Ar Ages Research Articles

A revised stratigraphic framework for Cretaceous sedimentary and igneous rocks at Mokka Fiord, Axel Heiberg Island, Nunavut, with implications for the Cretaceous Normal Superchron

New data and interpretations on geological relationships of igneous rocks at Mokka Fiord, Axel Heiberg Island, Nunavut, provide insight into the timing and nature of magmatism associated with the Sverdrup Basin and High Arctic Large Igneous Province (HALIP). Field relationships indicate that the igneous rocks, previously interpreted to be volcanic flows, are most likely an intrusive unit discordant to regional bedding. An intrusive origin helps resolve chronostratigraphic inconsistencies in previous work. The host rocks are palynologically constrained to be late Barremian to late Aptian in age and are interpreted to be Paterson Island or Walker Island member of the Isachsen Formation. If the igneous body is intrusive, it’s previously reported Ar–Ar age (102.5 ± 2.6 Ma) is no longer in conflict with accepted stratigraphic interpretations and probably reflects the emplacement age of the intrusion. Lingering uncertainties in interpreting the normal and reverse magnetic polarities determined in the previous work remain, and both are considered viable. Although this uncertainty precludes definitive conclusions on the significance of paleomagnetic data at Mokka Fiord, examination of the stratigraphic, paleomagnetic, and geochronologic relationships there highlight potential for the study of excursions, or reversed magnetic polarity subchrons, in the Cretaceous Normal Superchron elsewhere in the HALIP.

Isotopic and Petrologic Investigation, and a Thermomechanical Model of Genesis of Large-Volume Rhyolites in Arc Environments: Karymshina Volcanic Complex, Kamchatka, Russia

This study examines the largest center of silicic magmatism, 4-0.5 Ma Karymshina Volcanic Complex, which also includes the largest 25x15 km Karymshina caldera in Kamchatka. A series of rhyolitic tuff eruptions at 4Ma were followed by the main eruption at 1.78Ma produced an estimated 800 km3 of rhyolitic ignimbrites followed by ring-tracture high-silica rhyolitic post-caldera extrusions. We here present results a of geologic, petrologic, and isotopic study of this complex, and present new Ar-Ar ages, and isotopic values of rocks. Basement of the complex is formed by 4-3.5 Ma rhyolitic ruffs, the oldest pre-1.78Ma caldera ignimbrites and intrusions include a diversity of compositions from basalts to rhyolites. All rocks are crystal-rich with quartz, plagioclase, biotite, and amphibole phenocrysts. Temporal trends in δ18O, 87Sr/86Sr, 144Nd/143Nd indicate values comparable to neighboring volcanoes, increase in homogeneity, and temporal increase in mantle-derived Sr and Nd with increasing differentiation over the last 4 million years. Data are consistent with a batholithic scale magma chamber formed by primarily fractional crystallization of mantle derived composition and assimilation of Cretaceous and younger crust, driven by basaltic volcanism and mantle delaminations. Rhyolite-MELTS crystallization models favor shallow (2 kbar) differentiation conditions and varying quantities of assimilated amphibolite partial melt and hydrothermally-altered silicic rock to reproduce the compositions seen at Karymshina Eruptive Center. Results of thermomechanical modeling with a typical 0.001km3/yr eruption rate of hydrous basalt into a Kamchatkan arc crust produces two magma bodies, one near the Moho and the other engulfing the entire section of upper crust. Basalt is getting trapped in the lower portion of the upper crustal magma body, which exists as partially molten to solid state. Differentiation products of basalt periodically mix with the resident magma diluting its crustal isotopic signatures. At the end of the magmatism crust is getting thicker by 8 km. Modeling show that the only way to generate large spikes of rhyolitic magmatism is through delamination of cumulates in the lower crust. The paper also present chemical dataset for Pacific ashes from ODDP 882 and 883 and compares them to Karymshina ignimbrites and two other other Pleistocene calderas studied by us earlier.

Mineralogy, geochemistry and40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation

AbstractMost continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.

40Ar-39Ar ages and petrogenesis of middle Eocene post-collisional volcanic rocks along the Izmir-Ankara-Erzincan suture zone, NE Turkey

The central and northeastern of Turkey were shaped by the transition from collisional to post-collisional tectonics and coeval magmatism during Early Cenozoic. However, the temporal and spatial evolution of the magmatism during the middle Eocene remains elusive. In this paper, we provide petrological, isotopic and radiometric data from two middle Eocene volcano-sedimentary successions, from the northern (Almus) and southern (Yıldızeli) parts of the Izmir–Ankara–Erzincan Suture Zone, and discuss their generation in the post-collisional setting. Our results indicate that the volcanic units from these regions display similar stratigraphic and petrological evolution during the narrow window of activity (44.4–44.8 Ma), which is constrained by Ar–Ar dating. The lower parts of the volcano-stratigraphic sections contain hawaiite, and trachybasalts that have a mildly alkaline, nepheline-normative character (V1a subseries), together with clinopyroxene-hornblendite xenoliths. The middle portions of the sections contain basaltic andesitic to dacitic lavas that become tholeiitic/calc-alkaline in character (V1b subseries). In the middle to upper parts of the successions, tholeiitic/calc-alkaline basaltic andesite (V2a subseries) and nepheline-normative mildly alkaline basalts (V2b subseries) become dominant, marking the end of the volcanic episode.All of these units display negative high field strength element values, and enriched large-ion lithophile and light rare earth element values, which mimic subduction-related signatures. The Na-rich character, high Ba/Rb and low Rb/Sr ratios, together with FC3MS values (wt.% FeO/CaO-3 * MgO/SiO2), of these series indicate the presence of amphibole and pyroxene minerals in the source area. Also, Dy/Yb values from the most basic suites, V1a and V2b, are supportive of a spinel-bearing source area. Low-degree partial melting (∼1–5%) modelling of cumulate clinopyroxene-hornblendite xenoliths in the lava series provided the observed geochemical patterns in our most basic lava samples. Petrological observations indicate that the V1a and V1b subseries were shaped by fractional crystallization and assimilation-related modifications. In addition, the V2a subseries displays basic magma replenishment, with inversed clinopyroxene cores and mantle/rim transects. The V2b subseries were most likely shaped by fractional crystallization of homogenized basic magma mush.Post-collisional magmatism across the region was therefore generated by thinning of the lithospheric mantle during the middle Eocene.

Petrogenesis and mantle source characteristics of volcanic rocks on Jeju Island, South Korea

Volcanic rocks on Jeju Island, South Korea, are dominantly alkaline suites (basalt to trachyte) with small amounts of intercalated tholeiitic (TH) basaltic rocks. The alkaline suites can be subdivided into high-Al alkaline (HAA) and low-Al alkaline (LAA) sub-suites. We determined the major- and trace-element abundances, and Sr–Nd–Pb–Mg isotopic compositions for 60 samples from four boreholes and nine outcrops on Jeju Island to better understand their petrogenesis and mantle source characteristics. The samples yield whole-rock40Ar/39Ar ages of ~968–24 ka. The HAA suite formed slightly earlier than the other suites. On a primitive-mantle-normalized trace-element diagram, Jeju basaltic rocks exhibit typical oceanic island basalt (OIB)-like large-ion lithophile element (LILE) enrichments without significant Nb or Ta depletions. No meaningful correlation was observed between SiO2 and Sr–Nd–Pb–Mg isotope compositions. However, some samples, especially the TH and HAA suites, exhibit positive Eu and Sr anomalies, coupled with elevated 87Sr/86Sr, Δ7/4Pb, and Pb/Ce ratios, indicating an important role of ancient anorthosite assimilation during its evolution. Furthermore, they show negative K and Rb anomalies. The basaltic rocks have fractionated Nb/Ta and Zr/Hf ratios with negative Ti anomalies. They have elevated Dy/Yb ratios within the range of melts derived from garnet lherzolite, but have Yb contents much higher than those of garnet lherzolite melts, instead plotting close to those of eclogite-derived melts. They also have high Fe/Mn ratios (>~60) and FeO/CaO–3MgO/SiO2 values (>0.45), similar to melts from pyroxenite/eclogite sources. The Sr–Nd–Pb isotopic compositions display a good linear array between depleted mid-ocean-ridge basalt (MORB) mantle and enriched mantle type 2 (EM2). There are no discernible 143Nd/144Nd, Δ8/4Pb, and δ26Mg isotopic differences among the three suites, implying their derivation from a compositionally homogeneous source by variable degrees of melting. The Mg isotopic compositions yield δ26Mg values of −0.53‰ to −0.20‰, extending to a much lower value than normal mantle (δ26Mg = −0.25‰ ± 0.04‰). These observations suggest the presence of recycled rutile-bearing eclogite, carbonate, and sedimentary components in the mantle source of the Jeju volcanic rocks together with a peridotite component. No significant spatiotemporal changes in the mantle sources and melting conditions were observed.

The Olimpiadinskoe Gold Deposit (Yenisei Ridge): Temperature, Pressure, Composition of Ore-Forming Fluids, δ34S in Sulfides, 3He/4He of Fluids, Ar–Ar Age, and Duration of Formation1

Abstract —New thermobarogeochemical and isotope-geochemical data are presented, which show the intricate and long history of the formation of the unique Olimpiadinskoe gold deposit with predicted gold reserves of >1000 tons on the Yenisei Ridge. Metal-bearing oxidized water–carbon dioxide and reduced carbon dioxide–hydrocarbon fluids participated (at the same time or successively) in the formation of the deposit at 220–470 °C and 0.6–2.5 kbar. Fluids of gold-bearing mineral assemblages include CO2, hydrocarbons, and S-, N-, and halogen-containing compounds capable of transporting ore elements, including gold. Highly mobile carbon dioxide–hydrocarbon fluids were responsible for the appearance of disseminated gold mineralization in large bodies of quartz–carbonate–mica schists serving as geochemical barriers in the Olimpiadinskoe deposit. The deposit formed in the period from 817 to 660 Ma, which fits the time interval from crystallization to cooling (868–721 Ma) of the most proximal multiphase Chirimba granitoid pluton. The hydrothermal activity of the fluids that formed the Olimpiadinskoe deposit lasted at least 100–150 Myr year.

Peraluminous granitoid magmatism from isotopically depleted sources: The case of Jing'erquanbei pluton in Eastern Tianshan, Northwest China

The Eastern Tianshan region is a critical segment of the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic belts in the world. Several pulses of isotopically depleted Late Palaeozoic calc‐alkaline and high‐K calc‐alkaline granitoid magmatism are known to be associated with the collisional orogeny in the CAOB. However, no systematic studies have been conducted on the peraluminous granites that were formed during the Late Palaeozoic collisional orogeny in Eastern Tianshan. Here, we present major and trace element characteristics, whole‐rock Sr–Nd isotopes, and zircon U–Pb and Lu–Hf data from the Jing'erquanbei pluton in the eastern part of Eastern Tianshan. Zircon grains from the granite yield a weighted mean U–Pb age of 250 ± 4 Ma, which is consistent with the previously determined 40Ar–39Ar age of muscovite in the associated ore body, suggesting Late Permian magmatism. The pluton shows a wide range of Sr isotope ratios (87Sr/86Sr = 0.705777–1.308836) but depleted Nd (εNd(t) = +6.71 to +8.21) and Hf (εHf (t) = +12.6 to +14.6) isotopic values. The Nd and Hf isotopic values are also consistent with the Nd and Hf isotopic values of the Carboniferous mafic volcanics (εNd(t) = +5.50 to +9.66, εHf (t) = +6.3 to +13.2) in Eastern Tianshan. The whole‐rock Nd model age (TDM2 = 350–480 Ma) and zircon Hf model age (TDM2 = 342–451 Ma) as well as the Carboniferous inherited zircon (341 ± 5 Ma) suggest that original magma was derived from young and depleted juvenile crust source. The granite is characterized by high SiO2 (74.50%–75.80%), enriched alkali contents with high sodium values (K2O + Na2O = 5.65%–8.20%, Na2O/K2O = 1.24–2.74), concentration of peraluminous elements (Al2O3/(CaO + Na2O + K2O) [A/CNK] = 1.18–1.24), high large‐ion lithophile element content, low Nb/Ta (9.97–10.64) ratio, variable Rb/Sr (0.18–16.11) ratios, and negative Eu anomalies (Eu/Eu* = 0.05–0.81). These features suggest that some minerals, such as biotite, plagioclase, and apatite, underwent fractional crystallization during the formation and evolution of the pluton. Considering the regional magmatic activity as well as tectonic evolution, we speculate that peraluminous granites of the Huangshan–Jing'erquan area of Eastern Tianshan were generated during the postcollisional extension in the Late Permian. Dehydration and melting of the Carboniferous mafic volcanics through heat input from mafic–ultramafic magma in the Middle Permian is considered to have generated the peraluminous granites.

Mineralogical constraints on the thermal history of martian regolith breccia Northwest Africa 8114

Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.We report an approximate 40Ar-39Ar maximum age of 1.13–1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days.Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.

Age constraints on high-pressure/low-temperature metamorphism and sedimentation in the Luk Ulo Complex (Java, Indonesia)

Pre-Cenozoic blueschist- and eclogite-facies rocks occur widely scattered in the central Indonesian region, possibly representing lateral equivalents of the same subduction zone. High-pressure/low-temperature (HP/LT) metamorphism is thought to have occurred during the Early Cretaceous, which however is only constrained by K–Ar ages. This study is part of a systematic re-investigation of the regional geochronological record of various metamorphic rocks from this belt and focuses on the Luk Ulo Complex in central Java. Rubidium–Sr multipoint isochrons of one eclogite, two epidote-glaucophane schists and one epidote amphibolite yielded Early Cretaceous (Aptian) ages of 119–117 Ma (± 0.6–0.8 Ma). These ages are similar to previously reported K–Ar results, indicating that the existing data had not significantly been compromised by extraneous Ar contamination. Relatively low peak metamorphic temperatures (ca. 400 °C) during the HP/LT stage suggest that the Rb–Sr isotopic system was not significantly affected by diffusive loss during cooling. The Rb–Sr ages most likely constrain the time of mica growth or recrystallization during high-pressure metamorphism. The matching ages of high-pressure and lower pressure rocks are attributed to inheritance, but mixing of rocks that originally had formed at the same time at different depths of the subduction complex cannot completely be ruled out yet. Rubidium–Sr white mica ages of eclogite- and blueschist-facies rocks from central Java are younger than those of similar rocks from the Western Sulawesi tectonic province (130–120 Ma), revealing a previously unknown age difference that had been hidden by the poorer precision of the K–Ar dataset. At this point, it remains unclear whether this age difference is related to processes operating within the same or in different subduction zones.The zircon populations of clastic metasedimentary rocks from the Luk Ulo Complex are dominated by a prominent Jurassic to Carboniferous population (ca. 320–180 Ma) with a distinct Permo-Triassic peak (ca. 270–200 Ma). Another less pronounced Paleozoic age signal shows up at 450–380 Ma. Proterozoic and Archean zircon grains are very rare. At least some data points at the lower end of the age spectrum (ca.135–117 Ma) most likely are related to in-situ metamorphic processes. The youngest zircon grains of unambiguous detrital origin show up at ca. 160 Ma, indicating a Late Jurassic maximum depositional age. Additional detrital zircon data for the Barru Complex (Western Sulawesi) further substantiate previous findings documenting a complex age spectrum with contributions of Mesozoic (Triassic), Paleozoic (Permian, Carboniferous, Devonian to Ordovician), Meso- to Paleoproterozoic subpopulations and rare Archean zircons. The youngest zircons indicate a maximum depositional age of ca. 255–250 Ma. New and existing U–Pb zircon data indicate that the pre-metamorphic basin histories recorded in the Luk Ulo and Barru complexes are linked to different source terrains and depositional systems.

Geochronology and Zr-in-rutile thermometry of high-pressure/low temperature metamorphic rocks from the Bantimala complex, SW Sulawesi, Indonesia

The timeframe of eclogite- and blueschist-facies metamophism in the central Indonesian region is poorly defined. Relative to its geographic extent, the regional coverage of isotopic age data is insufficient for an unambiguous interpretation of the geologic evolution of the Central Indonesian Accretionary Collision Complex. This study reports new geochronological constraints for high-pressure/low-temperature (HP/LT) rocks from the Bantimala Complex, SW Sulawesi, which further substantiate the importance of Cretaceous HP/LT metamorphism for the larger region. Multipoint Rb–Sr mineral isochrons of eclogites and glaucophane-rich rocks display a narrow range of white mica ages (130–120 Ma), interpreted to postdate peak-HP conditions, which is considerably smaller and more precise than the previously published K–Ar age spectrum (137–113 Ma). The close similarity with K–Ar ages suggests that these analyses were only slightly contaminated, if at all, with an extraneous Ar component, which did not significantly compromise the geological relevance of the existing dataset. An important result of this study is the emerging evidence for regional age differences between HP/LT rocks from SW Sulawesi (130–120 Ma) and Java (ca. 118 Ma). The existence of different subduction zones cannot be excluded but P–T and age differences between these occurrences can be reconciled with a model suggesting formation in the same subduction complex. Zircon is very rare in the studied HP/LT rock suite, but two glaucophane-rich rocks were suitable for U–Pb zircon dating and indicate Late Triassic–Early Jurassic protolith ages (ca. 205–185 Ma). The zircon population of a retrogressed eclogite records a more complex history including evidence for igneous crystallization, metamorphic recrystallization or growth, inheritance and variable degrees of Pb-loss. Metamorphic zircon rims mostly yielded U–Pb ages in the 135–110 Ma range that broadly confirm metamorphic ages determined by K–Ar and Rb–Sr white mica geochronology. Zircons that are significantly older (2.55–1.4 Ga and 400–260 Ma) than the protolith age are interpreted to represent detrital grains from subducted sediments that had been incorporated in mafic melts directly derived from a contaminated mantle source. The presence of continentally-derived detrital zircons that have presumably been included during melt formation in the mantle have not yet been described for any eclogite- and/or blueschist-facies rock of basaltic or gabbroic parentage.

Neoproterozoic magmatic and metamorphic events in the Cuchilla Dionisio Terrane, Uruguay, and possible correlations across the South Atlantic

The Cuchilla Dionisio (or Punta del Este) Terrane of the southern Dom Feliciano Belt, eastern Uruguay, experienced a prolonged Neoproterozoic to Cambrian tectonothermal evolution. Zircon and monazite age data from orthogneisses of the basement of the terrane (Cerro Olivo Complex) provide evidence of widespread magmatism at c. 780 Ma and high-grade, up to granulite-facies metamorphism between 655 and 632 Ma. Isotope data imply that the orthogneiss protolithic melts formed in a within-plate, rift-related setting by Neoproterozoic (re-) melting of lower crustal material that was extracted from the mantle during the Palaeoproterozoic. We show that the terrane extends further north than previously thought. The presence of inherited Meso- and Palaeoproterozoic zircon grains indicates the existence of older crustal components in the terrane. The ages of the detrital zircon grains and their Hf isotope signatures cannot be explained by a Río de la Plata Craton provenance but are consistent with a southern African Kalahari Craton origin. Ar-Ar ages of 628 and 625 Ma were obtained on magnesiohornblende in amphibolite from the northern Cuchilla Dionisio Terrane, whose protolith formed in an Ediacaran back-arc basin. These ages are interpreted as the time of amphibolite-facies metamorphism. Thus, crustal convergence between the Kalahari (or Congo?) and the Río de la Plata Craton and subduction of the intervening oceanic basin must have been underway between 655 and 625 Ma.The early Neoproterozoic basement of the Cuchilla Dionisio Terrane is part of the southern African Namaqua Province that was separated from the Kalahari Craton during Rodinia break-up and opening of the southern Adamastor Ocean in the Tonian. Closure of this ocean in the late Cryogenian to early Ediacaran was associated with arc magmatism and high-temperature metamorphism of the Cuchilla Dionisio Terrane basement. A back-arc basin formed and was closed between this arc and the western African margin. Relics of this basin were obducted onto the Cuchilla Dionisio Terrane at c. 625 Ma. Late Neoproterozoic intrusive igneous rocks from the Cuchilla Dionisio and the Coastal terranes were emplaced in a supra-subduction setting (at least in Uruguay) and have the same Hf isotope signature.

Nature, age and emplacement of the Spongtang ophiolite, Ladakh, NW India

The Spongtang ophiolite (Ladakh, NW India) constrains the nature of oceanic lithosphere before Indo-Asia collision and key stages in the development of the Himalayas. We report whole-rock 40 Ar/ 39 Ar and in situ zircon 238 U– 206 Pb ages from its crustal and upper and lower mantle sequences. Major and trace elements from harzburgite minerals suggest that the ophiolite formed at a mid-ocean ridge-type spreading centre, whereas published spinel compositions from Spongtang dunites are consistent with a suprasubduction-zone setting. Rare earth element-in-two-pyroxene thermometry for the harzburgite yields 1058 ± 13°C whereas temperature from solvus-based two-pyroxene and olivine–spinel thermometry is lower (to 656°C). The distribution suggests that the mantle section of the ophiolite cooled at rates of 100° Ma −1 or slower. Based on ages, major and trace element geochemistry, and geospeedometric estimates, we model the origin of the Spongtang ophiolite as forming within a mid-ocean ridge-type spreading centre with a spreading rate >2 cm a −1 in the Neotethyan Ocean, possibly from the Late Triassic to Jurassic. By the Early Cretaceous, the ridge experienced increasing influence of subduction beneath the Spongtang oceanic lithosphere owing to a subduction polarity reversal. Based on 238 U– 206 Pb ages of the youngest Cenozoic zircon grain, latest obduction occurred between 64.3 ± 0.8 and 42.4 ± 0.5 Ma, in accordance with 56.7 ± 5.2 Ma whole-rock 40 Ar/ 39 Ar ages. Supplementary material: Excel files with details of electron microprobe and inductively coupled plasma mass spectrometry (ICP-MS) analyses, argon isotopic whole-rock and secondary ion mass spectrometry (SIMS) analyses, and the T REE calculations, including an inversion diagram showing regression through measured REE distributions in cpx and opx (from Liang et al. 2013), are available at https://doi.org/10.6084/m9.figshare.c.4261856

“Rejuvenated” Globular Phyllosilicates in the Riphean Deposits of the Olenek Uplift (North Siberia): Structural Identification and Geological Significance of Rb–Sr and K–Ar Age Data

The complex mineralogical–geochemical and isotope–geochronological study of globular phyllosilicates (GPS) of glauconite–illite series from Upper Proterozoic deposits of the Olenek Uplift (North Siberia) was performed. The Rb–Sr and K–Ar dates of the post-diagenetic transformation stages of GPS from the Osorkhayata and Arymass formations of the Solooly series were obtained. These dates along with the model cation distribution in the structure of minerals and comparison of the model results with the Mossbauer and IR spectroscopic data give grounds to assume that the post-diagenetic transformation of crystalline structure of GPS and their chemical and isotope compositions occurred during the Pre-Debengda hiatus (1280–1250 Ma).

Blueschist facies overprint of late Triassic Tethyan oceanic crust in a subduction–accretion complex in north-central Anatolia, Turkey

The late Cretaceous accretionary complex along the İzmir–Ankara–Erzincan suture zone, northern Turkey, includes various types of metamorphic rock together with radiolarite, sandstone, mudstone, serpentinite, basalt and limestone. Meta-plagiogranite blocks (up to 5 m in diameter) and a meta-gabbro slice (800 × 500 m) cross-cut by meta-plagiogranite are observed in a matrix of serpentinite, mudstone and radiolarian chert. These meta-gabbros and meta-plagiogranites show subduction-related geochemical characteristics. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons from two meta-plagiogranites yielded 222.3 ± 1.5 and 227.2 ± 1.6 Ma crystallization ages. This study shows that the meta-gabbro and meta-plagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late Triassic lower crustal rocks of the subducted Tethyan oceanic crust. They are the oldest lower crustal rocks observed in the İzmir–Ankara–Erzincan suture zone. Both the meta-gabbros and meta-plagiogranites have Na–Ca amphiboles, indicating blueschist facies metamorphism. However, phengites from a blueschist facies rock yielded an 40 Ar/ 39 Ar age of 104.3 ± 0.7 Ma, indicating that blueschist metamorphism occurred during the Albian. All these data suggest that the oceanic crust formed during the Norian, metamorphosed during the Jurassic to early Cretaceous and then dismembered during accretionary complex formation in the late Cretaceous. Supplementary material: A complete description of the analytical methods ( electron microprobe, LA-ICP-MS zircon U–Pb and 40 Ar/ 39 Ar age analyses, pseudo-section modelling and THERMOCALC) are available at https://doi.org/10.6084/m9.figshare.6509984 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Gold Mineralization in Izu Peninsula, Central Japan, during Crustal Extension in Response to Double Subduction

AbstractIzu Peninsula in central Japan, the northern tip of the Izu‐Bonin arc, hosts numerous epithermal Au–Ag vein deposits of low‐sulfidation style. All have similar vein textures, mineralogy, and alteration. Geochemical data from fluid inclusions in vein quartz, the mineralogy and mineral chemistry of alteration, and stable isotope data indicate that auriferous hydrothermal activity occurred under subaerial conditions. The K–Ar ages of auriferous vein minerals are <1.5 Ma, indicating that the mineralization took place after extensive submarine volcanism for the host rocks. These observations suggest that Au–Ag mineralization was synchronous with the development of an extensional regime of the Izu block after its collision with the Honshu arc after 1.5 Ma. This collision resulted in the shifting of the Izu block far from the trench to the rear position, and the subduction of the Izu block along the Suruga trough to the west and along the Sagami trough to the east. The reararc position of the Izu block and double subduction resulted in crustal extension, upwelling of asthenospheric mantle, and tholeiitic magmatism reflected by mafic dyke swarms and subsequent monogenetic volcanic activity in the Izu peninsula. The timing of the Au mineralization in the Izu Peninsula during the beginning of lithospheric extension is similar to that of the Sado Au–Ag deposit on Sado island in the Japan Sea. Two mineralization events coincide with extensive tholeiitic mafic volcanism and injections of dyke swarms related to the back‐arc opening of the Japan Sea. The geological setting of the Au–Ag mineralization in Izu and Sado is also similar to that of the epithermal Au–Ag deposits in northern Nevada, where mineralization was contemporaneous with crustal extension and tholeiitic mafic magmatism derived from the asthenospheric mantle. This study suggests that epithermal Au mineralization at shallow crustal depths is a product of large‐scale lithospheric evolution.

The Red Beds series in the Erta Ale segment, North Afar. Evidence for a 6 Ma-old post-rift basin prior to continental rupturing

The Erta Ale rift segment, North Afar, is regarded as the most mature rift part within the entire Afar rift system. Very little is known about its deformation history because of limited exposures of geological records in its inner floor, except volcanics of the Erta Ale chain, and the poorly-known Red Beds series along the flanks of the depression. An integrated study, combining sedimentological, geochemical, 40Ar-39Ar radiometric and tectonic approaches, has been devoted to Red Beds series flanking the depression to the SW. Our new results allow to argue that (1) the >300 m-thick Red Beds exposed section comprises alluvial deposits that enclose (2) basaltic lavas and related sill intrusions that both yield ~6 Ma 40Ar-39Ar ages and display similar geochemical affinities, (3) the Red Beds series locally overlap unconformably basement bounding terrains, and (4) are involved in a limited number of low-displacement normal faults that recorded a modest amount of extension (<6%), (5) isotope contents of Red Beds volcanics indicate crustal contamination, without any contribution of the Afar plume, by contrast to the younger Erta Ale magmatism which represents the more recent Afar-plume related event in the Erta Ale segment. Combining these results leads us to regard the Red Beds series as part of an alluvial basin that post-dated a major rift event to which are attributed to (1) the present-day Ethiopian fault-scarp, (2) a concealed sedimentary depocenter at depth in its hanging wall, and (3) prominent crustal thinning. Riftward migration and axial focusing of strain during Miocene-Present times is also argued, while later flexuring of the entire Red Beds basin is assigned to magmatic loading during the axial emplacement of the Erta Ale volcanic chain. Lastly, emphasis is put on the large-scale segmentation of the Afar system into the tectonically-accreted Erta Ale rift segment to the north, and the magmatically-accreted Central segment to the south.

U-Pb, Rb-Sr and Ar-Ar systematics of the ungrouped achondrites Northwest Africa 6704 and Northwest Africa 6693

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar, and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013; Hibiya et al., 2019). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76 + 0.22/−0.30 Ma for NWA 6704 and 4562.63 + 0.29/−0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784 ± 0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543 ± 46 Ma (initial 87Sr/86Sr = 0.699013 ± 0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576 ± 250 Ma for NWA 6704 (Hibiya et al., 2019), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age (Sanborn et al., 2019), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199 ± 32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997 ± 0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5 ± 2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al., 2012).

Characterization of a carbonate karstic aquifer flow system using multiple radioactive noble gases (3H-3He, 85Kr, 39Ar) and 14C as environmental tracers

Groundwater age in a carbonate karstic aquifer was assessed using a multiple tracer method that enables identification of modern groundwater (recharged after 1955; using 3H-3He, 85Kr CFCs, SF6,), older components (39Ar, 14C) and quantification of the mixing between them. Twelve wells were sampled in the Eastern Mountain Aquifer (EMA) of Israel along two trajectories, from the recharge area in the mountains, to the natural outlets in the Dead Sea area. The concentration of the dissolved 39Ar in the groundwater decreased from 96 to 12% along the trajectories, indicating recent recharge upstream, and groundwater aged more than 800y downstream. Other tracers present a similar general trend of decreasing concentrations with distance from the recharge area at two distinct rates, suggesting two different groundwater flow velocities in the two different groundwater flow paths. In most of the wells, pronounced mixing was observed according to the presence of young (after 1955) and older water components. The fraction of the young water was quantified by tritium (3H) and by the combination of 3H and 85Kr measurements and found to be between 1 and 67%. The wide age distribution is likely caused by the karstic nature of the aquifer with pronounced dispersion and exchange between highly permeable flow channels and stagnant water stored in the rock matrix. Another mixing mechanism is vertical leakage from the upper to the lower sub-aquifer and vice versa according to the groundwater head differences between the two sub-aquifers. Mixing, diffusive exchange and water rock interaction lead to a reduction of 14C in DIC, resulting in an apparent half-life of ∼900 y instead of 5730y for radioactive decay only. This is concluded from the comparison of 14C and 39Ar ages.

Asynchronous formation of the adjacent epithermal Au-Cu and porphyry Cu-Mo deposits in the Zijinshan orefield, southeast China

Adjacent high-sulfidation epithermal (HSE) and porphyry deposits often have similar Cu-Au metal association, suggesting they are cogenetic and formed contemporaneously. The Zijinshan orefield (ZOF) is the largest Au producer in China and contains a series of HSE Au-Cu and porphyry Cu-Mo deposits. This study presents new (1) muscovite 40Ar-39Ar ages for the HSE deposit in this area, new (2) zircon laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages for felsic igneous rocks associated with the epithermal and porphyry mineralization, and (3) new zircon Lu-Hf isotopic data for these intrusions, all of which provides insights into why these adjacent epithermal Au-Cu and porphyry Cu-Mo deposits have different metal associations.The age data indicate that the alunite-dickite alteration related to the HSE Au-Cu mineralization formed before ca. 110 Ma and the main HSE Au-Cu mineralizing event occurred at ca. 113 Ma. In comparison, the adjacent porphyry Cu-Mo mineralization formed at ca. 104 Ma, ca. 9 Myr after the HSE event. The volcanic rocks associated with the HSE Au-Cu mineralization yield zircon εHf (t) values from −4.1 to 0.3, whereas the Cu-Mo mineralized porphyry has lower zircon εHf (t) values (−4.5 to −1.2), suggesting that the magmas that formed the latter intrusion were derived from a source containing more crustal material than the magmas associated with the HSE mineralization. The differences in the timing of mineralization and the sources of the magmas indicate that the adjacent HSE and porphyry deposits in the ZOF formed from different magmatic-hydrothermal systems that controlled the different metal associations of the mineralizing systems. This means that future mineral exploration within the ZOF should target porphyry Au-Cu deposit at depths that is genetically related to the HSE Au-Cu deposit. The results also show that not all adjacent porphyry and epithermal deposits, especially those with different metal associations, are cogenetic.

Post-spreading volcanism in the central South China Sea: insights from zircon U–Pb dating on volcaniclastic breccia and seismic features

Post-spreading volcanism in the central South China Sea (SCS) is of broad interest and has been well-studied using mainly geophysical data. The zircon U–Pb dating of volcaniclastic breccia cores recovered by the International Ocean Discovery Program (IODP) Expedition 349 provides important insights into the characteristics and origin of the volcanism in conjuction with the evolution of the SCS and related regional tectonic events. The results show that two volcanic eruptions were dated as 11.29 ± 0.53 Ma at Site U1431 in the East sub-basin and 7.56 ± 0.28 Ma at Site U1434 in the Southwest sub-basin; both ages are in good agreement with the shipboard ages derived from biostratigraphic and paleomagnetic datums and are a little older than the published K–Ar ages of dredged samples from nearby seamounts. Furthermore, by integrating the IODP borehole data and regional multichannel seismic data, the distribution of the post-spreading volcanism in the central SCS is mapped through geological time. The volcanism was mainly concentrated in the Late Miocene and did not occur immediately after the cessation of seafloor spreading at ~ 15.5 Ma. The origin of the post-spreading volcanism in the central SCS is discussed and we propose that it resulted from the post-spreading cooling and subsidence of the SCS, the changed subduction direction of the Philippine Sea plate, the rapid uplift of the southeastern Tibetan Plateau in the Late Miocene and potential Hainan mantle plume activities.