Extensive normal faulting during exhumation revealed by the spatial variation of phengite K–Ar ages in the Sambagawa metamorphic rocks,central Shikoku,SW Japan

Metamorphic rocks experience change in the mode of deformation from ductile flow to brittle failure during their exhumation. We investigated the spatial variation of phengite K–Ar ages of pelitic schist of the Sambagawa metamorphic rocks (sensu lato) from the Saruta River area, central Shikoku, to evaluate if those ages are disturbed by faults or not. As a result, we found that these ages change by ca 5 my across the two boundaries between the lower‐garnet and albite–biotite, and the albite–biotite and upper‐garnet zones. These spatial changes in phengite K–Ar ages were perhaps caused by truncation of the metamorphic layers by large‐scale normal faulting at D₂ phase under the brittle‐ductile transition conditions (ca 300°C) during exhumation, because an actinolite rock was formed along a fault near the former boundary. Assuming that the horizontal metamorphic layers and a previously estimated exhumation rate of 1 km/my before the D₂ phase, the change of 5 my in phengite K–Ar ages is converted to a displacement of about 10 km along the north‐dipping, low‐angle normal fault documented in the previous study. Phengite ⁴⁰Ar–³⁹Ar ages (ca 85 to 78 Ma) in the actinolite rock could be reasonably comparable to the phengite K–Ar ages of the surrounding non‐faulted pelitic schist, because the K–Ar ages of pelitic schist could have been also reset at temperatures close to the brittle–ductile transition conditions far below the closure temperature for thermal retention of argon in phengite (about 500–600°C).     

Phengite K-Ar ages of schists from the Sanbagawa southern marginal belt, central Shikoku, southwest Japan: Influence of detrital mica and deformation on age

Abstract K–Ar age determinations were carried out on phengite separates from pelitic schists collected systematically from the Sanbagawa southern marginal belt and the associated area. The petrography and phengite chemistry by electron probe micro-analyzer (EPMA) revealed the existence of detrital white micas in the schist that have an extremely older age (108 Ma) in comparison with the neighboring schists (88 Ma) without any detrital mica. The ages become gradually older from the north ( ca 78 Ma) to the south ( ca 90 Ma) except for some samples that contain detrital micas and/or have been reactivated thermally by intrusives. The age is interpreted as an exhumation-cooling age that has been controlled by the ductile deformation of the host rocks that have never experienced a culmination temperature higher than 350°C which corresponds to the closure temperature of the K–Ar phengite system. The southward aging of the recorded ages in the extensive chlorite zone of the central Shikoku, from the Dozan river area of the north ( ca 65 Ma) to the study area of the south ( ca 85 Ma) through the Asemi river area ( ca 75 Ma), is explained in terms of increasing exhumation/cooling rates of the host rocks from north to south. The phengite K–Ar ages in the pelitic schists from the Kyomizu tectonic zone, which is classically considered as a remarkable thrusting shear zone, have no significant difference in comparison with that of the neighboring schists. This fact suggests that the latest stage of brittle deformation during exhumation/uplift has not significantly affected the ages of phengite in the schists.     

Phengite geochronology of crystalline schists in the Sakuma–Tenryu district,central Japan

The Sakuma–Tenryu district consists mainly of pelitic and basic schists. Its metamorphic sequence has been divided into two units, the Shirakura and the Sejiri units. We carried out K–Ar analyses of phengite separates and X‐ray diffraction analyses of carbonaceous materials from the pelitic schists of both units. The age–d₀₀₂ relationships show that the ages become older (66–73 Ma) in the Shirakura unit and younger (57–48 Ma) in the latter with increasing metamorphic temperature. The former has a positive relationship observed in the Sanbagawa meta‐Accretionary Complex (meta‐AC) (Sanbagawa metamorphic belt sensu stricto) in central Shikoku and the latter, a negative one in the Shimanto meta‐AC (a subunit of traditional Sanbagawa belt) of the Kanto Mountains. These contrasting age–temperature relationships are due to different tectonic styles relating to the exhumation of the metamorphic sequences. The duration from the peak metamorphism to the closure of the phengite K–Ar system was significantly different between the two metamorphic sequences: longer than 31 my in the Sanbagawa meta‐AC and shorter than 13 my in the Shimanto meta‐AC. The different natures of subducted plate boundaries may cause the different exhumation processes of metamorphic belts.     

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.     

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).     

K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan

Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and ⁴⁰Ar/³⁹Ar dating of phengite show all the pelitic schist clasts from low- to high-grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. ⁴⁰A/³⁹Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.     

Sm-Nd and Zircon U-Pb ages from garnet-bearing eclogites, NE Oman: constraints on High-P metamorphism

Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and ⁴⁰Ar/³⁹Ar ages of 82-79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet-garnet leachate-whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97-94 Ma) as well as the subsequent obduction onto the margin (80-70 Ma). A U-Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism.     

40Ar/39Ar ages of blueschist facies pelitic schists from Qingshuigou in the Northern Qilian Mountains, western China

Abstract   Pelitic schists from Qingshuigou in the Northern Qilian Mountains of China contain mainly glaucophane, garnet, white mica, clinozoisite, chlorite and piemontite. Isotopic age dating of these schists provides new constraints on the formation of the high-grade blueschists at Qingshuigou. White mica ⁴⁰Ar/³⁹Ar ages range from 442.1 to 447.5 Ma (total fusion age of single grain) and from 445.7 to 453.9 Ma (integrated age of white mica concentrates). These ages (442.1–453.9 Ma) represent the peak metamorphic ages or cooling ages of the blueschists during exhumation shortly after peak metamorphism. The ⁴⁰Ar/³⁹Ar dates in the present study are similar to ages previously reported for eclogites and blueschists in the area; this suggests that both the eclogites and pelitic sediments underwent high-grade metamorphism during the same subduction event. From this chronological evidence and the presence of well-developed Silurian remnant-sea flysch and Devonian molasse, it is concluded that the Northern Qilian Ocean had closed by the end of the Ordovician, and rapid orogenic uplift followed in the Devonian.     

Blueschist-facies metamorphism during Paleozoic orogeny in southwestern Japan: Phengite K–Ar ages of blueschist-facies tectonic blocks in a serpentinite melange beneath early Paleozoic Oeyama ophiolite

Blueschist-bearing Osayama serpentinite melange develops beneath a peridotite body of the Oeyama ophiolite which occupies the highest position structurally in the central Chugoku Mountains. The blueschist-facies tectonic blocks within the serpentinite melange are divided into the lawsonite–pumpellyite grade, lower epidote grade and higher epidote grade by the mineral assemblages of basic schists. The higher epidote-grade block is a garnet–glaucophane schist including eclogite-facies relic minerals and retrogressive lawsonite–pumpellyite-grade minerals. Gabbroic blocks derived from the Oeyama ophiolite are also enclosed as tectonic blocks in the serpentinite matrix and have experienced a blueschist metamorphism together with the other blueschist blocks. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist-facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~ 10°C/km). Phengite K–Ar ages of 16 pelitic and one basic schists yield 289–327 Ma and concentrate around 320 Ma regardless of protolith and metamorphic grade, suggesting quick exhumation of the schists at ca 320 Ma. These petrologic and geochronologic features suggest that the Osayama blueschists comprise a low-grade portion of the Carboniferous Renge metamorphic belt. The Osayama blueschists indicate that the 'cold' subduction type (Franciscan type) metamorphism to reach eclogite-facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum-Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician.     

Thermobaric structure of the Kokchetav ultrahigh-pressure–high-pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO₂ (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.     

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit,Kamuikotan metamorphic rocks,central Hokkaido,Japan: constraints on metamorphic histories

Accurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.     

A jadeite–quartz–glaucophane rock from Karangsambung, central Java, Indonesia

High-pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk–Ulo Complex) with fault-bounded slices of shale, sandstone, chert, basalt, limestone, conglomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K–Ar ages. Small amounts of eclogite, glaucophane rock, garnet–amphibolite and jadeite–quartz–glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite–garnet–glaucophane–phengite–quartz equilibrium, peak pressure and temperature of the jadeite–quartz–glaucophane rock are P  = 22 ± 2 kbar and T  = 530 ± 40 °C. The estimated P–T conditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was ∼ 7.0 °C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz–mica schists at lower or middle crustal depths.     

Tectono-metamorphic history of the Tacagua ophiolitic unit (Cordillera de la Costa, northern Venezuela): Insights in the evolution of the southern margin of the Caribbean Plate

Abstract   The southern margin of the Caribbean Plate is well exposed in the Cordillera de la Costa of northern Venezuela, where amalgamated terranes consisting of continental and oceanic units occur. In the Cordillera de la Costa, metamorphosed oceanic units crop out along the coast near Caracas. Among them, the Tacagua unit is characterized by metaserpentinites and metabasites showing mid-oceanic ridge basalt geochemical affinity. These lithologies, representative of a disrupted ophiolite sequence, are associated with metasediments consisting of calcschists alternating with pelitic and psammitic schists, whose protoliths were probably represented by deep-sea hemipelagic and turbiditic deposits. In the Tacagua unit, a polyphase deformation history has been reconstructed, consisting of four folding phases from D1 to D4 . Geological setting suggests an involvement of the Tacagua unit in the processes connected with a subduction zone. The following deformations (from D2 to D4 ) observed in the field might be related to the exhumation history of the Tacagua unit. The late deformation history consists of an alternation of deformation phases characterized by displacement parallel ( D2 and D4 phases) and normal ( D3 phase) to plate boundary between the Caribbean and South America Plates. All lines of geological evidence suggest that the whole evolution of the Tacagua unit was acquired in a setting dominated by oblique convergence, in which alternation of strike-slip and pure compressional or pure extensional tectonics occurred through time.     

Duplex of metamorphic units including ultramafic rocks in the central region of Tokunoshima,Southwest Japan

Geological observations in the central part of Tokunoshima in the Amami Islands, Southwest Japan, reveal that discrete layers of serpentinite, dioritic gneiss, and amphibolite are intercalated into pelitic schist and these rock bodies form a northwest‐dipping tectonic stack. A subhorizontal psammitic schist layer overlies them. These rocks underwent ductile deformation that is denoted by penetrative foliation and mineral lineation. Microstructures of the sheared metamorphic rocks and serpentinite indicate top‐to‐the‐east, ‐southeast or ‐south (hanging‐wall up) displacements. The en echelon array of rock bodies is interpreted as a duplex with the psammitic schist layer on its top and the pelitic schist layer on its bottom. It is inferred that the serpentinite‐bearing duplex was formed due to the tectonic erosion and the subsequent accretionary growth operated in a Cretaceous or older subduction zone. Tokunoshima has been considered to belong to the Shimanto Belt. However, regional low‐pressure and high‐temperature type amphibolite‐facies metamorphism and related ductile deformation have not been recognized in the other areas of the Shimanto Belt. There is no metamorphic rock occurrence comparable to that of Tokunoshima in the neighboring islands. The metamorphic rocks in Tokunoshima can be correlated to any of low‐pressure/temperature type metamorphic regions in Kyushu.     

40Ar/39Ar geochronology and exhumation of mylonitized metamorphic complex in Changle-Nanao ductile shear zone

New⁴⁰Ar/³⁹Ar plateau ages from rocks of Changle-Nanao ductile shear zone are 107.9 Ma(Mus), 108.2 Ma(Bi), 107.1 Ma(Bi), 109.2 Ma(Hb) and 117.9 Ma(Bi) respectively, which are concordant with their isochron ages and record the formation age of the ductile shear zone. The similarity and apparent overlap of the cooling ages with respective closure temperatures of 5 minerals document initial rapid uplift during 107–118 Ma following the collision between the Min-Tai microcontinent and the Min-Zhe Mesozoic volcanic arc. The⁴⁰Ar/³⁹ Ar plateau ages, K-Ar date of K-feldspar and other geochronologic information suggest that the exhumation rate of the ductile shear zone is about 0.18–1.12 mm/a in the range of 107–70 Ma, which is mainly influenced by tectonic extension.     

Isotope systematics in minerals: biotite rejuvenation and exchange during Alpine metamorphism

The Hercynian schists and granites of the Helvetic basement (Aar Massif) in the Central Swiss Alps are overprinted by low-grade Alpine metamorphism and associated deformation. Rb/Sr and K/Ar ages of biotites are systematically reset by the later greenschist facies event. This rejuvenation is apparently controlled by temperature, mineral chemistry and grain size and independent of deformation and exsolution. Thermally activated volume diffusion best explains this behaviour with Ar and Sr having similar activation energies, and diffusion rates for Sr showing a greater dependence on biotite composition. Feldspars and apatite also show increasing equilibration to the Alpine event and together with Alpine equilibria in the micas reveal important changes in the isotopic compositions of exchanging reservoirs. These spatial and temporal variations must be assessed before true mineral ages can be calculated. Generally the higher-grade equilibria show less radiogenic exchange reflecting increased dilution by external fluids of radiogenic components released at the main front of isotopic resetting.Rb/Sr ages of Alpine muscovites in the granites suggest a maximum possible age of metamorphism of 25 Ma in this low-grade area. This implies significant diachroneity in the timing of peak metamorphism in the Central Alps, with the relatively delayed metamorphism in this area a reflection of the later initial thickening and delayed uplift of the Helvetic basement.     

Phase equilibria and metamorphic evolution of garnet‐mica‐plagioclase gneisses from the Qiliping terrane in the western Dabie Orogen,central China

The extensive gneisses in the high‐pressure and ultrahigh‐pressure metamorphic terrane in the Dabie‐Sulu orogen usually show no evidence of eclogite‐facies metamorphism. The garnet‐mica‐plagioclase gneisses from the Qiliping region in the western Dabie Orogen, comprise garnet, phengite, biotite, plagioclase, quartz, rutile, ilmenite, chlorite, epidote, and hornblende. The garnet porphyroblasts, with inclusions of quartz, epidote, and rutile, exhibit slight compositional zonations, from core to mantle with an increase in pyrope and a decrease in spessartine, and from mantle to rim with a decrease in pyrope and grossular and an increase in spessartine. The high‐Si phengite indicates that the gneisses may be subjected to a high‐pressure metamorphism. By the P–T pseudosections calculated in a system NCKMnFMASHTO (Na₂O‐CaO‐K₂O‐MnO‐FeO‐MgO‐Al₂O₃‐SiO₂‐H₂O‐TiO₂‐O) for two representative samples, the metamorphic P–T path, reconstructed by the compositionally zoned garnet, shows that the prograde metamorphism is characterized by a temperature increase with a slight pressure increase from the conditions of 17.6 ± 1.5 kbar at 496 ± 15°C to the peak‐pressure ones of 21.8 ± 1.5–22.7 ± 1.5 kbar at 555 ± 15–561 ± 15°C; the early retrograde stage is dominated by decompression with a temperature increase to the maximum of 608 ± 15–611 ± 18°C at 10.3 ± 1.5–11.0 ± 1.5 kbar; and the late retrograde one is predominated by pressure and temperature decreases. The mineral assemblages in the prograde metamorphism are predicted to contain garnet, glaucophane, jadeite, lawsonite, phengite, quartz, rutile, and/or chlorite, which is different from those observed at present. Such high‐pressure metamorphism can partly be reconstructed by the P–T pseudosection in combination with the high‐Si phengite and garnet compositions in the core and mantle. This provides an important constraint on the subduction and exhumation of the terrane during the continent–continent collision between the Yangtze and Sino‐Korean cratons.     

Phengite compositions and post-nappe high-pressure metamorphism in the pennine zone of the French Alps

A systematic study of relict highly substituted phengite micas (Si_3.6–3.7) formed in the less deformed and less recrystallized rocks of the internal zone of the French Alps shows that silica content “isograds” for phengites cut across major thrust boundaries. This indicates that a high-pressure metamorphism was post-nappe in the orogenic sequence. Little substituted, secondary phengites and those found in highly recrystallized rocks reflect metamorphic conditions which occurred during the later mountain building phases of the orogeny.Variations of mica phengite content within mineral facies zones indicate rather important differences in pressure along the Pennine zone in going from low- to high-temperature mineral facies.     

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb–Sr and fission-track ages of gneissose granitoid and numerical modeling

Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.     

Discovery of khondalite series from the western segment of Altyn Tagh and their petrological and geochronological studies

The khondalite series, which are characterized by aluminum-rich gneisses (schists) consisting of sillimanite-garnet-biotite-monzonite gneiss, garnet-biotite-monzonite gneiss, graphite-sillimanite-biotite schist, and garnet-amphibole two-pyroxene granulites occurring as lenses and layers within gneisses (schists), were discovered in Tula area of western segment of Altyn Tagh. The petrology and geochemistry indicate that the protoliths of aluminum-rich gneisses (schists) are aluminum-rich pelitic and pelitic arenaceous sedimentary rocks, the protoliths of basic granulites are continental tholeiitic basalts. Therefore, the khondalite series may be produced at continental margin. They had suffered granulitic facies metamorphism with peak temperatures of 700-850℃ and pressures of 0.8-1.2 GPa. The U-Pb and Pb-Pb isotopic dating of zircons provided the ages of 447-462 Ma representing the ages of peak granulitic metamorphism. The U-Pb dating of detrital zircons from aluminum-rich gneisses yielded older upper intercept ages which reflect the times of older materials derived from source rocks of the gneiss protoliths.