Takeshi Imayama

Okayama University of Science · Research Institute of Natural Sciences

The Western Arunachal Himalaya (WAH), part of NE Himalaya, is characterised by complex tectonic history and diversity of rocks, offering unique insights into the mountain building processes. This research aims to delineate the northeastern extension of the Zimithang Thrust (ZT) in the upper Greater Himalayan Sequence (GHSU) along the Tawang-Bumla t...

The magmatic and metamorphic evolutions of the Muslim Bagh Ophiolite (MBO) can help understand the relationship among supra-subduction ophiolite, subduction initiation, and the spreading centre in the forearc. Here, we investigated the whole-rock chemistry, mineral composition, P-T conditions, and K-Ar ages of the MBO and its associated units. The...

The Unazuki Belt, Southwest Japan, is a part of the Hida Belt, which is characterized as a plutono-metamorphic complex with a continental affinity formed between the late Paleozoic and early Mesozoic. The Unazuki Belt is known to be an important tectonic unit for the tectonic correlation between the proto-Japan and East Asian continents as it recor...

The origins and age distribution of the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical for understanding the pre-Himalayan history. Although the protoliths to the UHP Tso Morari eclogites in Ladakh, NW Himalaya are believed to be the Permian Panjal volcanics, the geochronological evidence is absent. Here, w...

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Ma...

Early Cretaceous Nidar Ophiolite Complex (NOC, eastern Ladakh) is associated with the north-dipping supra-subduction of the Neo-Tethyan Ocean along the Indus suture zone. The supra-subduction zone ophiolite formed in the forearc setting records the magmatic response to the subduction initiation, but the magmatic evolution in the NOC is poorly const...

Futamatayama volcano is a Quaternary stratovolcano located 6 km north of the Nasu volcano group in southern Fukushima Prefecture, NE Japan. We investigated the eruption history and magmatic processes of the volcano using geological, petrological, and geochronological [thermoluminescence (TL) dating] analyses. The eruptive activity of the volcano ca...

We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite‐ or pelite‐matrix mélange, which are distributed at the highest structural level of the high‐P/T type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet‐epidote amphibol...

In the Bhagirathi River Transect of the Garhwal Himalaya, India, the existence of the Jhala Normal Fault (JNF) and its movement sense are disputed. The JNF has been considered either as part of the South Tibetan Detachment System (STDS) or as a distinct, more southerly discontinuity within the Higher Himalayan Crystalline Sequence (HHCS). Field stu...

The origins and age distributions of the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks exposed along the Himalayas are essential to understanding the pre-Himalayan history. The Tso Morari Complex (TMC), located in the northwestern Indian Himalayas, consists of UHP eclogites and retrogressed HP amphibolites as boudins within the...

In the Bhagirathi River Transect of the Garhwal Himalaya, India, the existence of the Jhala Normal Fault (JNF) and its movement sense are disputed. The JNF has been considered either as part of the South Tibetan Detachment System (STDS) or as a distinct, more southerly discontinuity within the Higher Himalayan Crystalline Sequence (HHCS). Field stu...

The Ilam Nappe, consisting of the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal, was largely emplaced southward along the Main Central Thrust (MCT). Here we present the P–T–t path of the Ilam Nappe using conventional geothermobarometers, pseudosection modelling, and monazite petrochronology. Amphibole-bearing migmatite just above...

The 'channel flow' concept is generally associated with the collisional mountain belts (such as the Himalaya) to explain the exhumation of deeper crustal materials. According to the concept, the top part of the subducting plate gets 'molten' and tries to return to the surface following the 'pipe flow' mechanism via a combination of Poiseuille-and C...

The Main Central Thrust (MCT) is one of the major tectonic discontinuities stretching about ~2500 km along the length of the Himalaya and it plays important role during the tectono-metamorphic evolution of Himalayan orogen. This contribution aims to find out the element mobility and loss or gain of chemical mass during mylonitization and migmatizat...

The Nidar ophiolite in the Ladakh, NW India formed in supra subduction zone during the Early Cretaceous. The chemical composition of hornblende in dolerite, basalt and andesite consists mainly of hornblende and actinolite. Hornblende coexisted with clinopyroxene and formed during magma crystallization. On the other hand, actinolite, which replaces...

The pattern and timing of tectonic discontinuity and partial melting in the Garhwal Himalaya, resulting from the India-Asia collision, are crucial factors for understanding the evolution of the Himalayan orogen, but these parameters remain poorly constrained. Middle-to high-grade metamorphic rocks from the Main Central Thrust (MCT) zone and the Hig...

Metamorphic P–T conditions, monazite chemical ages, and zircon U–Pb ages from the gneisses exposed in the Oki belt, Japan, were integrated to unravel the multi-stage metamorphic history of the belt. Microstructural observations combined with obtained P–T conditions and metamorphic ages reveal three distinct stages of metamorphism: M1, M2, and M3. T...

In the Himalayan orogen, the microstructures of migmatite formed by the biotite dehydration melting were investigated. Garnet and potassium feldspar grown in equilibrium with the melt have irregularly arranged inclusions. Quartz in the matrix has a cathode luminescence (CL) zoning with grey-CL core surrounded by bright-CL rim. In contrast, quartz i...

The eastern Wolhyeonri complex in the southwestern Gyeonggi Massif, South Korea, which has a close affinity with the northern margin of the South China Craton, primarily consists of Neoproterozoic igneous and sedimentary rocks that were metamorphosed by the middle Paleozoic thermal event. In this study, we present the results from thermobarometry,...

Three types of amphibolites were found around a large serpentinite body in the basement of the Ohmachi Seamount in the Izu-Bonin-Mariana arc: epidote-albite amphibolite, epidote-garnet amphibolite, and garnet-zoisite amphibolite. The epidote-albite amphibolite shows a mid-ocean ridge basalt geochemical affinity based on whole-rock chemistry and Nd...

The metamorphic pressure–temperature (P–T) conditions across the Main central thrust (MCT) in the Arun area have been investigated. The MCT marks the tectono-metamorphic boundary between the overlying high-grade High Himalaya crystalline sequences (HHCS) and the underlying low-grade Lesser Himalaya sequences (LHS). The metamorphic rocks regionally...

Field surveys of the Tso Morari metamorphic complex in Ladakh, NW India were performed to collect ultrahigh-pressure metamorphic rocks and ophiolite igneous rocks. Garnet and omphacite are observed in the eclogite dyke in the center of the Tso Morari metamorphic complex. On the other hand, the granitic gneisses of the host rock have a remarkable de...

Several type volcanic rocks occurred in the Muslim Bagh (MB) area, which were developed mainly in the Cretaceous period at a continental active margin of the northwestern India subcontinent: Bibai-basalt intruded into the sediment from the Indian subcontinent, Bagh Complex(BC) consisting of melange and basalt-chert unit, and metamorphosed basalts i...

We investigated alkali series volcanic rocks newly discovered in the southern Mongolia volcano-plutonic belt of the Central Asian Orogenic Belt (CAOB) and classified as basalt, basaltic trachyandesite, andesite, trachyandesite, and trachyte, mainly showing andesitic-dacitic compositions. In Harker diagrams, the TiO2, FeO, MnO, Al2O3, CaO, and MgO c...

The aim of this study is to addresses unresolved issues associated with the depositional and metamorphic ages of the Sanbagawa metamorphic rocks (central Shikoku, Japan). We performed laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb analysis on psammitic schists from the Sanbagawa metamorphic rocks (sensu stricto)...

Two undeformed metagranite samples were collected from the UHP Brossasco–Isasca Unit (BIU) of Dora–Maira Massif, Italy to carry out the laser step–heating ⁴⁰Ar/³⁹Ar analyses of individual biotite crystals. The metagranites occur as undeformed domains (m– to ten of meters in size) within strongly deformed augen–gneiss. They still preserve their medi...

Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age d...

Early Oligocene partial melting and prolonged low-pressure–low-temperature (low-P/T) metamorphism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced at...

Early Oligocene partial melting and prolonged low-pressure-low-temperature (low-P/T) metamor-phism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced a...

北西インドUttarakhand地域のBhagirathi川流域に沿って野外調査を実施して，低変成度から高変成度までの変成岩類や優白質花崗岩類などを採取した．高変成度岩からなる高ヒマラヤ帯結晶質岩は，低変成度岩からなる低ヒマラヤ帯を主中央断層帯に沿って覆いかぶさっている．高ヒマラヤ帯内部では，複数のせん断帯が発見され，逆転温度構造が繰り返し観察される．高ヒマラヤ帯上部では，部分溶融したミグマタイトが，優白質の花崗岩類に貫入されており，２段階の部分溶融イベントが推定される．今後は，定量的な圧力－温度条件や変形・変成年代を推定して，せん断帯の活動時期や変成岩類の変成時期を詳しく明らかにしていく．

The Precambrian Salma eclogites on the Kola Peninsula, Russia, represent some of the oldest eclogites in the world; however, there has been much debate regarding whether the timing of their eclogite facies metamorphism is Archean (2.72–2.70 Ga) or Paleoproterozoic (1.92–1.88 Ga). New microstructural observations, pressure-temperature (P-T) analyses...

The Himalayan range was formed and uplifted in association with the southward migration of plate boundary thrusts that separate the Himalaya into four belts. Initially, during 50-35 Ma, the Tibetan marginal mountain range was uplifted after slab break-off of the Tethyan oceanic plate, which was subducted under the Asian continent to depths of up to...

Whole rock chemistry from thirteen leucogranite samples in far-eastern Nepal shows peraluminous compositions and low CaO contents, indicating that they are S-type granites. These leucogranites are subdivided into two-mica, tourmaline-bearing, and garnet-bearing leucogranites. Two-mica leucogranites are characterized by higher Sr and Ba contents and...

Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet-biotite gneiss and sillimanite-garnet-biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888-1871 Ma) at P–T conditions of 760–820...

Metabasites exposed in far-eastern Nepal provide an important insight into the metamorphic evolution of the Himalayan orogen independent from data obtained on metapelites. The P–T conditions and formation process of mafic granulite intercalated within Early Oligocene migmatites and two amphibolites surrounded by Early Miocene metapelites were infer...

Whole-rock Rb-Sr analyses of metasedimentary rocks were carried out to characterize the protolith among the Himalayan tectonic units in far-eastern Nepal. The different lithologic units yield the different Sr model ages: Higher Himalayan Crystalline Sequence (HHCS: 1630-710 Ma), Main Central Thrust (MCT) zone (2500-1650 Ma), and Lesser Himalaya Seq...

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18–16 Ma in the High Himalaya. In garnet-bearing sillimanite-muscovite-biotite leucogranite, most monazite grains have...

The timing of partial melting and the pressure–temperature (P–T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P–T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic...

Common occurrences of high-P/T metamorphic oceanic rocks in serpentinite melange suggest that deep portions of subduction channel could act as conduits both of subduction and exhumation transports. Metamorphic analysis of these rocks can provide information on modes of material transport along the subduction interfaces. Serpentinites exposed on the...

The timing of partial melting and the pressure-temperature paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, trace element compositions, and P-T pseudosection analysis. Two metamorphic rims in zircon from the migmatites formed during Himalayan thermal events can be distinguis...

The Oligocene volcanism in central Hokkaido may have taken place as the volcanic arc of the Eurasian continental margin prior to the formation of the Japan Sea back-arc basin. The Oligocene volcanic rocks in the Minami-Naganuma Formation recovered from the Nanporo drill cores consist of alkali olivine basalt, basaltic andesite, and andesite, and is...

Petrological study was carried out for the first time on the metabasites of the Lesser Himalaya in central Nepal. The metabasites are mostly tholeiitic basalts emplaced in the elastic sediments as supracrustal dikes and sills, and later metamorphosed together with the host rocks. They contain almost a constant mineral assemblage of Ca-amphiboles +...

The Ohmachi Seamount in the Izu-Bonin frontal arc is one of the very rare localities where rocks from a deep subduction zone are exposed on the modern sea floor. Submersible and dredge results revealed that the basement serpentinite body is accompanied by small amounts of amphibole schist (six float stones less than 20 cm in diameter were collected...

北海道中央部の前期中新世火山岩のうち，苫小牧市の勇払油ガス田の浅層貯留岩を構成する滝の上層火山岩と，それに対比されると考えられる火山岩類のK-Ar法による年代測定を行った．勇払油ガス田の滝の上層火山岩，馬追丘陵地域と栄地域に分布する同層準火山岩は，いずれも約14-21 Maの年代を示す．これらのうち勇払油ガス田と馬追丘陵地域の火山噴出物は陸成環境で定置した．勇払油ガス田の滝の上層火山岩のLIL・HFS元素の含有量とSr，Nd同位体組成は，東北本州弧で生じた日本海形成に伴う中期中新世火山岩と類似するが，近接して産出する火山岩はより高いLIL・HFS元素の含有量，より枯渇していないSr, Nd同位体組成を示す．後者は，東北本州弧の中期中新世火山岩には出現しないので，この時期の北海道中央部は東北本...

Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak-T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high-grade Higher Himalayan Crystalline Sequences (HHCS) over the low-grade Lesser Himalaya Sequences (LHS) in far-eastern Nepal. Maximum recorde...

ABSTRACT: Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak-T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high-grade Higher Himalayan Crystalline Sequences (HHCS) over the low-grade Lesser Himalaya Sequences (LHS) in far-eastern Nepal. Maxim...

The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks...