Arc-trench System Research Articles

The influence of a subduction component on magmatism in the Okinawa Trough: Evidence from thorium and related trace element ratios

The Okinawa Trough (OT) is a back-arc, initial continental marginal sea basin located behind the Ryukyu Arc-Trench System. Formation and evolution of the OT have been intimately related to subduction of the Philippine Sea Plate (PSP) since the late Miocene; thus, the magma source of the trough has been affected by subduction components, as in the case of other active back-arc basins, including the Lau Basin (LB) and Mariana Trough (MT). We review all the available geochemical data relating to basaltic lavas from the OT and the middle Ryukyu Arc (RA) in this paper in order to determine the influence of the subduction components on the formation of arc and back-arc magmas within this subduction system. The results of this study reveal that the abundances of Th in OT basalts (OTBs) are higher than that in LB (LBBs) and MT basalts (MTBs) due to the mixing of subducted sediments and EMI-like enriched materials. The geochemical characteristics of Th and other trace element ratios indicate that the OTB originated from a more enriched mantle source (compared to N-mid-ocean ridge basalt, N-MORB) and was augmented by subducted sediments. Data show that the magma sources of the south OT (SOT) and middle Ryukyu Arc (MRA) basalts were principally influenced by subducted aqueous fluids and bulk sediments, which were potentially added into magma sources by accretion and underplating. At the same time, the magma sources of the middle OT (MOT) and Kobi-syo and Sekibi-Syo (KBS+SBS) basalts were impacted by subducted aqueous fluids from both altered oceanic crust (AOC) and sediment. The variable geochemical characteristics of these basalts are due to different Wadati-Benioff depths and tectonic environments of formation, while the addition of subducted bulk sediment to SOT and MRA basalts may be due to accretion and underplating, and subsequent to form mélange formation, which would occur partial melting after aqueous fluids are added. The addition of AOC and sediment aqueous fluid to MOT and KBS+SBS basalts is therefore the result of cold subducted slab dehydration combined with a rapid subduction rate (82mm/a), leading to the migration of fluids into the mantle wedge. The presence of these attributes is likely because the OT was a back-arc, initial continental marginal sea basin.

Morpho-sedimentary features and sediment dispersal systems of the southwest end of the Ryukyu Trench: a source-to-sink approach

The southwestern Ryukyu Trench near Taiwan is an ideal place for source-to-sink studies because of the short sediment transport route between the terrestrial sediment source in Taiwan and the marine sink in the Ryukyu Trench. Bathymetric and seismic reflection data and core samples from the area around the southwestern Ryukyu Trench were used to identify features of the trench–arc system, including submarine canyons, the trench wedge, bathymetric ridges, and forearc basins, which together form two distinct sediment dispersal systems: a longitudinal (trench-parallel) system and a transverse (trench-normal) system. The longitudinal sediment dispersal system carries sediments eroded from the Taiwan orogenic belt eastward, primarily along the Hualien Canyon and a channel–terminal fan system at its mouth, and deposits them in the southwestern end of the Ryukyu Trench. The transverse sediment dispersal system carries sediments eroded from the Ryukyu Islands downslope and deposits them in the Hoping, Nanao, East Nanao, and Hateruma forearc basins, behind the barrier formed by the E–W-trending Yaeyama Ridge on the trench-slope break. The presence of pyrrhotite, a characteristic component of sediments sourced from Taiwan, in a seafloor sample from the Ryukyu Trench and its absence in a sample from the East Nanao forearc basin support the view that the southwestern Ryukyu Trench is longitudinally fed by sediment derived from Taiwan, whereas the trench-slope forearc basins receive sediment transported transversely downslope from the Ryukyu Islands.

（講演取消）白亜紀前弧盆地に記録された後背の経年変化：東北方白亜系砂岩の砕屑性ジルコンU-Pb年代

（講演取消）白亜紀前弧盆地に記録された後背の経年変化：東北方白亜系砂岩の砕屑性ジルコンU-Pb年代

太平洋型造山帯要素研究におけるジルコンU-Pb 年代測定の貢献：顕生代日本弧-海溝系の例

太平洋型造山帯要素研究におけるジルコンU-Pb 年代測定の貢献：顕生代日本弧-海溝系の例

The birth of the Xigaze forearc basin in southern Tibet

The stratigraphic succession of a forearc basin provides crucial information on the history of a convergent plate margin. In particular, it helps to establish the origin of the underlying ophiolites and to unravel the earliest evolutionary stage of arc-trench systems, which remain poorly understood. The Xigaze forearc basin in southern Tibet is one of the best examples of a fossil forearc basin. This study illustrates detailed stratigraphic and high-precision SIMS U–Pb zircon geochronological and Hf isotopic data from the Chongdui Formation, representing the very base of the Xigaze forearc-basin succession, and reconstructs when and how the basin was formed. The Chongdui Formation includes tuffaceous chert and siliceous mudrocks deposited directly on top of pillow basalts of the Xigaze ophiolite and conformably overlain by volcaniclastic turbidites. Tuff layers are interbedded throughout the unit, and their U–Pb zircon ages range from 119 to 113 Ma in the lower member and from 113 to 110 Ma in the upper member, broadly consistent with the established radiolarian biostratigraphy. U–Pb ages and Hf isotope signatures of zircons contained in both tuff layers and turbiditic sandstones indicate clear affinity with magmatic rocks of the Lhasa terrane. Direct depositional and chronostratigraphic relationship with the underlying oceanic crust, dated between 131 and 124 Ma, proves that the Xigaze ophiolite is the basement of the Xigaze forearc basin. After an initial prolonged stage of starved siliceous sedimentation, influx of terrigenous detritus began at 113–110 Ma, reflecting the onset of topographic growth and erosion of the Lhasa terrane in response to intense magmatic activity. Formation of the ophiolitic basement during the early stage of subduction and the subsequent topographic growth of the arc source induced by subduction-related magmatism are thus two critical factors for the birth of the Xigaze forearc basin. Similar stratigraphies were identified in the Great Valley and Luzon Central Valley forearc basins, suggesting that the initial geodynamic evolution of the Xigaze forearc basin may be common to many other forearc basins worldwide.

SPACE REGULARITY MANIFESTATION OF THE TEMPORAL VARIATION OF SEISMIC PARAMETERS: POSSIBILITY FOR THE STRONG SEISMIC ACTIVITY ASSESSMENT

In the present work the detailed analysis of the space-time variation, of the seismic energy released and b-value was performed, in order to study the space regularity manifestation of the temporal variation of seismic parameters, to test the reliability of the results and to compare the obtained temporal profiles in relation to the strong earthquake activity. The study was carried out in the subduction zone along the western part of the Hellenic trench arc system. The earthquakes data, witch occurs in the five adjacent local areas in the period 1980 -2007 were used. The reliability of the results is tested positively for “internal” dubiety, against independent seismic data sets from adjacent local areas, being in common the processing method, and the properties of seismic catalogue. It was found that in the majority of the cases (23 of 32) the significant temporal variation changes, considered as anomalies, can be related to the preparation process of the strong earthquakes Ms>5.7, acting as intermediate term precursors. Based on these findings the quality index of the successful intermediate term earthquake predictions reach up to 71.9%. It is observed that the temporal variation of the seismic energy released anomalies were time shifted in respect to the central areas, toward to N-NE and S-SE direction. Moreover it is found that the strong earthquakes occurrence also shows immigration, in respect to the central areas, along the concatenation of the adjacent local areas. These two observations may reveal the formation of a tectonic wave, in the broader area south of Zakynthos Island, Its velocity was estimated to be as 100 to 150 km/year

Reconstruction of the Cretaceous Paleo-Japan continental arc-trench system reconsidered from Cretaceous geologic records in the Japanese Islands

Reconstruction of the Cretaceous Paleo-Japan continental arc-trench system reconsidered from Cretaceous geologic records in the Japanese Islands

Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3, 1.0, and 0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust: (1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic (2.9–2.3 Ga), (2) net increase in volume of the continents during Paleo- to Mesoproterozoic (2.3–1.0 Ga), and (3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic (after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries. In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.

Basin Analysis of the Albian–Santonian Xigaze Forearc, Lazi Region, South-Central Tibet

Abstract The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

U[sbnd]Pb zircon geochronology constraints on the ages of the Tananao Schist Belt and timing of orogenic events in Taiwan: Implications for a new tectonic evolution of the South China Block during the Mesozoic

The Tananao Schist Belt is a low-pressure metamorphic complex comprised of three lithological units of marble, schist formation, and granite, in ascending order of proportion. Previous studies have found that the schist formation was formed during the Mesozoic. However, there is a lack of geochronological data to corroborate the schist protolith and metamorphic ages. In this study, we have used UPb zircon geochronology to provide a time frame for the creation of the schist formation and metamorphism, as well as a new tectonic model.Twenty-three schist and eleven meta-magmatic samples were used for UPb dating by LA-ICP-MS. Results from the youngest peak age of detrital zircon indicate that the schist formed in a new depositional age of 120–110Ma and, therefore, is different from the previously estimated age of the Paleozoic-Mesozoic. Additionally, the block-in-matrix schist indicates an age that ranges from 270 to 80Ma and was inferred from the chaotic deposits to be a metamorphic mélange. The Tananao Schist Belt appears to represent an arc-trench system that formed during the Cretaceous in the South China Block margin. Moreover, the youngest UPb age of 80Ma from the leucogranite dike and schist, constrains the upper age limit for a metamorphism that is younger than the previously accepted age of 100–90Ma.The contact layer between Permian-Triassic marble and the overlying early Cretaceous schists remains a chloritoid bed that is, therefore, considered to be a paleosol at the unconformity, which formed over an extended duration of 60±30Ma. The stratigraphic contact indicates a long period of erosion during the Jurassic and suggests that a tectonic event occurred. However, we propose that two important metamorphic events took place prior to, and following, the schist formation during the Jurassic and late Cretaceous orogenic events.

Late Aptian palaeoclimatic turnovers and volcanism: Insights from a shallow-marine and continental succession of the Apennine carbonate platform, southern Italy

This study reports an integrated facies analysis carried out on a new section from the Upper Aptian of the SE Matese Mts. (Civitella Licinio, northern Campania, S. Italy), which includes several decimeter-to-meter scale lacustrine intervals straddling a meter-scale plant-rich Plattenkalk. These intervals are assigned to the late Gargasian–early Clansayesian by their stratigraphic relationship with the upper Salpingoporella dinarica acme as well as lithologic and biostratigraphic supra-regional key-markers found in the Apennine Carbonate Platform (ApCP), including the Orbitolina Level. We infer that the monogeneric parautochthonous plant remains (Frenelopsis sp.) were deposited in a supratidal-to-paralic coastal mudflat close to a restricted, shallow-marine lagoon, at the verge of an arid–semiarid climatic phase. The Plattenkalk was rapidly replaced by persistent lacustrine lithofacies at the onset of a major climatic turnover (Glacial Trigger). The overlying shallow-marine deposits are capped by a deeply incised palaeokarstic surface and, in turn, by unconformable middle Clansayesian transgressive shallow-marine deposits.Interlayering of reliable lithologic indicators of palaeoclimate and High-Resolution Event Stratigraphic (HIRES) correlation of the Civitella Licinio succession with the astrochronologically tuned Mt. Faito reference section of the ApCP bring evidence of rapidly fluctuating hydroclimatic conditions (wet and arid phases) in the short (100ky) and long (400ky) eccentricity time band. HIRES and astrochronological age assignment to crucial key markers and humid/arid phases at Civitella Licinio permit to further refine the detailed stratigraphy of the Mt. Faito reference section and to assign the upper S. dinarica acme of the ApCP to a numerical age of 118.55–118.50My. SEM and EDS analyses document common to abundant windblown volcanic particles (glass shards and sanidine crystals) throughout the Frenelopsis-rich Plattenkalk (~118.3 to ~118.2My) and the encasing units, especially in the overlying brackish–lacustrine deposits. Palaeogeographic and palaeoclimatic reconstructions suggest that the pyroclastic material was probably transported by northeasterly trade winds from orogenic volcanic centers located within the Dinaridic–Carpatho-Balkan arc–trench system and have been supplied to the ApCP for a time span of ~3My during the Late Aptian. A preliminary tephrostratigraphic correlation of the Upper Aptian volcaniclastic levels of the ApCP with the bentonite levels of the Vocontian Basin is proposed.

On the tectonic problems of the southern East China Sea and adjacent regions: Evidence from gravity and magnetic data

In this paper, two sets of gravity and magnetic data were used to study the tectonics of the southern East China Sea and Ryukyu trench-arc system: one data set was from the ‘Geological-geophysical map series of China Seas and adjacent areas’ database and the other was newly collected by R/V Kexue III in 2011. Magnetic and gravity data were reorganized and processed using the software MMDP, MGDP and RGIS. In addition to the description of the anomaly patterns in different areas, deep and shallow structure studies were performed by using several kinds of calculation, including a spectrum analysis, upward-continuation of the Bouguer anomaly and horizontal derivatives of the total-field magnetic anomaly. The depth of the Moho and magnetic basement were calculated. Based on the above work, several controversial tectonic problems were discussed. Compared to the shelf area and Ryukyu Arc, the Okinawa Trough has an obviously thinned crust, with the thinnest area having thickness less than 14 km in the southern part. The Taiwan-Sinzi belt, which terminates to the south by the NW-SE trending Miyako fault belt, contains the relic volcanic arc formed by the splitting of the paleo Ryukyu volcanic arc as a result of the opening of the Okinawa Trough. As an important tectonic boundary, the strike-slip type Miyako fault belt extends northwestward into the shelf area and consists of several discontinuous segments. A forearc terrace composed of an exotic terrane collided with the Ryukyu Arc following the subduction of the Philippine Sea Plate. Mesozoic strata of varying thicknesses exist beneath the Cenozoic strata in the shelf basin and significantly influence the magnetic pattern of this area. The gravity and magnetic data support the existence of a Great East China Sea, which suggests that the entire southern East China Sea shelf area was a basin in the Mesozoic without alternatively arranged uplifts and depressions, and might have extended southwestward and connected with the northern South China Sea shelf basin.

古生代中葉日本の弧-海溝系の破片：高知県西部の黒瀬川帯の花崗岩と砂岩のジルコンU-Pb年代測定

古生代中葉日本の弧-海溝系の破片：高知県西部の黒瀬川帯の花崗岩と砂岩のジルコンU-Pb年代測定

Reconstruction of the Cretaceous continental arc of Japan on a viewpoint of arc-trench system

Reconstruction of the Cretaceous continental arc of Japan on a viewpoint of arc-trench system

Structure and petrology of the Nagaland-Manipur Hill Ophiolitic Mélange zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

The Nagaland-Manipur Hill ophiolite belt in NE India represents the southern extension of the Neotethyan Yarlung-Zhangbo suture zone in Southern Tibet, and connects this on-land exposure of the late Mesozoic collision front in the north with a modern trench-arc system in the Andaman Sea region in the south. Ophiolitic subunits in the Nagaland-Manipur Hill area in the Indo-Myanmar Ranges occur as blocks or thrust sheets within a melange with a serpentinite or fine-grained greywacke matrix, and are spatially associated with eclogitic and blueschist rock assemblages. This ophiolitic melange zone is tectonically sandwiched between an older (Triassic - Cretaceous) accretionary prism complex (Nimi Flysch) to the east and a younger (Late Cretaceous -Miocene) accretionary wedge (Disang Flysch) to the west. The Nagaland-Manipur Hill ophiolitic melange is thus part of a progressively westward migrated subduction-accretion complex, and it represents a typical subduction channel melange evolved during the fast subduction of the Neotethyan oceanic lithosphere beneath Asia - Sundaland. The exhumation of this melange was facilitated by return flow within a subduction channel at the plate interface between the downgoing Indian continental margin, subducting at a shallow angle during the Eocene, and the Triassic - Early Cretaceous accretionary prism of the overlying Burma plate. The Upper Eocene - Miocene Pokhpur sedimentary strata uncomformably overlying the ophiolitic melange and the accretionary prism metasedimentary rocks constitute a wedge-top or slope-basin sequence, rather than a postcollisional molasse deposit. The occurrence of ophiolitic subunits in the Nagaland-Manipur Hill area represents an accretionary-type ophiolite, derived mainly from the downgoing plate, and does not characterize a Penrose-type, complete ophiolite sequence.

Present-day stress orientations in the Great Sumatran Fault in North Sumatra

Trench-parallel strike slip faults develop at lithospheric scale during oblique high-angle subduction. A “sliver” plate forms due to slip partitioning between the subduction plane (margin-normal slip) and the strike slip fault (margin-parallel slip). This process ultimately controls the location of volcanoes and earthquakes. The Great Sumatran Fault (GSF) is a showcase of this tectonic configuration located in the Sumatran section of the Sunda arc-trench system. Kinematics of the large-scale structures of the Sumatra section of the Sunda trench are well understood and tensional and compressional domains have been identified at the regional scale. However, detailed understanding of the stress distribution is still lacking yet essential for evaluating the seismic hazard potential in order to mitigate the impact of the large, hazardous earthquakes associated with this system. In this contribution, we study the present-day stress orientations of the Great Sumatran Fault at its northern section (NGSF). We deduced the state of (paleo)stress along structural features observed at two scales; (a) at meso-scale, analyzing ASTER GDEM data, and (b) at outcrop-scale, with field data measurements. We focus on the leading edge of northwestward propagating continental sliver deformation exposed on land, i.e. the northernmost tip of Sumatra (between 4,5°N and 6°N), where the NGSF bifurcates into its two major branches. These two fault branches form two structural highs bounding a graben basin in the onshore, continuing into the Pulau Weh Island in the east, and the Pulau Aceh Archipelago in the west. Given their location at the present day deformation front, these islands provide a unique opportunity to compare the sub-recent stress field with present day stresses, contributing to the understanding of the stress field evolution during northwestwards propagation of the Sumatran forearc continental sliver.

Late Palaeogene emplacement and late Neogene–Quaternary exhumation of the Kuril island-arc root (Kunashir island) constrained by multi-method thermochronometry

The Kuril islands constitute a volcanic island arc-trench system, stretching from eastern Hokkaido (Japan) to Kamchatka (Russia) along the northwestern Pacific subduction system. The current arc consists of several volcanic islands mainly with Neogene basement and capped by several, predominantly andesitic, active subduction stratovolcanoes. Kunashir Island is the southwestern-most island of the arc, just off the Hokkaido coast and represents the study area in this paper. The island is composed of a Lower Complex of mainly late Miocene to Pliocene volcanic rocks, covered by an Upper Complex of younger (basaltic) andesitic lava flows and tuffs on which currently four active volcanic edifices are built. In the Lower Complex sub-volcanic and deeper-seated intrusives of the so-called Prasolov and Dokuchaev magmatic complexes are found. More differentiated, tonalitic–granodioritic rocks were collected from these small intrusive bodies. An early Oligocene zircon LA-ICP-MS U/Pb age of 31 Ma for the Prasolov Complex was obtained, showing that the basement of Kunashir Island is older than previously thought. Thermochronometry (apatite fission-track and U-Th-Sm/He and zircon U-Th/He analyses) further shows that the magmatic basement of the island was rapidly exhumed in the Pleistocene to present levels in a differential pattern, with He-ages ranging from 1.9 to 0.8 Ma. It is shown that the northern section of the island was hereby exhumed more intensely.

Cambrian plutonism in Northeast Japan and its significance for the earliest arc-trench system of proto-Japan: New U–Pb zircon ages of the oldest granitoids in the Kitakami and Ou Mountains

In order to clarify the early history of Japan, particularly during the Early Paleozoic, pre-Devonian granitoids in the South Kitakami Belt (SKB), NE Japan were dated by U–Pb zircon age by laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). Two samples of diorite/tonalite from the Nagasaka area (Shoboji Diorite) and two of mylonitic tonalite from the Isawa area (Isawagawa Tonalite) yielded late Cambrian ages (500–490Ma) for the primary magmatism. These ages newly identify a ca. 500Ma (late Cambrian) arc plutonism in central NE Japan, which has not been recognized previously and has the following geological significance. The Cambrian granitoids are the oldest felsic plutonic rocks in NE Japan, which are independent of the previously known ca. 450Ma (latest Ordovician) Hikami Granite in SKB. The Cambrian granitoids are extremely small in size at present but likely had a much larger distribution primarily, at least 30km wide and potentially up to 80km wide in a cross-arc direction. Their southerly extension was recognized in the Hitachi area (ca. 200km to the south) and in central Kyushu (ca. 1500km to SW). They likely represent remnants of the same mature arc plutonic belt in Early Paleozoic Japan, which developed in western Panthalassan (paleo-Pacific) margin, as well as the Khanka block in Far East Russia. The extremely small size of the Cambrian granitoids at present can be best explained by intermittent, severe tectonic erosion since the Paleozoic. This Cambrian arc granitoid belt likely developed from Primorye possibly to eastern Cathaysia (South China) via Japan.

Present state of knowledge and new geochemical constraints on the central part of the Mexican Volcanic Belt and comparison with the Central American Volcanic Arc in terms of near and far trench magmas

Abstract: A brief critical review is presented of the geological, geophysical, geochemical, and plate tectonic evidence for the origin and evolution of the central part of the Mexican Volcanic Belt (C-MVB). Databases were constructed for the C-MVB from new and published data as well as for the Central American Volcanic Arc (CAVA), other continental and island arcs, continental rifts or extensional areas, and collision zones. Conventional procedures as well as statistical techniques of multidimensional tectonic discrimination diagrams and significance tests were used to interpret the data. A subdivision of the databases for the C-MVB and CAVA into near the trench or front-arc and far from the trench or back-arc magmas helped to demonstrate the lack of slab input for the C-MVB and the presence of a classic arc-trench system for the CAVA. The use of software IgRoCS, DODESSYS, UDASYS, TecD, and TecDIA provided the following conclusions: 1) basic magmas in the C-MVB that originated in the mantle without the involvement of the subducted slab represent a continental rift setting; 2) C-MVB acid and intermediate magmas that originated, respectively, from the crust and mixed mantle-crustal sources do not show a clear influence of the present subducted slab and represent the tectonic setting of the older crustal and mixed mantle-crustal source rocks; and 3) all rocks in the CAVA were derived from the involvement of the subducted slab and represent a classical continental arc system related to the subduction of the Cocos plate beneath the Caribbean plate, whereas CAVA back-arc rocks originated mainly in the mantle, probably with much less involvement of the subducted slab. Reinterpretation of the Sr, Nd, Pb, and Hf isotopic data supports these inferences. I propose a new tectono-petrogenetic model based on the continuation of the extensional tectonics related to the triple rift system in the western part of the MVB.

Neoproterozoic arc-trench system and breakup of the South China Craton: Constraints from N-MORB type and arc-related mafic rocks, and anorogenic granite in the Jiangnan orogenic belt

The early Neoproterozoic Jiangnan orogenic belt, located in the southeastern margin of the Yangtze Block, is considered to mark the suture zone between the Yangtze and Cathaysia Blocks. The eastern part of this belt consists of the Huaiyu and Jiuling terranes, separated by the NE Jiangxi fault zone which represents the back-arc marginal sea. The western part of the Jiangnan orogenic belt is characterized by the Jiangnan arc and back-arc sea. Neoproterozoic mafic–ultramafic rock suites are exposed along both the Jiangshan-Shaoxing-Pingxiang-Shuangpai and the NE Jiangxi suture zones, whereas I-, S- and A-type granites occur widely in the Jiangnan region. Here, we present new LA–ICP–MS zircon U–Pb data, Hf isotopes and whole rock geochemistry for the mafic rocks and granite from the Jiangnan orogenic belt. The N-MORB (normal mid-ocean ridge basalts) type basalts from the Nanqiao of Liuyang area in the west Jiangnan orogenic belt show low K2O contents (0.74–0.16wt.%), belonging to tholeiitic series, and display LREE (light rare earth element) depleted REE (rare earth element) patterns, with positive Pb, Sr anomaly and minor negative Ti anomalies. The other mafic suites in the Daolinshan, Yiyang and Liuyang areas exposed in the eastern and western segments of the Jiangnan orogenic belt show distinctive geochemical signatures of volcanic arc, with zircon U–Pb ages of 863±6 Ma. Most of these rocks show negative Nb, Ta, Zr, Hf and Sr anomalies and have LREE enriched REE patterns, with minor negative Eu anomaly. These features coincide with those of the arc and fore-arc rock assemblages in the eastern Huaiyu Terrane and adjacent areas. Anorogenic granites of 790±5Ma are also found in the Daolinshang area. Our Hf isotopic data further reveal that the zircons from both the dolerite and A-type granite in the Daolinshan area possess positive ɛHf(t) values, yielding latest Mesoproterozoic to early Neoproterozoic model ages. Combined with previous studies, our data confirm the existence of an arc-trench system in the Jiangnan orogenic belt at ca. 863Ma. Therefore, we infer that the amalgamation of the Yangtze and Cathaysia Blocks in the eastern part occurred at some time after ca. 863 Ma. This was followed by breakup at ca. 790Ma which was triggered by mantle upwelling, as evidenced by the Daolinshan A-type granite with positive ɛHf(t) values