Ar/ 39 Ar eruption ages of Turkana Basin tuffs: millennial-scale resolution constrains palaeoclimate proxy tuning models and hominin fossil ages

The Turkana Basin in NW Kenya and SW Ethiopia hosts remarkable fossil-rich sediments that are central to our understanding of early hominin evolution, with interbedded volcanic tuffs providing critical time markers. However, the resolution of existing Early Pleistocene–Pliocene ages is limited to c. 20–60 kyr, inhibiting the evaluation of climatic and environmental drivers of evolution. We present high-precision, single-feldspar 40 Ar/ 39 Ar age and elemental data for four stratigraphically significant tuffs. These samples exhibit variably dispersed age distributions correlated with feldspar compositional trends, interpreted to indicate the partial retention of inherited 40 Ar related to crustal ‘cold storage’ and rapid melt infiltration preceding eruption. We evaluated various statistical methods and calculated astronomically calibrated Bayesian age estimates of 1879.1 ± 0.6 ka (±2.4 ka including external errors) for the Kay Behrensmeyer Site (KBS)/H2 Tuff, 1837.4 ± 0.9 ka (±2.4 ka) for the Malbe/H4 Tuff, 1357.5 ± 1.8 ka (±2.5 ka) for the Chari/L Tuff and 1315.4 ± 1.9 ka (±2.5 ka) for the Gele Tuff. Our results permit refined age constraints for important early Homo fossils, including the cranium KNM-ER1813 ( Homo habilis ) and various Homo erectus fossils. The KBS Tuff age also provides an important calibration locus for orbital tuning of palaeoclimate proxy records, revealing the complex interplays between palaeoclimate and geological drivers of sedimentation. Supplementary material : The supplementary tables and figures include previously published ages (Table S1), sample and irradiation information (Table S2), electron probe microanalytical results for tuff glasses (Table S3, Figs S2 and S3) and feldspars (Table S4), 40 Ar/ 39 Ar analytical data (Tables S5 and S6, Fig. S4), orbital tuning model parameters (Table S7), a summary of previous and revised hominin ages (Table S8) and a worked Bayesian estimation example (Fig. S1) and are available at https://doi.org/10.6084/m9.figshare.c.6602994