Subduction Accretion Research Articles

Proterozoic mafic dyke swarms of Bundelkhand Craton, North India: A connection to Columbia supercontinent

The present paper addresses petrography, geochemistry and Ar‐Ar geochronology of a significant number of mafic dykes from the Paleo‐ to Neoarchean Bundelkhand Craton in central India. The majority of the dykes are NW‐SE oriented (with a few NE‐SW and ENE‐WSW) with tholeiitic, sub‐alkaline and basalt to basaltic andesite composition. The trace element geochemistry of these dykes indicates an island arc setting during emplacement. The Ar‐Ar mineral dating (plagioclase) of three representative dykes reveals an emplacement age between 1.53 and 1.46 Ga. This finding and earlier reports (2.1–1.73 Ga) point to sustained mafic magmatism throughout the Bundelkhand Craton in a preferred structural orientation between 2.1 and 1.46 Ga. Mafic magmatism was episodic and can be linked to the perpetual subduction accretion processes between the central Indian Archean continents during the development of the Columbia supercontinent. The mafic dykes were emplaced at 45° to the maximum compression direction (E‐W), that is, along the line of no finite longitudinal strain. This time equivalent widespread NW‐SE and NE‐SW trending mafic dyke system is also relatable along the adjacent continents (Singhbhum, Bastar) and thus opened up a new paradigm for the dyke's emplacement across the Indian cratons.

Early Ordovician to Early Permian tectonic evolution of the northern Western Kunlun Orogen (NW China): Insights from the Omixia Complex and surrounding rocks

The Palaeozoic tectonic evolution of the Western Kunlun Orogen plays an important role in deciphering the Tethyan tectonic evolution. This study provides new geochronology, geochemistry and Lu–Hf isotopic data of igneous rocks from the Omixia Complex in the eastern section of the northern Western Kunlun, as well as detrital zircon ages from turbidites and meta‐sediments north of it. From the Omixia Complex, the determined ages of five ultramafic–mafic rock samples are approximately 470, 456, 429, 401 and 382 Ma. Two samples of acidic rocks from the same complex yielded ages of approximately 438 and 378 Ma. One pegmatitic plagiogranite sample exhibits feature of accretionary arc granites, while other samples show geochemical characteristics of island arc tholeiite and E‐MORB. Additionally, four turbidite matrix and two limestone samples, with a major peak around ca. 480–500 Ma, have the youngest zircon ages ranging from ca. 481 to 387 Ma, paralleling the age range of igneous rocks in the ophiolitic mélange. The εHf(t) values of these samples reveal a broad spectrum of crustal and mantle processes. The youngest zircon ages of five meta‐sedimentary rock samples north of the Omixia Complex range from ca. 581 to 535 Ma, with peak ages concentrated around ca. 0.9–1.0 Ga. Their provenance characteristics differ from the turbidite matrix in the southern Omixia Complex and from the meta‐sediments in the northern Tiklik terrane with peak ages of ca. 0.8 Ga. A younger limestone sample yielded youngest zircon age of ca. 294 Ma, which is unconformably overlain the Omixia Complex and surrounding older rocks. Based on these new results, combined with previous data, we propose a new tectonic model for the eastern section of the northern Western Kunlun Orogen, suggesting a continuous evolution process of multi‐terrane subduction–accretion collage from the Early Ordovician to the Middle Devonian in the Paleo‐Tethys Ocean, which evolved into an Andean‐type active margin in the Early Permian, contributing to the substantial continental growth of the southern Tarim Craton.

Geochronology and Geochemistry of Granitic Pegmatites from Tashidaban Li Deposit in the Central Altun Tagh, Northwest China

The Central Altun orogenic system is a result of the amalgamation of multiple micro-continental blocks and island arcs. This complex system originated from subduction–accretion–collision processes in the Proto-Tethys Ocean during the Early Paleozoic. Research has reported the discovery of several Li-Be granitic pegmatite deposits in the Central Altun Block, including the North Tugeman granitic pegmatite Li-Be deposit, Tugeman granitic pegmatite Be deposit, Tashisayi granitic pegmatite Li deposit, South Washixia granitic pegmatite Li deposit, and Tamuqie granitic pegmatite Li deposit. The Tashidaban granitic pegmatite Li deposit has been newly discovered along the northern margin of the Central Altun Block. Field and geochemical studies of the Tashidaban granitic pegmatite Li deposit indicate: (1) Spodumene pegmatites and elbaite pegmatites, as Li-bearing granitic pegmatites that form the Tashidaban granitic pegmatite Li deposit, intrude into the two-mica schist, and marble of the Muzisayi Formation of the Tashidaban Group. (2) Columbite–tantalite group minerals and zircon U-Pb dating results indicate that the mineralization age of Tashidaban Li granitic pegmatites is 450.2 ± 2.4 Ma with a superimposed magmatic event at around 418–422 Ma later. (3) Whole-rock geochemical results indicate that the Kumudaban rock sequence belongs to the S-type high-K to calc-alkaline granites and the Tashidaban Li granitic pegmatites originated from the extreme differentiation by fractional crystallization of the Kumdaban granite pluton.

Heavy mineral indicators of ridge subduction beneath the Bainaimiao arc in the southeastern Central Asian Orogenic Belt

As one of the major subduction-accretionary systems in the southeastern Central Asian Orogenic Belt, the Bainaimiao arc presents a valuable opportunity for investigating the subduction–accretion process of accretionary orogens. It is located adjacent to the northern margin of the North China Craton and developed arc-related deposits during the Silurian–Devonian period. In this paper, we employed heavy mineral geochemistry and U–Pb ages of zircon and rutile, to determine the provenance of the Silurian–Devonian deposits and reconstruct the paleogeography of the source area. The stratigraphic units involved include the Xuniwusu Formation of Early Silurian and the Xibiehe Formation of Early Devonian.Both the Xuniwusu and Xibiehe formations exhibit heavy mineral assemblages dominated by zircons and tourmalines. Nevertheless, the Xibiehe Formation contains Cr-spinels and garnets which are absent in the Xuniwusu Formation. Geochemical analyses of the detrital Cr-spinels exhibit characteristics of a supra-subduction zone ophiolite. The garnets originated from middle- to high-grade metamorphic rocks, as well as mafic and felsic rocks. The tourmalines in both formations primarily indicate a metasedimentary rock source, with a small percentage of Li-poor granitoid species in the Xuniwusu Formation. The U–Pb age spectra of both detrital zircon and rutile in the Xuniwusu Formation show multiple peaks. The age spectra consist of Precambrian ages, indicative of a recycled origin, as well as Paleozoic ages, suggesting a magmatic origin. In contrast, the U–Pb age spectra of zircon and rutile in the Xibiehe Formation display a unimodal age peak at ca. 455 and 490 Ma, respectively. These differences suggest a provenance shift from the Xuniwusu to Xibiehe formations. The Xuniwusu Formation was sourced from the Bainaimiao arc itself, whereas the Xibiehe Formation has a mixed origin of the Bainaimiao arc and Ondor Sum accretionary complex. The provenance change correlates with the unconformity between the two formations, suggesting a forearc uplift event. In combination with the previous research on the Tulinkai ophiolite, it can be inferred that the forearc uplift was triggered by ridge subduction of the Paleo-Asian Ocean plate at ca. 450–430 Ma. This research provided a new perspective on the ridge subduction of the oceanic slab in fossil orogens.

Spatial and Temporal Exhumation of the Northeastern China: Insights From Low Temperature Thermochronology

ABSTRACTThe northeastern (NE) China has undergone a long‐term tectonic history involving subduction–accretion of the Siberian Craton, North China Craton, and Western Pacific Plate, providing an ideal target for understanding the interactions between cratons and the Pacific oceanic lithosphere. However, the Mesozoic‐Cenozoic geological and topographic evolutions of the NE China remain unclear. Here, we present new apatite fission track data from Precambrian metamorphic rocks in the Lesser Hinggan Mountains. Apatite fission track dating yielded a group of central ages ranging from 130 ± 18 to 151 ± 11 Ma with mean track lengths varying between 13.67 ± 0.13 and 14.49 ± 0.08 μm. Thermal modeling results revealed a rapid cooling event at ca. 165–140 Ma, which was related to compressional deformations driven by shallow subduction of the Paleo‐Pacific Ocean Plate beneath the northeastern China. Furthermore, regional low temperature thermochronological datasets in NE China show additional three rapid exhumation stages, including the Early Jurassic (ca. 200–180 Ma), Early Cretaceous (ca. 120–100 Ma), and Late Cretaceous (ca. 80–60 Ma). They were associated with closure of the Mudanjiang Ocean, slab rollback of the Paleo‐Pacific Plate, and collision between the Okhotomorsk Block and southeastern Asia. As the Cenozoic, slow exhumation has emerged throughout the northeastern China.

Kahi mélange complex in Kurram and Waziristan, NW Pakistan: An integrated approach for tectonic implications to India–Afghan suturing

The Kahi mélange complex in the Waziristan and Kurram sector (N–W Pakistan) is one of the several Neotethyan ophiolite–mélange complexes exposed along the westernmost margin of the Indian plate, preserving significant information on pre, syn and post–collision tectono–stratigraphic evolution of collision tectonics between the Indian plate and the Afghan block. It consists of highly dismembered ophiolitic and sedimentary units. Petrographic and field observations of mafic and ultramafic rocks within the ophiolitic unit, reveal a complete, but dismembered ophiolite suite similar to the Waziristan Ophiolite (WO) to its southwest. The sedimentary units consist of syn–orogenic deposits, turbidites and slope sediments of Campanian age as constrained by the Late Cretaceous radiolarians (Stichomitra communis Squinabol) and planktonic foraminifera (Rodotruncana Calcarata). From the structural point of view, the dismembered ophiolitic units represents the highest tectonic edifice. It’s overthrusting onto the Campanian sedimentary unit constraints the onset of the collision between the Indian plate and Afghan block to the Late Cretaceous. The subsequent obduction of combined ophiolite–sedimentary units over the brecciated Late Paleocene Lockhart Formation of Indian plate margin constraints the time of the end of the collisional stage. The unconfirmable deposition of the marine Patala Formation, Late Paleocene–Early Eocene in age, above the Kahi mélange complex documents the closure of Neotethys in the Early Eocene. In conclusion, our findings document that the Kahi mélange complex represents a typical polygenetic mélange formed by the superposition of two different dismembered units (the ophiolitic and the sedimentary units) coupled as a result of at least two tectonic events in a subduction–accretion setting related to the collision between the Indian plate and the Afghan block.

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Source and Migration of Fluids in a Meso-Tethyan Subduction Zone: Fluid Inclusion Study of Syn-Mélange Veins from the Mugagangri Accretionary Complex

The Mugagangri Group (MG), located at the southern margin of the Qiangtang terrane in Tibet, is a crucial research target for understanding the subduction and accretion history of the Meso-Tethys Ocean. Extensional crack-seal veins restricted within sandstone blocks from the broken formation in the MG (Gaize) formed synchronously in the mélange formation. The primary inclusions trapped in the veins recorded multiple pieces of information during the formation of the accretionary wedge. To precisely constrain the MG subduction–accretion processes, we investigated the trapping temperature, salinity, density, and composition of the fluid inclusions within the crack-seal veins derived from the broken formation in the MG (Gaize). The primary inclusions indicate that the crack was sealed at ~151–178 °C. The salinity of the primary inclusions exhibited a well-defined average of 3.3 ± 0.7 wt% NaCl equivalent, slightly lower than the average of seawater (3.5 wt%). There were no nonpolar gases, and only H2O (low salinity) was detectable in the primary inclusions. These characteristics suggest that the syn-mélange fluids were a type of pore fluid in the shallow subduction zone, with the principal source being pore water from sediments overlying the oceanic crust. Because of mineral dehydration and compaction, the pore fluids became more diluted with H2O and fluid overpressure owing to a pore fluid pressure that was greater than the hydrostatic pressure. Subsequently, the creation of cracks through hydraulic fracturing provided a novel pathway for the flow of fluids which, in turn, contributed to the décollement step-down and underthrusting processes. These fractures acted as conduits for fluid movement and played a crucial role in facilitating these peculiar occurrences of quartz veins. The depth (~5 km) and temperature estimates of the fluid expulsion align with the conditions of the décollement step-down, thereby leading to the trapping of fluids within the sandstone blocks and their subsequent underplating to the accretionary complex. In our preferred model, such syn-mélange fluids have the potential to provide valuable constraints on the subduction–accretion processes occurring in other accretionary complexes.

Subduction–Accretion History of the Paleo-Pacific Plate Beneath the Eurasian Continent: Evidence from the Tongjiang Accretionary Complex, NE China

Detrital zircons in the matrix of an accretionary complex play an important role in providing evidence to reconstruct oceanic plate subduction and accretion processes. The Nadanhada accretionary complex (NAC) dominated by the Yuejinshan, Raohe and Tongjiang accretionary complexes provides significant geological evidence to better understand the Paleo-Pacific subduction–accretion process. Most previous studies have focused on the Yuejinshan and Raohe accretionary complexes, while those of the Tongjiang accretionary complex on the north side have focused on blocks. In this study, we present zircon U–Pb dating and Hf isotopic data for the matrix of metasedimentary rock in the Tongjiang accretionary complex. The analysis results show that the zircons in the fine silty mudstone, phyllonite and fine argillaceous siltstone define the youngest weighted mean ages (youngest detrital zircon ages) of 261.4 ± 2.9 Ma (247 Ma), 175.2 ± 4.9 Ma (169 Ma) and 168.6 ± 2.1 Ma (162 Ma), respectively, and yield a younging trend of the accretion materials from west to east. Provenance analysis indicates that the matrix was mainly sourced from the neighboring Jiamusi and Xingkai blocks. Based on previous results of the Permian and Late Triassic blocks in the Yuejinshan region, the Permian and Early Jurassic blocks in the Tongjiang region, and the Late Triassic and Early–Middle Jurassic blocks in the Raohe region, as well as the lower limit of the depositional age of the Late Triassic matrix in the Yuejinshan region and the Middle Jurassic and Early Cretaceous matrices in the Raohe region, we propose that the NAC may record the Late Permian–Triassic, Jurassic and Early Cretaceous oceanic accretion events, representing the westward subduction and accretion process of the Paleo–Pacific Ocean Plate.

Long-lived subduction retreating led to continental rifting along the northern Gondwana: Insights from Devonian igneous rocks and ophiolite in the Beishan orogenic collage

The Beishan orogenic collage is an ideal setting for investigating the temporal and spatial relationships between extensional tectonics in the Central Asian Orogenic Belt and the rifting of Gondwana. This study presents new geochemical and geochronological data for Devonian igneous rocks and the Zhangfangshan ophiolitic mélange in the southern Beishan region. The Zhangfangshan ophiolitic mélange contains a Late Devonian gabbro block (ca. 369 Ma) with extremely depleted zircon Hf isotopic compositions (εHf[t] = +16.5 to +22.4). The Lucaogou diorites (ca. 367 Ma) have low MgO and compatible element contents, low Sr/Y and La/Yb ratios, and positive zircon εHf(t) values, indicative of derivation by dehydration melting of Mesoproterozoic mafic igneous rocks in a high-temperature and medium- to low-pressure environment. The Lucaogou K-feldspar granites (ca. 374 Ma) and Dundunshan silicic volcanic rocks (ca. 375 Ma) have A-type granite affinities, and were probably derived from magmas formed by mixing between Neoproterozoic orthogneiss-derived melts and depleted mantle-derived mafic magmas in a high-temperature shallow-crustal environment. Compiled geochronological data for subduction-related magmatism and supra-subduction zone-type ophiolites are consistent with protracted subduction–accretion until the early Carboniferous along the northern margin of the Dunhuang Block. Slab retreat in this long-lived subduction zone led to the generation of oceanic lithosphere in the southern Beishan region. Furthermore, Devonian extensional tectonics in the southern Beishan area coincided with the opening of the Paleo-Tethys Ocean, indicating that protracted subduction retreat caused the separation of the Dunhuang Block from East Gondwana.

Geochemistry, geochronology and Sr–Nd–Hf isotopes of Paleozoic granitoids in the Chinese Altai, NW China: constraints on the conversion from subduction–accretion to syn-/post-collision

The Altai region of China features abundant granitoids, which can be used to reconstruct the tectonic evolution of the orogen. We conducted petrological, geochronological, geochemical and isotopic analyses on several representative granitoids from the Altai region. U‒Pb dating revealed ages of 444 ± 3 Ma for gneissic granites, 406 ± 8 Ma for syenogranites, 280 ± 6 Ma for diorites, 278 ± 3 Ma for two-mica monzonitic granites and 269 ± 3 Ma for muscovite granites. The Ordovician gneissic granites have I-type arc-related element characteristics and are derived from volcanic magmas extracted from the mantle wedge and then metasomatized by subducted sediment in a continental arc. The Devonian syenogranites show dual I-type arc and within-plate granite geochemical signatures, suggesting derivation from volcanic magmas extracted from the lithospheric mantle metasomatized by subducted fluid/melt in a back-arc basin. The Permian diorites are Mg-diorites derived from a mixed-magma source involving residual subducted basaltic oceanic crust and lithospheric mantle. The two-mica monzonitic granites are S-type granites originating from the crustal recycling of sedimentary rocks, whereas the muscovite granites are leucogranites resulting from the anatexis of ancient metasedimentary rocks. Their tectonic setting is syn-/post-collisional. By combining these results with previously published data, we propose that the Chinese Altai region experienced mid- to late Ordovician continental arc magmatism, early to mid-Devonian back-arc extension and early to mid-Permian arc–arc syn-/post-collision. Supplementary material: Supplementary Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.6420236

Spatial-temporal distribution, metallogenic mechanisms and genetic types of nephrite jade deposits in China

The nephrite jade deposits of different tectonic units in China exert significant commercial quality, which have attracted wide attention. However, these deposits have not been systematically summarized to date. Here, we investigate the major nephrite jade deposits in China to decipher their gemological and mineralogical characteristics, spatial-temporal distribution, and mineralization processes as well as to identify their geological settings and gemological properties, via integrating published geochronology, major and trace elements as well as H-O isotopes. The compiled data suggest that the major nephrite jade deposits in China can also be generally divided into green jade-type (GJ-type) and white jade-type (WJ-type) which covers different species, but most of them belong to the latter. The GJ-type nephrite deposits predominantly occur in ophiolite/ophiolitic mélange suites or are embedded into ultramafic serpentine jade orebodies. This type of nephrite jade mainly formed through the late auto-metamorphic metasomatism of serpentine or the spontaneous crystallization/precipitation along suture/shear zones that acted as pathways to migrate Ca-rich fluids during orogeny. In contrast, the orebodies of WJ-type nephrite jade are usually hosted at the contact zones between the dolomitic marbles and intermediate-felsic or mafic intrusive rocks, which were produced through metamorphism-metasomatism during post continent-continent collision, such as the Kunlun orogenic belt with about ∼1,300 km Hetian nephrite belt, and the eastern nephrite jade deposits along the Pacific Ocean. Our study highlights that the nephrite jade deposits in China originally formed during multiple tectonic stages related to the interactions between hydrothermal fluids and metamorphism under subduction accretion- and collision orogenesis-related settings. In addition, this study also provides insights into the genetic discrimination, mineral exploration, and occurrence characteristics of nephrite jade deposits as well as the evaluation and identification of nephrite jade quality.

Discovery of a >1,000 km Cambrian Eclogite‐Bearing High‐Pressure Metamorphic Belt in the Central Asian Orogenic Belt: Implications for the Final Closure of the Pan‐Rodinian Ocean

AbstractThe immense Central Asian Orogenic Belt (CAOB) records the transition from the Pan‐Rodinian Mirovoi Ocean to the Paleo‐Asian Ocean, but the final closure of the Mirovoi Ocean remains unexplored. Here we document two new eclogite occurrences, located between CAOB microcontinents and the Ediacaran–Cambrian island arc in Mongolia. Pseudosection modeling and geothermobarometry constrain peak conditions of ∼560°C and ∼2.2 GPa for the Urgamal eclogite and ∼715°C and ∼2.0 GPa for the Tsengel eclogite. Zircon U‐Pb dating results suggest the Urgamal eclogite‐facies metamorphism occurred at ∼522 Ma, and the granite gneiss enclosing the eclogite yielded ∼811 Ma crystallization age and ∼519 Ma metamorphic age. Detrital zircon age spectra of the paragneisses associated with the eclogite suggest a probable provenance from the adjacent Zavkhan microcontinent and the Neoproterozoic arc superimposed on the microcontinent. The Tsengel eclogite has a protolith age of ∼853 Ma and an eclogite‐facies metamorphic age of ∼522 Ma, and the associated migmatite gneiss record a ∼520 Ma migmatization event. The rock associations and geochemical affinities indicate that their protoliths represent mafic rocks formed in a continental rift setting and the Early Neoproterozoic subduction–accretion complex, respectively. Their similar tectonic position and metamorphic age suggest that these rocks are part of a single >1,000 km long Cambrian high‐pressure metamorphic belt. The rocks record closure of the Mirovoi Ocean within the CAOB and constrain this crucial event to ∼520 Ma along the western margin of the microcontinents in Mongolia.

Geochronology and geochemistry of gneiss and migmatite from the Korla Complex in the Quruqtagh block, NW China: Implications for Proterozoic crustal evolution of the northeastern Tarim Craton

The Korla Complex in the Quruqtagh block of the northeastern Tarim Craton records Proterozoic tectono–thermal events and provides a window to understand the role of Tarim during the Columbia–Rodinia supercontinent cycle. Here we present zircon U–Pb ages, Lu–Hf isotopic compositions and bulk–rock geochemical data of gneisses and leucosomes of migmatites from the Korla Complex. Zircon grains from the orthogneiss exhibit core–rim structure, nevertheless the core and rim domains have indistinguishable ages, and jointly defined the Paleoproterozoic metamorphism of 1842 ± 35 Ma, which was ascribed to assembly of the Tarim Craton to the Columbia supercontinent along interior accretionary orogen. The leucosomes of migmatites were formed at 814 ± 3 Ma, and the consistent Hf isotope characteristics of newly grown anatectic zircon and inherited zircon of their protolith, indicates that the migmatite was formed by in–situ crustal anatexis associated with subduction–accretion process. Therefore, it is inferred that the assembly of the Tarim Craton to the Rodinia supercontinent lasted at least until ca. 0.81 Ga. Since this accretionary orogenic event (ca. 0.81 Ga) occurred during the initial breakup stage of the Rodinia supercontinent (0.83 Ga), it indicates that the Tarim Craton was probably located at peripheral position in the Rodinia reconstruction with long-lasting peripheral subduction and accretion. Collectively, it is suggested that the Tarim Craton was involved in global scale tectonic evolution during the Columbia–Rodinia supercontinent cycle. In combination with previous studies, it is proposed that the Tarim–North China connection within the Paleoproterozoic Columbia supercontinent must have broken–up before the Tarim–South China affinity within the Neoproterozoic Rodinia supercontinent, ascribed to the breakup of Columbia supercontinent.

Cambrian–Silurian sediments in the southeastern Qilian Orogen, NE Tibetan Plateau: Constraints on crustal and tectonic evolution of microcontinents in the northern Proto-Tethys Ocean

The Qilian Orogen preserves abundant Early Paleozoic records, revealing the tectonic evolution of the northern Proto-Tethys Ocean on the periphery of the northern margin of eastern Gondwana. We present detrital zircon U–Pb and Hf isotopes, and whole-rock element and Nd isotopic geochemical data on the Early Paleozoic sedimentary rocks in the southeastern Qilian Orogen. Detrital zircon grains from these rocks are clustered into seven distinct age groups: 495–426 Ma, 710–501 Ma, 1043–726 Ma, 1347–1054 Ma, 2009–1609 Ma, 2874–2274 Ma, and >3100 Ma. Detrital zircon and geochemical results suggest that their provenances are Early Paleozoic subduction–accretion related rocks, Neoproterozoic basements of the Qilian Block, and recycled sediments from Precambrian units. The whole-rock εNd(t) values of the sedimentary samples range from –13.0 to 2.5, and the εHf(t) values of Neoproterozoic–Early Paleozoic detrital zircon grains range from –28.6 to 15. Hf and Nd isotopes indicate marked growth and reworking of the continental crust in the early Neoproterozoic and Ordovician–Early Silurian. As revealed by age spectra of detrital zircon, we infer that the Qilian Block was tectonically related to blocks on the northern margin of eastern Gondwana during the Early Paleozoic, such as the North Qinling, Indochina and South China blocks. We also suggested that the Qilian Block experienced double-sided subduction of the North and South Qilian oceans, and developed sedimentation in the inter-arc and back-arc basins during the Ordovician to earliest Silurian. Finally, sedimentation occurred in an Early Silurian retro-arc foreland basin and Middle Silurian foreland basin during the syn-collisional stage.

Tectonometamorphic evolution of the Trivandrum and Southern Madurai blocks in the Southern Granulite Terrane, south India: correlation with south-central Madagascar

AbstractDetailed structural investigations across the Achankovil terrane boundary shear zone (AKSZ) system show distinct differences in the geometry of superposed fold structures between the Trivandrum (TB) and Southern Madurai (SMB) blocks separated by the AKSZ. The metamorphic temperatures and pressures estimated from the SMB and TB have a similar range, that is, 600–880°C and 5–8 kbar. The similar clockwiseP-Tpaths retrieved by phase equilibrium modelling from both the blocks represent the last deformation and metamorphism shared between them during their accretion along the AKSZ. The distinct evolutionary history of the SMB and TB prior to their amalgamation is supported by the contrasting structural, metamorphic and chronological patterns, particularly the lack of prominent middle Neoproterozoic ages and the presence of Palaeoproterozoic ages in the TB, and vice versa in the SMB. The prominent 600–500 Ma monazite ages in the TB, SMB and AKSZ attest to their timing of accretion along the AKSZ. The study corroborates the S-directed subduction model proposed for terrane accretion along the AKSZ, and provides further insight into the subduction–accretion–collision tectonics associated with the late Neoproterozoic – Cambrian evolutionary history of this region.The tectonothermal histories and geochronology of the SMB and TB are compared to the once adjoined crustal domains of Madagascar within Gondwanaland. It is suggested that the TB is equivalent to the Androyan and Anosyan domains of southern Madagascar, the SMB is equivalent to the Antananarivo and Itremo–Ikalamavony blocks of the central Madagascar, and the sinistral AKSZ is contiguous with the Ranotsara Shear Zone in southern Madagascar.

Subduction-accretion complex with boninitic ophiolite slices and Triassic limestone seamounts: Ankara Mélange, central Anatolia

AbstractOphiolitic mélanges in Anatolia represent Mesozoic subduction-accretion complexes, which are unusually poor in land-derived coarse-clastic rocks. A segment of the ophiolitic mélange in the Beynam region south of Ankara was studied. The ophiolitic mélange consists of three accretionary units (AUs), which are distinguished by lithology, structure, age and geochemistry. At the base there is a serpentinite mélange, which is overlain by a semi-intact Upper Jurassic ophiolite with boninite geochemistry. The topmost AU consists of ocean-island-like alkali basalts with seamount-derived Triassic shallow-marine limestones and Jurassic radiolarian cherts, which are stratigraphically overlain by Upper Cretaceous fore-arc turbidites. The base of the fore-arc sequence is palaeontologically and isotopically dated to the early to middle Campanian (c. 81 Ma). Detrital zircons from the fore-arc sequence indicate a Late Cretaceous (87–81 Ma) magmatic arc as a source. The formation of the subduction-accretion complex was a two-stage process. The first stage took place during the Late Jurassic – Early Cretaceous, when supra-subduction type oceanic crust was generated, and subduction accretion was intra-oceanic. In the second stage during the Late Cretaceous the subduction jumped inboard, creating an Andean-type convergent margin, and the Jurassic oceanic crust was incorporated in the subduction-accretion complex. The lack of land-derived sandstones in the ophiolitic mélange can be attributed to the intra-oceanic subduction and to the limestone deposition in the upper plate during the main phase of subduction accretion in the Late Jurassic – Early Cretaceous.

Magnetotelluric signatures of Neoproterozoic subduction, and subsequent lithospheric reactivation and thinning beneath central South China

Broadband and long-period magnetotelluric (MT) soundings were collected along a 600-km-long EW-trending profile across the southwestern segment of the Jiangnan Orogenic Belt (JOB) to explore the possible remnants of ancient tectonic processes beneath the central South China Block (SCB). Beneath the JOB, the resistivity model reveals a west-dipping, trans-lithosphere conductor (<20 Ω ∙ m) embedded within the relatively thick (80–120 km) and highly resistive (>1000 Ω ∙ m) lithosphere, possibly associated with relics of the Neoproterozoic subduction zone between the Yangtze and Cathaysia blocks. The crustal portion of this conductor is interpreted as solid conducting materials (e.g., graphite, sulfide) that were emplaced into the crust during subduction and orogenesis, whereas its lithospheric mantle portion is more likely to be volatile-bearing minerals formed by subduction-related metasomatic processes. We attribute the greatly reduced asthenospheric resistivities (1~10 Ω ∙ m) below the JOB to low-degree melting of the early-formed metasomatized mantle. In contrast, the lithosphere of the Cathaysia Block is evidently thinned (60–100 km) and segmented by several subvertical, lithospheric-scale conductors (1~30 Ω ∙ m) that are likely associated with fossil fluid pathways along reactivated shear/fault zones. Notably, the thinnest lithosphere sits above a prominent low-resistivity (10 ~ 30 Ω ∙ m) anomaly in the asthenospheric mantle and is spatially coincident with the location of Late Jurassic basaltic outcrops. We interpret these features as signs of lithospheric extension and asthenospheric upwelling driven by roll-back of the Paleo-Pacific Plate during the Late Mesozoic. The results provide key constraints on subduction–accretion tectonics associated with the Neoproterozoic assembly of the SCB and the subsequent lithospheric evolution during the Phanerozoic.

Along-Strike Variation in Accretion, Non-accretion, and Subduction Erosion Recorded in Rocks of the Jurassic-Neogene Convergent Plate Margin of California

Post-subduction dextral faulting was restored to evaluate the spatial distribution of units of the Franciscan subduction complex of California that formed as a result of subduction accretion. The Franciscan and related rocks of western California exhibit significant along-strike variation in its recording of subduction-accretion processes. Most notably, two segments 830 km apart record subduction erosion associated with low-angle subduction events that took place at ca. 120 Ma and ca. 80–70 Ma in the north and south, respectively. This spatial relationship is not affected by restoration of post-subduction dextral slip because none of the slip passes between the two inboard tectonic windows. Between these segments the subduction complex records net accretion from ca.175 Ma to 12 Ma, but includes horizons recording non-accretion. None of the accreted units of the subduction complex are preserved over the entire length of the subduction complex. One unit, however, correlated on the basis of its structural level in the subduction complex and distinctive detrital zircon age spectra, accreted at about 80–83 Ma extends a strike length of 580 km, an amount increased to 850 km with restoration of post-subduction dextral faulting. The long-strike length of accretion of this unit demonstrates that detrital zircon age populations of subduction complex clastic rocks are poor indicators of strike-slip displacement. Some reaches of the subduction complex include schistose blueschist facies rocks (most Franciscan blueschist facies rocks are not schistose), whereas others do not, and some reaches lack blueschist facies rocks altogether. The significant along-strike variation in the Franciscan and related rock units reflects temporal and spatial differences in history of accretion, non-accretion, subduction erosion, and probably subduction dip. Similar time and space variation in processes and resultant geologic record should be expected for other subduction complexes of the world.

Provenance of Late Cretaceous accretionary complex within the Yarlung–Zangpo suture zone, Bainang, southern Tibet: Implications for the subduction–accretion of the Neo-Tethyan ocean

Accretionary complexes offer important source of information for studying the subduction history and source-to-sink system and neighboring terranes. In this study, we conducted field mapping, detrital zircon U–Pb age and trace element analyses, and electronic microprobe composition analysis of detrital Cr-spinel in sandstones from the Cretaceous Bainang accretionary complex in the central part of the Yarlung–Zangpo suture zone (YZSZ). The Bainang accretionary complex is located in a structural window that is mainly composed of Xigaze ophiolite, Late Triassic–Early Cretaceous radiolarite chert with a Late Cretaceous hemipelagic siliceous shale matrix, and Late Cretaceous sandstone. The obtained detrital zircon geochronological dataset confirms two spectra. Detrital zircons from the Canggasaba and Lianxiang sections are dominated by Precambrian grains with subordinate numbers of Mesozoic grains clustering at 243–178, 164–117, and 109–91 Ma, consistent with the mélange and trench-fill Rongmawa Formation, and all the trace elements of the Mesozoic grains match continental arc affinity. Provenance analysis suggests that detritus in these sections was mainly derived from the eastern part of the Gangdese arc and the central–northern Lhasa terrane. These results confirm the existence of the ancestral Lhasa River that transports sediments from the eastern Lhasa terrane to the subduction zone and further westward along the east–west axial system parallel to the trench. Conversely, the Qiajiu section is mainly dominated by Mesozoic detrital zircon grains, representing submarine fan deposits that were sourced from the adjacent Gangdese arc and transported through a local north–south-flowing river system. Detrital Cr-spinel compositions and detrital zircon trace elements from the Qiajiu section also revealed the contribution of ophiolites. Considering the abundance of Eurasia-affinity detrital zircons, this study supports the single subduction model that was developed along the southern margin of the Lhasa terrane.