Zircon U–Pb ages of plutonic rocks in the southern Abukuma Mountains: Implications for Cretaceous geotectonic evolution of the Abukuma Belt

Plutonic rocks in the southern Abukuma Mountains include gabbro and diorite, fine‐grained diorite, hornblende–biotite granodiorite (Ishikawa, Samegawa, main part of Miyamoto and Tabito, Kamikimita and Irishiken Plutons), biotite granodiorite (the main part of Hanawa Pluton and the Torisone Pluton), medium‐ to coarse‐grained biotite granodiorite and leucogranite, based on the lithologies and geological relations. Zircon U–Pb ages of gabbroic rocks are 112.4 ±1.0 Ma (hornblende gabbro, Miyamoto Pluton), 109.0 ±1.1 Ma (hornblende gabbro, the Hanawa Pluton), 102.7 ±0.8 Ma (gabbronorite, Tabito Pluton) and 101.0 ±0.6 Ma (fine‐grained diorite). As for the hornblende–biotite granodiorite, zircon U–Pb ages are 104.2 ±0.7 Ma (Ishikawa Pluton), 112.6 ±1.0 Ma (Tabito Pluton), 105.2 ±0.8 Ma (Kamikimita Pluton) and 105.3±0.8 Ma (Irishiken Pluton). Also for the medium‐ to fine‐grained biotite granodiorite, zircon U–Pb ages are 106.5±0.9 Ma (Miyamoto Pluton), 105.1 ±1.0 Ma (Hanawa Pluton) and the medium‐ to coarse‐grained biotite granodiorite has zircon U–Pb age of 104.5 ±0.8 Ma. In the case of the leucogranite, U–Pb age of zircon is 100.6 ±0.9 Ma. These data indicate that the intrusion ages of gabbroic rocks and surrounding granitic rocks ranges from 113 to 101 Ma. Furthermore, K–Ar ages of biotite and or hornblende in the same rock samples were dated. Accordingly, it is clear that these rocks cooled down rapidly to 300 °C (Ar blocking temperature of biotite for K–Ar system) after their intrusion. These chronological data suggest that the Abukuma plutonic rocks in the southern Abukuma Mountains region uplifted rapidly around 107 to 100 Ma after their intrusion.     

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district,central Shikoku,southwest Japan

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).     

Existence of multiple units with different accretionary and metamorphic ages in the Sanbagawa Belt,Sakuma–Tenryu area,central Japan

U–Pb ages of detrital zircons and white mica K–Ar ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the Sakuma–Tenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95 Ma, and the youngest age in the detrital zircons is 94.0 ± 0.6 Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 ± 0.9 Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica K–Ar ages of T1 and T5 are 69.8 ± 1.5 Ma and 56.1 ± 1.2 Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the Sakuma–Tenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high‐P/T Sanbagawa Metamorphic Belt.     

Zircon U–Pb dating of gabbro and diorite from the Bato pluton,northeast Japan

To constrain the timing of the tectonothermal events and formation process of a plutonic suite, U–Pb dating was carried out by laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence imaging on zircon grains extracted from the Bato pluton, northern Yamizo Mountains, Japan. The Bato pluton consists of gabbro and diorite. Zircon grains separated from a gabbro sample had a unimodal ²³⁸U–²⁰⁶Pb age (105.7 ±1.0 Ma). It was interpreted as the solidification age of the gabbro. Cathodoluminescence observation showed that the zircon grains from a diorite sample were characterized by anhedral cores, oscillatory zoned mantles, and dark rims. The ²³⁸U–²⁰⁶Pb age of the anhedral cores ranged from 2 165 Ma to 161 Ma, indicating the assimilation of surrounding sedimentary rocks. The ²³⁸U–²⁰⁶Pb ages of the oscillatory zoned mantles and dark rims are 109.0 ±1.3 Ma and 107.7 ±1.3 Ma, respectively. Observation under polarizing microscopy suggests that the anhedral cores occurred before plagioclase and hornblende, and the oscillatory zones around the anhedral cores had crystallized at the same time as the crystallization of biotite. Moreover, the dark rims formed at the same time as the crystallization of quartz and K‐feldspar. The formation process of the gabbro‐diorite complex in the Bato pluton was inferred as follows. (i) A mafic initial magma intruded into Mesozoic sedimentary rocks, and the assimilation of these sedimentary rocks led to geochemical variation yielding a dioritic composition. Subsequently, plagioclase and hornblende of the diorite were crystallized before 109.0 ±1.3 Ma. (ii) Biotite crystallized in the middle stage around 109.0 ±1.3 Ma. (iii) Quartz and K‐feldspar of the diorite were crystallized at 107.7 ±1.3 Ma. The gabbroic magma solidified (105.7 ±1.0 Ma) after solidification of the diorite.     

Hf–Nd isotope constraints on the origin of Dehshir Ophiolite,Central Iran

The peri‐Arabian ophiolite belt, from Cyprus in the west, eastward through Northwest Syria, Southeast Turkey, Northeast Iraq, Southwest Iran, and into Oman, marks a 3000 km‐long convergent margin that formed during a Late Cretaceous (ca 100 Ma) episode of subduction initiation on the north side of Neotethys. The Zagros ophiolites of Iran are part of this belt and are divided into Outer (OB) and Inner (IB) Ophiolitic Belts. We here report the first Nd–Hf isotopic study of this ophiolite belt, focusing on the Dehshir ophiolite (a part of IB). Our results confirm the Indian mid‐oceanic ridge basalt (MORB) mantle domain origin for the Dehshir mafic and felsic igneous rocks. All lavas have similar Hf isotopic compositions, but felsic dikes have significantly less‐radiogenic Nd isotopic compositions compared to mafic lavas. Elevated Th/Nb and Th/Yb in felsic samples accompany nonradiogenic Nd, suggesting the involvement of sediments or continental crust.     

Evidence for ∼80–75 Ma subduction jump during Anatolide–Tauride–Armenian block accretion and ∼48 Ma Arabia–Eurasia collision in Lesser Caucasus–East Anatolia

Orogens formed by a combination of subduction and accretion are featured by a short-lived collisional history. They preserve crustal geometries acquired prior to the collisional event. These geometries comprise obducted oceanic crust sequences that may propagate somewhat far away from the suture zone, preserved accretionary prism and subduction channel at the interplate boundary. The cessation of deformation is ascribed to rapid jump of the subduction zone at the passive margin rim of the opposite side of the accreted block. Geological investigation and ⁴⁰Ar/³⁹Ar dating on the main tectonic boundaries of the Anatolide–Tauride–Armenian (ATA) block in Eastern Turkey, Armenia and Georgia provide temporal constraints of subduction and accretion on both sides of this small continental block, and final collisional history of Eurasian and Arabian plates. On the northern side, ⁴⁰Ar/³⁹Ar ages give insights for the subduction and collage from the Middle to Upper Cretaceous (95–80 Ma). To the south, younger magmatic and metamorphic ages exhibit subduction of Neotethys and accretion of the Bitlis–Pütürge block during the Upper Cretaceous (74–71 Ma). These data are interpreted as a subduction jump from the northern to the southern boundary of the ATA continental block at 80–75 Ma. Similar back-arc type geochemistry of obducted ophiolites in the two subduction–accretion domains point to a similar intra-oceanic evolution prior to accretion, featured by slab steepening and roll-back as for the current Mediterranean domain. Final closure of Neotethys and initiation of collision with Arabian Plate occurred in the Middle-Upper Eocene as featured by the development of a Himalayan-type thrust sheet exhuming amphibolite facies rocks in its hanging-wall at c. 48 Ma.     

The petrogenesis and implications of deformed late Caledonian–early Hercynian granites in the Wuyi area,South China

Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (T_DM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have T_DMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.     

SHRIMP U–Pb zircon chronology of ultrahigh‐temperature spinel–orthopyroxene–garnet granulite from South Altay orogenic belt,northwestern China

Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U–Pb ages are reported from an ultrahigh‐temperature (UHT) spinel–orthopyroxene–garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China. This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan–Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al₂O₃ contents up to 9.3 wt%. The peak temperatures of metamorphism were >950°C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P–T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 ± 8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490–500 Ma. Since the granulite was metamorphosed at temperatures >900°C, exceeding the closure temperature of U–Pb system in zircon, a possible interpretation is that the 499 ± 8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a ～500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo‐Asian Ocean and associated accretion‐collision tectonics of the Siberian and Kazakhstan–Junggar Plates followed by rapid exhumation leading to decompression.     

Timescale of magma chamber processes revealed by U–Pb ages,trace element contents and morphology of zircons from the Ishizuchi caldera,Southwest Japan Arc

U–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.     

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 ⁴⁰K-⁴⁰Ar (or ⁴⁰Ar/³⁹Ar) 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 ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar 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 ²³⁰Th 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.     

Rhenium–osmium isotope and elemental behaviour during subduction of oceanic crust and the implications for mantle recycling

This study presents major-, trace-element, and rhenium–osmium (Re–Os) isotope and elemental data for basalts and gabbros from the Zermatt-Saas ophiolite, metamorphosed to eclogite-facies conditions during the Alpine orogeny. Igneous crystallisation of the gabbros occurred at 163.5 ± 1.8 Ma and both gabbro and basalt were subject to ‘peak’ pressure–temperature (P–T) conditions of > 2.0 GPa and ~ 600 °C at about 40.6 ± 2.6 Ma.Despite such extreme P–T conditions, Re–Os isotope and abundance data for gabbroic rocks suggest that there has been no significant loss of either of these elements during eclogite-facies metamorphism. Indeed, ¹⁸⁷Re–¹⁸⁷Os isotope data for both unaltered gabbros and gabbroic eclogites lie on the same best-fit line corresponding to an errorchron age of 160 ± 6 Ma, indistinguishable from the age of igneous crystallisation. In contrast, metamorphosed basalts do not yield age information; rather most possess ¹⁸⁷Re/¹⁸⁸Os ratios that cannot account for the measured ¹⁸⁷Os/¹⁸⁸Os ratios, given the time since igneous crystallisation. Taken with their low Re contents these data indicate that the basalts have experienced significant Re loss (∼ 50–60%), probably during high-pressure metamorphism. Barium, Rb and K are depleted in both gabbroic and basaltic eclogites. In contrast, there is no evident depletion of U in either lithology.Many ocean-island basalts (OIB) possess radiogenic Os and Pb isotope compositions that have been attributed to the presence of recycled oceanic crust in the mantle source. Published Re–Os data for high-P metabasaltic rocks alone (consistent with this study) have been taken to suggest that excessive amounts of oceanic crust are required to generate such signatures. However, this study shows that gabbro may exert a strong influence on the composition of recycled oceanic crust. Using both gabbro and basalt (i.e. a complete section of oceanic crust) calculations suggest that the presence of ≥ 40% of 2 Ga oceanic crust can generate the radiogenic Os compositions seen in some OIB. Furthermore, lower U/Pb ratios in gabbro (compared to basalt) serve to limit the ²⁰⁶Pb/²⁰⁴Pb ratios generated, while having a minimal effect on Os ratios. These results suggest that the incorporation of gabbro into recycling models provides a means of producing a range of OIB compositions having lower (and variable) ²⁰⁶Pb/²⁰⁴Pb ratios, but still preserving ¹⁸⁷Os/¹⁸⁸Os compositions comparable to HIMU-type OIB.     

Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea

Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO₂ contents ranging from ～72 to 73 wt%, K₂O/Na₂O ratios of 1.04–1.23 and low MgO/FeO^t values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al₂O₃/Na₂O + K₂O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)_N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.7104–0.7106 and identical ¹⁴³Nd/¹⁴⁴Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of ?12. The depleted mantle model ages (T_DM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.522–18.552, ²⁰⁷Pb/²⁰⁴Pb = 15.642–15.680 and ²⁰⁸Pb/²⁰⁴Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.     

U–Pb zircon age from the radiolarian‐bearing Hitoegane Formation in the Hida Gaien Belt,Japan

The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian‐bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian‐bearing Hitoegane Formation, Japan, was undertaken. The Hitoegane Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well‐preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the Hitoegane Formation, which is the boundary between the Pseudospongoprunum tauversi to Futobari solidus–Zadrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the Ludlowian to Pridolian. The last occurrence of F. solidus is considered to be Pragian based on the reinterpretation of a U–Pb sensitive high mass‐resolution ion microprobe (SHRIMP) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the Kurosegawa belt, Southwest Japan. Thus the F. solidus–Z. tenuis assemblage can be assigned to the Ludlowian or Pridolian to Pragian. The present data also contribute to establishing overall stratigraphy of the Paleozoic rocks of the Fukuji–Hitoegane area. According to the Ordovician to Carboniferous stratigraphy in this area, Ordovician to Silurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early Devonian. And the quiet Carboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the Middle Permian.     

U–Pb zircon geochronology of the Hida gneiss and granites in the Kamioka area,Hida Belt

A new U–Pb dating and oxygen isotope analysis of zircons collected from a granitic mylonite and an undeformed granite in the Kamioka area, in the Hida Belt of southwest Japan, was conducted using a sensitive high‐resolution ion microprobe (SHRIMP) to restrict the timing of the mylonitization in the Funatsu Shear Zone, which is situated on the eastern and southeastern margins of the Hida Belt. Here, undeformed granite intrudes into the granitic mylonite deformed by mylonitization in the Funatsu Shear Zone. The granitic mylonite and the undeformed granite yielded U–Pb zircon ages of 242.6 ±1.9 Ma and 199.1 ±1.9 Ma, respectively. The granitic mylonite and the undeformed granite also yielded zircon oxygen isotope ratios (δ¹⁸O_VSMOW) of 7.74 ±0.37 ‰ and 5.74 ±0.17 ‰, which suggests that these rocks are derived from different magmas. Therefore, the timing of the mylonitization in the Funatsu Shear Zone is constrained to be at least 242.6–199.1 Ma, which is consistent with other data from the Tateyama area. The U–Pb zircon ages of the banded gneiss in the Kamioka area also reveals that the protolith is a sedimentary rock deposited at approximately 256 Ma, and regional metamorphism occurred at 245.0 ±6.6 Ma, which indicates that the mylonitization in the Funatsu Shear Zone occurred after the metamorphism in the Hida Belt. These geochronological and geochemical data give new insight into the relationship between the Hida Belt and the eastern margin of the Asian continent: the geochronological and geochemical data in this study support the possibility that the Funatsu Shear Zone is comparable with the Cheongsan Shear Zone located at the center of the Ogcheon Belt on the Korean Peninsula.     

The subduction of the Paleo‐Pacific Plate to the Jiamusi Block: Evidence from the Early Mesozoic sedimentary rocks of the eastern Jiamusi Block

In order to provide references of the subduction process of the Paleo‐Pacific Plate beneath the Jiamusi Block, this paper studied the clastic rocks of the Nanshuangyashan Formation using modal analysis of sandstones, mudstone elements geochemistry, and detrital zircon U–Pb dating. These results suggest the maximum depositional age of the Nanshuangyashan Formation was between the Norian and Rhaetian (206.8 ±4.6 Ma, mean standard weighted deviation (MSWD) = 0.17). Whole‐rock geochemistry of mudstone indicates that source rocks of the Nanshuangyashan Formation were primarily felsic igneous rocks and quartzose sedimentary rocks, which were mainly derived from the stable continental block and a magmatic arc. Detrital zircon analysis showed the Nanshuangyashan Formation samples recorded four main age groups: 229–204 Ma, 284–254 Ma, 524–489 Ma and 930–885 Ma, and the provenances were attributed to the Jiamusi Block and a Late Triassic magmatic arc near the study area. Furthermore, the eastern Jiamusi Block was a backarc basin, affected by the subduction of the Paleo‐Pacific Plate in the Late Triassic, but the magmatic arc related to the subduction near the study area finally died out due to tectonic changes and stratigraphic erosion.     

Petrogensis and tectonic setting of the Yemaquan granite from the iron‐polymetallic ore area of Qimantag,Eastern Kunlun Mountains,Qinghai–Tibet Plateau

Being a part of the Paleo‐Tethys Ocean, closing of the Buqingshan‐Anyemaqen oceanic basin left a rich geologic record in the East Kunlun Orogenic Belt. The genesis and tectonic setting of the granites including quartz monzodiorite, granodiorite and mozogranite is discussed in light of the geochemical and U–Pb chronological data obtained. U–Pb dating studies on zircon from the quartz monzodiorite and monzogranite of the research area yielded ages of 220.11 ± 0.49 Ma ((Mean Square Weighted Deviates) MSWD = 0.046) and 223.33 ± 0.54 Ma (MSWD = 0.14), respectively, by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICP–MS) method. According to sedimentological and structural investigations, the Paleo‐Tethys Ocean in the Qimantag region began to close at about 235 Ma, and completely disapperared at about 220 Ma. The three types of granites in this study are considered to intrude the syn‐ to post‐collisional stages. The quartz monzodiorite and granodiorite belong to the I‐type granite whereas the monzogranite is of the S‐type granite. These two types of granites were formed by different ways of partial melting: first, partial melting of the lower crust took place as a result of asthenosphere upwelling triggered by break‐up of the leading edge or tearing of the descending oceanic slab. Subsequently partial melting of the middle–lower crust was caused by the underplating of basaltic magma formed by partial melting of the mantle wedge fluxed by fluids liberated by the oceanic slab dehydration. The magma responsible for the formation of S‐type granites appears to have originated from partial melting of the upper crustal material at a shallower level with a clear signature of continental crust.     

New 40Ar/39Ar ages for selected young (<1 Ma) basalt flows of the Newer Volcanic Province,southeastern Australia

The Pliocene-Holocene Newer Volcanic Province (NVP) of southeastern Australia is an extensive, relatively well-preserved, intra-plate basaltic lava field containing more than 400 eruptive centres. This study reports new, high-precision ⁴⁰Ar/³⁹Ar ages for six young (300–600 ka) basalt flows from the NVP and is part of a broader initiative to constrain the extent, duration, episodicity and causation of NVP volcanism. Six fresh, holocrystalline alkali basalt flows were selected from the Warrnambool-Port Fairy area in the Western Plains sub-province for ⁴⁰Ar/³⁹Ar dating. These flows were chosen on the basis of pre-existing K-Ar age constraints, which, although variable, indicated eruption during a period of apparent relative volcanic quiescence (0.8–0.06 Ma).⁴⁰Ar/³⁹Ar ages were measured on multiple aliquots of whole rock basalt samples. Three separate flows from the Mount Rouse volcanic field yielded concordant ⁴⁰Ar/³⁹Ar age results, with a mean eruption age of 303 ± 13 ka (95% CI). An older weighted mean age of 382 ± 24 ka (2σ) was obtained for one sample from the central Rouse-Port Fairy Flow, suggesting extraneous argon contamination. Two basalt flows from the Mount Warrnambool volcano also yielded analogous results, with an average ⁴⁰Ar/³⁹Ar age of 542 ± 17 ka (95% CI). The results confirm volcanic activity during the interval of relative quiescence. Most previous K-Ar ages for these flows are generally older than the weighted mean ⁴⁰Ar/³⁹Ar ages, suggesting the presence of extraneous ⁴⁰Ar. This study demonstrates the suitability of the ⁴⁰Ar/³⁹Ar incremental-heating method to obtain precise eruption ages for young, holocrystalline alkali basalt samples in the NVP.     

U–Pb ages of granitoids around the Kofu basin: Implications for the Neogene geotectonic evolution of the South Fossa Magna region,central Japan

The Japanese archipelago underwent two arc–arc collisions during the Neogene. Southwest Honshu arc collided with the Izu‐Bonin‐Mariana arc and the northeast Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu‐Bonin‐Mariana arc and the southwest Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been mainly estimated through biotite and hornblende K–Ar dating, here, we perform U–Pb dating of zircon to determine the igneous ages more precisely. In most cases, the secondary post‐magmatic overprint on the zircon U–Pb system was minor. Based on our results, we identify four groups of U–Pb ages: ca 15.5 Ma, ca 13 Ma, ca 10.5 Ma, and ca 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu‐Bonin‐Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu‐Bonin‐Mariana arc, suggesting that the arc–arc collision started by ca 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear‐arc setting in parallel with the other granodiorites of the third group. The U–Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.     

U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 5.0/-6.0 Ma, and the other orthogneiss sample con- tains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 12/?17 Ma and 516 ± 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 10/?8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Pre- cambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.     

Multiple combustion metamorphic events in the Goose Lake Coal Basin,Transbaikalia, Russia: First dating results

Combustion metamorphic (CM) rocks (clinker and paralava) occur in abundance in the eastern and southern margins of the Goose Lake in Western Transbaikalia and form five fields. The sections we studied in natural outcrops exposed in numerous gullies and in quarries comprise the full range of CM varieties from low-grade to fused paralavas and clinkers. The tridymite-plagioclase-cordierite and tridymite-cordierite paralava and clinker have medium to high K/Ca ratios (∼2.5–4.5 wt.%) with K restricted to K-rich (∼4–6 wt.% K₂O) high-silica glass, making the bulk samples suitable for ⁴⁰Ar/³⁹Ar dating.Regional-scale combustion metamorphic events were triggered by reactivation of faults in the Goose Lake Basin causing repeated erosion of gently dipping coal-bearing sediments that exposed coal beds to oxidation resulting in their spontaneous ignition. Geological evidence indicates that the earliest natural coal fire and formation of CM rocks occurred at the end of the Early Cretaceous. Geological and preliminary geochronological data indicate that large-scale coal fires occurred in the Early Pleistocene (no later than 1.8 ± 0.4 Ma ago) and in Late Pleistocene (0.02 ± 0.01 Ma and 0.03 ± 0.03 Ma).