Ar Ages Research Articles

Short-lived alkaline magmatism related to the Réunion plume in the Deccan Large Igneous Province: inferences from petrology, 40 Ar/ 39 Ar geochronology and palaeomagnetism of lamprophyre from the Sarnu-Dandali Alkaline Igneous Complex

Abstract Available geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40 Ar/ 39 Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40 Ar/ 39 Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 ± 1.5 Ma and 68.17 ± 1 Ma. However, palaeomagnetic results show a VGP at 32.31° N and 298.52° E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 ± 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 ± 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB-type enriched (metasomatized) mantle source with an involvement of phlogopite. This finding suggests that the pre-Deccan ages of c. 68–69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67–65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.

Genesis and fluid evolution of the Huangtan Au-Cu deposit in the Kalatag district, Eastern Tianshan, NW China: Constraints from geology, geochronology, fluid inclusions, and H-O-S-Pb isotope geochemistry

The newly discovered Huangtan Au-Cu deposit is located in the central Dananhu - Tousuquan arc of Eastern Tianshan, southern Central Asian Orogenic Belt (CAOB). It is the first Au-dominated volcanogenic massive sulfide (VMS) polymetallic deposit in the Eastern Tianshan. Veinlet-disseminated and massive orebodies are hosted within Early Silurian pyritic phyllic tuff and volcanic breccia and controlled by a secondary fracture zone of the Kalatag fault with extensive hydrothermal alteration. Four primary alteration/mineralization stages have been recognized as follows: (1) Early ore stage (S1), forming mainly ore-barren fine quartz veins with minor gold-bearing sulfides; (2) main ore stage (S2), forming mainly thick quartz veins with abundant coarse-grained subhedral pyrite (S2-1), and plentiful chalcopyrite, sphalerite, barite and Au-bearing sulfide veins (S2-2); (3) late ore stage (S3), which is characterized by plenty of barren quartz–calcite veins with few sulfides; and (4) supergene stage (S4), accompanied by abundant oxide mineralization, including malachite, jarosite and other supergene minerals. From S1 to S3, microthermometric data of fluid inclusions show homogenization temperatures of 263–379 °C (mean = 308 °C), 188-292 °C (mean = 240 °C), and 118-198 °C (mean = 158 °C), respectively, and salinities of 5.3–14.2 (mean = 10.8) wt.% NaCl equiv., 2.8–10.7 (mean = 7.6) wt.% NaCl equiv., and 0.3–14.1 (mean = 2.6) wt.% NaCl equiv., respectively. The ore-forming fluids are characterized by middle-low temperature, low salinity, relatively reduced condition, and an H2O-NaCl ± CO2 ± CH4 system.The δ34S values (−5.25‰ to 0.50‰) and Pb isotopic ratios (206Pb/204Pb = 17.868–19.495, 207Pb/204Pb = 15.446–15.575, and 208Pb/204Pb = 37.350–38.491) suggest that the ore-forming materials came predominantly from a deep-seated magma source with a minor contribution of lower continental crust. The δ18OH2O and δDV-SMOW values of fluid inclusions in each metallogenic stage range from −6.1 to 5.6‰ and −66.8 to −53.9‰, respectively, suggesting the dominant role of magmatic water mixed with convectively circulating heated seawater during fluid evolution. Fluid cooling dilution, local boiling, and fluid mixing were considered as the main mechanisms of metal precipitation. The 40Ar-39Ar plateau age of hydrothermal muscovite from the late ore stage (S3) is 414.4 ± 0.4 Ma, which represents the upper limit age of shallow hydrothermal alteration and Au-Cu mineralization. It is also consistent with the Early Silurian polymetallic metallogenic event in the Kalatag district. The auriferous Huangtan and adjacent Cu-Zn-rich Huangtupo VMS deposits show obvious ore-forming element differences, and constitute a unique VMS metallogenic system in the Eastern Tianshan Orogenic Belt (ETOB), which provides an important research object and new insight for ore prospecting in the peripheral Gobi Desert area.

An Automated System for MeasuringIn SituK–Ar Ages

A completely automated system for measuringin situK–Ar ages using laser ablation for argon extraction and potassium measurement, and mass spectrometry for argon determination is described. The system consists of a laser system, an optical spectrometer, a vacuum line, a noble gas mass spectrometer and control software. The automated prototype was designed with ease of use in mind, specifically planned for dating many samples at low cost and reasonable precision. This instrumentation can perform one hundred K–Ar analyses every 24 h with the current setup, including blanks, reference materials and unknowns. Measurement results for the reference materials HDB‐1 biotite, GL‐O glauconite, LP‐6 biotite, Mica‐Mg phlogopite, Alder Creek sanidine and two tektites (moldavite and indochinite) are presented. It is shown that very different sample matrices require specific calibrations. Uncomplicated sample preparation, fast analytical times and uncertainties typically below 5% (1 standard deviation), depending on the age and potassium content, are the hallmarks of the system, which is suitable for many exploratory geology applications (e.g. geological mapping, mineral provenance and dating of drill cores).

Magmatic-hydrothermal origin for Carlin-type Au deposits: Evidences from in-situ S-Pb isotopic compositions of sulfides, the Chang’an Au deposit, southern Ailaoshan tectonic zone

The Chang’an Au deposit is a large Au deposit located in the south of the Ailaoshan-Red River tectonic zone, southwestern margin of the South China block. The deposit exhibit characters similar to Carlin-type Au deposits in Nevada and Dian-Qian-Gui “Golden Triangle”, including tectonic evolution (deformed and rifted cratonic margins), orebody controlled by lithology and structure and invisible Au ionically bound in arsenian pyrite and pyrite. Mineralization is controlled by a strike-slip fault and occurs as barren milky quartz veins, disseminated gold-bearing pyrite and arsenopyrite in the Early Paleozoic sandstone, siltstone and limestone. Gold is invisibly and ionically bound in arsenopyrite and pyrite or locally occurs as isolated native gold crystals (micron scale). Wall rock alteration includes sulfide, silicic, sericite and decarbonized alterations. Three hydrothermal mineralizing stages (I, II, III) can be identified: milky quartz-pyrite (I), quartz-sericite-pyrite-arsenopyrite (II) and dolomite-calcite-quartz (III). Different from those deposit in Dian-Qian-Gui, numerous plutons intruded in and around the mine. The syenite dikes are pyritized, thus, formed prior to the mineralization while lamprophyre dikes are unaltered and crosscut by the syenite dikes. The syenite dikes yielded zircon U − Pb ages of ca. 35.8 Ma and the lamprophyre yielded similar Ar-Ar age of ca. 36.6 Ma, largely constraining the age of Au mineralization. In situ S–Pb isotopic compositions for ore pyrite and arsenopyrite were measured by fs-LA-MC-ICP-MS. Sulfides from Chang’an show a uniform range from –6.4‰ to + 4.6‰, a typical characteristic of magmatic-derived S. Such sulfur isotopic compositions are similar to those of sulfides from Carlin-type Au deposits, but different from those of sulfides from orogenic-type deposit, which commonly exhibit a wide range of δ34S values. Pb isotopic ratios of pyrite and arsenopyrite are similar to those of the intrusive rocks in the deposit with more radiogenic Pb, suggesting that the metal is probably sourced from the coeval intrusive rocks and sedimentary wall rocks. Combined with the low Au concentrations of the sedimentary wall rocks, Au in the Chang’an deposit are probably sourced from the intrusive rocks. Differences between the Chang’an deposit and typical orogenic-type Au deposit include: (1) Orebodies in the Chang’an deposit exhibit transitional contact relationship with the host rocks and boundaries between them are defined by cut-off grade while these boundaries in typical orogenic-type deposits are clearly defined. (2) Sulfidized and decarbonized alterations are quite significant in the Chang’an deposit but commonly absent in typical orogenic-type deposits. (3) The Chang’an deposit have lower CO2 contents of ore-forming fluids and more shallow ore-forming depth relative to typical orogenic-type deposits. Based on the ore deposit geology, dating and isotopic data, the Chang’an mineralization is similar to typical Carlin-type gold deposit, formed by ore fluids originated from a deep magmatic S source in an post-collisional intracontinental setting at ca. 36 Ma.

First identification of a Cathaysian continental fragment beneath the Gagua Ridge, Philippine Sea, and its tectonic implications

AbstractThe tectonic history of the Philippine Sea plate is an essential piece in understanding the tectonic evolution of Southeast Asia, but it is still unclear and controversial. We present the first geochemical data obtained from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR lavas exhibit geochemical signatures typical of subduction-related arc magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related geochemical signatures support the formation of GR lavas in the vicinity of an arc during the Early Cretaceous induced by subduction of the oceanic plate along East Asia. The ages of trapped zircon xenocrysts within the GR lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of zircons recovered from the Cathaysian block, southern China. Our results imply that the GR basement is partially composed of continental material that rifted away from the Eurasian margin during opening and spreading of the Huatung Basin. The depleted mantle wedge-derived magmas evolved and picked up the continental zircons during ascent. The youngest zircon ages and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly constrain an Early Cretaceous age for the Huatung Basin. Our study provides further evidence that the Huatung Basin is a remnant of a Mesozoic-aged ocean basin that dispersed from southern China during the Cretaceous. Transport of continental slivers by growth and closure of marginal seas along the East Asia margin may have been more prevalent than previously recognized.

A test of models for recent lithosphere foundering or replacement in the Canadian Cordillera using peridotite xenolith geothermometry

We examine the thermal history of spinel lherzolite xenoliths to test models suggesting recent replacement of mantle lithosphere beneath the Canadian Cordillera. The xenoliths are hosted in basanite lavas and bedded pyroclastic deposits with an ArAr age of 465 ± 22 ka at Mt. Timothy, British Columbia, Canada. The xenolith suite is mostly fertile spinel peridotite (Fo89.5–91.5, bulk Al2O3 2–5.6 wt%) showing enrichment in highly incompatible elements. Geothermometry on coexisting ortho- and clinopyroxene using Ca-Mg-Fe (TBKN) and REE (TREE) exchange yield similar temperature distributions (950–1100 °C) indicating a sampling between 32 and 55 km depth along regional geotherm. The differences in TREE and TBKN (ΔTREE-BKN) observed in the samples are statistically insignificant – with a maximum of 80 °C, similar to other subcontinental mantle xenoliths, and far less than observed for rapidly cooled mantle exhumed in oceanic settings (> 150 °C). The maximum ΔTREE-BKN values can be used to place limiting values on the cooling rate of the lithosphere, as functions of the grain size of ortho- and clinopyroxene. The observed ΔTREE-BKN requires a cooling rate < 10−5 °C /Myr for the xenoliths, far less than that expected for foundering or replacement of lithosphere in the past 50 Myr from asthenosphere (>10 °C /myr). The slow cooling rates are more consistent with a view that the mantle lithosphere of the Canadian Cordillera is ancient, has been maintained to be thin and hot for at least the past 30 Myr, and not cooled from asthenosphere in the past ~50 Myr. Extension of our result to other xenolith suites throughout the Canadian Cordillera can further test this conclusion.

40Ar/39Ar dating of high temperature geothermal systems: First attempt on hydrothermally altered pyroxenes from the Saintes archipelago (Lesser Antilles arc, Guadeloupe)

We investigate the potential of 40Ar-39Ar systematics in secondary K-bearing minerals developed at the expense of hydrothermally altered volcanic pyroxenes from a fossil geothermal system located in the Saintes archipelago (Lesser Antilles magmatic arc).Our work reports the first example of dating hydrothermal muscovite crystallization developed during pseudomorphic transformation of pyroxenes within altered rhyodacites at temperatures above 300 °C. The white micas display a well-defined Ar/Ar plateau age of 2.59 ± 0.12 Ma, interpreted as the best age estimate for high temperature fluid circulation in the Saintes geothermal paleo-reservoir. A model of muscovite substitution by illite during cooling supports the interpretation that illitization perturbs the K-Ar and the Ar-Ar ages only when muscovite is highly illitized and/or if illitization is significantly younger than muscovite crystallization. The whole temporal dataset implies a fast cooling rate (>150 °C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system.

Shortening and exhumation of Sierra de Catorce in northeastern Mexico, in light of 40Ar/39Ar illite dating and (U-Th)/He zircon thermochronology

The Sierra de Catorce along the eastern margin of the Mesa Central in central northern Mexico exposes Triassic to Neogene rocks in two deeply incised canyons in the northern portion of the range, permitting a 3D view of structures and mineral fabrics associated with the Mexican Orogen. Triassic siliciclastic units, recording a penetrative deformation, are unconformably covered by less-deformed Jurassic volcaniclastic and fluvial deposits and Upper Jurassic shale and limestone. The shales acted as a regional décollement horizon, facilitating Late Cretaceous and Early Cenozoic thin-skinned shortening, related to the Sierra Madre Oriental fold and thrust belt, and accommodating translation of Cretaceous carbonate in the hanging wall. The post-tectonic Cenozoic cover consists of alluvial deposits and remnants of basaltic flows and cinder cones. The Sierra de Catorce allows for an opportunity to study the structural, temporal, and thermal evolution of internal portions of the Sierra Madre Oriental fold and thrust belt, employing geometric and kinematic structural analysis, optical and Scanning Electronic Microscopy fabric characterization, X-Ray diffraction of clays, 40Ar-39Ar illite dating, and zircon (U–Th)/He dating. Structural cross-cutting relationships point to at least three distinct Late Cretaceous-Paleogene shortening episodes, with the first being associated with initial thrust activity to the area and a second one characterized by intense shortening deformation, while a third and final episode of shortening deformation is only observed on a kilometric scale. 40Ar-39Ar illite ages from bed-parallel sheared layers and décollement zone suggest that after shortening within the Triassic units, bed-parallel shear was localized within Cretaceous carbonate ~91 Ma. Intense shearing occurred along a major décollement horizon within Upper Jurassic calcareous shale at ~69 Ma, resulting in the folding of Cretaceous strata in the hanging wall. Finally, illite 40Ar-39Ar ages recorded late folding of Upper Cretaceous turbidites at ~52 Ma, accompanied by a rapid exhumation, as evidenced by ~50 Ma zircon-He ages. This multiphase history of contractional deformation records the growth and propagation of the internal portion of the orogenic wedge of the Mexican Fold-Thrust belt (MFTB).

Constraints on duration, age and migration of the feeder systems of the Madagascan Flood Basalt Province from high-precision 40 Ar/ 39 Ar chronology

Abstract The Late Cretaceous magmatism in Madagascar is correlated with the break-up between Madagascar and Greater India, with a presumed track of a hotspot from Madagascar towards the Marion Island and with an anoxic event in the Late Cretaceous. The lava succession and associated dyke swarms and sills of western Madagascar (Mailaka area) represent a volumetrically important area of the igneous province, where dykes with random orientation, several igneous intrusions and a flood basalt to rhyodacite sequence do occur. The magmas have a tholeiitic and weakly alkaline affinity. Using plagioclase separates, we obtained two plateau 40 Ar/ 39 Ar ages, and an inverse isochron age statistically indistinguishable, ranging from 92.9 ± 3.8 to 91.2 ± 1.3 Ma (2 σ ). These ages indicate that tholeiitic and alkaline rocks were erupted in the same age span. In addition, these ages are close to the Cenomanian–Turonian (C–T; 93.9 ± 0.2 Ma) boundary and are indistinguishable from the U–Pb ages available for the capping rhyodacitic unit of the Mailaka lava succession. A filtered compilation of eight ages for northern and central-western Madagascar rocks suggests a duration for the magmatic activity in this part of Madagascar province of the order of c. 3 Ma. If the western Madagascar magmatism is plume related, the plume head would need to have been located near the Mailaka area at c. 93 Ma. The geochemistry of the mafic lavas and dykes of western Madagascar is barely distinguishable from mid-ocean ridge basalt (MORB), with an increasing crustal contamination towards the evolved rocks, and does not constrain input of typical components derived by plume magmatism.

Graben type calderas: The Bolaños case, Sierra Madre Occidental, Mexico

Graben calderas are volcano-tectonic structures that use the faults of a graben as main vents from which magma is massively erupted from a shallow magma chamber during collapse of intra-graben blocks. These eruptions are generally silicic and explosive and form large volume ignimbrite sheets without a previous Plinian major depressurization event. Graben calderas can be associated to tectonic settings ranging from pure extension to strike-slip transtension, including complete grabens, half-grabens or pull-apart grabens. The Sierra Madre Occidental (SMO) in western Mexico includes several of these graben type calderas with the corresponding large volume ignimbrite sheets that result from fissure type eruptions along the graben's border and interior faults. The Bolaños graben constitutes one of the best examples of these tectonically controlled collapse calderas and is the second largest caldera of the world after Toba caldera. It is a 90 × 25 km rectangular caldera and the vent of the 25.37 ± 0.36 Ma silicic Alacrán ignimbrite, with a minimum Dense Rock Equivalent volume of 2650 km3 (3800 km3 rock volume). Post-collapse silicic domes were emplaced just after the Alacrán ignimbrite from 25.02 ± 0.33 Ma to 23.94 ± 0.33 Ma, with a total minimum lava volume of 171 km3. Both ignimbrite and domes account for at least 2800 km3 rhyolitic magma output from Bolaños graben caldera, without considering the distal deposits, and in particular the co-ignimbrite ash-cloud deposit. We describe the characteristics of the Bolaños graben caldera focusing on its major products, the Alacrán ignimbrite and the post-collapse dome volcanism, providing the geological frame of the Bolaños graben area documented with 40Ar-39Ar ages. We finally propose a conceptual model to explain the dynamics of the Bolaños graben that can be applied to other similar volcano-tectonic depressions of the Sierra Madre Occidental and elsewhere in the world with similar geological settings.

Geochemistry and geochronology of intermediate volcanic rocks from the Compostela area, Nayarit, Mexico: Implications for petrogenesis and tectonic setting

The Compostela area is located in the western Trans‐Mexican Volcanic Belt, which consists of volcanic rocks that produced during the Pliocene to Recent volcanism. In this paper, we present petrography, whole‐rock major‐ and trace‐element concentrations, Ar–Ar ages, and Sr–Nd isotopic data of volcanic rocks from the Compostela area in the south of the city of Tepic, Nayarit, Mexico. These volcanic rocks are of intermediate composition and belong to the transitional series: basaltic trachyandesites [52.72–53.94 wt% SiO2; 0.69–2.53 wt% MgO] and subalkaline series: low‐Si [55.11–60.94 wt% SiO2; 0.79–2.74 wt% MgO] and high‐Si [61.60–62.71 wt% SiO2; 0.36–0.80 wt% MgO] andesites. The 40Ar/39Ar dating of two basaltic trachyandesites yields plateau ages of 1.05 ± 0.15 Ma and 1.07 ± 0.17 Ma and the andesites yielded a plateau age of 2.42 ± 0.36 Ma. These results indicate that the studied rocks were formed during the Pleistocene in two discrete episodes. The basaltic trachyandesites show enriched light rare earth elements patterns relative to high rare earth elements [(La/Yb)N = 5.81–8.07] with negligible Eu anomalies. The andesites display enriched large‐ion lithophile elements (Ba, K) with anomalies of Nb (Th/Ce) and Ti in the three groups identified in the subalkaline series. The basaltic trachyandesites appear in a tight cluster of initial 87Sr/86Sr ratios (0.703519–0.703882) as compared to the basaltic andesite 87Sr/86Sr (0.704073) and positive ɛNd(t) values of +5.6 to +3.4, respectively, indicating that the basaltic trachyandesites were derived from intermediate magmas from a shallow mantle source. Geochemical modelling reveals that both basaltic trachyandesite and andesite rocks were derived by a process of crystal fractionation accompanied by assimilation of crustal rocks at the lower or middle level. Geochemical ratios and multidimensional discrimination diagrams, combined with the cross‐section of the subduction zone indicates that basaltic trachyandesites were generated from a rift and/or Ocean Island Basalt‐type source, whereas andesites were generated from a slab‐derived source in a subduction environment.

Transpression in the Sanandaj-Sirjan Zone (Zagros Orogen, Iran) during the Jurassic and Early Cretaceous: Evidence from the North Shahrekord Shear Zone

Deformation style associated with the early stage of subduction of the Neo-Tethys under Iranian microcontinent is poorly understood. Utilizing field observations, microscopic thin-section interpretation, and recently published 40Ar-39Ar ages, we conducted a structural analysis of the North Shahrekord Shear Zone. The study area is located in the central Sanandaj-Sirjan Zone in western Iran, in an area that includes high-grade eclogite-bearing and low-grade metamorphic units of the North Shahrekord Metamorphic Complex. Two generations of deformation were observed in the study area, including D1 and D2. D1 structures are rare and involve folds and axial-plane foliations (S1), which have been almost obliterated by D2. D2 is characterized by a dextral transpressional zone, where strain is partitioned into distinct simple shear and pure shear domains. The pure shear domain is observed mainly in low-grade zone and some high-grade metamorphic rocks, and is characterized by NW-striking close to isoclinal folds (F2) and axial-plane foliations (S2). Folds are mostly inclined and their vergence is towards the southwest and northeast in the northern and southern parts of the study area, respectively. In the central part of the area, F2 includes approximately upright folds. These mesoscopic folds are associated with map-scale folds. The simple shear domain is restricted to the high-grade metamorphic rocks of the North Shahrekord Shear Zone, defined by NW-striking steeply dipping mylonitic foliation with low-angle stretching lineation. Kinematic indicators, such as quartz sigmoids, feldspar porphyroclasts, mica fish, and C- and C′-type shear bands, reveal a dextral strike-slip movement with a minor reverse component along the North Shahrekord Shear Zone. Recrystallized phengite from mylonitic eclogite and metagranite have been recently dated (40Ar-39Ar) at 178–170 Ma and ~110 Ma, respectively, indicating that the Sanandaj-Sirjan Zone was dominated by dextral transpression phases in the Early-Middle Jurassic and Early Cretaceous. The transpressional phases are interpreted to have occurred in response to the oblique subduction of the Neo-Tethys Ocean under the Iranian microcontinent.

Xenoliths in Late Cretaceous to Early Paleocene adakites of the Eastern Pontides Orogenic Belt, NE Turkey

Late Cretaceous to Early Paleocene hypabyssal intrusives near Bayburt in northeastern Turkey, in the Eastern Pontides Orogenic Belt (EPOB), host a variety of xenoliths, including clinopyroxenite, hornblendite, hornblende- and plagioclase-rich gabbro, quartz diorite and amphibole-rich diorite, quartz syenite, and amphibolite. We report major and trace element data for both host rocks and xenoliths, along with zircon U-Pb and amphibole 40Ar/ 39Ar ages, zircon trace element data, mineral compositions, and Sr-Nd-Pb-Hf isotopic data. Host rock compositions range from trachyandesite to dacite and rhyodacite, but major element (e.g. Mg-number) and trace element systematics (e.g. La/YbN) indicate that they are not related by fractional crystallization. Host rocks belong to the high-K calc alkaline and calc-alkaline series, and have the signature of arc rocks (depleted Nb, Ta, Ti, and elevated Pb). Trace element data (Sr/Y and La/Yb) indicate that the dacitic rocks are adakitic. Host rock ages cluster around 70 ± 2 Ma; one intrusion contains zircons that suggest a ~62 Ma crystallization age. Hornblendite xenoliths show ages of 76, 78, and 81 Ma, whereas quartz syenites range from 66 to 70 Ma. The younger ages (62, 66, 70 Ma) fall in an apparent magmatic gap, and document the oldest adakitic magmatism in the EPOB. Mineral compositions indicate initiation of crystallization in the host rocks at ~9 kb and 1250 °C, with progressive crystallization continuing to ~2 kb and 800 °C. Host rocks and xenoliths have similar ranges of 87Sr/86Sri (0.7044–0.7062), 143Nd/144Ndi (−1.5 < εNd < 1.5), and Pbi (18.28 <206Pb/204Pb < 18.68) compositions. The clinopyroxenite, hornblendite, and gabbro xenoliths are broadly cognate with the host rocks, and are cumulate fragments from an arc-root magma system. The quartz diorite, quartz syenite, and amphibolite xenoliths have different histories. Some of the quartz syenites are near eutectic compositions in the H2O saturated q-or-ab system at ~5 kb. The host rocks and xenoliths derive from crustal magma chambers and dominantly crustal melting in the back-arc region of the EPOB.

An atom trap system for 39Ar dating with improved precision.

Cosmogenic 39Ar dating is an emerging technique in dating mountain glacier ice, mapping ocean circulation, and tracing groundwater flow. We have realized an atom-trap system for the analysis of the radioactive isotope 39Ar (half-life = 269 years) in environmental samples. The system is capable of analyzing small (1-5kg) environmental water or ice samples and achieves a count rate of 10 atoms/h for 39Ar at the modern isotopic abundance level of 8 × 10-16. By switching frequently between counting 39Ar atoms and measuring the stable and abundant isotope 38Ar, drift effects in the trapping efficiency are largely suppressed, leading to a more precise measurement of the isotope ratio 39Ar/38Ar. Moreover, cleaning techniques are developed to alleviate cross-sample contamination, reducing the background 39Ar count rate down to <0.5 atoms/h. These advances allow us to determine the 39Ar age in the range of 250-1300 years with precisions of <20%.

Geology and geochronology of the Tokuzbay gold deposit in the Chinese Altai: A case study of collision-related orogenic gold deposits in Central Asian Orogenic Belt

Orogenic gold deposits are one of the most important gold deposits in the world. However, it is controversial that orogenic gold deposits are only generated at subduction-accretion or both in collisional setting. The Altai Orogen experienced a complex subduction accretion–collision history, including magmatism and metamorphism, and forming giant gold deposits. The Tokuzbay gold deposit consists of quartz vein ores and disseminated ores hosted in dikes. The ore bodies are controlled by secondary shear zone of the NW-, NNW-trending Ma’erkakuli Fault and occurred in meta-sediments, i.e., meta-sandstone and phyllite of Devonian Altai Formation, and diorite dikes and granite porphyritic dikes. Wall rock alteration is represented by silicification, pyritization, and muscovite alteration. The ores are restricted by ductile and brittle deformation in the shear zone. Large amounts of native gold and auriferous tellurium were precipitated during a transition from ductile to brittle regime. These characteristics indicate that the Tokuzbay gold deposit is an orogenic gold deposit in the Altai Orogen. The zircon LA-ICP-MS U-Pb ages of the ore-bearing granite porphyritic dikes and diorite dikes are 390 ± 2.5 Ma (MSWD = 0.87) and 378 ± 4 Ma (MSWD = 0.21), respectively, indicating that the magmatic rocks were emplaced during subduction of the Paleo Asian Ocean beneath Altai Orogen. Muscovite samples associated with gold from the auriferous quartz vein and disseminated ores hosted in diorite dikes returned Ar-Ar ages of 292.7 ± 1 Ma (MSWD = 1.3) and 293.9 ± 1.2 Ma (MSWD = 0.7), indicating that gold mineralization in quartz vein is synchronized with disseminated ores. The gold mineralization is restricted to the Early Permian and overlaps with the collision between Altai Orogen and Junggar Plate. The ore-hosting magmatic rocks related to subduction are more than 100 million years earlier than the ore-forming events. Combined with the regional tectonic evolution and the shear characteristics of the gold district, it is well proved that sinistral shearing trigged by the collision of Junggar Plate and Altai Orogen contributed to gold mineralization of the Tokuzbay deposit. Considering the Altai gold deposits in Kazakhstan, it is revealed that Permian collision between Junggar Plate and Altai Orogen triggered a large number of orogenic gold mineralization.

Unravelling metamorphic ages of suture zone rocks from the Sabzevar and Makran areas (Iran): Robust age constraints for the larger Arabia–Eurasian collision zone

AbstractIn this study we establish a time frame for metamorphic processes affecting Neotethyan suture zones at the northern (Sabzevar) and southern (Makran) margin of the Central Iranian Microcontinent. For this purpose, we have applied Rb–Sr and U–Pb geochronology to blueschist‐ and amphibolite facies rocks from these occurrences. So far, no isotopic ages are available for high‐P/low‐T rocks from the Sabzevar area which are exposed in the Govain and Chili mountains near Soltanabad. Three newly dated blueschists from the Govain mountain yielded Rb–Sr ages of 56.52 ± 0.32, 57.20 ± 0.32, and 55.12 ± 0.33 Ma respectively. These ages provide the first unambiguous evidence for late Palaeocene to early Eocene high‐P metamorphism in the Sabzevar zone and confirm the accuracy of earlier age assignments that were derived indirectly. The clearly confirmed age difference between late Palaeocene–early Eocene blueschists of the Govain mountain and kilometre‐scale dismembered HP granulites with Early Cretaceous (Albian) metamorphic ages from outcrops c. 60 km to the west of Soltanabad further corroborates interpretations that these rocks are not related to the same subduction zone complex. The blueschists record closure of an Albian to Late Cretaceous back‐arc basin, whereas the granulites formed due to subduction of older oceanic lithosphere. Uranium–Pb zircon dating of an amphibolite from the Iranshahr region in the Makran Zone yielded an intercept age of 87.8 ± 1.8 Ma and weighted mean age of 88.1 ± 1.7 Ma. Rb–Sr mineral isochrons for seven blueschist‐ and amphibolite facies samples from the same area generated very consistent ages of c. 84 Ma (± &lt;1–3 Ma), regardless of metamorphic grade. The apparent contradiction that rocks recording different P–T conditions have identical metamorphic ages either indicates tectonic mixing of rocks during exhumation that originally had formed at the same time at different depths of the subduction system, or very rapid exhumation so that differences in closure temperatures became indistinguishable. The new data are similar, but much more precise, than previously reported K–Ar ages and suggest that most of the existing data had not been significantly compromised by contamination with extraneous Ar. Potassium–Ar indications for older blueschist‐ and/or amphibolite facies metamorphism (&gt;85 Ma) have not yet been confirmed but can be reconciled with the larger geological frame implying that northward subduction of the Arabian plate beneath the Bajgan–Durkan continental block started shortly after c. 120 Ma.

A cryptotephra from the Upper Pleistocene volcanism of the Bas-Vivarais in the sedimentary infilling of the Chauvet-Pont d’Arc cave (Ardèche, France)

A cryptotephra belonging to the Upper Pleistocene volcanism of the Bas-Vivarais was identified for the first time in the sedimentary infilling of the entrance area of the Chauvet-Pont d'Arc cave. This slightly reworked tephra fall is characterized by its heavy minerals assemblage, among which magnesian olivine, enstatite, and chromian diopside issued from peridotites and olivine and diopside of basaltic origin. This composition likely refers to a phreatomagmatic eruption of a maar-crater. The 14 C datings of the cryptotephra beds are contemporaneous with the Aurignacian settlement and paintings, that is, an age slightly older or close to 36 ka. Among the maar-craters from the Bas-Vivarais, the Ray-Pic maar have the closest 40 Ar/ 39 Ar age (36.2 ± 11.3 / 32.2 ± 11.1 ka). By contrast, the Vestide-du-Pal, the biggest maar-crater of this country, seems to have a younger age, but the 40 Ar/ 39 Ar dating was measured on a basalt which could have occurred later and been the source of the tephra-fall, since its tephras contain the same minerals of unknown origin as those found in Chauvet Cave. This discovery strengthens the previous hypothesis according to which the humans living in the Chauvet-Pont d'Arc area during the Upper Paleolithic have witnessed eruptions from the Bas-Vivarais volcanic field and other phenomena.

Cretaceous to Miocene NW Pacific Plate Kinematic Constraints: Paleomagnetism and Ar–Ar Geochronology in the Mineoka Ophiolite Mélange (Japan)

AbstractThe Mineoka Ophiolite Mélange is located at the intersection of the Pacific, Philippine Sea, Eurasian, and North American plates. The Mineoka ophiolite origin is disputed, and it has been ascribed to a fully subducted plate or part of the Pacific and Philippine Sea plates. In this paper, we present a kinematic reconstruction of the Mineoka Ophiolite Mélange and its relation with the Pacific Plate, based on new paleomagnetic data and bulk‐rock 40Ar/39Ar ages of basaltic rocks. In addition to standard analyses for paleolatitudes, we performed a Net tectonic rotation analysis on sheeted dolerite dikes to infer the paleospreading direction that formed the ophiolite. The analysis shows that 85–80 Ma MOR pillow basalts erupted at a paleolatitude of N ∼16°, whereas ∼50 Ma basalts formed at N ∼34°. Net Tectonic Rotation analysis suggests that the spreading direction was NE 60°. Ar–Ar ages yielded 53–49 Ma for MORBs and 41–35 Ma for island‐arc basalts. The formation of this ophiolite occurred in the back‐arc spreading of the Nemuro–Olyutorsky arcs of the NW Pacific. It infers that the final consumption of Izanagi below Japan instigated a subduction jump and flipped its polarity. Subduction initiated parallel to the ridge, and a piece of the original back‐arc crust got trapped near the Japan trench during the northwards motion of the Philippine Sea Plate. The contrasting motion between the Pacific and the Philippine Sea plates generated a highly unstable setting followed by a subduction zone that left a small‐sized and short‐lived plate (“Mineoka”), surrounded by subduction zones.

Geology, geochronology and tectonic setting of the Chaihulanzi gold deposit in Inner Mongolia, China

The Chaihulanzi is a Precambrian metamorphic rock-hosted gold deposit in the northern margin of the North China Craton (NCC). The local outcropping lithologies include mainly Archean metamorphic rock, Paleoproterozoic carbonatite, and Phanerozoic diorite, granite, aplite and mafic to felsic dikes. Due to the lack of data, the deposit was variably proposed to be orogenic, intrusion-related, or mesothermal type, and the mineralization was loosely constrained to be Paleozoic or Mesozoic. Based on field geology, petrological/mineralogical observations, whole-rock geochemistry, and zircon U-Pb-Hf isotopes of ore-related aplite, and sericite Ar-Ar dating of gold-ore veins, we discussed the ore-causative magmatism, and the age and tectonic setting of gold mineralization. Our results show the presence of an early (native gold-dominated) and a late (gold-silver telluride-dominated mineralization) stage: The early-stage gold mineralization was genetically linked to the Middle-Late Permian aplite (LA-ICP-MS zircon U-Pb age: 264.5 ± 1.3 Ma), whose ages are similar to the gold ore veins (sericite Ar-Ar age: 263.67 ± 0.99 Ma). The distinct negative zircon εHf (-18.31 to -15.45) and model age (TDMC = 2.1–2.4 Ga) indicate a crustal magma source, probably from partial melting of the NCC basement. The whole-rock trace element compositions and zircon U-Pb ages, together with the tectonic evolution of northern NCC margin imply a post-collisional extension setting (after the Paleo-Asian ocean closure) for the gold mineralization. The newly-discovered late-stage quartz-carbonate ore veins occur as fracture infill, and crosscut early-stage quartz-sericite veins and Mesozoic volcanic wallrocks, implying that the late-stage mineralization occurred in the Mesozoic or after. The two-stage gold mineralization overprinting makes Chaihilanzi and its periphery highly prospective for gold exploration.

Reactivation of minor faults in a blind active fault area: A case study of the aftershock area of the 2000 Western Tottori Earthquake, Japan

The 2000 Western Tottori Earthquake (Mw 6.6, Mjma 7.3) in Japan occurred in an area where no active faults are known to exist. We examined the distribution, occurrence, paleostress field, and K–Ar ages of minor faults in the aftershock region of this earthquake to understand the tectonics and deformation process in a blind active fault area. Five stress stages were identified: NW–SE compressional strike-slip and NE–SW extensional normal faulting stress field in Stage 1; clockwise rotation of ~ 40° between 18 and 16 Ma in Stage 2; N–S compressional strike-slip and N–S extensional normal faulting stress field in Stage 3; stress state with σHmin (σ3) oriented NE–SW in Stage 4; and E–W compressional strike-slip faulting stress field in Stage 5. The results from the relationship between stress fields and K–Ar ages of fault indicate that most minor faults in the epicentral area of the 2000 Western Tottori Earthquake were formed along pre-existing joints during rifting and opening of the Japan Sea during the late Oligocene. Some minor faults show signs of reactivation under the present stress field. However, a large active fault with a clear topographic expression has not yet been developed due to the small displacement of minor faults. These findings indicate that detailed structural analysis of minor faults and reconstruction of stress history are important for seismic hazard assessment in areas where fault activities are initial stages.