Search Results (23)

Zircon serves as a robust tracer for crustal recycling processes owing to its wide stability under diverse geological conditions. Its cryptic occurrence within ophiolites offers valuable insights into regional paleotectonic evolution. In this study, we identify a few zircon xenocrysts in both peridotite and basalt units from the Neoproterozoic South Anhui Ophiolite (SAO) in the southeastern Yangtze Block, South China. Zircon xenocrysts within the peridotite yield U-Pb ages ranging from ca. 2.7 to 1.0 Ga (n = 21), with three peaks of 2.8–2.5 Ga, 2.2–1.8 Ga, and 1.2–1.0 Ga. Comparative analysis of age spectra suggests these xenocrysts likely originated from recycled subducted continental materials within the Yangtze Block. In the basaltic rocks, zircon xenocrysts exhibit ages of ca. 2.1–0.9 Ga (n = 27), with peaks of 1.1–0.9 Ga, 1.5–1.4 Ga, and 2.1–1.7 Ga. These zircons are interpreted to have been inherited from wall rocks through crustal contamination during magma ascent, as their age spectra closely resemble those of the surrounding basement strata. Collectively, these findings support that the SAO possibly formed in a back-arc basin setting, characterized by significant crust–mantle interactions. Full article

Identifying the tectonic setting of rocks is essential for gaining insights into the geological contexts in which these rocks were formed, aiding in tectonic plate reconstruction and enhancing our comprehensive understanding of the Earth’s history. The application of machine learning algorithms helps identify complex patterns and relationships between big data that may be overlooked by binary or ternary tectonomagmatic discrimination diagrams based on basalt compositions. In this study, three machine learning algorithms, i.e., Support Vector Machine (SVM), Random Forest (RF), and eXtreme Gradient Boosting (XGBoost), were employed to classify the basalts from seven diverse settings, including intraplate basalts, island arc basalts, ocean island basalts, mid-ocean ridge basalts, back-arc basin basalts, oceanic flood basalts, and continental flood basalts. Specifically, for altered and fresh basalt samples, we utilized 22 immobile elements and 35 major and trace elements, respectively, to construct discrimination models. The results indicate that XGBoost demonstrates the best performance in discriminating basalts into seven tectonic settings, achieving accuracies of 85% and 89% for the altered and fresh basalt samples, respectively. A key innovation of our newly developed tectonic discrimination model is the establishment of tailored models for altered and fresh basalts. Moreover, by omitting isotopic features during model construction, the new models offer broader applicability in predicting a wider range of basalt samples in practical scenarios. The classification models were applied to investigate the Carboniferous to Permian evolution in the Western Tianshan Orogen (WTO), revealing that the subduction of Tianshan Ocean ceased at the end of Carboniferous and the WTO evolved into a post-collisional orogenesis during the Permian. Full article

In order to better constrain the specific depositional age and provenance of the Danzhou Group and understand the geological evolution of the Jiangnan Orogenic Belt, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from the Gongdong and Hetong formations of the Danzhou Group in the Longsheng area of the Western Jiangnan Orogenic Belt. Detrital zircons from the Gongdong Formation yield three age populations of 2658–2517 Ma, 2427–1678 Ma and 891–781 Ma, and the youngest ages suggest that the sedimentation began after ca. 783 Ma. U-Pb ages of detrital zircons from the Hetong Formation yield major populations at 2769–2502 Ma, 2492–2100 Ma, and 991–731 Ma, and the youngest ages redefine the maximum depositional age of this unit is 760 Ma, much younger than previously considered. Thus, the upper part of the Hetong Formation in the Longsheng area is newly subdivided into the Sanmenjie Formation, which is characterized by a large amount of 765–761 Ma volcanic rocks. The dominant 991–731 Ma detrital zircons for all samples were likely sourced from the Neoproterozoic igneous rocks of the southeast margin of the Yangtze Block. The subordinate 2494–1678 Ma detrital zircons were probably sourced from the Cathaysia Block. Minor amounts of 2769–2502 Ma detrital zircons may have been sourced from the Yangtze Block. Detrital zircons from the Gongdong Formation have mainly negative εHf (t) values (−1.1 to 21.8, 90%), suggesting that the detritus of the Gongdong Formation is dominated by the recycling of old crustal materials. The εHf (t) values of detrital zircons from the Hetong Formation have a large spread of −22.2 to +9.7, indicating that the source material of the Hetong Formation includes both the juvenile crustal materials and the recycled ancient crustal materials. The above age populations and Hf isotopic characteristics are consistent with the magmatic rocks in the Jiangnan Orogenic Belt and the Southeast Yangtze Block. Taking into account the lithostratigraphic features, provenances, and depositional ages, the Danzhou Group in the Western Jiangnan Orogenic Belt was deposited in a back-arc basin. Full article

Studying the basement characteristics and tectonic evolution of the proto-Yap Izu–Bonin–Mariana arc system can provide essential clues for understanding the subduction process in the Western Pacific Ocean. The Yap arc is a part of the Izu–Bonin–Mariana (IBM) arc, but the origin and formation time of the metamorphic basement rocks of the Yap arc still need to be determined. In this paper, we present the first systematic and detailed chronological, mineralogical, whole-rock major element and trace element research on metamorphic basement rocks from four stations in the Southern Yap arc and discuss the origin of the metamorphic basement rocks in the Southern Yap arc. The results show that the single mineral U-Pb ages of titanites and ⁴⁰Ar/³⁹Ar ages of amphiboles in the Southern Yap arc are 19.3 and 19.4 Ma, respectively, which are consistent within the error ranges, and these ages represent the metamorphic age of the basement rocks. The amphibolite is mainly produced via the metamorphism of back-arc basin basalt (BABB) and forearc basalt (FAB). The temperature and pressure conditions of this metamorphism are 446.5–641.6 °C and 0.24–0.73 GPa, the facies series of which is the medium P/T series. This study provides important data for understanding the Yap subduction system and the early evolution of the proto-IBM Arc. Full article

The discovery of eclogite outcrops in the East Kunlun Orogen Belt (EKOB) has confirmed the existence of an Early Paleozoic HP-UHP metamorphic belt. However, the protoliths and metamorphic histories of widespread metabasites remain poorly constrained. We collected three types of metabasites from the central part of EKOB. We present an integrated study of petrography, whole-rock geochemistry, Sr-Nd isotopes, estimated P–T conditions, and zircon U-Pb isotope ages. The results show that amphibolites and retrograde eclogites have clockwise P–T paths with peak conditions of, respectively, 11–12 kbar and 675–695 °C, and 21.5–22.2 kbar and 715–750 °C. Zircon dating of metabasites from Dagele yields Late Ordovician (~449 Ma) to Early Silurian (~440 Ma) protolith ages and Early Devonian (~414 Ma) amphibolite facies metamorphic ages. Retrograde eclogites from east Nuomuhong have a protolith age of ~902 Ma and metamorphic ages of ~418 Ma, consistent with other eclogites from East Kunlun. Our data suggest that the protoliths of Dagele metabasites represent arc-type magmatism during the subduction of a small back-arc oceanic basin. Instead, the protoliths of retrograde eclogites are Neoproterozoic tholeiitic basalts emplaced into continental crust and subsequently deeply subducted. We develop a new model for Early Paleozoic subduction and collision in the East Kunlun region, emphasizing the role of ‘dominant’ and ‘secondary’ suture boundaries. This model helps explain the ages and metamorphic histories of the metabasites studied here and offers new perspectives on the evolution of the Proto-Tethys Ocean. Full article

The Northeast China Block is a major component of the Central Asian Orogenic Belt, and its tectonic evolution has attracted much research attention. Ordovician strata are important in reconstructing the tectonic evolution of the Northeast China Block. This paper presents the results of sedimentological, zircon U–Pb, and geochemical analyses of sandstones of the Luohe Formation in the Wunuer area, Northern Great Xing’an Range, Northeast China. Lithological data, sedimentary structures, and grain-size analysis indicate that the Luohe Formation was deposited in a shallow marine environment. Detrital zircon U–Pb dating yields age peaks of 463, 504, 783, 826, 973, and 1882 Ma for sandstones from the Luohe Formation. The youngest zircon grain age of 451 ± 6 Ma represents the maximum depositional age of the Luohe Formation. The peak age at 463 Ma is consistent with the timing of post-collisional magmatism and the formation of the Duobaoshan island arc, while the peak at 504 Ma is consistent with the timing of magmatic activity related to the collision between the Erguna and Xing’an blocks. The peaks at 788, 826, 973, and 1882 Ma correspond to magmatism in the Erguna block, these ages indicate that the sandstones of the Luohe Formation were derived mainly from the Erguna block. Sandstone modal compositional analysis indicates that the provenance of the Luohe Formation was mainly a magmatic arc. The geochemical compositions of the sandstones suggest that the source rocks have continental island arc signatures. Based on the depositional age, sedimentary environment, provenance, and regional geology, it is concluded that the Luohe Formation was deposited in a back-arc basin setting during the formation of the Duobaoshan island arc–basin system in response to subduction of the Paleo-Asian oceanic plate. Full article

A synthesis of the evolution of the Neoproterozoic belts or orogens surrounding the São Francisco craton (SFC) in northeastern and southeastern Brazil is presented. Emphasis is placed on recognizing the superposition of sedimentary basins, from rift to passive margin to retroarc and foreland, as well as identifying three diachronic continental collisions in the formation of the SFC. The Tonian passive margin occurs in the southern Brasília Belt with the Vazante, Canastra, and Araxá Groups. During the Tonian, island magmatic arcs and basins developed in front and behind these arcs (fore- and back-arcs). Subsequently, in the Cryogenian–Ediacaran, a retroarc foreland basin developed with part of the Araxá Group and the Ibiá Group, and finally, a foreland basin developed, which was filled by the Bambuí Group. A tectonic structure of superimposed nappes, with subhorizontal S_1–2 foliation, formed between 650 and 610 Ma, is striking. In the northern Brasília Belt, there is the Stenian passive margin of the Paranoá Group, the Tonian intrusion of the Mafic–Ultramafic Complexes, and the Mara Rosa Island magmatic arc, active since the Tonian, with limited volcanic–sedimentary basins associated with the arc. A thrust–fold belt structure is prominent, with S1 foliation and late transcurrent, transpressive tectonics characterized by the Transbrasiliano (TB) lineament. The Cryogenian–Ediacaran collision between the Paranapanema and São Francisco cratons is the first collisional orogenic event to the west. In the Rio Preto belt, on the northwestern margin of the São Francisco craton, the Cryogenian–Ediacaran Canabravinha rift basin is prominent, with gravitational sediments that represent the intracontinental termination of the passive margin that occurs further northeast. The rift basin was intensely deformed at the Ediacaran–Cambrian boundary, as was the Bambuí Group. On the northern and northeastern margins of the São Francisco craton, the Riacho do Pontal and Sergipano orogens stand out, showing a comparable evolution with Tonian and Cryogenian rifts (Brejo Seco, Miaba, and Canindé); Cryogenian–Ediacaran passive margin, where the Monte Orebe ophiolite is located; and Cordilleran magmatic arcs, which developed between 620 and 610 Ma. In the Sergipano fold belt, with a better-preserved outer domain, gravitational sedimentation occurs with glacial influence. A continental collision between the SFC and the PEAL (Pernambuco-Alagoas Massif) occurred between 610 and 540 Ma, with intense deformation of nappes and thrusts, with vergence to the south and accommodation by dextral transcurrent shear zones, such as the Pernambuco Lineament (PE). The Araçuaí belt or orogen was formed at the southeastern limit of the SFC by a Tonian intracontinental rift, later superimposed by a Cryogenian–Ediacaran rift–passive margin of the Macaúbas Group, with gravitational sedimentation and glacial influence, and distally by oceanic crust. It is overlain by a retroarc basin with syn-orogenic sedimentation of the Salinas Formation, partly derived from the Rio Doce cordilleran magmatic arc and associated basins, such as the Rio Doce and Nova Venécia Groups. A third continental collision event (SF and Congo cratons), at the end of the Ediacaran (580–530 Ma), developed a thrust–fold belt that deforms the sediments of the Araçuaí Belt and penetrates the Paramirim Corridor, transitioning to the south to a dextral strike-slip shear zone that characterizes the Ribeira Belt. Full article

Clinopyroxene, one of the primary rock-forming minerals in mafic rocks, is the major host of lithophile elements in the mantle lithosphere and plays a crucial role in understanding mantle evolution and rock petrogenesis. Taking the Bingdaban ophiolite as an example, this study employed electron probe microanalysis and in situ trace element analysis to investigate the geochemistry of clinopyroxene in gabbros to determine the magma series and evolution, constrain the physicochemical conditions of the magmatic processes and explore the petrogenesis and tectonic setting. Representative gabbro samples were subjected to zircon U–Pb isotopic analysis, yielding an age of 424.3 ± 5.9 Ma. Geochemical investigations revealed that the Bingdaban gabbros exhibit tholeiitic composition, suggesting a genesis associated with enriched mid-ocean ridge basalt (E-MORB). Mineralogical analyses indicated that the clinopyroxene in the gabbros was Mg-rich, Fe-poor, and alkali-poor, representing a subalkaline series. The compositional end members of clinopyroxene were calculated as Wo_38.9–48.0En_30.9–48.1Fs_10.4–24.4, indicating a predominance of diopside with a minor amount of augite. Temperature–pressure conditions imply that these rocks formed in a high-temperature, low-pressure, and shallow-source environment. Compositional estimates of the melt in equilibrium with clinopyroxene are consistent with the overall characteristics of the host rock, reflecting an E-MORB setting. The Bingdaban gabbro likely originated from an initially depleted mantle source that later received an input of enriched mantle material, indicating formation in either an initial oceanic or immature back-arc basin tectonic setting. Full article

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB. Full article

The Liaohe Group, which is a significant lithostratigraphic unit within the Paleoproterozoic Jiao-Liao-Ji Belt situated between the Longgang and Liaonan-Nangrim blocks, comprises the Langzishan, Li’eryu, Gaojiayu, Dashiqiao, and Gaixian formations, which are characterized mainly by a clastic-rich sequence with an interlayered bimodal-volcanic sequence, carbonate-rich sequence, and (meta-)pelite-rich sequence. Currently, the tectonic background and evolution of the Liaohe Group remain contentious. Based on the study of detrital zircon geochronology and the zircon trace element characteristics in the Langzishan and Li’eryu formations in the North Liaohe Group in the Lianshanguan area, NE China, this paper reveals the formations’ provenances, depositional ages, and relationships with Paleoproterozoic granitoids (the Liao-Ji granites). The present results, in conjunction with previous studies, indicate that the depositional age of the Langzishan Formation is 2136 Ma and that of the Li’eryu Formation is 1974 Ma. The provenances of the Langzishan Formation and the Li’eryu Formation are mainly characterized by Neoarchean-to-early-Paleoproterozoic basement rocks (~2.6–2.4 Ga) and the Liao-Ji granites (~2.2–2.0 Ga), respectively. Moreover, the coeval mafic and metasedimentary rocks of the Liaohe Group exhibit characteristics of an extensional environment, which is represented by the tectonic setting of a back-arc basin. Notably, the Upper Langzishan Formation records a prominent shift in sedimentary environment from a passive continental margin to an active continental margin. In terms of the tectonic evolution of the North Liaohe Group and the Jiao-Liao-Ji Belt, our proposed model suggests that the Archean basement rocks in the northern part of the continental block, along with a limited contribution from the Paleoproterozoic Liao-Ji granites, served as the primary sources for the Langzishan Formation. Subsequently, the rapid deposition of the Li’eryu Formation was influenced by intense magmatism and subsequent erosion of the subduction-related magmatic arc (the Liao-Ji granites) within a back-arc basin environment. Lastly, the deposition of clastic materials from the Longgang blocks and the Liao-Ji granites resulted in the formation of the Gaojiayu, Dashiqiao, and Gaixian formations. Full article

The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U–Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186–190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182–183 Ma. The content of MgO in the SSPP is relatively high, but the SiO₂, Al₂O₃, CaO and TiO₂ content and ΣREE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong–Nujiang suture zone. The SSPP rocks have relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7091–0.7131) and positive Hf(t) values (11.2–13.8), with εNd(t) values varying from −1.1 to 9.4. The YHP has relatively low εHf(t) values (3.5 to 6.9), with the Nd–Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr–Nd and Lu–Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong–Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong–Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed. Full article

The Late Paleozoic is an important epoch of coal-bed accumulation in the North China, particularly in its western segment, where a world-class coal field has been found and exploited for three decades. Coal-bearing layers recorded rapid Late Carboniferous to Early Permian sea-level rise that led to the evolution of the shallow marine tidal flat basin, followed by regression that resulted in the formation of deltaic facies. However, little attention has been paid to the tectonic factors that assisted in the coal accumulation processes, which have been demonstrated worldwide to be pivotal. In this study, we evaluate the significant influence of southward subduction of the middle segment of the Paleo-Asian Ocean (PAO) during Carboniferous to Early Permian deposition of coal-bearing neritic to deltaic successions in the western North China Block (NCB). We pinpoint a direct link between basin-filling evolution of the northwestern Ordos Basin and the progressive uplift of the Inner Mongolia continental arc triggered by the south-dipping subduction of the Solonker ocean plate. Sedimentary facies variation and paleocurrent reconstruction indicate sedimentary routing from the north to northeast. The petrology and detrital zircon geochronology of sampled sandstones indicate evolving provenances from a dominated cratonic basement to a mixed orogen and continental arc. The Carboniferous samples are generally quartz arenites and contain a large number of Archean to Paleoproterozoic zircons with small amounts of Paleozoic arc pluton components, indicating sediment supply dominated by the western NCB Precambrian basement. In contrast, the youngest sample of the Shanxi Formation is sub-litharenite, the age spectrum of which is characterized by the dominance of ca. 450 and 270 Ma ages, indicative of a high influx of Paleozoic arc detritus. This shift in sandstone petrology, detrital zircon distribution, and depositional setting allows us to link the shoaling of the Ordos Basin with mountain building and the unroofing of the continental arc, which enhances our understanding of both the subduction of the PAO and coal accumulation in the NCB. Full article

In this study, we investigated the structural evolution of the Vértessomló (VT) Thrust and the Környe-Zsámbék (KZ) Fault, which are located in the Transdanubian Range in the center of the Miocene Pannonian back-arc basin. Our study is based on surface and well data. The Transdanubian Range was located on the Adriatic passive margin during the Late Triassic, where a thick succession of platform carbonates was deposited. Intercalations of intraplatform basin deposits occur in the eastern part of the study area. South-directed thrusting and the formation of the VT Thrust took place during the Cretaceous, related to the Austroalpine orogeny. Asymmetric half-grabens were formed during the Eocene in the hanging wall of the segmented dextral normal KZ Fault. The geometry and kinematics of the KZ Fault were influenced by the pre-existing VT Thrust located in the Mesozoic basement of the Paleogene sub-basins. These Eocene half-grabens suffered mild inversion due to the dextral reverse reactivation of the VT Thrust and the KZ Fault during the Oligocene–Early Miocene. The geometry of Miocene normal faults indicates that the VT-KZ Fault system was an active transfer fault during the Miocene extension of the Pannonian Basin, as well. We found a positive correlation between the rheology of the Triassic basement and the mode of Paleogene fault reactivation. Our results show that reactivation of the pre-existing thrust took place along that segment, where the Triassic basement is made up of homogeneous platform carbonates. In contrast, a diffuse fault zone developed, where the Triassic basement is represented by the weak layers of intraplatform basins. Full article

As one of the youngest back-arc basins, the evolutionary behavior of magmatic volatiles in the Eastern Manus Basin has been poorly studied. Recently, apatite has received widespread attention for its powerful function in recording information on magmatic volatiles. In this paper, by determining the major element compositions and primary volatile abundances (F, Cl, SO₃) of apatites in dacite, we analyze the compositions of volatiles before magma eruption in the Eastern Manus Basin, as well as their indications for magmatic oxygen fugacity and petrogenesis, so as to improve the study about the evolution of magmatic volatiles in this region. Experimental data indicate that apatite saturation temperatures range from 935 to 952 °C. All the apatites are magmatic apatites with F contents of 0.87−1.39 wt.%, Cl contents of 1.24−1.70 wt.%, and SO₃ ≤ 0.06 wt.%. Analysis reveals that the apatites in this study crystallized from volatile-undersaturated melts, so their chemical compositions can be used as indicators of magmatic compositions. According to calculations, the minimum S concentrations of the host melts range from 2−65 ppm or 8−11 ppm. The crystallization and separation of magnetite caused the reduction state of melts, and the relatively low oxygen fugacity (ΔFMQ = −0.2 ± 0.9) caused low SO₃ contents in apatites. In addition, F and Cl contents of the host melt were calculated to be 185−448 ppm and 1059−1588 ppm, corresponding to the H₂O contents of 1.4−2.1% and 1.2−1.5% (error ± 30−40%), respectively. The high Cl/F ratio and H₂O contents of samples indicate the addition of slab-derived fluids in the mantle source region of the Eastern Manus Basin. High F contents of the melts may be influenced by F-rich sediments, as well as the release of F from lawsonite and phengite decomposition. High Cl appears to originate from the dual influence of subduction-released fluids and Cl-rich seawater-derived components. Further, it is estimated that 14−21% of the total Cl concentrations in melts were added directly from subduction-released fluids, or higher. Full article

The Jinshajiang–Ailaoshan–Song Ma orogenic belt (JASB), as a vital segment of the eastern Paleo-Tethyan tectonic zone, is one of the most important zones in which to study the Paleo-Tethyan tectonic evolution. We have undertaken an integrated geochronological, petrological, and geochemical study of mafic rocks from the JASB to reveal the subduction and closure processes of the eastern Paleo-Tethyan Ocean during the Permian to Triassic. In conjunction with previous magmatic and metamorphic records in the JASB, three important tectonic stages are identified: (1) Early Permian to Early Triassic (ca. 288–248 Ma). Most of the Early Permian to Early Triassic mafic rocks have normal mid-ocean ridge basalt (N-MORB)- or enriched MORB (E-MORB)-like rare earth elements (REE) and trace element-normalized patterns with positive εNd(t) and εHf(t) values and negative Nb and Ta anomalies. Their La/Nb ratios and εNd(t) values show that approximately 3%–15% of slab-derived fluid accounts for the generation of these rocks. These characteristics suggest that the mafic rocks formed in an arc/back-arc basin setting at this stage. Additionally, the Early Permian mafic rocks are mainly exposed in the Jomda–Weixi–Yaxuanqiao–Truong Son magmatic rock belt (JYTB) on the western side of the JASB, indicating that the westward subduction of the Jinshajiang–Ailaoshan–Song Ma Paleo-Tethys Ocean (JASO) began in the Early Permian. Middle Permian mafic rocks are exposed in the Ailaoshan-Day Nui Con Voi metamorphic complex belt and the JYTB on both sides of the JASB. We propose that the bipolar subduction of the JASO occurred in the Middle Permian and ended in the Early Triassic. (2) Middle Triassic (ca. 248–237 Ma). The mafic rocks at this stage have LREE- and LILE-enriched patterns, negative Nb and Ta anomalies and negative εNd(t) values. Their variable εHf(t), εNd(t) values and La/Nb ratios show that these mafic rocks were highly affected by crustal material (ca. 16%). Considering the Middle Triassic high-pressure (HP) metamorphism and massive Al-enriched felsic magmatism in the JASB, these rocks may have formed in a collisional setting between the South China Block (SCB) and the North Qiangtang–Simao–Indochina Block (QSIB) during the Middle Triassic. (3) Late Triassic (ca. 235–202 Ma). The mafic rocks at this stage have negative εNd(t) and εHf(t) values and show terrestrial array characteristics. The εNd(t) values and La/Nb ratios show that approximately 30% of crustal components account for the generation of these rocks. Combined with the contemporaneous bimodal magma and metamorphism during the Late Triassic, we suggest that these rocks may have formed in a postcollisional extensional setting associated with magma diapir. Full article