Subduction Accretion Research Articles

Contrasting subduction–exhumation paths in the blueschists of the Anarak Metamorphic Complex (Central Iran)

AbstractThe Anarak Metamorphic Complex, localized in Central Iran, is a fossil accretionary wedge composed of several tectonometamorphic units. Some of these, the Chah Gorbeh, the Morghab and the Ophiolitic complexes, contain mafic rocks that have been metamorphosed at high-pressure–low-temperature conditions. Such units have been stacked together and later refolded during the final stages of exhumation. Structural analysis at the mesoscale recognized at least three deformation events. Microstructural analyses, mineral chemistry and thermodynamic modelling reveal that the mafic schists followed contrasting P–T paths during their tectonometamorphic evolutions. In the schists of the Chah Gorbeh and Ophiolitic complexes an early greenschist-facies stage was later overprinted by blueschist-facies phase assemblages with suggested peak conditions of 390–440°C at 0.6–0.9 GPa for the meta-basalt within the Ophiolitic Complex and 320–380°C at 0.6–0.9 GPa for the blueschists of the Chah Gorbeh Complex. P–T conditions at metamorphic peak were 410–450°C at 0.78–0.9 GPa for the Morghab blueschists, but they are reached before a greenschist-facies re-equilibration. Compositional zoning of amphiboles and epidotes of this greenschist-facies stage suggests a renewed pressure increase at the end of this metamorphic stage. Based on these data we reconstructed a clockwise P–T path for the Morghab mafic schists and a counter-clockwise path for the Chah Gorbeh blueschists and ophiolitic meta-basalts. Such contrasting metamorphic evolutions of tectonic units that were later accreted to the same wedge are indicative of the complex tectonic dynamics that occur within accretionary–subduction complexes.

The Cenerian orogeny (early Paleozoic) from the perspective of the Alpine region

In the Alps, relicts of pre-Variscan basement are composed of metagreywackes and metapelites (partly migmatic) with intercalated amphibolites and sheets of Cambro–Ordovician peraluminous metagranitoids. Such gneiss terranes are the result of an orogenic type, which was globally widespread in early Paleozoic times. It caused the formation of several 100 km wide cratonized subduction–accretion complexes (SACs) hosting peraluminous arcs at the periphery of Gondwana. “Cenerian orogeny” is a newly suggested term for these early Paleozoic events, which culminate in the Ordovician. The justification for a separate name is given by three characteristics, which are significantly different compared to the Cadomian, Caledonian and Variscan orogenies: the age, the paleogeographic position and the tectonic setting. Other parts of the southern and central European crust might also have been generated by the cratonization of peri-Gondwanan SACs during the Cenerian orogeny.

Chapter 3 Regional context of the geology of the Andaman–Nicobar accretionary ridge

Abstract The Andaman–Nicobar accretionary ridge forms the eastern boundary of the Bay of Bengal and is presently being constructed by accretion and underplating of sediments off-scraped from the obliquely colliding Bengal Fan. Net accretion is relatively low ( c. 28%) with the rest subducted mostly into the upper mantle. Although subduction initiated along the margin at c. 95 Ma, large-scale subduction accretion likely accelerated during the Early Miocene by which time wedge top basins had formed. Prior to that time, sediment off-scraped during the Eocene against the ophiolitic backstop was probably derived from the adjacent magmatic arc of Burma. There was also some erosion of continental sources at that time, probably from the Sibumasu Block which forms the western edge of Sundaland. The scale of this accretion was small and potentially interrupted by times of tectonic erosion during the Palaeogene. The influence of continental erosion increased into the Oligocene, potentially accompanied by modest flux from the Indus–Yarlung Suture via the Irrawaddy River. Drainage capture in eastern Tibet in the Early Miocene and opening of the Andaman Sea, probably in the Late Miocene, has removed these source areas to the Andaman Trench.

Fossil oceanic subduction zone beneath the western margin of the Trans-North China Orogen: Magnetotelluric evidence from the Lüliang Complex

The Trans-North China Orogen (TNCO) is a major collisional belt along which the Western and Eastern Blocks of the North China Craton were amalgamated through subduction–accretion–collision tectonics during Paleoproterozoic. The Lüliang Complex along the western margin of the TNCO is thought to be the trace of an ancient suture zone. Here we employ magnetotelluric array to build a lithosphere-scale high-resolution 3-D resistivity model for this region. Between 36°N and 38°N, E-W trending slices of the resistivity model show relatively high-conductivity (∼10–100Ωm) lithospheric mantle of the Western Block that extends eastward beneath the relatively high-resistivity lithosphere of the TNCO at depths from 80 to 180km. We interpret this east-dipping upper mantle conductivity contrast as the lithosphere-scale suture between the Western Block and TNCO that formed through eastward subduction. The enhanced conductivity of the fossil oceanic subduction zone might be a reflection of the reduction in grain size of olivine grains in mantle peridotite by the localized shearing stress associated with the subduction, combined with the interconnecting high-conductivity minerals along grain boundaries formed through metasomatism by slab-derived fluids. Similar subduction-related boundaries are absent in the resistivity models beneath the northern Lüliang Complex near 39°N, where prominent high-conductivity anomalies indicating asthenosphere upwelling and lower crustal underplating are seen. Such mantle architecture could result from the subduction of an oceanic ridge or slab window, as also proposed for the origin of the subduction-related Luyashan charnockite suite in this region. Our data also suggest that the Phanerozoic tectonic and magmatic events in this region only locally modified the Precambrian lithospheric tectonic framework without total destruction. We propose a 3-D model to illustrate the contrasting Paleoproterozoic tectonic evolution between the northern and southern domains of the Lüliang Complex and the TNCO during the transition stage between the termination of oceanic subduction and the subsequent continental collision.

Lithological, structural, and geochemical characteristics of the Mesoarchean Târtoq greenstone belt, southern West Greenland, and the Chugach – Prince William accretionary complex, southern Alaska: evidence for uniformitarian plate-tectonic processes

The Mesoarchean Târtoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite–trondhjemite–granodiorite suites. Field observations, along with geochemical and zircon U–Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ± 4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach – Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid–greenstone terrains share many geological characteristics of Phanerozoic subduction–accretion complexes such as the Alaskan and Altaid subduction–accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite–trondhjemite–granodiorite generation in subduction–accretion complexes.

Early Mesoproterozoic arc magmatism followed by early Neoproterozoic granulite facies metamorphism with a near-isobaric cooling path at Mount Brown, Princess Elizabeth Land, East Antarctica

Mount Brown is a unique inland outcrop between Prydz Bay and Denman Glacier that provides useful insights into the tectonic evolution of the Indian Ocean sector of Antarctica. The bedrock in this area is dominated by felsic orthogneisses with subordinate amounts of mafic granulites, anatectic paragneisses and pegmatite veins. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating reveals the emplacement of mafic granulite and felsic orthogneiss protoliths at ca. 1490–1400Ma, sedimentation of paragneiss precursors after ca. 1250Ma, and subsequent high-grade metamorphism accompanied by partial melting at ca. 920–900Ma. The trace element geochemistry of these early Mesoproterozoic mafic–felsic igneous rocks indicates that they formed in a continental arc setting. Nd isotopic compositions for these rocks yield initial εNd values ranging from +2.8 to −6.6 and Nd depleted mantle model ages clustering between 2.4 and 1.7Ga, implying a significant crustal formation in the Paleoproterozoic. Petrographic textures, mineral compositions, and pressure–temperature (P−T) pseudosection calculations for mafic granulite and paragneiss in the system NC(K)FMASHTO [Na2O–CaO(–K2O)–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3] system suggest that early Neoproterozoic metamorphism reached peak P−T conditions of 830–870°C and 7–8kbar, followed by near-isobaric cooling to 760–830°C and 7–8.5kbar. The age spectra and characteristics of geological events at Mount Brown are similar to those in the Rayner Complex, thereby supporting an eastward continuation of the Rayner orogen to Wilhelm II Land. Combined with existing data, we infer that a protracted tectonic evolution between the Indian craton and East Antarctica (possibly the Ruker craton) might have occurred for a period of ca. 600Myr from long-lived oceanic subduction–accretion at ca. 1500–1000Ma to final collision at ca. 1000–900Ma.

Major Advances in the Study of the Precambrian Geology and Metallogenesis of the North China Craton: A Review

AbstractThe North China Craton (NCC) is one of the most ancient cratons in the world and records a complex geological evolution since the early Precambrian. In addition to recording major geological events similar to those of other cratons, the NCC also exhibits some unique features such as multistage cratonization (late Archaean and Palaeoproterozoic) and long‐term rifting during the Meso–Neoproterozoic. The NCC thus provides one of the best examples to address secular changes in geological history and metallogenic epochs in the evolving Earth. We summarize the major geological events and metallogenic systems of the NCC, so that the evolutionary patterns of the NCC can provide a better understanding of the Precambrian NCC and facilitate comparison of the NCC with other ancient continental blocks globally.The NCC experienced three major tectonic cycles during the Precambrian: (1) Neoarchaean crustal growth and stabilization; (2) Palaeoproterozoic rifting–subduction–accretion–collision with imprints of the Great Oxidation Event and (3) Meso–Neoproterozoic multi‐stage rifting. A transition from primitive‐ to modern‐style plate tectonics occurred during the early Precambrian to late Proterozoic and is evidenced by the major geological events. Accompanying these major geological events, three major metallogenic systems are identified: (1) the Archaean banded iron formation system; (2) Palaeoproterozoic Cu–Pb–Zn and Mg–B systems and (3) a Mesoproterozoic rare earth element–Fe–Pb–Zn system. The ore‐deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities.

Multichannel Seismic Imaging of the Rivera Plate Subduction at the Seismogenic Jalisco Block Area (Western Mexican Margin)

During the TSUJAL marine geophysical survey, conducted in February and March 2014, Spanish, Mexican and British scientists and technicians explored the western margin of Mexico, considered one of the most active seismic zones in America. This work aims to characterize the internal structure of the subduction zone of the Rivera plate beneath the North American plate in the offshore part of the Jalisco Block, to link the geodynamic and the recent tectonic deformation occurring there with the possible generation of tsunamis and earthquakes. For this purpose, it has been carried out acquisition, processing and geological interpretation of a multichannel seismic reflection profile running perpendicular to the margin. Crustal images show an oceanic domain, dominated by subduction–accretion along the lower slope of the margin with a subparallel sediment thickness of up to 1.6 s two-way travel time (approx. 2 km) in the Middle American Trench. Further, from these data the region appears to be prone to giant earthquake production. The top of the oceanic crust (intraplate reflector) is very well imaged. It is almost continuous along the profile with a gentle dip (<10°); however, it is disrupted by normal faulting resulting from the bending of the plate during subduction. The continental crust presents a well-developed accretionary prism consisting of highly deformed sediments with prominent slumping towards the trench that may be the result of past tsunamis. Also, a bottom simulating reflector (BSR) is identified in the first half a second (twtt) of the section. High amplitude reflections at around 7–8 s twtt clearly image a discontinuous Moho, defining a very gentle dipping subduction plane.

Crustal structure of the northern Menderes Massif, western Turkey, imaged by joint gravity and magnetic inversion

In western Anatolia, the Anatolide domain of the Tethyan orogen is exposed in one of the Earth’s largest metamorphic core complexes, the Menderes Massif. The Menderes Massif experienced a two-stage exhumation: tectonic denudation in the footwall of a north-directed Miocene extensional detachment, followed by fragmentation by E–W and NW–SE-trending graben systems. Along the northern boundary of the core complex, the tectonic units of the Vardar–Izmir–Ankara suture zone overly the stage one footwall of the core complex, the northern Menderes Massif. In this study, we explore the structure of the upper crust in the northern Menderes Massif with cross-gradient joint inversion of gravity and aeromagnetic data along a series of 10-km-deep profiles. Our inversions, which are based on gravity and aeromagnetic measurements and require no geological and petrophysical constraints, reveal the salient features of the Earth’s upper crust. We image the northern Menderes Massif as a relatively homogenous domain of low magnetization and medium to high density, with local anomalies related to the effect of interspersed igneous bodies and shallow basins. In contrast, both the northern and western boundaries of the northern Menderes Massif stand out as domains where dense mafic, metasedimentary and ultramafic domains with a weak magnetic signature alternate with low-density igneous complexes with high magnetization. With our technique, we are able to delineate Miocene basins and igneous complexes, and map the boundary between intermediate to mafic-dominated subduction–accretion units of the suture zone and the underlying felsic crust of the Menderes Massif. We demonstrate that joint gravity and magnetic inversion are not only capable of imaging local and regional changes in crustal composition, but can also be used to map discontinuities of geodynamic significance such as the Vardar–Izmir–Ankara suture and the West Anatolia Transfer Zone.

A review of the Paleozoic tectonics in the eastern part of Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen in the world, which is responsible for considerable Phanerozoic juvenile crustal growth. The NE China and its adjacent areas compose the eastern segment of the CAOB, which is a key area for providing important evidence of the CAOB evolution and understanding the NE Asian tectonics. The eastern segment of the CAOB is composed tectonically of four micro-blocks and four sutures, i.e. Erguna block (EB), Xing'an block (XB), Songliao-Xilinhot block (SXB), Jiamusi block (JB), Xinlin-Xiguitu suture (XXS), Heihe-Hegenshan suture (HHS), Mudanjiang-Yilan suture (MYS) and Solonker-Xar Moron-Changchun-Yanji suture (SXCYS). The EB and XB were amalgamated by westward subduction, oceanic island accretions and final collision in ca. 500Ma. The XB and SXB were amalgamated by subduction-related Early Paleozoic marginal arc, Late Paleozoic marginal arc and final collision in the late Early Carboniferous to early Late Carboniferous. The JB probably had been attached to the SXB in the Early Paleozoic, but broken apart from the SXB in the Triassic and collided back in the Jurassic. The closure of Paleo-Asian Ocean had experienced a long continue/episodic subduction–accretion processes on margins of the NCC to the south and the SXB to the north from the Early to Late Paleozoic. The final closure happened along the SXCYS, from west Solonker, Sonid Youqi, Kedanshan (Keshenketengqi), Xar Moron River through Songliao Basin via Kailu, Tongliao, Horqin Zuoyizhongqi, Changchun, to the east Panshi, Huadian, Dunhua, Yanji, with a scissors style closure in time from the Late Permian-Early Triassic in the west to the Late Permian-Middle Triassic in the east. The amalgamated blocks should compose a united micro-continent, named as Jiamusi-Mongolia Block (JMB) after Early Carboniferous, which bounded by Mongo-Okhotsk suture to the northwest, Solonker-Xar Moron-Changchun suture to the south and the eastern margin of JB to the east.

The petrogenesis of sodic granites in the Niujuanzi area and constraints on the Paleozoic tectonic evolution of the Beishan region, NW China

Ordovician to Devonian sodic granites dominate the newly recognized Luotuojuan composite granite in the Lebaquan–Luotuojuan–Niujuanzi region of Beishan, along the southern margin of the Central Asian Orogenic Belt in NW China. The granites include sodic (K2O/Na2O>0.5) tonalites with low Y (<7ppm), Yb (<0.7ppm), high Sr/Y (>68) that formed during at least two events at c. 435 and c. 370–360Ma. Their compositions are consistent with high-pressure melting of basaltic crust, although relatively non-radiogenic Nd isotope compositions (εNd(t)+0.9) require some crustal assimilation. The interpretation that these granites reflect melts of a subducted slab (i.e. adakite) is supported by independent local and regional geological evidence for an oceanic subduction–accretion setting, including a long history of calc-alkaline magmatism and the identification of a series of early Paleozoic ophiolite belts. Other sodic granites forming the Luotuojuan composite granite are mainly quartz-diorite and granodiorite formed between c. 391 and c. 360Ma. These rocks are not adakites, having Sr concentrations and Sr/Y ratios too low and Y and Yb concentrations too high. They are low- to medium-K calc-alkaline rocks more typical of magmas derived through melting in a subduction modified mantle wedge. Compositional changes from sodic to potassic granites, over time frames consistent with subduction processes, suggest at least two separate cycles, or pulses, of hot subduction in the Lebaquan–Luotuojuan–Niujuanzi region. Although early Paleozoic adakites have been inferred to exist elsewhere in the Beishan region, many of the reported adakitic rocks have compositions inconsistent with melting of subducted oceanic lithosphere and so tectonic interpretation of hot subduction might not be valid in these cases. A study of regional granite data also shows not only that adakite magmatism does not extend into the Permian but that if subduction–accretion processes extended into the late Paleozoic, no typical subduction-related magmatism was preserved. New and published Nd isotope data from regional granites also requires at least the local presence of Proterozoic basement, or microcontinental slivers, in the evolution of the Beishan region.

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99Ma intraplate basaltic rock suite hosted in a subduction–accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431–99Ma; n=31; peak at 256Ma), meta-granitoid-derived (ca. 449–113Ma; n=24; peak at 251Ma) and gabbro-derived (436–242Ma; n=15; peaks at 264 and 244Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605–741Ma; n=40) exhibits a prominent age peak at 2520Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone–forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian–Triassic composite granitoid–gabbroic pluton (8–20km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean–continent convergent margin.

The early Paleozoic tectonic evolution of the Russian Altai: Implications from geochemical and detrital zircon U–Pb and Hf isotopic studies of meta-sedimentary complexes in the Charysh–Terekta–Ulagan–Sayan suture zone

The Charysh–Terekta–Ulagan–Sayan suture zone was regarded as a tectonic boundary separating two distinct subduction–accretion systems in the Central Asian Orogenic Belt (CAOB). In the north, magmatic arcs, such as the Gorny Altai terrane, formed in the southwestern periphery of the Siberian continent, whereas in the south, arc-prism systems, such as the Altai–Mongolian terrane, formed around the so-called Kazakhstan–Baikal composite continent with Gondwana affinity. When did these two systems amalgamate and whether the metamorphic complexes in the suture zone represent Precambrian micro-continental slivers are critical for our understanding of the accretionary orogenesis and crustal growth rate in the CAOB. A combined geochemical and detrital zircon U–Pb–Hf isotopic study was conducted on the meta-sedimentary rocks from the Ulagan (also referred to Bashkaus) and Teletsk Complexes in the suture zone. The results indicate that the protoliths of these rocks were dominated by immature sediments deposited in a time period between 500 and 420Ma. Thus, Precambrian micro-continental slivers may not exist in the suture zone and even in the whole Altai Orogen.The meta-sedimentary rocks from the Ulagan Complex yield geochemical compositions between those of common intermediate and felsic igneous rocks, implying that these kinds of rocks possibly served as dominant sources. Detrital zircons from this complex consist of a major population of ca. 620–500Ma, a subordinate one of ca. 931–671Ma and rare grains of ca. 2899–1428Ma. This age spectrum is compatible with the magmatic records of the western Mongolia. We propose that the Ulagan Complex possibly represents part of a subduction–accretion complex built upon an active continental margin of the western Mongolia in the early Paleozoic. The remarkable similarities in source nature, provenance, and depositional setting to the early Paleozoic meta-sedimentary rocks from the northern Altai–Mongolian terrane imply that the Ulagan Complex was possibly fragmented from this terrane.The meta-sedimentary rocks from the Teletsk Complex show similar detrital zircon populations but contain higher proportions of mafic sediments and have more depleted whole-rock Nd isotopic compositions. Our data suggest that the detritus mostly came from the same source as that for the Ulagan Complex but those from the Gorny Altai terrane also contributed. This implies that the Gorny Altai and Altai-Mongolian terranes possibly amalgamated prior to the early Devonian rather than in the middle Devonian to early Carboniferous as previously thought. Thus, the widespread Devonian to early Carboniferous magmatism within these two terranes was possibly generated in a similar tectonic setting. Moreover, the dominant Neoproterozoic to early Paleozoic detrital zircons from the Teletsk Complex yield largely varied ɛHf(t) values of −23.8 to 12.4, indicating that crustal growth and reworking are both important in the accretionary orogenesis.

Imprints of diverse mantle deformational episodes in the Cauvery Suture Zone, South India

This study attempts to understand the upper mantle deformation pattern beneath the northern part of Southern Granulite Terrain (SGT), located to the south of Archean Dharwar craton in south India. The study region comprises the tectonic blocks – Nilgiri, Biligiri Rangan, Shevaroy and Madras blocks – separated from the northern part of Madurai block by the Cauvery Suture Zone (CSZ), associated with subduction–accretion–collision tectonics. Shear wave splitting observed on high quality XK(K)S (SKS, SKKS, PKS) waveforms from 119 teleseismic events recorded by 19 broadband seismic stations, mostly within the CSZ and the northern part of Madurai block, is utilized in this study. The 122 splitting measurements obtained, show an average delay time of 0.8s between the fast and slow waves. Strong lateral variations in the fast polarization azimuths (FPA) are observed, with the splitting parameters at the majority of stations showing a dependence on back azimuth. Importantly, excluding the Madras block, a poor correlation between the FPA and the present day absolute plate motion (APM) direction is observed. This dominance of non-APM strain reflects complex anisotropy preserved in the lithosphere. By plotting the fast polarization trends based on the ray piercing points at a depth of 150km, three distinct splitting patterns associated with the (i) Biligiri Rangan block and the eastern part of the Nilgiri block, (ii) eastern part of the CSZ and the northern Madurai block (NMB), and (iii) Madras block and its vicinity within the CSZ, are observed. The strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions, two episodes of ocean closure and the collision that followed, plate motion related strain or a combination of these. Similar deformation patterns in the northern part of Madurai block might be related to remnants of subducted oceanic lithosphere.

Crustal nature and origin of the Russian Altai: Implications for the continental evolution and growth of the Central Asian Orogenic Belt (CAOB)

The Central Asian Orogenic Belt is a gigantic tectonic collage of numerous accreted terranes. However, its geodynamic evolution has been hotly debated primarily due to incomplete knowledge on the nature of these enigmatic terranes. This work presents new detrital zircon U–Pb and Hf isotopic data to constrain the crustal nature and origin of the Russian Altai, a critical segment of Altai-Mongolian terrane. The youngest zircon 206Pb/238U ages of 470Ma constrain that the Terekta Formation, previously envisaged as Precambrian basement, was actually deposited after the Middle Ordovician. As for the three more sedimentary sequences above the Terekta Formation, they have youngest zircon 206Pb/238U ages of 425Ma, 440Ma and 380Ma, respectively, indicating their depositions likely in the Late Silurian to Devonian. From all analyses, it is noted that many zircon U–Pb ages cluster at ca. 520Ma and ca. 800Ma, and these zircons display oscillatory zoning and have subhedral to euhedral morphology, which, collectively, suggests that adjacent Neoproterozoic to Paleozoic igneous rocks were possibly dominant in the sedimentary provenance. Additionally, a few rounded Archean to Mesoproterozoic zircon grains are characterized by complex texture, which are interpreted as recycling materials probably derived from the Tuva–Mongolian microcontinent. Precambrian rocks have not been identified in the Russian Altai, Chinese Altai and Mongolian Altai so far, therefore, Precambrian basement may not exist in the Altai-Mongolian terrane, but this terrane probably represents a large subduction–accretion complex built on the margin of the Tuva–Mongolian microcontinent in the Early Paleozoic. Multiple episodes of ridge–trench interaction may have caused inputs of mantle-derived magmas to trigger partial melting of the newly accreted crustal materials, which contributed to the accretionary complex. During accretionary orogenesis of the CAOB, formation of such subduction–accretion complex is likely ubiquitous, indicating continental crust growth by both lateral accumulation and vertical basaltic injection.

Tectonic evolution of the eastern Central Asian Orogenic Belt: Evidence from zircon U–Pb–Hf isotopes and geochemistry of early Paleozoic rocks in Yanbian region, NE China

We present the first evidence of an early Paleozoic terrane in the southern Yanbian region, NE China. We used LA-ICP-MS zircon U–Pb and Hf isotope techniques to analyze one plagioclase gneiss and two garnet-bearing two-mica quartz schists from the early Paleozoic Jiangyu Group, as well as two tonalites that intruded the Jiangyu Group. The tonalites yield weighted mean 206Pb/238U zircon crystallization ages of 423 and 422Ma. Zircons from the Jiangyu Group gneiss and two schist samples yield maximum depositional ages of 439±4, 443±2, and 443±5Ma, respectively. These constraints, together with the age of the tonalite intrusion, indicate that the Jiangyu Group was deposited between 443 and 423Ma (i.e., Silurian). In addition, detrital zircon age spectra of the three Jiangyu Group samples exhibit prominent age peaks at 442, 473, 513, 565, 600, 635, 671, 740, 1000, and 1162Ma, as well as secondary peaks between 1344 and 3329Ma. The occurrence of the prominent Meso- and Neoproterozoic detrital zircon age populations for the Jiangyu Group, combined with the corresponding zircon Hf isotopic data, reveals that the Jiangyu Terrane has a tectonic affinity with northeastern Gondwana. The early Paleozoic magmatism, as suggested by the medium-K calc-alkaline I-type tonalite intrusion and Jiangyu Group detrital zircon age spectra, corresponds to coeval subduction–accretion events along the southern margin of the eastern Central Asian Orogenic Belt (CAOB). Accordingly, we propose that the Jiangyu Group is part of an exotic terrane that rifted from northeastern Gondwana, drifted northward, and ultimately became involved in the early Paleozoic tectonic evolution of the southern margin of the eastern CAOB after the Early Cambrian.

Subduction-related metasomatic mantle source in the eastern Central Asian Orogenic Belt: Evidence from amphibolites in the Xilingol Complex, Inner Mongolia, China

The Central Asian Orogenic Belt (CAOB) formed mainly in the Paleozoic due to the closure of the Paleo-Asian oceanic basins and accompanying prolonged accretion of pelagic sediments, oceanic crust, magmatic arcs, and Precambrian terranes. The timing of subduction–accretion processes and closure of the Paleo-Asian Ocean has long been controversial and is addressed in a geochemical and isotopic investigation of mafic rocks, which can yield important insight into the geodynamics of subduction zone environments. The Xilingol Complex, located on the northern subduction–accretion zone of the CAOB, mainly comprises strongly deformed quartzo-feldspathic gneisses with intercalated lenticular or quasi-lamellar amphibolite bodies. An integrated study of the petrology, geochemistry, and geochronology of a suite of amphibolites from the complex constrains the nature of the mantle source and the tectono-metamorphic events in the belt. The protoliths of these amphibolites are gabbros and gabbroic diorites that intruded at ca. 340–321Ma with positive εHf(t) values ranging from +2.89 to +12.98. Their TDM1 model ages range from 455 to 855Ma and peak at 617Ma, suggesting that these mafic rocks are derived from a depleted continental lithospheric mantle. The primitive magma was generated by variable degrees of partial melting of spinel-bearing peridotites. Fractionation of olivine, clinopyroxene and hornblende has played a dominant role during magma differentiation with little or no crustal contamination. The mafic rocks are derived from a Late Neoproterozoic depleted mantle source that was subsequently enriched by melts affected by slab-derived fluids and sediments, or melts with a sedimentary source rock. The Carboniferous mafic rocks in the northern accretionary zone of the CAOB record a regional extensional event after the Early Paleozoic subduction of the Paleo-Asian Ocean. Both addition of mantle-derived magmas and recycling of oceanic crust played key roles in significant Late Carboniferous (ca. 340–309Ma) vertical crustal growth in the CAOB. Amphibolite–facies metamorphism (P=0.34–0.52GPa, T=675–708°C) affected these mafic rocks in the Xilingol Complex at ca. 306–296Ma, which may be related to the crustal thickening by northward subduction of a forearc oceanic crust beneath the southern margin of the South Mongolian microcontinent. The final formation of the Solonker zone may have lasted until ca. 228Ma.

The tectonic setting of early Permian bimodal volcanism in central Inner Mongolia: continental rift, post-collisional extension, or active continental margin?

ABSTRACTThe petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive εNd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The εNd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.

パミールにおける新生代の地質構造発達史に関する最近の研究

The Pamir is a mountain region in the westernmost part of the Himalayan–Tibetan orogen. It extends 〜300 km from north to south and 〜300–400 km from west to east. The Pamir lies on a double subduction zone, where two buoyant continental plates have subducted several hundred kilometers deep into the asthenosphere. The southern plate is the northward-dipping Hindu Kush slab, and the northern plate is the southward-dipping Pamir slab. The subduction of the Hindu Kush slab began at 〜8 Ma. This narrow part of the Indian plate subducts to a depth of 〜600 km and has a neck at a depth of between 250 and 300 km, causing lithospheric thinning. The Pamir slab, a part of the Asian plate, probably started subduction at 〜25 Ma accompanied by slab rollback (possibly with the back-arc extension), subduction erosion, subduction accretion, and marginal slab-tear faulting. This slab, subducting to depths of 〜400–450 km, forms a broad arcuate shape split at the center due to a vertical tear extending from a depth of 〜200 to 〜400 km. The current geometry of both slabs implies the possible occurrence of tectonic events, such as slab break-off, in the near geological future. The metamorphic and exhumation history of the crystalline basement domes in the Pamir reveal regional tectonic evolution since the Cenozoic India–Asia collision. Prograde metamorphism of the mid–lower crust, driven by crustal shortening/thickening, continued from 45 to 25 Ma. Subsequently, retrograde metamorphism and exhumation of the mid–lower crust began during the period 25–15 Ma. This change suggests a transition from crustal shortening/thickening to crustal extension. A possible cause of this transition is that the Pamir slab obtained sufficient gravitational potential to enable the gravitational collapse of the crust, mainly due to the isostatic uplift of the orogen, which was triggered by the break-off of the Indian plate during 25–15 Ma. Subsequently, activation of extensional exhumation in the southwest and eastern Pamir was synchronous with magmatism in the eastern Pamir at 〜10 Ma. At present, the Pamir region is characterized by an active E–W extension of the Kongur Shan footwall to the east and a westward lateral extension of the western Pamir into the Tajik–Afghan Basin.

Four Cordilleran paleorivers that connected Sevier thrust zones in Idaho to depocenters in California, Washington, Wyoming, and, indirectly, Alaska

Upper Cretaceous sandstones from 17 localities from California to southeastern Alaska (United States) contain unexpectedly large populations of detrital zircons with Proterozoic U-Pb ages, with age peaks at 1800–1650 and 1380 Ma. These peaks are indicative of a sediment source region in the southern part of the Proterozoic Belt Supergoup basin in central Idaho, which hosts 1800–1650 Ma detrital zircons and which was intruded by rift-related 1380 Ma bimodal plutons and sills. Belt rocks were strongly uplifted and eroded during Late Cretaceous Sevier shortening and fed four paleoriver systems. The Lemhi Pass–Hawley Creek river system flowed east and sourced the Beaverhead-Harebell-Pinyon nonmarine megafan in the Cordilleran foreland basin. The Kione River flowed southwest to northern California, where it sourced a very large, ca. 82–80 Ma, ∼600-m-thick delta and submarine fan complex within the northern Great Valley forearc basin. Considerable Kione detritus also transited the forearc basin to reach the Franciscan trench, sourcing a pulse of deposition and subduction accretion in central California and even part of southern California. The Swakane River flowed northwest out of Idaho into Washington, sourcing the protolith for the high-grade Swakane gneiss. More speculatively, a Yakutat River may have flowed northwest and deposited Yakutat strata in a trench off Washington or British Columbia, before those rocks were translated north to southeastern Alaska. Recognition of a major source area in central Idaho for zircons with an uncommon age of 1380 Ma helps constrain the ca. 85–65 Ma paleogeography and paleotectonics of major sectors of the North American convergent margin orogen.