Eastern Paleo-Tethys Research Articles

Extraordinarily high organic matter enrichment during the late Guadalupian in northwestern Sichuan basin, China

Shale with extraordinarily high organic matter content (EHOMC) formed in the margin of the Sichuan carbonate platform during the late Guadalupian in the eastern Paleo-Tethys. So far, not much consideration has been given to the enrichment of EHOMC. Here, integrated lithological and geochemical analyses of the top Maokou Formation black shales in the northwest Sichuan basin were undertaken to investigate the EHOMC. The black shales were divisible into three units (i.e., Units 1 to 3 in ascending order); EHOMC occurred in Unit 2. Unit 1 was deposited during the early stage of the transgression under the small-scale upwelling in an anoxic environment with lower paleo-productivity than Unit 2, considering the moderate Cd/Mo ratios and the concentration of Mo, U, V, P, Cu, and Zn. Unit 2 exhibited high Cd/Mo ratios and high concentrations of elements (Mo, U, V, P, Cu, and Zn), suggesting that the shales with EHOMC were deposited through an intense upwelling in an euxinic-sulfidic environment with high paleo-productivity. Unit 3 demonstrated moderate paleo-productivity and anoxic-suboxic conditions, while the nutrient elements were derived through increased terrestrial detrital input. Unit 2 exhibited the highest total organic matter content (TOC) (17.72–32.58 wt%); it is inferred that the enrichment of EHOMC was mainly caused by a euxinic-sulfidic environment, while high paleo-productivity was another key factor. Besides, the euxinic-sulfidic environment and high paleo-productivity in Unit 2 occurred approximately during the intense upwelling, the occurrence of the high sea-level, and the biological extinction during the end of Guadeloupe. These factors together promoted the enrichment of EHOMC in Unit 2.

Lower Viséan (Lower Carboniferous) rugose corals from the Donets Basin (Ukraine)

Rugose corals are described from the lower Viséan of the Donets Basin. The new coral Dorlodotidae fam. nov. and the species Cyathoclisia sukhensis sp. nov. are here introduced and the presence of the genera Amygdalophyllum and Ceriodotia are established for the first time in the Donets Basin. The priority of the generic name Protolonsdaleia over Eolithostrotionella is discussed. The lower Viséan rugose coral assemblage of the Donets Basin differs from those of the upper Tournaisian and upper Viséan. Global and regional events may have led to abrupt changes in sedimentation conditions at the end of the Tournaisian time and at the beginning of the late Viséan. The colonial rugose corals Siphonodendron, Dorlodotia, Protolonsdaleia and Ceriodotia appear in the early Viséan of the Donets Basin as in many other regions of Eurasia. The similarity of the lower Viséan coral assemblage of the Donets Basin with that of the Western Paleotethys is noted but at the same time, there are significant differences with the regions of the Eastern Paleotethys. This is due to the presence of a possible paleogeographic barrier located to the eastern part of Laurussia hindering the migration of corals.Keywords: Mississippian, Viséan, corals, paleobiogeography, Ukraine.

Formation of the Middle Permian Danzhou Monzogranite in Northern Hainan Island (South China): Insight into Evolution of the Eastern Paleo-Tethys

Formation of the Middle Permian Danzhou Monzogranite in Northern Hainan Island (South China): Insight into Evolution of the Eastern Paleo-Tethys

Origin of the Multiple-Sourced Cherts in Maokou Carbonates in Sichuan Basin, South China

Cherts have been thought to originate from biosilicification, terrestrial inputs and hydrothermal activity. The study of cherts is helpful in understanding the paleo-ocean environment and tectonic–sedimentary processes. Large amounts of cherts occur widely in the Maokou Formation in the Sichuan Basin, which may be largely connected to the Permian Chert Event (PCE). However, the source of silica and the formation process of cherts remain debated. Here, we analyze the petrographic and geochemical features of the cherts from the Guadalupian Maokou Formation (~268–259 Ma) in six sections in the Sichuan Basin. Two main types of cherts, nodular and bedded, are recognized in the Maokou Formation. The formation of nodular cherts was mainly affected by hydrothermal fluids, whereas the bedded cherts are mainly of biogenetic origin. The Emeishan large igneous province (ELIP) caused the activation of deep faults, accompanied by intense hydrothermal activities. Correspondingly, the cherts of significant hydrothermal origin developed near the active deep faults. The intensified hydrothermal activities may provide extra silica supplies and flourish the silica-secreting organisms by the associated volcanogenic upwellings that facilitated the enrichment of cherts. The study of Maokou cherts can help to record the volcanic- and silicon-related biological activities in the eastern Paleo-Tethys Ocean and can provide significant implications for chert enrichment in analogous settings.

Detrital zircon U Pb age distributions and Hf isotopic constraints of the Ailaoshan-Song Ma Suture Zone and their paleogeographic implications for the Eastern Paleo-Tethys evolution

During the Late Paleozoic to Cenozoic, with the closure of the Tethys oceans, the East and/or Southeast Asia was amalgamated by individual blocks. In order to understand the Paleogeography of the Eastern Paleo-Tethys, a systematic U Pb dating on 11 samples have conducted to constrain the age and provenance of the elements forming the Ailaoshan-Song Ma mélange. These samples consist of the ophiolitic mélange zone (6 samples), the sedimentary cover of the Indochina block (4 samples), and the post-orogenic continental deposits (1 samples). The achieved detrital zircon age distribution, and the corresponding in-situ Hf isotopic recorder allow us to discuss the provenances of the materials involved in the tectonic processes during the subduction and collision. Three units (M1, M2, M3), which stand for the different segment of the suture zone , could be separated based on the differences in zircon age clusters and Hf isotopic values. These differences might be due to the variety of provenances. Combined with the previous works, especially our work on the Song Chay mélange zone, we concluded that both upper and lower plates are potential provenances for the mélanges between South China block and Indochina block. Along the suture zone, the component of the mélange presents a significant heterogeneity.

Thailandina and Neothailandina and their family Thailandinidae salvaged: a valid taxonomic group of peculiar Permian fusuline Foraminifera

The fusuline genera Thailandina Toriyama and Kanmera, 1968 and Neothailandina Toriyama and Kanmera, 1968 were established by Toriyama and Kanmera (1968) based on material from the Khao Phlong Phrab section of the Permian Rat Buri Limestone in central Thailand that is currently assigned to the Khao Khad Formation of the Saraburi Group (Ueno and Charoentitirat, 2011). These fusuline genera are peculiar in having parachomata and replaced tests by secondary mineralization. Moreover, Neothailandina was described to have a test with transverse septula, considered to be characteristic for Neoschwagerinidae. Based on these remarkable test features, Toriyama and Kanmera (1968) newly introduced the subfamily Thailandininae to accommodate these two new genera and assigned it to the Neoschwagerinidae, despite the lack of septula in Thailandina. Later, Kobayashi et al. (2010) argued that Thailandina and Neothailandina are just a mixed grouping of several known genera of schwagerinids, verbeekinids, and neoschwagerinids that are too altered by recrystallization to be recognizable, and rejected the taxonomic validity of these two genera as well as Thailandininae.The Khao Phlong Phrab section represents one of the standard late Cisuralian−Guadalupian (late early−middle Permian) fusuline successions in the eastern Paleotethys (Zhang and Wang, 2018) and contains not only Thailandina and Neothailandina but also abundant schwagerinid, verbeekinid, and neoschwagerinid fusulines (Toriyama, 1975; Fig. 1). I investigated the original specimens described by Toriyama and Kanmera (1968) and Toriyama (1975) from the section that are housed in the Department of Earth and Planetary Sciences of Kyushu University, Japan. I found that most of the grounds for Kobayashi et al.'s (2010) arguments to regard the thailandinin genera as taxonomically invalid are not supported by observations on these specimens as explained in the account that follows. In this taxonomic note, I propose that Thailandina and Neothailandina, and their family Thailandinidae, should be retained as valid taxonomic groups.

A new marattialean fern, Pectinangium xuanweiense sp. nov., from the Lopingian of Southwest China

A new species of adpressed marattialean fern with vegetative and fertile pinnules, Pectinangium xuanweiense sp. nov., is described from the Lopingian (upper Permian) in eastern Yunnan and western Guizhou provinces, Southwest China. The fronds of this new species are at least bipinnate, with linear to lanceolate ultimate pinnae. The pinnules are alternately arranged and pecopterid-type with pinnate venation. Fertile pinnules show recurved margins and abaxially bear a row of synangia on each side of the midvein. Synangia are 0.7 mm long and 0.4 mm wide on average, arranged in a density of approximately 16 per centimeter. Each synangium consists of four sporangia that are completely fused in well-regulated rectangular/ovoid shapes in the distal view. Sporangia are exannulate, 370 μm long and 200 μm wide on average. The sporangial walls are composed of at least three layers of elongated rectangular to polygonal cells. In situ spores are circular to oval, 25–(30)–35 μm in diameter, with trilete marks and microverrucate sculptures. Ultrastructural investigations indicated that the exine encompasses a perine, sexine, and nexine. In situ spores correspond to Verrucosisporites if found as sporae dispersae, and thus indicate that P. xuanweiense sp. nov. belongs to the Acitheca miltonii Group (palynological grouping) of Paleozoic marattialeans. Our study provides insight into the diversity of late Permian marattialeans in the tropics of the eastern Paleo-Tethys ocean.

Ocean redox changes from the latest Permian to Early Triassic recorded by chromium isotopes

Multiple observations have shown that widespread oceanic anoxia may have played an important role in the end-Permian mass extinction and delayed biotic recovery in the Early Triassic. However, it has not been reached a consensus on the temporal and spatial changes in oceanic redox conditions at that time. Chromium (Cr) isotopes have been widely used to trace redox processes in surface environments. Here, we present the Cr isotope compositions (δ53Cr) of the Upper Permian–Lower Triassic carbonate ramp from the Xiakou section in South China (eastern Paleotethys). We observed largely fractionated δ53Cr values (1.15‰ to 2.16‰) in the latest Permian, which are higher than most values among Phanerozoic sedimentary carbonates. Such high values may result from Cr reduction facilitated by the development of a reducing environment during the latest Permian. The high Cr isotope ratios were followed by an abrupt negative excursion of δ53Cr values (from average 1.78‰ to average −0.13‰) across the extinction horizon, which may be attributed to a rapid expansion of oceanic anoxia in the eastern Paleotethys. Persistent negative δ53Cr values covering the Early Triassic indicate that the ocean may not have recovered from anoxia for an extended period. Our Cr isotope results support a stepwise deterioration of oceanic redox environments, which has been previously proposed as a possible cause of the most severe biotic catastrophe in the Phanerozoic.

Provenance analysis of Late Triassic turbidites in the eastern Songpan–Ganzi Flysch Complex: Sedimentary record of tectonic evolution of the eastern Paleo-Tethys Ocean

The Xikang Group represents the main stratigraphic sequence of the Songpan–Ganzi complex (SGC), a flysch basin with a wide distribution and monotonous lithology. Strong tectonic deformation and accumulation of folds have led to destruction of the original sedimentary sequence and sedimentary structure of the strata, such that there exists no reasonable sedimentary model that can explain the massive flysch. Herein, we investigated in detail the sedimentary structures, paleocurrent direction, particle sizes, major and trace elements, and magmatic zircon and detrital zircon U–Pb ages of the typical sections of the Xikang Group in the eastern SGC. Our analysis led to the following findings: 1) The U–Pb zircon ages from the Galazi (215.6 ± 1.1 Ma), Sudi (214.5 ± 1.5 Ma and 221.1 ± 1.5 Ma), and Gulu (221.3 ± 1.2 Ma) plutons, which intrude the Xikang Group, and detrital zircon samples from the uppermost part of the Xinduqiao Fm. indicated the youngest mean age (YC1σ[2+]) of 222.4 ± 2.5 Ma. This in turn indicates a pre-221 Ma depositional age for the Xikang Group. 2) The provenance was derived from the North China Block and Qiangtang Terrane, Qinling–Dabie orogenic belt, South China Block, and East Kunlun–Qaidam, and the construction of the flysch in the SGC belongs to the multi-source or line source provided by the peripheral orogenic belt. 3) Trace elements suggested that the Xikang Group may have been deposited in a sedimentary basin associated with a continental island arc. 4) The sedimentary environment in this area belonged to the distal fan of a submarine, and the new sedimentation age and sedimentation rate indicate that the Carnian pluvial event may have been the trigger mechanism. 5) The formation of the flysch deposition in SGC related to the Paleo-Tethys Ocean underwent a scissor-like closure from east to west.

Strontium isotope evolution of Middle Permian seawater in the Sichuan Basin, South China: Possible causes and implications

Middle Permian 87Sr/86Sr variations are recorded by the marine carbonates in the Qianfo section and the Yundadi3 well in the Sichuan Basin of the Yangtze Platform, China. Petrographic and geochemical examinations of micritic carbonates confirmed the preservation of near-primary 87Sr/86Sr signatures that can be utilized to construct a reliable high-resolution Sr isotope profile of the equatorial eastern Paleo-Tethys for global correlations. The high 87Sr/86Sr ratios associated with a eustatic regression event in the Late Capitanian (late Middle Permian) imply a predominant contribution from terrigenous materials. We observed extremely low 87Sr/86Sr ratios (ca. 0.7068 to 0.7069) in the 40 m-thick Capitanian interval, confirming the significant chemostratigraphic episode called the Capitanian Minimum. Globally low 87Sr/86Sr values in the Middle Permian were probably caused by weak continental weathering rates, suppressed riverine flux into the ocean during Pangean assembly, and increased hydrothermal flux into the ocean associated with the opening of the Neotethys during the Middle Permian. Based on the 87Sr/86Sr values of micritic carbonates from the Qianfo section and the Yundadi3 well, large amounts of nonradiogenic Sr may have been derived from strong submarine hydrothermal activity related to the Emeishan LIP, and areas closer to volcanic activity were more strongly affected. These combined factors were responsible for the observed low 87Sr/86Sr ratios in the Sichuan Basin.

Oceanic lithosphere heterogeneity in the eastern Paleo-Tethys revealed by PGE and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite

Platinum group elements (PGE) and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite (SW China) were investigated in this study, in order to constrain the evolution of the lithospheric mantle beneath the Jinshajiang-Ailaoshan Ocean, which was a branch of the eastern Paleo-Tethys. The Jinshajiang peridotites have whole-rock compositions (e.g., MgO = 32.7–38.1 wt.%; Al2O3 = 0.67–1.30 wt.%) and spinels with moderate Cr# values (0.4–0.6) similar to those of abyssal peridotites, which indicate moderate degrees of partial melting (15%–20%). These peridotites exhibit U-shaped chondrite-normalized REE patterns that could be caused by hydrothermal alteration or melt-rock interaction after mantle melting. In addition, Pd concentrations and (Pd/Ir)N ratios of the Jinshajiang peridotites increases with decreasing Al2O3 concentrations. These negative correlations cannot be explained by simple partial melting but record a melt-rock reaction event after mantle melting. This study therefore demonstrates the efficiency of PGE in detecting the melt-rock reaction process relative to whole-rock major and trace elements. The suprachondritic 187Os/188Os ratios (0.1272–0.1374) further indicate that the later percolating melt derived from a mantle domain with distinct 187Os-enriched isotopic compositions. In comparison with peridotites in the Ailaoshan ophiolite belt, which were not significantly affected by melt percolation, this study further highlights that the lithospheric mantle compositions beneath different segments of the same ocean basin are highly variable and might be controlled by distinct mantle processes in response to different rifting mechanisms.

U-Pb Isotope Geochronology of Syntectonic Granites from Hainan Island, South China: Constraints on Tectonic Evolution of the Eastern Paleo-Tethys Ocean

Since the recognition of the Indosinian orogeny in northern Vietnam, Triassic orogens have been widely identified around the western and southwestern boundaries of the South China block. The paleo-Tethys sutures stretch from west to southeast, from Jinshajiang-Ailaoshan via NE Vietnam to Hainan Island; these sutures exhibit and develop voluminous Permian-early Triassic magmatism and numerous high-strain shear zones. As ophiolites related to the paleo-Tethys are lacking on Hainan Island, the eastward extension of the Indosinian orogeny and subduction and closure time of the paleo-Tethys Ocean on Hainan Island remain controversial. Here, an integrated kinematic and geochronological study has been conducted on two shear zones, called the Xiaomei and Mangsan shear zones. U-Pb zircon dating yields an age of early Triassic (252–251 Myr) for Xiaomei syntectonic granites which formed in the same tectonic setting and presented the similar nanoparticles to the Indosinian granites from Ailaoshan ductile shear zone, including the strawberry-like and flower-like nanoparticles. The NE-trending Mangsan shear zone represented by the gneissic granites with middle Permian ages (264–262 Myr) formed in the same tectonic setting as the Wuzhi-shan granites that were proposed as I-type granites. These middle Permian gneissic granites with arc affinity may represent an arc setting related to the NW subduction of the paleo-Tethys. The analyses reveal that granites with late Triassic ages (235–232 Myr) in the Xiaomei shear zone have the characteristics of A-type granites. The late Triassic extensional events on Hainan Island may be related to the subduction of the paleo-Pacific Plate beneath the East Asian continent.

Tectonic control of Guadalupian-Lopingian cherts in northwestern Sichuan Basin, South China

Abstact Cherts have been considered to be formed mainly through processes involving biosilicification, riverine inputs and hydrothermal activities. In the Permian, massive cherts are widely distributed in South China, bearing significant implications for the reconstruction of paleogeography and paleoceanography of the eastern Paleo-Tethys Ocean. The origin of the Permian cherts in South China, however, remains inconclusive due to a lack of systematic petrographic and geochemical study. In this article, we analyze the petrologic and geochemical characteristics of the cherts collected from five outcrops in the northwestern Sichuan Basin, South China. These cherts are found from the Upper Guadalupian Maokou Formation (ca. 268–259 Ma) and Upper Lopingian Dalong Formation (ca. 254–252 Ma). Field and microscopic observations suggest that cherts from the Maokou Formation occur mainly in nodular forms, and consist of microcrystalline quartz, silicified bioclasts and siliceous fossil debris. Both petrographic observation and geochemical analysis suggest that the Maokou cherts are mainly of hydrothermal origin near deep faults in the southwest and of biogenetic origin with less hydrothermal influence in the northeast of the studied area. The Dalong cherts occur in both nodular and bedded forms, with more abundant siliceous fossil debris than the Maokou cherts. Major, trace and rare earth element data support that the Dalong cherts are influenced by both biosilicification and hydrothermal activities. The spatial heterogeneity of the cherts origin in South China is largely controlled by tectonic activities associated with the Emeishan large igneous province (ELIP) event and the closure of the Paleo-Tethys Ocean. The troughs formed by the extensional tectonic activity linked to the ELIP could provide accommodation space and activate deep faults allowing the rise of silicon-rich hydrothermal fluids from the deep and nutrient-rich water from volcanogenic upwellings. Subsequent cooling may have facilitated the precipitation of large amounts of silica and the formation of cherts in the study region. This work demonstrates that the origin of the Guadalupian-Lopingian cherts in South China is predominantly controlled by tectonic and biological activities, which sheds light on the global “Permian Chert Event”.

Granite-Related Tin Metallogenic Events and Key Controlling Factors in Peninsular Malaysia, Southeast Asia: New Insights from Cassiterite U-Pb Dating and Zircon Geochemistry

Abstract Permian-Triassic granites and associated tin deposits are widespread in the Eastern and Western belts of Peninsular Malaysia. The ages and key controlling factors of tin mineralization, however, are poorly constrained. Cassiterite separates from the Sintok and Rahman tin deposits in the Western belt, and Bandi, Setahum, Lembing, and Cherul tin deposits in the Eastern belt have U-Pb ages of 218.9 ± 3.4 and 226.8 ± 7.6 Ma, and 213.1 ± 3.9, 270.6 ± 4.6, 282.7 ± 4.6, and 281.3 ± 3.5 Ma, respectively. These ages directly constrain the tin mineralization in Peninsular Malaysia to two separate periods: 290 to 270 Ma and 230 to 210 Ma. Zircon crystals from tin-bearing granites in the Cherul and Sintok deposits have U-Pb ages of 276.0 ± 1.9 and 221.9 ± 0.6 Ma, respectively, consistent with the cassiterite U-Pb ages within uncertainties. Zircon crystals from barren granites of the Kuantan pluton in the Eastern belt have a U-Pb age of 260.5 ± 0.7 Ma, which is between the two tin mineralization periods. Zircon from these barren granites have εHf(t) values from −5.4 to 3.6, two-stage Hf model ages (TDM2) from 1.4 to 1.0 Ga, and Ce4+/Ce3+ ratios from 40 to 120. By comparison, zircon crystals from the tin-bearing granites have low εHf(t) values (−9.7 to −3.2) and Ce4+/Ce3+ ratios (4–67) and high TDM2 (1.8–1.4 Ga). Zircon ages, Hf isotopes, and trace elements indicate that the tin-bearing granitic magmas in Peninsular Malaysia had relatively low oxygen fugacity and were derived from reworking of Paleo- to Mesoproterozoic sedimentary rock-dominated crust in response to the Paleo-Tethyan subduction and continental collision. This study confirms that the nature of magma sources and redox states of magmas were key in the formation of the tin-rich granites and associated tin deposits and that the granite-related tin mineralization in Peninsular Malaysia was closely related to the evolution of the eastern Paleo-Tethys.

Mixed seawater and hydrothermal sources of nodular chert in Middle Permian limestone on the eastern Paleo-Tethys margin (South China)

Middle Permian limestone and marl of the Maokou Formation deposited along the eastern Paleo-Tethys margin (South China) host abundant chert nodules that formed in association with the global Permian Chert Event (PCE). An enhanced understanding of the origin of the chert may help elucidate Middle Permian oceanic and diagenetic conditions. However, the mechanisms responsible for the formation of chert, especially the source of silica, remain controversial. Maokou chert nodules, which are made up of quartz and subordinate calcite, hematite, pyrite, and fluorite, and rimmed by F-bearing sepiolite laminae, appear to have formed mainly by the replacement of precursor nodular limestone by silica. The nodular limestone may have formed by differential compaction of interbedded limestone and marl, after which carbonate was replaced gradually outward from nodule centers by dissolved silica released from the dissolution of siliceous skeletal grains (including spicules and radiolarians) in host marl during early diagenesis. The introduction of high-T hydrothermal fluids into ocean water is evidenced by the presence of F-bearing minerals (fluorite and sepiolite), a hybrid rare earth element (REE) + Y pattern, and a wide range of δ30Si values (−0.4‰ to +0.7‰) documented from the Maokou nodular chert. Conservative estimation indicated that Middle Permian seawater may have had a δ30Si composition (+0.7‰ to +1.8‰) similar to that of modern open ocean water, suggesting that seawater was the dominant silica source of nodular chert. Mixing calculations based on REE + Y geochemistry suggest additional contributions of ca. 0.05–0.2% high-T hydrothermal fluid and ca. 1–10% of continental-derived freshwater to ocean water. Such increased influx of silica to seawater may have been related to the early rifting induced by Emeishan volcanism and intensified chemical weathering during an interglacial period, which played an important role in promoting the flourishing of siliceous organisms and subsequent chert enrichment during the Middle Permian.

Anisian granodiorites and mafic microgranular enclaves in the eastern Kunlun Orogen, NW China: Insights into closure of the eastern Paleo–Tethys

Granitoids and their mafic microgranular enclaves (MMEs) provide important insights into magmatic activity and tectonic evolution associated with accretionary orogens. Here we investigate the origin of Middle Triassic granodiorites and MMEs in the central segment of the eastern Kunlun Orogen (EKLO) in NW China, which is an important region to evaluate the history of closure of the Paleo–Tethys, through zircon U–Pb geochronology, major and trace element geochemistry, and Sr–Nd–Hf isotopes. Zircon U–Pb ages indicate the coeval formation of granodiorites and their MMEs during the Anisian (ca. 245 Ma). The granodiorites are calc‐alkaline and metaluminous with high SiO2 contents and Mg# values. They are characterized by enrichment of LREEs and LILEs, and depletion of Nb, Ta, and Ti. Their Sr–Nd –Hf isotopic compositions indicate that the magmas were derived from enriched mantle and Paleoproterozoic basement. The monzodioritic‐dioritic MMEs exhibit low SiO2 contents and high Al2O3 and MgO with high Mg# values, enrichment in LREEs and LILEs, and depletion in HFSEs with negative Nb, Ta, Ti, and P. In contrast, the gabbroic MMEs show higher Mg# values and Cr contents. Both types of MMEs have Sr–Nd–Hf isotopic compositions similar to the host granodiorites, and originated from mixing of lower crust‐ and enriched mantle‐derived magmas. The mantle‐derived sources were metasomatized by slab‐derived melts. The Middle Permian to Middle Triassic magmatism, metamorphism, sedimentation, and tectonics in the EKLO suggest northward subduction of the Paleo–Tethys Ocean as represented by the A'nyemaqen Ocean. The magma mixing that generated the Anisian granodiorites and MMEs was likely associated with slab break‐off of the Paleo–Tethys Ocean. The formation of the Anisian magmatic rocks in the EKLO suggest that the cessation of subduction of the Paleo–Tethys Ocean and onset of collision took place during the late stage of Middle Triassic.

Rift- and subduction-related crustal sequences in the Jinshajiang ophiolitic mélange, SW China: Insights into the eastern Paleo-Tethys

Abstract The Paleozoic Jinshajiang ophiolitic mélange in southwest China marks an important branch ocean (i.e., the Jinshajiang Ocean) of the Paleo-Tethys. Basic-intermediate rocks are widespread features in the mélange; their formation age is well known, but the petrogenesis has not been well studied, which means that the evolutionary history of the Jinshajiang Ocean is not well constrained. To understand the nature of the mélange and the ocean, we present a set of elemental and isotopic data from two typical crustal sequences in two areas of the Jinshajiang ophiolitic mélange, Zhiyong and Baimaxueshan. The basalts in the ca. 343 Ma Zhiyong crustal sequence show mid-ocean-ridge basalt–like geochemical compositions with Nb/La ratios of 0.98–1.15 and εNd(t) values of +6.5 to +7.7, indicating that the basalts formed in the spreading ridge of the ocean. In contrast, the 283 Ma Baimaxueshan crustal sequence consists of gabbros and basaltic-andesitic lavas, which have an arc affinity with Nb/La ratios of 0.54–0.67 and εNd(t) values of +5.1 to +6.5. The geochemical differences were not caused by crustal assimilation but reflect mantle metasomatism by fluids dehydrated from the subducting slab. Therefore, we propose that the Zhiyong and Baimaxueshan crustal sequences formed in seafloor spreading and subduction settings, which were related to the opening and closure of the ocean, respectively.

Subduction polarity of the Ailaoshan Ocean (eastern Paleotethys): Constraints from detrital zircon U-Pb and Hf-O isotopes for the Longtan Formation

Abstract The Paleotethys Ailaoshan Ocean separated the South China and Indochina blocks during the late Paleozoic. Uncertainty remains regarding subduction of this ocean—whether it was subducted eastward beneath the South China block or westward beneath the Indochina block. In this study, we present new detrital zircon U-Pb age, and Hf and O isotope data from the Longtan Formation, which was recognized to be deposited before the ocean closed. Our results show that the formation can be divided into three units: Unit 1 is distributed west of the suture and dominates the area; it contains major age peaks at 290–250 Ma and minor multiple old age peaks. Unit 2 consists of a minor distribution west of the suture, and it shows a dominant 250 Ma age peak; old zircons are very few or not present. Their Hf and O isotopic signatures are similar to those of unit 1. Unit 3 is distributed east of the suture and is characterized by a single distinct ca. 240 Ma age peak with almost no Precambrian zircons. We interpret that units 1 and 2 were likely deposited in a back-arc and forearc basin, respectively, and a volcanic arc developed on the eastern margin of the Indochina block, similar to the present-day northeastern Japan arc. Meanwhile, unit 3 was likely deposited in a forearc basin on the western margin of the South China block. Therefore, the Ailaoshan Ocean may undergone bipolar subduction both westward and eastward beneath the Indochina and South China blocks, respectively.

Late Carboniferous ophiolites from the southern Lancangjiang belt, SW China: Implication for the arc–back-arc system in the eastern Paleo-Tethys

Abstract The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. In this manuscript, we present a newly discovered ophiolite in the Yakou-Banpo area of the southern Lancangjiang belt in the Sanjiang area, SW China. The ophiolite displays a complete Penrose-type ophiolite sequence, composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity. Furthermore, positive Ɛ Nd (t) (+4.5 to +6.7) and zircon Ɛ Hf (t) values (+12.4 to +14.3), as well as mantle-like δ 18 O values (~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. SHRIMP zircon U Pb dating yield weighted mean ages of 305 ± 3 Ma, 310 ± 2 Ma, and 313 ± 6 Ma. Hence, the Yakou-Banpo ophiolite belt formed during the Late Carboniferous (313–305 Ma) and records the existence of a mature ocean basin. We propose that an arc – back-arc system developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

Late Permian back-arc extension of the eastern Paleo-Tethys Ocean: Evidence from the East Kunlun Orogen, Northern Tibetan Plateau

The East Kunlun Orogen is located in the northern part of the Tibetan Plateau and has received significant attention in tracing the evolution of Paleo-Tethys Ocean. In this study, we present U-Pb zircon geochronology, geochemistry and Sr-Nd isotopic data on the Kengdenongshe tholeiitic gabbro in the eastern part of the orogen. The gabbro is characterised by high and variable MgO and Fe2O3, as well as low K2O and TiO2 contents, typical of low-K tholeiitic series. The rock shows enrichment in large-ion lithophile elements, depletion in high field strength elements, with rare earth element signature similar to that of a back-arc basin basalt. The rock displays relatively low and uniform (87Sr/86Sr)i ratios and high εNd(t) values. These geochemical and isotopic features are interpreted to represent magma derivation from partial melting of an enriched asthenospheric source. Our model envisages that the initial melting started in the garnet-facies (~4%) and continued in the spinel-facies (~6%). The geochemical and isotopic systematics of the gabbro are consistent with magma generation in a back-arc basin setting triggered by the subduction of the Paleo-Tethys Ocean. In conjunction with the data on mafic magmatic suites in the region, we trace a progressive change in source from asthenosphere to increasingly enriched mantle wedge. Our study proposes a transition in the geodynamic setting of the east Paleo-Tethys Ocean from back-arc extension during the Late Permian to continued subduction during the Late Permian and Middle Triassic.