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Late brittle extension is a common feature in orogenic belts, and its role in mountain building processes is still the subject of debate. Its timing relationship with crustal thickening, the building of topography, basin infill, and rock exhumation are of key importance in determining whether it is a major factor in orogenic development or merely causes near-surface secondary effects. We examined this question in relation to the active arc-continent collision of Taiwan, studying its structural evolution by integrating new and critical geochronological results for tensile vein filling of hinterland metamorphic terrane with syn-collision deposition records. Acceleration of rock exhumation and molasse deposition was found to be coeval with the initiation of brittle tensile structures at ca. 1.6 Ma, which was long overdue as continental subduction started well before 6.5 Ma in central to northern Taiwan. The topographic mountain of Taiwan was thus constructed when the upper crust of the thickened orogenic prism turned extensional, as orographic elevation and relief are prerequisites for molasses production. Syn-collisional brittle extension is therefore proposed as a possible facilitator of both augmented extrusive exhumation and the formation of orography.

Abundant earthquakes clustered within a particular zone often reflect an active geological feature, such as clustering seismicity along a fault zone and a huge number of volcanic-earthquakes around the erupting conduit. Herein we perform a double-difference tomographic inversion and relocate the seismicity at the long-resting Tatun volcano group (TVG) in northern Taiwan. A dramatic improvement of the earthquake location model surprisingly show that, from 2014 to 2017, two clustered seismic zones are identified in the TVG. One major group of events (>1000) persistently clustered within a ~500 m diameter vertical conduit with a ~2 km height. The clustering seismicity conduit is just located nearby Dayoukeng, one of the strongest fumaroles in the TVG, and is connected to a fracture zone characterized by low Vp/Vs in the shallow crust. The other group of events is clustered within a sphere-like zone beneath Mt. Chihsin around the depths between 0.5 km and 2 km. Both seismic zones are probably triggered by the significantly volcanic gases and fluids ascending from the deep magma reservoir. Combined with a variety of results from literature, the seismicity conduit near the strong fumarole is the evidence for an active volcano and also identifies a likely pathway for ascending magma if the TVG erupts again in the future. But possibility of developing different magma pathways at other clustered seismic zones such as beneath Mt. Chihsin may not be totally excluded.

Ophiolite-bearing belts mark convergent plate suture zones and are central in reconstructing plate configuration and structures of mountain belts. The active Taiwan mountain belt, product of ongoing convergence between the Eurasian and Philippine Sea plates, has dismembered ophiolitic rocks exposed on both sides of the Longitudinal Valley suture: blueschist blocks in schistose Yuli Belt to the west, and the Eastern Taiwan Ophiolite (ETO) in the unmetamorphosed Lichi Mélange to the east. How these ophiolitic materials correlate with the consumed South China Sea and Luzon forearc oceanic lithospheres between the colliding Chinese continental margin and the Luzon Arc remained speculative. We present zircon U-Pb age results from both ophiolitic rocks in the Yuli Belt and the ETO to pinpoint the formation ages of respective oceanic crusts, and 40Ar/39Ar dates to unravel the tectonothermal history. The zircon U-Pb ages of ~15 Ma for all analyzed samples indicate a common South China Sea origin for ophiolitic materials across the plate suture, since oceanic crust of the Philippine Sea Plate near Taiwan is either Eocene or older. The 40Ar/39Ar ages of Yuli meta-ophiolitic rocks might suggest blueschist metamorphic overprint at ~9 Ma. The 40Ar/39Ar results are generally identical to zircon U-Pb ages for the ETO rocks, concurrent to their olistostromal origin in the forearc basin after being tectonically transported from the trench interface to the eastern retrowedge within the Hengchun accretionary prism. A working tectonic model for the Taiwan arc-continent collision is put forward with emphasis on the fate of the South China Sea during the mountain building processes.

The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying 40Ar/39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S–C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ 40Ar/39Ar dating on muscovite yield ages mainly within 42–38 Ma with calculated closure temperatures of 435–330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset 40Ar/39Ar ages. The K-feldspar 40Ar/39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42–30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology.

In Peninsular Malaysia, Late Cretaceous (95-76 Ma) plutons are found within the Eastern Belt. Their compositions are diverse, ranging from biotite granite, hornblende granodiorite, diorite, and gabbro. The Aur Pluton is the easternmost body and is exposed in the Johore Archipelago, southeast of Peninsular Malaysia. The rock types vary from diorite, quartz monzodiorite and granodiorite. Ar-Ar stepheating age of the diorite and zircon U/Pb age of the granodiorite yielded ages of 77.9 ± 2 Ma and 79 ± 0.7 Ma, respectively. The overall major and trace element trends are consistent with two magmatic pulses, i.e., diorite-quartz monzodiorite and granodiorite. The diorite-quartz monzodiorites are characterized by high Fe2O3 (6.7–8.3 wt.%), CaO (5.5–7.6 wt.%), MgO (2.9–4.1 wt.%) and high but variable Sr concentrations (570–1300 ppm). The granodiorites have significantly higher total alkali content (8.49–8.91%), with high K2O concentrations (3.8–4.6%) and low Fe2O3 (4.7–6.5%), CaO (2.7–3.9%) and MgO (1.4–2.0%) concentrations. Low Nb/U and distinct Pb anomalies suggest that both rock types are likely derived from the crust. Furthermore, elevated LREE and trace element contents for the granodiorites, e.g., FTh (10–20 ppm), Pb (10–30 ppm), and U (2–5 ppm), also indicate that upper crustal components are involved in the magma generation. The dioritic rocks are chemically classified as calc-alkaline volcanic arc (VAG) rocks, but they also have field relationships and mineralogies typically associated with an active continental margin (ACM).

The continental crust of southeast Asia underwent from thickening, thinning to almost rifting during the Mesozoic era as the active continental margin transformed into a passive one. Such crustal thinning history is well-preserved in the Kinmen Island, as the lower crustal granitoids retrograded and rapidly exhumed to surface that were crosscutted by mafic dike swarm. Kinmen Island is situated on the SE coast of Asia, featured by the widespread Cretaceous magmatism as the Paleo-Pacific plate subducted and rollbacked underneath the South China block. Although these complex magmatism are well reported and studied, their associated structural evolution and plate kinematics have not been clearly deciphered. Detailed field mapping, structural measurement, and petrographic analysis of the Kinmen Island were conducted. Up to five deformation events accompanied with five relevant magmatic episodes as well as their corresponding kinematic setting are reconstructed. The ∼129 Ma Chenggong Tonalite (G1) preserved all deformation events identified in this study, which marks the lower bound timing of all reported events. D1 formed a gneiss dome with the Taiwushan Granite (∼139 Ma) at the core bounded by moderately dipping gneissic foliation (S1) as crust extended. D2 formed subhorizontal S-tectonite (S2) with further exhumation of D1 gneiss dome due to middle-to-lower crustal flow associated with further crustal thinning. D3 formed a sinistral ENE-WSW striking steeply S dipping shear belts with well-developed S/C/C’ fabrics. The moderately E-plunging lineation on C surface indicates its transtensional nature. Widespread garnet-bearing leucogranite (G2) associated with decompressional melting showed long lasting intrusion prior to D2 until post D3. D4 was the intrusion of biotite-bearing Tienpu Granite (∼100 Ma; G3) that truncated G1, G2, and all fabrics, which was followed by the intrusion of E-W striking, steeply dipping biotite-bearing pegmatite (G4) as the crust further extended. The youngest deformation event (D5) was NE-SW striking subvertical mafic dike swarm (G5; 90–76 Ma) due to mantle upwelling through significantly thinned crust. By integrating the structural evolution and the previously reported strain pattern, we delineate the slab rollback direction of the Paleo-Pacific plate, which changed from northeastward (129∼114 Ma) to southeastward (107∼76 Ma). This plate kinematic movement switched during 114–107 Ma.

In this article, we present a study of seawater 129I/127I time-series data from several coastal sites in Taiwan, including Yehliu Geopark (north–northeast coast), Kaohsiung (southwest coast), and Zhuwei Fishing Village (northwest coast). The objective of this study was to document how 129I/127I responds to known seasonal variations in the surface ocean currents that carry 129I to each of these sites. The responses were shown to be quite distinct. The Zhuwei site, across Mainland China, had elevated spring and summer 129I/127I values, with abrupt peaks that reflected transient cross-strait currents that carry seawater with elevated 129I to the northwest coast of Taiwan. The Yehliu site, which receives all of its seawater from the Kuroshio Current, had relatively low and uniform 129I/127I values year-round. The Kaohsiung site showed a summertime minimum that likely stems from upwelling upstream induced by oceanic eddies associated with the Kuroshio intrusion. To our knowledge, these are the first continuous coastal 129I/127I time-series data published for Asian waters and the first to show large, abrupt, and regional 129I/127I changes in the surface seawater. We also documented 129I/127I values from multiple surface ocean sites in the South China Sea (SCS), including a vertical profile from the South East Asia Time-Series Station that extends to a depth of 3,700 m. The 129I from both coastal Taiwan and surface waters of the SCS is >98% anthropogenic, primarily released to the environment as a byproduct of nuclear fuel reprocessing.

The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40Ar/39Ar ages of these dikes with the following strikes N98°–N112°, and N114°–N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti (TiO2 ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly (0.86–0.99) and slightly fractionated REE patterns ((La/Yb)N = 2.5–9.1; YbN = 9.5–19.9). The ratios of Th/Ta (1.3–11.4) and Ce/Pb (5.2–58.5) suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 °C (calculated from olivine compositions) and pressures of 6.9 kbar (calculated for pyroxene minerals). Calculated initial 87Sr/86Sr (0.70040–0.70260) and ɛNd(t) = +2.1 to −3.5 at 1575 Ma, also point to a crustal contamination with the most primitive samples showing TDM values of 1946 Ma and 2154 Ma. The low values of La/Ba (

Timing of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40Ar/39Ar plateau ages of ~158-159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ~155-165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128-125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic-Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the 40Ar/39Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. 40Ar/39Ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures (

In this study, we analyzed the carbon and nitrogen stable isotopic composition of human bone collagen in 33 individuals found at the Fantzuyuan site in Taiwan in order to investigate the dietary patterns of this Iron Age group. Forty-three faunal collagen samples were also analyzed to ascertain the variability of baseline isotopic signatures in the area. Mean δ13C and δ15N values of − 12.5 ± 0.7‰ and 8.1 ± 0.5‰, respectively, were found in 26 human individuals. In conjunction with archaeological evidence, this study showed that human diet at this site derived mainly from terrestrial animals, with a minor component derived from marine shellfish. No significant difference in isotopic compositions was detected between male and female adults or between adults and juveniles. However, six individuals had dietary patterns that were different from others, which probably reveals that they had special social status and/or non-local origins.

This paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the Coastal Range of Taiwan, the Northern Luzon Arc. The rocks, recovered from four main volcanoes, vary from low-K tholeiitic to medium-K calc-alkaline basalts to dacites. The rocks are typical of arc magmatic products, exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements. Our new 40Ar/39Ar age data constrain the youngest eruption time in each of the four volcanoes, i.e., from north to south, at ~ 7.2 Ma (Yuemei), ~ 4.2 Ma (Chimei), ~ 6.2 Ma (Chengkuang'ao) and ~ 8.5 Ma (Tuluanshan), respectively. These data indicate that volcanism in the Northern Luzon Arc did not cease progressively from north to south, as previously alleged. The high and broadly uniform Nd isotope ratios [eNd = + 10.1 to + 8.8] and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge. Their overall geochemical variations are ascribed to magma chamber processes. The effects of magmatic differentiation and crustal contamination differ among each volcano, most likely owing to the discrepancy of residence time in individual magma chambers. Consequently, we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to ~ 5% crustal contamination with a continental fragment split off from the Eurasian margin.

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm−1 and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm−1. These data combined with previously reported diamond band data (1331–1337 cm−1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureil...

Mica porphyroblasts, a common metamorphic microstructure, are analyzed in the slate belt of northern Taiwan where large fish-like growths are found within a meta-pyroclastics. With constraints on the time-temperature history from deposition through peak metamorphic state to exhumation, in-situ 40Ar/39Ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm-scale mica porphyroblasts grown on a pressure-solution seam. Because the peak metamorphic temperature and the porphyroblast formation condition (∼250 °C) remained far below the closure temperature of the K-Ar radioisotope system in muscovite, and the absence of muscovite in the mafic protolith, the dating results likely document the growth of the mica porphyroblast fabrics. The syn-kinematic nature of the analyzed porphyroblasts is confirmed by the ∼6 to ∼2.5 Ma growth ages, suggesting that the host rock was continuously deformed during the earlier two-thirds of the Taiwan Orogeny. The pattern of fiber growth, in contrast to outward-decreasing ages normally observed in peripheral recrystallization, appears random and resembles void fills in boudin openings. We postulate that syntaxial crack-seal following tensile micro-boudinage, along with slips on sub-grain boundaries, as a viable mechanism for the development of mica porphyroblasts and fish especially in lower-grade metamorphic rocks.

Carbon and nitrogen stable isotope analyses were conducted on Neolithic Yuan-Shan faunal bone collagen to reconstruct a site-specific dietary isotope baseline, and to evaluate the contribution of potential food resources to the diet of Yuan-Shan people. The mean δ 13 C and δ 15 N values of terrestrial mammals were − 17.7 ± 3.6‰ and 5.4 ± 1.3‰, respectively. The mean δ 13 C and δ 15 N values of freshwater fish were − 20.8 ± 2.2‰ and 7.7 ± 2.0‰; while the marine fish had the highest mean δ 13 C (− 11.8 ± 1.2‰) and δ 15 N values (12.1 ± 1.7‰). Combined with archeological evidence and previous isotopic data derived from Yuan-Shan human bone collagen, we suggest that the subsistence strategies of Yuan-Shan people were broad. They relied on rice cultivation, hunting, fishing and gathering food resources from the wild; in addition, they probably raise pigs, as early as 4200 yr BP. Although the Yuan-Shan people relied on broad-spectrum food resources, the foods they mainly consumed were terrestrial herbivores and freshwater fish based on the result of multi-source mixing model. Besides, marine fish, shellfish, and C 3 plants also contributed substantially to their diet. By comparison, the contribution of C 4 plants was minor.

Constraining the timing of brittle deformation, such as in high-angle normal or strike-slip faulting, is technically challenging. In this study we present a new approach of age determinations from cataclastic K-feldspars of brittle faults developed in rocks of the Tan–Lu fault zone and from Louzidian and Liaonan metamorphic core complexes in NE China. The attempt is based on field investigations, microstructural observations, back-scattered electron (BSE) imaging, electron-microprobe analyses, X-ray diffraction, scanning electron microscope (SEM) results, and 40Ar/39Ar step-heating of K-feldspar separated from brittle fault zones of cataclastic mylonite, granite and gneiss. SEM and BSE images of individual K-feldspar crystals reveal distinct grain deformation features, including grain-scale faults (fractures and microfractures) that occur parallel or perpendicular to each other. The 40Ar/39Ar dating results provide well-defined plateau spectra for the cataclastic K-feldspar grains in the Tan–Lu fault zone (74.5 ± 1.3 Ma), and of brecciated gneisses and mylonites that formed at depth in the Louzidian and Liaonan metamorphic core complexes (~120–129 Ma and 116 ± 2 Ma). These data suggest complete resetting of the Ar isotope system of the cataclastic K-feldspars, allowing to constrain the timing of their deformation.

International audience; Deep tectonic processes are key integral components in the evolution of mountain belts, while observations of their temporal development are generally obscured by thermal resetting, retrograde alteration and structural overprinting. Here we recorded an integrated rock time-temperature history for the first time in the pro-wedge part of the active Taiwan arc-continent collision starting from sedimentation through cleavage-forming state to its final exhumation. The integrated thermal and age results from the Raman Spectroscopy of Carbonaceous Material (RSCM) method, zircon U-Pb laser ablation dating, and in-situ 40Ar/39Ar laser microprobe dating suggest that the basal accretion process was crucial to the development of the Taiwanese orogenic wedge. The basal accretion process commenced early in the mountain building history (∼6 Ma) and gradually migrated to greater depths, as constrained by persistent plate convergence and cleavage formation under nearly isothermal state at similar depths until ∼ 2.5 Ma recorded in the early-accreted units. Such development essentially contributed to mountain root growth by the increased depth of the wedge detachment and the downward wedge thickening during the incipient to full collision stages in the Taiwan mountain belt.

The Doba gabbro was collected from an exploration well through the Cretaceous Doba basin of southern Chad. The gabbro is composed mostly of plagioclase, clinopyroxene and Fe–Ti oxide minerals and displays cumulus mineral textures. Whole-rock40Ar–39Ar step-heating geochronology yielded a Late Permian plateau age of 257 ± 1 Ma. The major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios (i.e. La/YbN> 7; Sm/YbPM> 3.4; Nb/Y > 1; Zr/Y > 5) indicative of a basaltic melt derived from a garnet-bearing mantle source. The moderately enriched Sr–Nd isotopes (i.e. ISr= 0.70495 to 0.70839; ɛNd(T)= −1.0 to −1.3) fall within the mantle array (i.e. OIB-like) and are similar to other Late Permian plutonic rocks of North-Central Africa (i.e. ISr= 0.7040 to 0.7070). The enriched isotopic composition of the Doba gabbro contrasts with the more depleted compositions of the spatially associated Neoproterozoic post-Pan-African within-plate granites. The contrasting Nd isotope composition between the older within-plate granites and the younger Doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the Saharan Metacraton.

Two ruby-related basaltic fields were recently discovered in the southeast region of Kenya, exposed in the Nguu and Ngulai Hills vicinities. These fields host abundant deep-seated xenoliths, including corundum-bearing granulites. The basalts are alkaline affinity having compositions from foidrite to basanite. The Ngulai basalts have a wider range of SiO2 (38.2 wt.%–44.8 wt.%) covering those of the Nguu basalts (38.7 wt.%–42.3 wt.%). This overlapping behavior also holds for other major oxides and trace elements, e.g., Al2O3, Na2O, K2O, Cr, Ni, Rb and Ga. The overall OIB-like incompatible patterns with strong K depletion and slight spike of Ti enrichment signatures imply low degrees of partial melting of the upper mantle region source induced under a mantle plume-related process. The K-depletion signature indicates a residual K-bearing phase still retained in the source domain. Chondrite-normalized REE patterns exhibiting strong LREE enrichment without Eu anomalies suggest that plagioclase fractionation is insignificant. New 40Ar/39Ar ages indicate eruption events occurred during the Pleistocene times, which are around 2 Ma for the Ngulai basalts and 0.9 to 1.6 Ma for the Nguu basalts. Clinopyroxene-basalt thermobarometric calculations yield the equilibrium P-T ranges of ∼8-29 kbar and 1 200–1 450 °C.

Much debate surrounds the temporal-thermal-structural evolution of the >1000-km NW-SE-trending Ailao Shan–Red River (ASRR) shear zone. This mainly reflects the contradictory interpretations of the timing and P-T conditions for the occurrence of left-lateral shearing event due to a diverse data set of mineral ages. Our new microstructural, petrological, and geochronological data from the southern section of the Diancang Shan (DCS) block along the west wall of the ASRR support slow cooling from ∼300°C to ∼150°C from the Middle Eocene to the Late Miocene, accompanied by left-lateral shearing activity. This conclusion is based on the following: (1) The lower greenschist facies assemblages of biotite replaced by muscovite or chlorite, the albitized K-feldspar, and the stable association of K-feldspar + chlorite rather than muscovite + biotite are universally observed for the synkinematic Sn+1 and mylonitic S/C fabrics from both quartzofeldspathic and amphibolite domains. (2) New 40Ar/39Ar data for K-fe...

Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by 40Ar/39Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation 40Ar/39Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of publ...

Granitoids are the predominant geological constituents of South China and important for the understanding of the formation and evolution of continental crust. We describe field structural features of four small-scale granitic plutons and their country rocks in southern Jiangxi Province and present new zircon U-Pb and mica 40Ar/39Ar ages that are used to discuss the tectonic-magmatic evolution and thermal overprint events, that is, the tectonic rejuvenation of continental crust of the Cathaysia Block. The muscovite-bearing granite of Mashuang pluton shows sub-north-south mylonitic foliation and stretching/mineral lineation that shallowly plunges to the northeast, generally consistent with fabrics in the host rocks that are composed of widely migmatized metasediments. Zircons and muscovites yielded U-Pb and 40Ar/39Ar ages at 457 ± 4 and 199 ± 1 Ma, which are considered to represent the ages of emplacement and postmylonitic thermal overprint, respectively. The undeformed granites of Tianjiu and Junme...

The Song-Ma suture zone including the “classical Song Ma suture” and the Nam Co antiform between Indochina and South China blocks in northern Vietnam is a crucial area for understanding the tectonic evolution of Southeast Asia. To explore their natures and evolutionary history, this paper reports new petrological studies and SIMS zircon U-Pb age data for the Song Ma ophiolites, metabasites and fine-grained quartzite. The Song Ma ophiolites consist mainly of peridotite (Fo90-91, En92), basalt and gabbro, and experienced greenschist- to lower amphibolite-facies ocean-floor metamorphism. Lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite. We identified two types of zircon in the altered pyroxenite: I uniform metamorphic (70 vol.%), and II, zircon having inherited magmatic cores and metamorphic rims (30 vol.%). All U-Pb analyses of both zircon types yield an upper intercept age of 340 ± 29 Ma that we interpret as the protolith age. Type I zircon and Type II metamorphic rims together yield a mean 206Pb/238U age of 280 ± 2 Ma, with rodingite forming broadly coevally at 283 to 282 Ma. Metabasalt (plagioclase-amphibole schist) has a MORB-type geochemical affinity. Magmatic zircon from a plagioclase-amphibole schist yields a protolith age of 315 ± 4 Ma. High-pressure eclogite, garnet-phengite-quartz schist, and garnet-hornblende rock are distributed in the northwestern Nam Co antiform. Zircon crystals from both eclogite and garnet hornblende rock have low Th/U ratios of 0.01 to 0.07 and 0.02 to 0.06, and yield metamorphic ages of 231 ± 8 Ma and 228 ± 3 Ma, respectively. The ages of detrital zircon grains from fine-grained, banded quartzite from the Nam Co antiform range from Paleozoic to Proterozoic with a peak at 1.7 to 1.8 Ga. All available data indicate a three-stage tectonic evolution: (1) 340 to 280 Ma: the Paleotethys that separated the Indochina and South China blocks was characterized by oceanic crust formation at 340 to 315 Ma and ocean-floor metamorphism at 283 to 280 Ma, (2)

Basaltic dykes of Peninsular Malaysia are confined to the Eastern Belt (Indochina/East Malaya block) as compared with the Western Belt (Sibumasu Block). The dyke intruded through a crustal fracture formed by stress developed from the evolution of two offshore basins (Malay and Penyu basins) east of Peninsular Malaysia. The Ar–Ar dating from the present study combined with the previous geochronological data indicate that the ages of dykes range from 79 ± 2 Ma to 179 ± 2 Ma. Thus it is difficult to correlate the dykes with the closure of Tethys during Permo-Triassic time because of the younger age of the dykes. The majority of the dykes exposed in the Eastern Belt may have been attributed to the difference of crustal thickness between the Eastern and Western belt of Peninsular Malaysia. A thicker Western Belt crust (13 km more than both Eastern and Central belts) is difficult to rupture with normal plate tectonic stress and therefore serves to contain the rise of a mantle derived melt. The chemistry indicates the basalts are olivine to quartz normative and are of the continental within-plate category.

The greenstone belts of northeast Burkina Faso in the West African Craton (WAC) comprise Birimian meta-sedimentary and mafic–ultramafic complexes that are intruded by granitoids and late dolerite dykes. The granitoid–greenstone belt assemblages underwent at least two phases of deformation that have resulted in their principal NW and NE trends. We performed a detailed 40Ar/39Ar dating together with zircon U–Pb ages in order to unravel the thermo-chronological history of the studied granitoids. Our previous zircon U–Pb data indicated that the granitoids were emplaced between 2138 ± 7 and 2181 ± 7 Ma. This period is well known for crustal growth in the WAC. The emplacement of granitoids was concomitant with D1 or the Tangaean event (2130–2170 Ma). Amphibole and micas yield two 40Ar/39Ar age groups; the first group (2046 ± 12–2132 ± 13 Ma) constrained the D2 deformation event, and the second group (1918 ± 24 to 1984 ± 12 Ma), constrained the D3 or Wabo-Tempelse event. Considering closure temperatures of the isotopic systems, the U–Pb ages and the present 40Ar/39Ar dating results suggest that after emplacement, the granitoids cooled to 315 C at ca. 2020 Ma with a mean cooling rate of 6.5 C/Ma; corresponding to a mean uplift rate of 0.65 km/Ma. The additional K-feldspar 40Ar/39Ar data indicate a later slow cooling stage with a rate of 0.54 C/Ma, corresponding to a mean uplift rate of 0.054 km/Ma, subsequent to the end of the major crustal growth in the WAC at ∼2000 Ma

Ultrapotassic rocks are a common, but volumetrically minor,hallmark of post-collisional magmatism along the Alpine–Himalayan orogenic belt. Here, we document the occurrenceof ultrapotassic volcanic rocks from the Eslamy peninsula, NWIran in the Arabia–Eurasia collision zone. Our results indicatethat magma genesis involved melting of phlogopite- and apa-tite-bearing peridotites in the sub-continental lithosphericmantle at ~11 Ma. These peridotites likely formed by metaso-matism involving components derived from subducted sedi-ments during Neotethyan subduction. The ~11 Maultrapotassic volcanism was preceded by a magmatic gap of~11 Ma after the cessation of arc magmatism in NW Iran andArmenia, thus likely representing the initiation of post-colli-sional magmatism. The age coincides with the onset of colli-sion-related magmatic activity and topographic uplift in theCaucasus–Iran–Anatolia region, and also with other regionalgeological events including the closure of the eastern Tethysgateway, the end of Arabian underthrusting and the start ofescape tectonics in Anatolia.

Mantle xenoliths provide direct information about lithospheric evolution and asthenosphere–lithosphere interaction, and therefore precise dating of the host basalts which carried the xenoliths is important. Here we report 40Ar/39Ar geochronology of phlogopite separates from five spinel lherzolite xenoliths collected from the North China Craton (Hannuoba of Hebei Province, Sanyitang of Inner Mongolia Autonomous Region and Hebi of Henan Province), as well as the groundmass of the host basalts. Argon extraction was performed by conventional step heating technique and ultra-violet laser ablation microprobe (UVLAMP) technique. 40Ar/39Ar incremental heating results on groundmass yielded geologically meaningless ages. However, conventional step heating on phlogopites produced Miocene cooling ages, identical to the eruption ages obtained from the K–Ar dating methods of the Hannuoba and Sanyitang basalts. Adopting procedures to exclude potential influence of excess radiogenic Ar from a deep fluid source on a phlogopite separate from lherzolite yielded results with a good agreement of ages suggesting that the argon isotopes are distributed homogenously in this mineral, with no influence of excess argon. Phlogopites from Hebi yield ages between 6.43 and 6.44 Ma which are slightly older than those obtained from K–Ar method on whole-rocks. The discrepancy in the K–Ar ages obtained from the altered whole-rock samples suggests partial loss of 40Ar. As a consequence, phlogopite Ar–Ar ages are considered more accurate than that of the whole-rocks. These results suggest that 40Ar/39Ar chronology of phlogopite provides reliable and precise 40Ar/39Ar ages of host basalts.

Buruanga Peninsula forms the westernmost part of Panay Island, Central Philippines and is a part of the Palawan Continental Terrane (PCT), which was formerly attached to south-eastern China. It acted as the leading edge of the continental fragment and collided with the Philippine Mobile Belt (PMB) followed by convergence beneath the latter. Dating of the collision is crucial for understanding the evolution of the archipelago. Samples collected from Buruanga Peninsula were dated using U–Pb, 40 Ar/ 39 Ar and fission track dating (FTD) techniques to constrain the timing of the tectonic events related to the collision of the PMB with the PCT. These techniques have enabled us to obtain ages over a range of closure temperatures from about 700 °C to about 110 °C. Paleoproterozoic and Permian zircon U–Pb ages from Saboncogon Formation emphasize derivation of the western part of Buruanga Peninsula from SE China; zircon and apatite fission track ages of 51 Ma and 16 Ma, respectively, constrain the exhumation of this formation. The age data suggest tectonic events at ~ 14 Ma, ~ 11–12 Ma and about 7–8 Ma following intrusive activity at about 18 Ma. Uplift and exhumation at ~ 14 Ma are thought to be the result of subduction of low-density crustal rocks, at 11 Ma to be the result of isostatic uplift as a consequence of crustal thickening and at ~ 8 Ma to be due to the isostatic re-equilibration of the sediments overlying the former suture. Hence, collision is constrained to have started at about 14–15 Ma and to have ended before 8 Ma. Multi-element patterns of the 18 Ma Patria-Diorite from Buruanga Peninsula show enrichment in LILE (Rb, Sr, and K) and LREE and depletion in HFSE elements (Ti, Nb, and Ta) similar to those from Luzon volcanics and the volcanic rocks of Negros Island. These arc-signatures indicate a subduction related environment for the emplacement of this intrusive body and show that the diorite belongs to the PMB. The age constraints of the present study neither support the concept of an Early Miocene collision nor the assumption of a Late Miocene collision, thus, have a significant bearing on the controversial issues regarding the evolution of the present day Philippine archipelago.

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1 Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122 ± 15 Ma and 2181 ± 7 Ma for the Birimian Na-rich granitoids and 2151 ± 10 Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition ( I Sr ( T ) ≤ 0.7007), positive epsilon neodymium values ( ɛ Nd ( T ) = +0.7 to +2.5) and Sm–Nd model ages ( T DM ) of 2363–2558 Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: ( I Sr ( T ) = 0.7003–0.7015, ɛ Nd ( T ) = +3.2 to +4.1, T DM = 2336–2552 Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15 Ga) of a large scale crustal growth event (2.2–2.0 Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

The source of intraplate basalts has long been a controversial topic, particularly in continental settings where ambiguity increases because both crustal contamination and crystal fractionation may mask important source characteristics. We present geochemical data to constrain the source and the chemical evolution of the continental intraplate magmas from the Aleppo Plateau and vicinity, NW Syria. New 40Ar/39Ar ages, coupled with published 40Ar/39Ar and K–Ar ages, reveal two discrete Miocene volcanic phases, ~ 19–18 Ma (Phase 1) and ~ 13.5–12 Ma (Phase 2), in the studied area. New chemical and isotopic compositions [87Sr/86Sr = 0.7036–0.7051, eNd = + 4.5 to + 1.1 and (187Os/188Os)t = 0.151–0.453] of the lavas reflect the unequivocal influence of crustal assimilation and fractional crystallisation (AFC). Despite the effects of the AFC processes, there still appear to be some differences between the most-primitive, least contaminated magmas of the two volcanic phases, interpreted as a result of source heterogeneity. Whereas the Phase 1 lavas, with relatively high Si, low Ti and trace-element contents, are consistent with partial melting of a largely peridotitic mantle source, the origin of the Phase 2 lavas is more complicated. The latter are characterised by a source component depleted in Si and enriched in Ti, Fe, Ca, P, alkalis, light and middle rare earth elements (REEs) relative to heavy REEs and with sub-chondritic Th–(U)/Nb, Pb/Ce and Zr/Sm. They approach compositions of experimental melts of amphibole-rich metasomatic veins. The compositional variations among the most primitive Phase 2 lavas are difficult to reconcile with varying degrees of partial melting of either the metasomatic veins or peridotite, but could be explained if partial melts of both lithologies were variably mixed, a scenario that could be sensibly envisioned as ascending (peridotitic) plume/asthenosphere derived melts assimilating highly fusible metasomatic veins during their traverse through the lithosphere. This process can be loosely quantified by trace-element forward partial melting modelling that suggests mixing of up to 80% metasomatic melts derived from ~ 40% melting of amphibole-rich metasomatic veins (which themselves were inevitably compositionally and mineralogically heterogeneous) with 20% plume/asthenospheric melts derived from ~ 7% melting of a garnet peridotite. Within the compressional framework of northern Arabia, invocation of diapiric material reasonably accounts for the generation of the intraplate basalts in Syria. Derivation of the Phase 2 hybrid melts was probably triggered by lateral flow of this diapiric material beneath the lithosphere subsequent to its arrival, with the migrating flow-front controlling the locus of volcanism. The increase in degree of Si-undersaturation with time for the Phase 1 and Phase 2 lavas is best explained by decreasing temperatures of this flow-front that resulted in less melt contribution from the diapiric mantle while the amphibole-rich veins within the lithosphere continued to be easily fusible, although we cannot totally exclude the possibility that the Phase 2 volcanism tapped a vein-richer domain which formed subsequent to the Phase 1 volcanism.

New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we find that island-arc tholeiite of the lower pillow lava sequence erupted first, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infill lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifice greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.