Episodic exhumation of accretionary complexes: Fission-track thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan

Abstract Apatite and zircon fission-track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (T_e) generally indicate ~10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C T_e exhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion-burial-exhumation process. Zircon data suggest two cooling episodes: ~70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and ~15 Ma cooling in the central Kii Peninsula. The former is consistent with ⁴⁰Ar/³⁹Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano-plutonic complexes contemporaneously placed inland. Available geo- and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphism concepts.     

An outline of the petrology, structure and age of the Pompangeo Schist Complex of central Sulawesi, Indonesia

Variably dismembered and metamorphosed accretionary complexes constitute the basement of much of the Indonesian island of Sulawesi. The most extensive of these is the Pompangeo Schist Complex, which crops out over ∼ 5000 km² in central Sulawesi, and is predominantly composed of interbanded phyllitic marble, calcareous phyllite, graphitic schist and quartzite; rocks of terrigenous to shallow marine origin. Along the eastern margin of the complex, schists are interthrust with unmetamorphosed Jurassic sandstone, which may represent parental material of the complex. The schists are unconformably overlain by pelagic sediments with an Albian–Cenomanian biostratigraphy. Synmetamorphic progressive deformation of the Pompangeo Schist Complex has resulted in repeated isoclinal folding and a strong transposition foliation striking north-northwest/south-southeast and dipping west, subparallel to the compositional banding of the complex; microstructural fabrics indicate a top-to-east sense of shear. On a regional scale the Pompangeo Schist Complex is lithostratigraphically coherent and an east-to-west metamorphic field gradient is recognizable, which, if continuous, represents a relatively low thermal gradient of ∼ 15 °C/km. K–Ar dating yielded ages of ca 111 Ma. Correlative metamorphic rocks appear to underlie the entire Neogene magmatic province, since they occur sporadically throughout western Sulawesi, including the Bantimala region of the South Arm. The Pompangeo schist metamorphism cannot be correlated with arc magmatism in western Sulawesi, which is of Neogene age. The Pompangeo and Bantimala schists, as well as other accretionary complexes in western Sulawesi, were probably generated in the same subduction system that was responsible for the extensive Mesozoic continental arc in central Kalimantan, at the eastern margin of Sundaland.     

Late Cretaceous-Cenozoic exhumation of the Yanji area, northeast China: Constraints from fission-track thermochronology

The Yanji area, located at the border of China, Russia, and Korea, where the Phanerozoic granitoids have been widely exposed, was considered part of the orogenic collage between the North China Block in the south and the Jiamusi–Khanka Massifs in the northeast. In this study, the cooling and inferred uplift and denudation history since the late Mesozoic are intensively studied by carrying out apatite and zircon fission-track analyses, together with electron microprobe analyses (EMPA) of chemical compositions of apatite from the granitoid samples in the Yanji area. The results show that: (i) zircon and apatite fission-track ages range 91.7–99.6 Ma and 76.5–85.4 Ma, respectively; (ii) all apatite fission-track length distributions are unimodal and yield mean lengths of 12–13.2 µm, and the apatites are attributed to chlorine-bearing fluorapatite as revealed by EMPA results; and (iii) the thermal history modeling results based on apatite fission-track grain ages and length distributions indicate that the time–temperature paths display similar patterns and the cooling has been accelerated for each sample since ca 15 Ma. Thus, we conclude that sequential cooling, involving two rapid (95–80 Ma and ca 15–0 Ma) and one slow (80–15 Ma) cooling, has taken place through the exhumation of the Yanji area since the late Cretaceous. The maximum exhumation is more than 5 km under a steady-state geothermal gradient of 35°C/km. Combined with the tectonic setting, this exhumation is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the late Cretaceous.     

Thermo-tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74-90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K-Ar ages and the Turonian-Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100-80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5-3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene-Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific-Eurasian plate and the demise of the Kula-Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.     

Fission track evidence on thermal history of Jiama polymetallic ore district,Tibet

It is a new attempt to study thermal evolution related to mineralization using the fission track (FT) method. Apatite and zircon fission track data are reported for 6 samples collected from Jiama ore district as well as its periphery. The FT ages of apatites in the ore district are (16.1±0.9) Ma and (18.8±1.1) Ma and reflect the age of late period of hydrothermal mineralizing event. Apatite FT age of (22.0±4.3) Ma and zircon FT age of (20.9±2.0) Ma are related to the early period of mineralization. Another zircon FT age of (341.6±79.1) Ma, inheriting mineral source characteristic, has no connection with the mineralization. Based on the thermal history analysis, the mineralization began before 25–22 Ma. Cooling rate in the ore district is 5–6°C/Ma averagely, in which a slow cooling occurred at 90–80°C. About 2.7 km has been denuded and the denudation rate is higher than the uplifting rate.

     

Apatite fission track constraints on the Neogene tectono-thermal history of Nimu area, southern Gangdese terrane, Tibet Plateau

Apatite fission track dating of five samples from Cenozoic volcanic strata in the Nimu District in the southern Gangdese Terrane exhibits single population grain ages with a single mean age and associated central ages ranging from 6.8 ± 0.6 Ma to 9.7 ± 1.2 Ma. Mean track lengths are between 12.9 ± 1.7 µm and 14.2 ± 2.3 µm with a single peak characteristic of a single thermal event. The newly documented ages coincide well with the age of high sedimentation rates in the North Tibet Basin that resulted from a 9–5 Ma compressional event. Track length modeling allows three stages to be identified in the sample cooling. The first stage (12–8 Ma) records a period of relative stability with little, if any, cooling at temperatures of 120–110°C suggesting this region had low relief. The second stage (8–2 Ma) reflects rapid cooling with temperatures decreasing from ∼110°C to surface temperatures of ∼15°C. This stage can be related to far-field effects of the Himalayan collision, which probably generated the surface uplift and relief that defines the present-day Gangdese Mountains. The mean uplift rate of this period is estimated to be 1.41–0.95 mm/y with total uplift reaching ∼5900 m. The final stage is related to surface evolution since the Pliocene.     

Recognition of an Early Triassic accretionary complex in the Nedamo Belt of the Kitakami Massif,Northeast Japan: New evidence for correlation with Southwest Japan

The Kitakami Massif of the Tohoku district, Northeast Japan, consists mainly of the South Kitakami Belt (Silurian–Cretaceous forearc shallow-marine sediments, granitoids, and forearc ophiolite) and the North Kitakami Belt (a Jurassic accretionary complex). The Nedamo Belt (a Carboniferous accretionary complex) occurs as a small unit between those two belts. An accretionary unit in the Nedamo Belt is lithologically divided into the Early Carboniferous Tsunatori Unit and the age-unknown Takinosawa Unit. In order to constrain the accretionary age of the Takinosawa Unit, detrital zircon U–Pb dating was conducted. The new data revealed that the youngest cluster ages from sandstone and tuffaceous rock are 257–248 Ma and 288–281 Ma, respectively. The Early Triassic depositional age of the sandstone may correspond to a period of intense magmatic activity in the eastern margin of the paleo-Asian continent. A 30–40 my interval between the youngest cluster ages of the sandstone and the tuffaceous rock can be explained by the absence of syn-sedimentary zircon in the tuffaceous rock. The new detrital zircon data suggest that the Takinosawa Unit can be distinguished as an Early Triassic accretionary complex distinct from the Early Carboniferous Tsunatori Unit. This recognition establishes a long-duration northeastward younging polarity of accretionary units, from the Carboniferous to Early Cretaceous, in the northern Kitakami Massif. Lithological features and detrital zircon spectra suggest that the Early Triassic Takinosawa Unit in the Nedamo Belt is comparable with the Hisone and Shingai units in the Kurosegawa Belt in Shikoku. The existence of this Early Triassic accretionary complex strongly supports a pre-Jurassic geotectonic correlation and similarity between Southwest and Northeast Japan.     

K-Ar ages of intrusive rocks in the Oban-Obudu massif and their significance for the tectonic and plutonic history of southeastern Nigeria

Abstract K-Ar ages have been obtained for mineral separates: plagioclases from two dolerites and biotites from one granite and four granodiorites in the Oban-Obudu massif of southeastern Nigeria. Dolerites in the Oban area give K-Ar plagioclase ages of 204.0 ± 9.9 Ma and 219.9 ± 4.7 Ma. The granite of the Obudu area yields a K-Ar biotite age of 507.6 ± 10.1 Ma whilst the granodiorites in the Oban area yield K-Ar biotite ages ranging from 474.6 ± 9.4 Ma to 511.8 ± 10.0 Ma. The dolerites are related to the tholeiitic basaltic magmatism at the early opening of the central Atlantic, and are compatible in age with the formation of the Ring Complexes and the rifting of the Benue Trough of Nigeria. The granites and granodiorites belong to the syntectonic Older Granite series in the Pan-African orogeny. The emplacement timing deduced from the reported Pb-Pb zircon age of 617 ± 2 Ma and the newly obtained biotite ages suggest that these older granites in southeastern Nigeria had a prolonged cooling history of ∼110 Ma.     

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.     

Denudation history of the Kiso Range,central Japan,and its tectonic implications: Constraints from low‐temperature thermochronology

Fission‐track (FT) and (U–Th–Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault‐block mountain range which has been uplifted since ca 0.8 Ma. We obtained nine zircon FT ages ranging 59.3–42.1 Ma, 18 apatite FT ages ranging 81.9–2.3 Ma, and 13 apatite He ages ranging 36.7–2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of <18 Ma. The younger ages are interpreted as a reflection of uplift of the Kiso Range because they were obtained only to the east of the Seinaiji‐touge Fault, and the event age estimated from apatite FT data is consistent with the timing of the onset of the Kiso Range uplift. On the basis of the distribution of the younger ages, we propose westward tilting uplift of the Kiso Range between the boundary fault of the Inadani Fault Zone and Seinaiji‐touge Fault, which implies a model of bedrock uplift that is intermediate between two existing models: a pop‐up model in which the Kiso Range is squeezed upward between the two faults and a tilted uplift model which assumes that the Kiso Range is uplifted and tilted to the west by the Inadani Fault Zone. The original land surface before the onset of uplift/denudation of the Kiso Range is estimated to have been uplifted to an elevation of 2700–4900 m. We estimated denudation rates at 1.3–4.0 mm/y and maximum bedrock uplift rates at 3.4–6.1 mm/y since ca 0.8 Ma. The Seinaiji‐touge fault is interpreted as a back thrust of the west‐dipping Inadani Fault Zone. The older group of apatite FT and He ages is interpreted to reflect long‐term peneplanation with a probable denudation rate of <0.1 mm/y.     

Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications

Abstract   The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U–Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event.     

The emplacement of in situ greenstones in the northern Hidaka belt: The tectonic relationship between subduction of the Izanagi–Pacific ridge and Hidaka magmatic activity

Greenstone bodies emplaced upon or into clastic sediments crop out ubiquitously in the Hidaka belt (early Paleogene accretionary and collisional complexes exposed in the central part of northern Hokkaido, NE Japan), but the timing and setting of their emplacement has remained poorly constrained. Here, we report new zircon U–Pb ages for the sedimentary complexes surrounding these greenstones. The Hidaka Supergroup in the northern Hidaka belt is divided into four zones from west to east: zones S, U, and R, which contain in situ greenstones; and zone Y, which does not. Detrital zircons in zones S, U, and R have early Eocene U–Pb ages (55–47 Ma) and these strata are intruded by early Eocene granites (46–45 Ma), indicating that they were deposited between 55 and 46 Ma. Therefore, in situ greenstones in the northern Hidaka belt can only be explained by the subduction of the Izanagi–Pacific Ridge during 55–47 Ma. In contrast, the deposition of zone Y (the Yubetsu Group, younging to the west) began by 73–71 Ma, indicating that the accretionary prism in front of the paleo-Kuril arc formed at the same time as that in the Idonnappu zone and grew continuously until 48 Ma. The plutonic rocks that intruded the Hidaka belt are roughly divided into three stages: (1) early Eocene granites intruded the northern Hidaka belt at 46–45 Ma, during subduction of the Izanagi–Pacific Ridge; (2) the upper sequence of the Hidaka metamorphic zone was metamorphosed by magmatism at 40–37 Ma associated with the collision of the paleo-Kuril arc and NE Asia; and (3) younger granites intruded the entire Hidaka belt at 20–17 Ma in association with asthenospheric upwelling caused by back-arc expansion.     

A high‐T metamorphic complex derived from the high‐P Suo metamorphic complex in the Omuta district,northern Kyushu,southwest Japan

New U–Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐P Suo metamorphic complex to a high‐T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐T metamorphism. The high‐P to high‐T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐P to high‐T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐T metamorphic field.     

The Dabie Orogen as the early Jurassic sedimentary provenance: Constraints from the detrital zircon SHRIMP U-Pb dating

The SHRIMP U-Pb ages of detrital zircon from the oldest Mesozoic strata, the Fanghushan Fomation, in the Hefei Basin range from 200 Ma to ca. 2500 Ma, which indicates that the Dabie Orogen as the early Jurassic sedimentary provenance was complex. The composition of the Dabie Orogen includes: the Triassic high pressure-ultrahigh pressure metamorphic rocks, of which the detrital zircon ages are from 234 Ma to 200 Ma; the rocks possibly related to the Qinling and Erlangping Groups representing the southern margin of the Sino-Korean craton in the Qinling and Dabie area, of which the detrital zircon has an age of 481-378 Ma; the Neo-proterozoic rocks originated from the Yangtze croton, of which the detrital zircon ages are 799-721 Ma old; and the rocks with the detrital zircon ages of ca. 2000 Ma and ca. 2500 Ma, which could be the old basement of the Yangtze craton.     

BURIAL AND EXHUMATION OF THE XIGAZE FORE-ARC BASIN FROM LOW TEMPERATURE THERMOCHRONOLOGICAL EVIDENCE

The Xigaze fore-arc basin is adjacent to the Indian plate and Eurasia collision zone. Understanding the erosion history of the Xigaze fore-arc basin is significant for realizing the impact of the orogenic belt due to the collision between the Indian plate and the Eurasian plate. The different uplift patterns of the plateau will form different denudation characteristics. If all part of Tibet Plateau uplifted at the same time, the erosion rate of exterior Tibet Plateau will be much larger than the interior plateau due to the active tectonic action, relief, and outflow system at the edge. If the plateau grows from the inside to the outside or from the north to south sides, the strong erosion zone will gradually change along the tectonic active zone that expands to the outward, north, or south sides. Therefore, the different uplift patterns are likely to retain corresponding evidence on the erosion information. The Xigaze fore-arc basin is adjacent to the Yarlung Zangbo suture zone. Its burial, deformation and erosion history during or after the collision between the Indian plate and Eurasia are very important to understand the influence of plateau uplift on erosion. In this study, we use the apatite fission track(AFT)ages and zircon and apatite(U-Th)/He(ZHe and AHe)ages, combined with the published low-temperature thermochronological age to explore the thermal evolution process of the Xigaze fore-arc basin. The samples' elevation is in the range of 3 860~4 070m. All zircon and apatite samples were dated by the external detector method, using low~U mica sheets as external detectors for fission track ages. A Zeiss Axioskop microscope(1 250×, dry)and FT Stage 4.04 system at the Fission Track Laboratory of the University of Waikato in New Zealand were used to carry out fission track counting. We crushed our samples finely, and then used standard heavy liquid and magnetic separation with additional handpicking methods to select zircon and apatite grains. The new results show that the ZHe age of the sample M7-01 is(27.06±2.55)Ma(Table 2), and the corresponding AHe age is(9.25±0.76)Ma. The ZHe and AHe ages are significantly smaller than the stratigraphic age, indicating suffering from annealing reset(Table 3). The fission apatite fission track ages are between(74.1±7.8)Ma and(18.7±2.9)Ma, which are less than the corresponding stratigraphic age. The maximum AFT age is(74.1±7.8)Ma, and the minimum AFT age is(18.7±2.9)Ma. There is a significant north~south difference in the apatite fission track ages of the Xigaze fore-arc basin. The apatite fission track ages of the south part are 74~44Ma, the corresponding exhumation rate is 0.03~0.1km/Ma, and the denudation is less than 2km; the apatite fission track ages of the north part range from 27 to 15Ma and the ablation rate is 0.09~0.29km/Ma, but it lacks the exhumation information of the early Cenozoic. The apatite(U-Th)/He age indicates that the north~south Xigaze fore-arc basin has a consistent exhumation history after 15Ma. The results of low temperature thermochronology show that exhumation histories are different between the northern and southern Xigaze fore-arc basin. From 70 to 60Ma, the southern Xigaze fore-arc basin has been maintained in the depth of 0~6km in the near surface, and has not been eroded or buried beyond this depth. The denudation is less than the north. The low-temperature thermochronological data of the northern part only record the exhumation history after 30Ma because of the young low-temperature thermochronological data. During early Early Miocene, the rapid erosion in the northern part of Xigaze fore-arc basin may be related to the river incision of the paleo-Yarlungzangbo River. The impact of Great Count Thrust on regional erosion is limited. The AHe data shows that the exhumation history of the north-south Xigaze fore-arc basin are consistent after 15Ma. In addition, the low-temperature thermochronological data of the northern Xigaze fore-arc basin constrains geographic range of the Kailas conglomerate during the late Oligocene~Miocene along the Yarlung Zangbo suture zone. The Kailas Basin only develops in the narrow, elongated zone between the fore-arc basin and the Gangdese orogenic belt. The southern part of the Xigaze fore-arc basin has been uplifted from the sea level to the plateau at an altitude of 4.2km, despite the collision of the Indian plate with the Eurasian continent and the late fault activity, but the plateau has been slowly denuded since the early Cenozoic. The rise did not directly contribute to the accelerated erosion in the area, which is inconsistent with the assumption that rapid erosion means that the orogenic belt begins to rise.     

Tectono-metamorphic evolution of the Cretaceous Shimanto accretionary complex, central Japan: Constraints from a fluid inclusion analysis of syn-tectonic veins

Abstract   Micro-thermometry of water-rich fluid inclusions from two syn-tectonic veins sets ( D1 and D2 veins) in the Otaki Group, part of the Cretaceous Shimanto accretionary complex of the Kanto Mountains, central Japan reveals the following tectono-metamorphic evolution. Combining the results of microthermometric analyses of fluid inclusions from D1 veins with an assumed geothermal gradient of 20–50°C/km indicates that the temperature and fluid pressure conditions during D1 were 270–300°C and 140–190 MPa, respectively. Peak metamorphic conditions during the development of D2 slaty cleavage involved temperatures in excess of 300°C and fluid pressures greater than 270 MPa, based on analyses of microthermometry of water-rich fluid inclusions from the D2 vein and illite crystallinity. The estimated fluid pressure increased by approximately 80 MPa from D1 accretionary processes to metamorphism and slaty cleavage development during D2 . Assuming that fluid pressure reached lithostatic pressure, the observed increase in fluid pressure can be accounted for by thrusting of the Jurassic Chichibu accretionary complex over the Cretaceous Shimanto accretionary complex. Following thrusting, both accretionary complexes were subjected to metamorphism during the latest Cretaceous.     

Radiometric ages of Alpine metamorphic rocks in the western and central Alps

Abstract The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia-Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U-Pb monazite (23-29 Ma), Rb-Sr muscovite (15–40 Ma) and biotite (15–30 Ma), K-Ar biotite (10-30 Ma), muscovite (15–25 Ma) and hornblende (25-35 Ma), and FT zircon (10-20 Ma) and apatite (5-15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U-Pb zircon age for a coesite-bearing whiteschist (metaquartzite) from the Dora-Maira represents the peak of ultra-high pressure metamorphism. Coesite-free eclogites and blueschists related to ultra-high pressure rocks in the Penninic crystalline massifs yield an ⁴⁰Ar-³⁹Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high P/T metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra-high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia-Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium P/T metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid-ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.     

Metamorphic and cooling history of the Shimanto accretionary complex,Kyushu, Southwest Japan: Implications for the timing of out‐of‐sequence thrusting

Illite crystallinity, K–Ar dating of illite, and fission‐track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out‐of‐sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono‐metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K–Ar ages and zircon fission‐track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years.     

Geochemical and geochronological constraints on the origin and emplacement of the Shimo-ondori diorites in Shikoku,Southwest Japan

In this study, the LA-ICP-MS zircon U–Pb dating of the Shimo-ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo-ondori diorites are suggesstive that they could be influential for the estimate of the early-Cretaceous tectonic evolution for the eastern Asian margin. Their whole-rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO₂ and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (ε_Hf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge-mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo-ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo-ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.     

Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians)

U–Pb Sensitive High‐Resolution Ion MicroProbe (SHRIMP) dating of zircon in combination with (U–Th)/He dating of zircon and apatite is applied to constrain the emplacement and exhumation history of the youngest granitic rocks in the Western Carpathians collected in the Central Slovakian Neovolcanic Field. Two samples of diorite from the locality Banky, and granodiorite from Banská Hodru?a yield the U–Pb zircon concordia ages of 15.21 ±0.19 Ma and 12.92 ±0.27 Ma, respectively, recording the time of zircon crystallization and the intrusions’ emplacement. Zircon (U–Th)/He ages of 14.70 ±0.94 (Banky) and 12.65 ±0.61 Ma (Banská Hodru?a), and apatite (U–Th)/He ages of 14.45 ±0.70 Ma (diorite) and 12.26 ±0.77 Ma (granodiorite) are less than 1 Myr younger than the corresponding zircon U–Pb ages. For both diorite and granodiorite rocks their chronological data thus document a simple cooling process from magmatic crystallization/solidification temperatures to near‐surface temperatures in the Middle Miocene, without subsequent reheating. Geospeedometry data suggest for rapid cooling at an average rate of 678 ±158 °C/Myr, and the exhumation rate of 5 mm/year corresponding to active tectonic‐forced exhumation. The quick cooling is interpreted to record the exhumation of the studied granitic rocks complex that closely followed its emplacement, and was likely accompanied by a drop in the paleo‐geothermal gradient due to cessation of volcanic activity in the area.