Ar Ages Research Articles

Geochronology and detrital zircons sources from the Sierra Albarrana Domain (SW Iberian Massif)

This study compiles the U-Pb and 40Ar/39Ar geochronology of the metasedimentary, migmatitic and granitic rocks of the Sierra Albarrana Domain, based on data recently published by the authors together with other bibliographic sources. Maximum depositional ages (MDA) and magmatic and metamorphic events ages are discussed. MDAs suggest that this domain was deposited during the middle Cambrian (ca. 511) and U-Pb ages of zoned magmatic and metamorphic zircon grains indicate that the main tectono-magmatic event is late Cambrian (granite emplacement; 481 ± 2 Ma) to early Ordovician (migmatization; 478 ± 2 Ma). The 40Ar-39Ar ages evidence a Cambro-Ordovician metamorphic event (ca. 482 Ma) and another related to the Variscan Orogeny (ca. 337-392 Ma). The most important detrital source corresponds to a Cambrian magmatic event (535-515 Ma), probably developed during the initial episodes of the early Paleozoic rifting affecting the N margin of Gondwana. The remaining Mesoproterozoic, Paleoproterozoic and Archean zircon grains would have been contributed by Paleoproterozoic basement and/or older continental crust recycled in the western sections of the Gondwana margin. We suggest that this domain is probably part of the autochthonous section of the Central Iberian Zone (CIZ) and derived from the Saharan Metacraton and/or Tuareg Shield.

New evidence for the Ontong Java Nui hypothesis

The formation of the Ontong Java Nui super oceanic plateau (OJN), which is based on the model that the submarine Ontong Java Plateau (OJP), Manihiki Plateau (MP), and Hikurangi Plateau (HP) were once its contiguous fragments, could have been the largest globally consequential volcanic event in Earth’s history. This OJN hypothesis has been debated given the paucity of evidence, for example, the differences in crustal thickness, the compositional gap between MP and OJP basalts and the apparent older age of both plateaus relative to HP remain unresolved. Here we investigate the geochemical and 40Ar-39Ar ages of dredged rocks recovered from the OJP’s eastern margin. Volcanic rocks having compositions that match the low-Ti MP basalts are reported for the first time on the OJP and new ~ 96–116 Ma and 67–68 Ma 40Ar-39Ar age data bridge the temporal gap between OJP and HP. These results provide new evidence for the Ontong Java Nui hypothesis and a framework for an integrated tectonomagmatic evolution of the OJP, MP, and HP. The isotopic data imply four mantle components in the source of OJN that are also expressed in present-day Pacific hotspots sources, indicating origin from (and longevity of) the Pacific Large Low Shear-wave Velocity Province.

Paleozoic VMS-type stratiform mineralization overprinted by Mesozoic vein-type mineralization in the Yushui copper deposit, eastern Guangdong, South China

The South China hosts a number of stratiform deposit in Late Paleozoic depression, including the Yushui copper deposit (which has a Cu grade up to 53% with an average grade of about 3.5%) in the Yong-Mei-Hui depression. The genesis type of Yushui copper ore is highly controversial due to the influence of post mineralization tectonic and magmatic activities, which affects the exploration of the deposits. Yushui copper deposit is mainly composed of stratiform ore body and stockwork orebody in the footwall, with typical VMS-type deposit characteristics and its mineralization age of 308 Ma. Small amounts of vein-type ore body can also be seen in the hanging wall dolomite of the stratiform ore body, which mainly develops along the joints and faults, and the mineral assemblage includes chalcopyrite, bornite, galena, sphalerite, tetrahedrite, furutobeite, and rare earth minerals (xenotime and bastnaesite). Hydrothermal sericite is also developed when vein-type ore body cut the sandstone in the footwall of stratiform ore body. Pristine euhedral xenotime and bastnaesite crystals from the Yushui Cu mineralized of veins ore body yield LA-ICP-MS weighted mean 206Pb/238U ages of 162.5 ± 6.7 Ma (1 s, MSWD = 11.8) and 164.6 ± 5.3 Ma (1 s, MSWD = 9.5), respectively. Hydrothermal sericite yields a plateau 40Ar–39Ar age of 163.19 ± 7.60 Ma, which is consistent with the xenotime and bastnaesite U–Pb age, suggesting that the vein-type Cu mineralized is generically related to the Middle-Late Jurassic hydrothermal event. These ages indicate that in addition to the major Late Paleozoic mineralization events that formed VMS-type stratiform ore body, there were also small-scale hydrothermal vein-type ore body formed during the Middle and Late Jurassic mineralization events that did not cause major disruptions to the earlier stratiform ore body. Combining with the chronological research of other copper deposits in the southeastern coastal area of China, this article believes that these 170 ∼ 155 Ma deposits were all due to the crust-mantle interaction caused by changes in the subduction angle of the ancient Pacific plate. Our study suggests that the southeastern margin of the South China has potential for the exploration of polymetallic hydrothermal mineralization related to Middle-Late Jurassic magmatic rock.

40Ar/39Arages ofL4,H5,EL6,and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008TC3)

AbstractThe Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC3, including ureilitic (70%–80%) and diverse non‐ureilitic materials. We determined the40Ar/39Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS‐D (EL6), as well as a clast of ureilitic trachyandesite MS‐MU‐011, also known as ALM‐A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the40Ar/39Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published40Ar/39Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum‐rich inclusion and is consistent with the interpretation that 2008 TC3was a polymict ureilite. With allowance for its 10‐Ma uncertainty, the 4549‐Ma40Ar/39Ar age of ALM‐A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative39Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of40Ar over the last 4.5 Ga. The other five such samples lost some40Ar at estimated times no earlier than 3800–4500 Ma bp. Clustering of ages in the low‐temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic‐ray exposure ages based on38Ar concentrations average 17 ± 5 Ma but may include some early irradiation.

New insights into the Piedra de Afilar Formation, Canelones department of Uruguay: Very-low-grade metamorphism, pressure solution, and age

A detailed microstructural, mineralogical, and geochronological study was carried out along a profile in the Piedras de Afilar Formation (PAF) at the González quarry and surroundings (Uruguay). Both the PAF and the Soca granite were affected by very-low-grade metamorphism and pressure-induced dissolution. These post-diagenetic processes resulted in intense corrosion and alteration indicated by sericitization, silicification, and crystallization of iron oxides. Major auto-metamorphism was identified between the intrusive gabbro and the PAF, with almost no effects on the wall rock. K–Ar ages of sericite indicate 609-604 Ma for the metamorphism and related pressure-solution in the PAF. This age is in agreement with the juxtaposition of the Edén or Nico Pérez terranes and the Piedra Alta Terrane along the Sarandí del Yí shear zone. K–Ar dating of the gabbro wall rock constrained a deposition age of the PAF between 1000 and ca. 734 Ma.

Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

New K–Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to better document the hot thermal anomaly which affected the upper mantle beneath the Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data, along with previously published ones from this and neighbor volcanic centers, suggest that this anomaly stagnated there, during at least the last 2 million years. Geochemical features from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma. Continental crust contamination in the San Rafael Block crustal basement occurred coupled with fractional crystallization processes, probably in magma chambers and across several cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro Nevado volcanic cone (1.34–1.22 Ma).

K-Ar dating, petrography, and geochemistry of diabase dikes from Sidakan area, northeastern Iraq: Implications for petrogenesis and Neotethyan tectonics

This study presents petrography, geochemistry, mineral chemistry and geochronology of diabase dikes from Sidakan area, northeastern Iraq. The area consists of basalts and diabase dikes alternating with sedimentary rocks, hosted by Walash Metavolcanic Group (WMG) as part of the Zagros Fold-Thrust Zone (ZFTZ). These rocks are dark grey to pale brown in color, fine to medium in grain size, and have ophitic and porphyritic textures. They consist of plagioclase and clinopyroxene as essential minerals and minor olivine as well as their alteration and low-grade metamorphism products including amphibole, albite, epidote, chlorite, serpentine, calcite and sericite, with ilmenite as an accessory mineral. The diabase dikes consist (as average wt.%) of 51.41 SiO2, 15.82 Al2O3, 9.95 FeOt, 6.58 MgO, 9.63 CaO, 3.42 Na2O, 0.87 TiO2, 0.17 MnO, 0.35 K2O, 0.09 P2O5, and 1.92 LOI. The Harker diagrams indicate that the diabases were formed from fractional crystallization of tholeiitic parent magma. The classification diagram Nb/Y vs. Zr/TiO2 indicated that the studied diabases have affinities of subalkaline basalts, while the SiO2 vs. K2O diagram showed that they are divided between the low-K basalt and basaltic andesite. The tectonic discriminant diagrams show that the diabases are oceanic island-arc tholeiites with N-MORB geochemical affinities. The K − Ar age of the diabases is Late Eocene (Bartonian-Priabonian) ranging between 34.6 ± 0.84 to 39.2 ± 1.81 Ma representing their intrusion/crystallization age and possibly the age of the formation of Oceanic Island Arc and/or the initial collision stage between the Arabian-Iranian microplates.

Geochronology, magma source, and geochemistry of igneous rocks in the northern Taebaeksan metallogenic region, South Korea

AbstractCretaceous igneous rocks are concentrated in the northern Taebaeksan metallogenic region of South Korea, some of which are related to Fe skarns and/or hydrothermal vein Au–Ag deposits. However, detailed studies on the emplacement age and magma source of igneous rocks supplying this metallogenic area are lacking. In this study, we investigated the emplacement age, magma sources, and geochemical characteristics of seven Cretaceous igneous rocks around the ore deposit, comparing them with previous studies. Zircon U–Pb and mica K–Ar age dating indicated that two magmatism events occurred in the Early Cretaceous (~113.7 ± 0.2 to 104.7 ± 0.5 Ma) and the Late Cretaceous (~85.8 ± 1.1 to 77.6 ± 0.4 Ma). Negative ɛHf(t) values (−2.57 to −22.05 approx.) and the calculated (2.55–1.33 Ga) suggested that the magma source of these igneous rocks was derived from the Proterozoic crust. Whole‐rock geochemical data indicated that the northern Taebaeksan igneous rocks are mostly high‐K calc‐alkaline series, enriched in light rare‐earth elements (LREEs) and large‐ion lithophilic (LIL) elements but depleted in high‐field‐strength (HFS) elements. Although igneous rocks in the northern Taebaeksan metallogenic region have broadly similar magma source characteristics, they can be classified into two groups: one comprises Early Cretaceous intermediate rocks, some of which are related to Au–Ag mineralization, whereas the other includes Late Cretaceous intermediate to felsic rocks, related to Au–Ag or Fe mineralization.

Petrogenesis of Early Cenozoic Sarıcakaya–Nallıhan Volcanism in NW Turkey: Implications for the Geodynamic Setting and Source Characterization of the Balkanatolia Magmatic Realm

Sarıcakaya–Nallıhan Volcanism was generated within the Balkanatolia Magmatic Realm between 48 and 44 Ma (by 40Ar–39Ar age determination) and is represented by three different volcanic units all displaying subduction-related geochemical signatures, such as depletion in HFSE and enrichment in LREE and LILE. The first unit (V1) consists of nepheline-normative, olivine basalts with OIB-like affinity. The second (V2) and third (V3) units are represented by more evolved compositions such as basaltic-andesitic, andesitic, and dacitic-rhyolitic lavas. Even the most basic lavas have elevated Mg# values (62–69), and they are far from representing the true mantle melts. Source characterization of Sarıcakaya–Nallıhan Volcanism reveals that there might be two possible mantle sources for the primary melts of the lavas: (i) metasomatized peridotitic mantle fluxed by sedimentary melts, or (ii) accreted mélange. The direct melting of the mélange-like lithologies is a more favorable mechanism for the Middle Eocene (44–40 Ma) magmatism in Balkanatolia since the Hf–Nd trace element, Nd isotopic systematics and petrological modelling efforts supported the latter. Overall, Early Cenozoic magmatism within this realm was characterized, first (58–44 Ma) by contractional and later (44–40 Ma) by extensional tectonics and the late-stage magmatic phase in the area was possibly controlled by melting of accreted mélange-like lithologies. The presented data indicate that mélange melting might be much more common than envisaged for the magmatism in the Alpine–Himalayan orogenic belt.

Metamorphic Ages of the Jurassic Accretionary Complexes in the Kanto Mountains, Central Japan, Determined by K–Ar Dating of Illite: Implications for the Tectonic Relationship between the Chichibu and Sanbagawa Belts

To determine the metamorphic ages of the accretionary complexes in the Northern Chichibu Belt in SW Japan, K–Ar dating was conducted using weakly metamorphosed sedimentary rocks collected from the Kanto Mountains, Central Japan. Whole-rock ages were obtained for chert and red shale samples, and the mineral ages of fine-grained illite with a grain size of less than 4 μm were obtained for chert, red shale, mudstone, acidic tuff, and basic tuff. The K–Ar ages of chert and red shale presented large variations, with systematically older ages compared to those of mudstone and tuff in the same strata. The influence of submarine hydrothermal activities on chert and red shale before subduction is a possible cause of this deviation. The illite samples, which were fractionated into four grain-size classes using a suspension method, yielded older ages and higher illite crystallinity (i.e., smaller values of Kübler’s crystallinity index) for larger grain-size classes. The peak metamorphic ages were determined from the K–Ar ages of the 3–4 μm class illite in mudstone and tuff. The Late Jurassic to the Earliest Cretaceous accretionary complex of the lowest structural unit (Kashiwagi Unit) was dated within a small range between 117–110 Ma, which is distinctly older than the K–Ar ages of white mica reported from the Sanbagawa Belt. The peak metamorphic age of acidic tuff (113 Ma) at the type locality of the Mikabu Greenstones indicates that the subducted Mikabu seamount is a constituent of the Kashiwagi Unit. The peak metamorphic ages of the Manba and Kamiyoshida Units were obtained as 132–107 Ma and 163–144 Ma, respectively. Major structural discontinuity is suggested within the Middle Jurassic accretionary complexes.

ODP459 Adiposity Rebound in Japanese Patients with Congenital Hypothyroidism Detected by Neonatal Screening

Abstract Background More than 40 years have passed since the introduction of newborn screening programs (NBS) for congenital hypothyroidism (CH), and there have been many reports on the long-term outcome of CH identified by NBS. There are some reports that CH is associated with early adiposity rebound and adult obesity. On the other hand, the association between AR and CH has not been revealed in Japan. Objective The aim of this study was to evaluate the timing of AR in individuals with CH detected by NBS in our institution, furthermore, to find out the risk factors of early AR in CH individuals. Patients and Methods The study included 212 CH children (105 female/107 males) who were ten years of age or older as of September 2021. We retrospectively collected height and weight data from one to ten years of age, and the timing of AR was visually determined. We examined the association between the AR age and the severity of hypothyroidism at NBS and at the first visit. Results The median AR age was 5.6 years for boys and 6.2 years for girls, which was unexpectedly not unexpectedly younger than that of the general Japanese population reported by Koyama (4.8 ± 1.4 years for boys, 4.7 ± 1.5 years for girls). Data from 115 patients (107 males, 105 females) whose physique data between the age of 15 and 18 were available showed that body mass index (BMI) was 21.1±3.2 kg/m2 for males and 20.6±2.7 kg/m2 for females, positively correlated with AR age, and not significantly different from BMI at 15-19 years of age (21.1±3.6 for males and 20.2±2.2 for females) in the 2019 National Nutrition and Health Survey (p=0.790, p=0.619). NBS-FT4 was slightly positively correlated to AR age (r=0.234, p=0. 0315). We found that the AR age may be earlier and adult BMI may be higher in CH patients without ossification of the distal femoral epiphyseal nucleus at first evaluation. Discussion Severe fetal hypothyroidism may cause an absence of mineralization of the distal femoral epiphyses in the neonatal period. Our results indicate that severe fetal hypothyroidism, even if diagnosed and treated at neonatal, contributes to the development of adult obesity. Conclusion The AR age and BMI at 15-18 years of CH are not significantly different from the general population. However, severe fetal hypothyroidism could result in adult obesity. Presentation: No date and time listed

Mineralogical, geochemical, and isotopic data of a new special agpaitic dyke, enriched in high field strength elements (Eastern Part of Baltic Shield, Russia)

We present the petrography, bulk major and trace elements, and isotopic compositions for the recently identified Mohnatye Roga agpaitic syenite dyke, located 30 km from the Kovdor carbonatite complex, Kola Peninsula, Russia. The dyke was identified below Quaternary deposits as the result of a gravimetric survey and subsequent drilling. A K–Ar age on lamprophyllite of 368 ± 9 Ma, coincides with the age of the Devonian Kola Alkaline Carbonatite Province (KACP). In the total alkalis vs. silica diagram, the Mohnatye Roga dyke exhibits a compositional trend from lamprophyre to peralkaline phonolite as well as mineralogical and geochemical affinities with the nearby Niva intrusion. The dyke is composed of orthoclase, Ti-rich aegirine-augite, aenigmatite, lamprophyllite-group minerals, K-amphibole, and astrophyllite with accessory noonkanbahite, calcite, ilmenite, rutile, barite, apatite, and sulfides. Late stage minerals include natrolite and ferripyrophyllite. A notable feature of this dуke is its extremely high agpaicity and enrichment in HFSEs (high-field strength elements). The concentrations of a number of HFSEs are higher in this dyke than in the Lovozero rare metal deposit. Mohnatye Roga dyke and Niva intrusion are examples of a gradual transition from melt to hydrothermal fluid. Isotopic data indicate a mantle source for the dyke, and seemingly an absence of contamination by continental crust during its formation. The geochemical and isotopic data suggest that the source of the KACP rocks, including the Mohnatye Roga dyke, is FOZO mantle which is believed to be related to plume magmatism. • Origin and age of the recently identified lamprophyllite-bearing agpaitic syenite Mohnatye Roga dyke (Kola peninsula) • Great similarity with the Niva intrusion and the link to Devonian Kola Alkaline Carbonatite province • Agpaitic SiO 2 -undersatured melts preserving the depleted FOZO plume signature • Enrichment in HFSEs of Mohnatye Roga dyke and comparison with gigant Lovozero and Khibiny deposits.

Laser-ablation ICP-MS zircon U-Pb ages for key Pliocene-Pleistocene tephra beds in unglaciated Yukon and Alaska

Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for 40 K- 40 Ar (or 40 Ar/ 39 Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (<1 Ma) tephra. Here, we demonstrate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2σ uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and 40 Ar/ 39 Ar ages: Biederman tephra (178 ± 17 ka), HP tephra (680 ± 47 ka), Gold Run tephra (688 ± 44 ka), Flat Creek tephra (708 ± 43 ka), PA tephra (1.92 ± 0.06 Ma), Quartz Creek tephra (2.62 ± 0.08 Ma), Lost Chicken tephra (3.14 ± 0.07 Ma), and GI tephra (542 ± 64 ka). We also present newly revised glass fission-track and 40 Ar/ 39 Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for 230 Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other stratigraphically important tephra in the region. • LA-ICP-MS method is well suited for rapid dating of very young zircon crystals. • Eight key tephra beds from Eastern Beringia were dated by zircon U-Pb method. • Young U-Pb ages are particularly sensitive to common-Pb and 230 Th corrections. • Bayesian modeling reduces subjectivity in calculating ages.

Exploring the Lithic Variability of the Mousterian (MIS 9–3) Open-Air Sites of the Pontine Plain (Central Italy)

This paper presents a preliminary study of the Middle Palaeolithic lithic industries recovered at six selected open-air sites in the Latium region (central Italy), part of a regional cultural complex known as the “Pontinian”. The pedo-stratigraphic analysis combined with 40Ar/ 39Ar age constraints on the sites permits us to provide for the first time a geomorphological and chronological (300–40 ka) framework for several open-air sites in central Italy. The temporal range is remarkably coincident with the Neanderthal presence in this region. Moreover, the lithic collections analysed include the oldest occurrence of a Mousterian industry in this region. The present work, therefore, provides the background context for future re-evaluation of the peculiar Pontinian techno-complex through a more in-depth technological approach and its comparison with the other Neanderthal occurrences in central Italy.

Repeated brittle reactivations of a pre-existing plastic shear zone: combined K–Ar and 40Ar–39Ar geochronology of the long-lived (&gt;700 Ma) Himdalen–Ørje Deformation Zone, SE Norway

AbstractBrittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the &gt;700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

What Caused the Inconsistency between Rb−Sr and 40Ar−39Ar Ages of Authigenic Illites?

What Caused the Inconsistency between Rb−Sr and 40Ar−39Ar Ages of Authigenic Illites?

Petrogenesis of submarine volcanic rocks dredged from Dokdo, Ulleungdo, and the neighboring seamounts in the East Sea: Constraints from mineral chemistry, geochemistry, and 40Ar/39Ar ages

Several seamounts, including the Dokdo (DD), Ulleungdo (UD) Anyongbok (AYB), Simheungtack (SHT), and Isabu, were formed from intraplate volcanism in the southwest of the East Sea back-arc basin during the Quaternary. In the present study, mineralogy and geochemistry data generated using submarine volcanic rocks collected from the DD, UD, AYB, and SHT locations were utilized to investigate characteristics of the source, evolution, and storage conditions of the associated magmas. Analyses of representative samples from the DD, UD, and AYB using the 40 Ar/ 39 Ar method yielded plateau ages of 1.566–2.658, 0.533–0.714, and 1.736 Ma, respectively. The dredged samples consist of trachybasalt, phonotephrite, trachyandesite, and trachyte and have oceanic island basalt (OIB)-like characteristics. According to numerical modeling using trace element data, the primary magma linked to the formation of these samples can originate from ≤5% partial melting of an enriched garnet–peridotite facies metasomatized by melt derived from a subducted slab. The composition of major oxides, from the trachybasalts to trachytes, and mass balance calculations suggest that the trachyandesites and trachytes were formed via fractional crystallization from a basaltic melt. These diverse rock compositions probably highlight pulses of magma that separated from the main reservoir in the shallow crust. Results from mineral–melt thermometry indicate that the olivine and plagioclase in these basaltic rocks crystallized at temperatures of 1213 ± 56 °C and 1163 ± 23 °C, respectively. Thermobarometric calculations using clinopyroxene–melt pairs indicate that the Quaternary volcanism was associated with two magma storage regions that correspondingly involved pressures of 791 ± 253 MPa (1098 ± 34 °C) and 187 ± 123 MPa (938 ± 77 °C). The primary magma was likely present in a reservoir beneath the Moho (17–32 km), and it evolved by fractional crystallization to produce rocks, such as trachybasalts and phonotephrites. Crystallization and differentiation of the basaltic magma that was transported to crustal reservoirs explain the evolved compositions. The trachytic magma generated were present at depths of 2–11 km, and crystallization produced a paragenetic assemblage comprising clinopyroxene, amphibole, plagioclase, biotite, apatite, and titanomagnetite. • Quaternary intraplate volcanisms occurred in the East Sea back-arc basin. • New 40 Ar/ 39 Ar ages of submarine volcanic rocks were obtained. • Basaltic rocks were originated from an enriched garnet-bearing mantle source. • Trachytes were formed by fractional crystallization from basaltic magma. • The Quaternary volcanism had at least two magma storage regions.

Late Cretaceous-early Paleogene magmatism in the Gyeongsang basin, southeast Korea and its implications for middle Paleogene climate change

• The youngest volcanic rocks in the Gyeongsang basin were formed at ∼ 56 Ma. • Paleogene magmatic activities indicate a transformation in tectonic regime. • Magmatic lull along east Asian and global cooling start from Cenozoic. The Gyeongsang basin is the largest volcanic-sedimentary basin in Korean Peninsula. It is characterized by widespread late Cretaceous-early Paleogene plutonic and volcanic rocks called Gyeongsang Supergroup, with its uppermost part of Yucheon group consisting mainly of andesitic to rhyolitic lavas and their derivative pyroclastic rocks. However, the precise geochronological framework of this supergroup remains elusive. In this study, we present new SIMS zircon U-Pb and whole-rock 40 Ar/ 39 Ar ages for the volcanic rocks of the Yucheon group and associated Bunam granites in the Juwangsan area, north of the Gyeongsang basin. Seven volcanic samples range in age from 71 to 56 Ma, while a granite sample has an age of 74 Ma. Combined with literature data, the Yucheon volcanic activities can be divided into two stages: 94–78 Ma and 71–56 Ma, comparable to 91–48 Ma of the granites in the basin. Actually, a gigantic igneous belt developed along the continental margin of NE Asia during the Mesozoic-Cenozoic, with a length over 10000 km and a width ∼ 1500 km. Exhibiting an eastward migration trend during 145–48 Ma and a magmatic tempo from flare-up to lull, this magmatic belt experiences a transformation in tectonic regime from active continental margin to back-arc due to subduction retreating of the Pacific subduction. The continental arc magmatic flare-up and associated contact metamorphism during Jurassic-early Paleogene may provide a huge amount of CO 2 for the global warming. With the magmatic lull after about 50 Ma, the cool Earth may be caused by less CO 2 inputs.

New geochronological and geochemical data for the Eocene shoshonitic trachyandesites in NW Iran: Constraints on their petrogenesis and tectonic setting

The Eocene magmatic flare-up in NW Iran provides valuable information on the tectonic evolution of the region. This study presents detailed geochronological and geochemical data from Eocene trachyandesitic rocks from the Ahar region of NW Iran. 40 Ar/ 39 Ar ages indicate that the Ahar trachyandesites were produced during the Eocene (44 Ma). The Ahar trachyandesites have a limited range of SiO 2 contents (53.4–54.4 wt%) and low MgO contents (1.57–2.78 wt%), and they belong to the shoshonitic series with high Na 2 O + K 2 O contents (9.1–9.8 wt%) and K 2 O/Na 2 O ratios (0.4–1). The Ahar trachyandesites are characterized by arc-like trace element patterns, including enrichment in large-ion lithophile and light rare earth elements and depletion in high field strength elements. They have depleted 87 Sr/ 86 Sr (i) ratios (0.704616–0.704736) and εNd (t) (+2.0 to +3.5) and εHf (t) (+6.6 to +6.8) values and high Th/La and Ba/La ratios, similar to those of previously studied Eocene K-rich mafic–intermediate rocks in NW Iran. The geochemical characteristics of the Ahar trachyandesites suggest that they originated from an enriched, fertile mantle source that was produced by the reaction between mantle wedge peridotite and oceanic slab-derived components (sediment-derived melts and slab-derived fluids) during the subduction of the Neo-Tethys Oceanic slab along the Zagros segment. Taking the regional tectonic evolution into consideration, we propose that NW Iran was in an extensional setting caused by the rollback of the Neo-Tethys Oceanic slab during the Eocene. Partial melting of the enriched, fertile mantle source would have produced the Ahar trachyandesites and other contemporaneous K-rich mafic–intermediate rocks in NW Iran. Accordingly, the Ahar trachyandesites and contemporaneous K-rich mafic–intermediate rocks provide a geochemical record of interaction between the crust and mantle in an oceanic subduction channel. • The Ahar shoshonitic trachyandesites were emplaced during the Eocene (44 Ma). • The mantle source of Ahar trachyandesites was enriched by slab-derived components. • The extensional setting for Eocene magmatism was triggered by the slab rollback.

Age, geochemistry and mantle source of the Alto Diamantino basalts: Insights on NW Paraná Magmatic Province

The Paraná-Etendeka is one of the largest and most thoroughly investigated large igneous provinces in the world formed in a very narrow time-range (i.e., 1.6–3.0 Ma) between 135 and 132 Ma. The South American portion of this province, i.e. the Paraná Magmatic Province, is mainly characterized by a mafic magmatism of coeval low- and high-Ti magmatic terms. This bimodal activity has been explained by different degrees of melting of a homogenous mantle source or by plume activity and metasomatism of the sub-continental mantle. Despite the large body of literature, the true origin of the mantle source and processes leading to this bimodal magmatism are still debated. In this work, we present the first geochronological, geochemical and isotopic data of basalts from Alto Diamantino (southern Mato Grosso, Brazil). These basalts have tholeiitic affinity and can also be classified in two different suites based on TiO 2 content (L-Ti < 3 wt%; H-Ti > 3 wt%) and Ti/Y (450 ppm), similarly to other volcanics from the Paraná Magmatic Province. Other geochemical parameters as Zr/Y and Nb/Yb ratios range from 3.78 to 8.60 and from 3.82 to 8.31, respectively. The L-Ti and H-Ti series have significant geochemical similarities, including 87 Sr/ 86 Sr ratios, with the Ribeira and Pitanga basalts of the Paraná Magmatic Province, respectively. The 40 Ar/ 39 Ar ages of the two series are statistically equivalent ranging between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. Our data suggest that the Alto Diamantino basalts constitute the northwestern most expression of the Paraná Magmatic Province. Sr and Nd isotope ratios suggest that the original source of the Alto Diamantino basalts was a EM-I metasomatized mantle characterized by small amounts of different components (DMM and EM-II). Positive Ba and lack of Pb anomalies, together with K/Rb ratio, indicate the presence of amphibole in the mantle source of the Alto Diamantino basalts. Th/Nb and TiO 2 /Yb ratios suggests that the compositional difference between the L-Ti and H-Ti basalts could be explained by different degree of melting at different depths of a heterogeneously metasomatized mantle source. The H-Ti group of basalts is compatible with low degrees of partial melting at depth while the L-Ti suite records larger degrees of melting. The metasomatic event occurred after the Mesoproterozoic, possibly during the Neoproterozoic formation of the Gondwana supercontinent. • Alto Diamantino basalts are northernmost part of Paraná Magmatic Province. • Low- and High-Ti basalts have similar geochemical affinity but different evolution. • Magmatic series are coeval between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. • Mantle source was refertilized after Mesoproterozoic subduction by hydrous melts. • L- and H-Ti basalts derived from different melting degree of a heterogeneous source.