Changning Menglian Research Articles

The initial magma–fluid evolution of the Laochang porphyry molybdenum deposit in Lancang, Yunnan, SW China: Insights from the in-situ analysis of mica and titanite

The Laochang polymetallic deposit is the only large deposit in the Changning–Menglian Junction Zone. The deposit features by deep porphyry-skarn Mo–(Cu) and shallow vein Pb–Zn–(Ag) mineralization. Researches on the deposit have yielded extensive data, however, details about the initial evolution from magma to ore-forming fluid remain unclear. This study centers on the in-situ analysis of biotite from the porphyry, as well as phlogopite and titanite from the A vein, leading to the following insights: (1) The biotite geothermobarometer suggests the porphyry intruded in a moderate depth (2.57–4.27 km), with average parameters as P = 93 MPa, T = 697 °C, and logfO2 = -16.78. (2) The A vein in the deposit can be further subdivided into A1 K-feldspar vein and A2 quartz vein. The phlogopite in the A1 vein formed at P = 98 MPa, T = 681 °C, and logfO2 = -17.29. The pressure of the A1 vein is slightly higher than the static pressure of wall rock, attributed to pressure increment from the initial exsolved fluid. Conversely, the temperature and oxygen fugacity experience a slight decrease, due to buffering of the adjacent tuff. (3) The in-situ LA–ICP–MS U–Pb dating of the titanite in the A1 vein yields 44.4 ± 1.5 Ma (MSWD = 1.8, n = 48), comparable to the emplacement age of the porphyry (44.6–44.4 Ma). It is slightly older than the main mineralization age (43.78 Ma), further delineated the initial evolution sequence from magma to ore-forming fluid. Our research suggests that the in-situ analysis of titanite is crucial for preciously tracking the magma–fluid system, as titanite records both time and temperature information.

Garnet and zircon petrochronology reveal coherent subduction and exhumation of eclogites and associated schists in the Changning–Menglian orogenic belt, southeast Tibet

Determining whether high-grade metamorphic eclogites and surrounding metasediments underwent a shared subduction-exhumation cycle or distinct metamorphic paths before their juxtaposition at shallow levels is crucial for deciphering metamorphic processes in orogenic belts. Constructing pressure-temperature-time (P–T-t) paths for both rocks is the most straightforward approach. However, current time constraints overwhelmingly rely on dating accessory mineral like zircon, rutile, and monazite, which may only record a portion of the entire metamorphic history. Here we present an integrated petrochronological analysis using garnet Lu–Hf and Sm–Nd, and zircon U–Pb on two eclogites and their surrounding micaschists from the Changning–Menglian orogenic belt in southeast Tibet. Pseudosection modelling and conventional thermobarometers suggest a clockwise P–T path for the eclogites, from ∼1.4 GPa/∼520 °C to ∼2.5 GPa/∼600 °C, followed by decompression to <∼0.7 GPa, and peak conditions of 2.0–2.2 GPa/570–620 °C for the micaschists. Lu–Hf dating for eclogites yields ages of ca. 238–241 Ma, indistinguishable from the Sm–Nd dates of ca. 239 Ma. However, the Lu–Hf ages for micaschists (ca. 249–251 Ma) are older than the corresponding Sm–Nd dates (ca. 240 Ma). The well-preserved growth zoning in garnet indicates a short growth period in eclogites and at least ∼10 Myr in micaschists, bracketing the timing of protracted the two distinct zircon growth phases at around 238 Ma and 250 Ma. The similarity in peak metamorphic conditions and the consistent high-pressure metamorphic ages suggest a coherent subduction and exhumation process for both eclogites and micaschists. The Lu–Hf ages of the micaschists constrain the subduction of the Lancang Group dates back to ca. 250 Ma. The narrow temporal gap between the initiation of subduction and the age of the protoliths implies a rapid shift from the formation of the protoliths to the subduction of oceanic slabs and the scraped-off sediments to depths exceeding 80 km during the Triassic—a hypothesis that warrants further exploration.

Bidirectional subduction of the Proto-Tethys Ocean: constraints from geochronology and geochemistry of S-type granites from Baoshan Block in western Yunnan (SW China)

Understanding the tectonic evolution of the Proto-Tethys Ocean is important for exploring the initial assembly of the Gondwana supercontinent. The closure of the Proto-Tethys Ocean represents the end of convergence along the northern edge of Eastern Gondwana. However, the timing and processes of the closing of the ocean have different interpretations. This work focuses on the Early Paleozoic granitic rocks in the Baoshan Block, SW China to constrain the tectonic evolution of the Proto-Tethys Ocean. Zircons from the Pinghe and Zhen'an granitic plutons yield concordant U–Pb ages from 489.8 to 467.7 Ma. The bulk-rock geochemical features suggest that these samples are high-K calc-alkaline, S-type granites enriched in light rare earth elements and depleted in heavy rare earth elements. With obvious negative Eu anomalies and high K 2 O/Na 2 O ratios (1.01–2.57), they are enriched in large ion lithophile elements and depleted in high field strength elements. These rocks were therefore derived from partial crustal melting associated with subduction of the converging plate. Previous studies on the Changning–Menglian suture zone have suggested that the Early Paleozoic magmatic activity in the Baoshan Block was related to the westward subduction of the Proto-Tethys Ocean. Combined with the Early Paleozoic subduction-related magmatic activity to the east, it is therefore suggested that the Changning–Menglian Ocean experienced bidirectional subduction.

Consistent garnet Lu–Hf and Sm–Nd ages indicate short‐lived high‐pressure metamorphism and rapid subduction in oceanic subduction belt: An example from the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

AbstractThe integration of garnet‐based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet‐bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh‐pressure metamorphic belt in southeast Tibet, in order to characterize tectono‐metamorphic events and determine the duration of Paleo‐Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well‐preserved Rayleigh‐fractionation‐style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M‐shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high‐pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic‐type and continental‐type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.

Triassic record of Paleo-Tethyan subduction: Evidence from prograde metamorphic P–T paths of the Baqing eclogite, eastern Qiangtang, central Tibet

ABSTRACT The prograde metamorphic P–T conditions of eclogites correspond to the subduction processes of oceanic plates. The conditions may be recorded in mineral inclusions and the compositional zoning of garnets, and they play significant roles in revealing the dehydration depth, geothermal structure of the subduction plate and in understanding arc magmatism. The P–T conditions of the Baqing eclogite have been determined previously using traditional geothermobarometers and Ti-in-zircon geothermometers. To obtain more precise prograde metamorphic P–T conditions and details of the Paleo-Tethyan subduction process, we carried out phase equilibrium modeling for a newly collected Baqing eclogite sample. The sample is characterized by well-developed prograde zonings in metamorphic garnets with increasing Mg and decreasing Mn contents from cores to rims, a peak metamorphic mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and a retrograde metamorphic mineral assemblage of amphibole + plagioclase that replaced omphacite. We calculated the peak metamorphic conditions of the Baqing eclogite using the Garnet–Omphacite–Phengite–Quartz thermobarometers to be 26.5 ± 1 kbar and 730 ± 60°C, consistent with the results (23.5–27.5 kbar, 730–750°C) calculated using a P–T pseudosection. The prograde metamorphic P–T conditions were constrained to be ~21.4 kbar and ~580°C using pyrope and grossular isopleths. These results are consistent with previous calculations for the Baqing eclogite. Eclogites of the Central Qiangtang metamorphic belt, the Baqing metamorphic complex, and Changning–Menglian metamorphic belt show parallel P–T paths with different geothermal gradients, that relate to their different protoliths but similar heating and burial processes during Triassic subduction of the Shuanghu Paleo-Tethyan oceanic plate.

Cover Image

The cover image is based on the Review Article Gabbroic eclogites formed during rapid and cold subduction of the Paleo-Tethys oceanic lithosphere in the Changning–Menglian orogenic belt, southeastern Tibetan plateau by Huining Wang et al., https://doi.org/10.1111/jmg.12694.

Gabbroic eclogites formed during rapid and cold subduction of the Paleo‐Tethys oceanic lithosphere in the Changning–Menglian orogenic belt, southeastern Tibetan plateau

AbstractThe Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan plateau separates Gondwana‐ from Eurasia‐derived continental blocks and marks the main suture of the Paleo‐Tethys, as evidenced by a variety of oceanic basalt‐derived eclogites. However, it is uncertain whether the belt contains high‐pressure rocks derived from gabbro, which is a key component of oceanic ophiolite. Here, we present a study of newly discovered gabbroic eclogites from the CMOB. These eclogites preserve relic gabbroic crystals (diopside, bytownite/anorthite and ilmenite) that survived metamorphism and occur in the form of inclusions within porphyroblasts. The eclogites have positive εNd(t) values of +1 to +8 and have an affinity to N‐MORB, with positive Eu anomalies and no depletions in high‐field‐strength elements (e.g. Nb, Ta, Zr and Hf). Cumulate gabbros generated in a mid‐ocean ridge setting are possible protoliths for the studied samples. The eclogite facies mineralogy is defined by the assemblage of garnet + omphacite + kyanite + talc + phengite + rutile, which was followed by the post‐kinematic crystallization of winchite and clinozoisite, and a later symplectite assemblage (diopside + sodic plagioclase + calcic amphibole + clinozoisite). Phase equilibrium modelling, average P–T thermobarometry and conventional mineral geothermobarometry constrain the P–T conditions for the peak‐stage and initial post‐peak‐stage metamorphism and symplectite formation to 25.6–27.1 kbar/595–637°C, 15.3–17.9 kbar/563–605°C and 5.5–7.3 kbar/470–500°C, respectively, consistent with a subduction depth of 75–85 km. Metamorphic zircons yielded a Triassic mean U–Pb age of 223.7 ± 2.9 Ma, which is interpreted to record the early‐stage decompressive overprinting. The similar paragenetic sequences, mineral evolution, peak P–T conditions and P–T–t paths for the gabbro‐ and basalt‐derived eclogites in the CMOB indicate that these rocks formed in the Paleo‐Tethys subduction regime. The lack of deformation, and the cold and rapid subduction history, contributed to the local preservation of gabbroic minerals and igneous textures under high‐pressure conditions in the studied rocks. The gabbroic eclogites provide insights into the detailed metamorphic evolution during the burial–exhumation cycle of ophiolites in the Paleo‐Tethyan regime.

Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo‐Tethyan Ocean

AbstractThe Changning–Menglian Belt represents the main Paleo‐Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana‐ and Eurasia‐derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo‐Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low‐Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine‐spinel Fe‐Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle‐melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)‐and primitive‐mantle‐normalized multi‐element patterns with slightly positive Nb‐Ta anomalies, but negligible Eu and Zr‐Hf anomalies. The volcanics have similar Sr‐Nd‐Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)‐like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB‐related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed‐system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB‐type mantle source (i.e., asthenospheric mantle) containing garnet‐spinel lherzolite material. Combing this new data with that from oceanic seamount volcano‐sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward‐migrating microcontinent of Lanping–Simao.

Construction mode of a Middle Permian sponge reef in theChangning–MenglianBelt, western Yunnan, China

The Changning–Menglian Belt in western Yunnan, China has long been acknowledged as a major suture zone of the Palaeo‐Tethys in south‐east Asia. Although bearing a wealth of environmental information for this vanished ocean, the Permo‐Carboniferous limestones of seamount resemblance in this belt have only been sketchily studied heretofore in terms of their sedimentary features. This study presents a detailed investigation on a Middle PermianArchaeolithoporella‐microbial‐sponge reef in the Damingshan Formation of these limestones. This reef could be ascendingly subdivided into five parts according to its petrographic and sedimentolotgic properties. The first, third, and fifth parts are characterized by the mixture ofArchaeolithoporella‐sponge boundstone, sponge cementstone, microbial boundstone, and skeletal wackestone. They are interpreted to be constructed in a moderate‐energy environment through the following processes: sponges in clusters were partly encrusted byArchaeolithoporella, and mostly toppled over after death; whereafter, spaces within frameworks formed by sponges and/orArchaeolithoporellawere occluded by syndepositional cements; during the processes, sponge clusters were bound together by microbial domes, and both acted as bafflers that trapped sediments. In contrast, the construction of the second and fourth parts occurred in a high‐energy environment and is almost devoid of microbial contribution: sponges that were encrusted byArchaeolithoporellatoppled over posthumously, and then spaces among them were filled by syndepositional cements, ultimately formingArchaeolithoporella‐sponge boundstone. The construction mode of the studied reef, especially sponge toppling and microbial domal frameworks in‐between sponge clusters, differs from that of most coeval sponge reefs, in which the framework is characterized by erect sponges and microbial encrustations onArchaeolithoporella. Our work not only enriches the sedimentary data of limestones in the Changning–Menglian Belt, but also would hopefully deepen the understanding of the growth model of Permian sponge reefs.

Petrogenesis of Triassic Caojian A-type rhyolites and associated I-type granites in the southeastern Tibetan Plateau: rejuvenation of crystal mush

AbstractThe orogenic process and crustal growth of the Changning–Menglian Palaeo-Tethys orogenic belt in the southeastern Tibetan Plateau is not fully understood. Triassic Caojian rhyolites and granites occur extensively in this orogenic belt and represent important constraints for this issue. This study aims to examine the relationships between the Triassic Caojian rhyolites and granites and to gain a better understanding of their possible petrogenesis. The study used zircon U–Pb geochronology, trace element analyses and Sr–Nd–Hf isotope data to better understand the relationships and possible origin of the rhyolites and granites. Recent zircon U–Pb ages indicated that the Caojian rhyolites were emplaced at 227.2 Ma, whereas age estimates for Caojian granites were slightly older (233.4–236.9 Ma). The Caojian rhyolites are enriched in large-ion lithophile elements and high-field-strength elements, with elevated FeOtot/MgO and Ga/Al ratios. However, they are significantly depleted in Ba, Sr, Eu, P and Ti. These geochemical characteristics indicate that they have an A-type affinity. Furthermore, the Caojian granites comprise biotite monzogranites and granodiorites and show unfractionated composition. Mineralogically, the Caojian granites were found to contain diagnostic I-type minerals such as hornblende. Geochemical data suggest that the petrogenesis of the Triassic Caojian rhyolites is characterized by rejuvenation of crystal mush represented by the Triassic Caojian granites. The necessary thermal input was supplied by mafic magma. This magmatic evolution was likely related to lithospheric delamination and upwelling of the asthenosphere during the Mid- to Late Triassic, forming post-collisional I-type granites and A-type volcanics in the Changning–Menglian Palaeo-Tethys orogenic belt.

Southeastern continuation of the Bangong-Nujiang suture zone: Constraints from Middle Jurassic–Early Cretaceous sedimentary rocks in the western Baoshan block, SW China

The Bangong-Nujiang Ocean (BNO) separated the Lhasa and Qiangtang blocks in Tibet during the Mesozoic. Whether it separated the Tengchong and Baoshan blocks and how it evolved remain debated. The sedimentary rocks from the Middle Jurassic Mengjia and Liuwan formations, Late Jurassic Longhai Formation, and Early Cretaceous Longkan Formation of the western Baoshan block serve to investigate these issues. Geochemical signatures indicate that their sediments were deposited on a passive continental margin, with a collisional provenance identified in the Late Jurassic–Early Cretaceous formations. All the formations show similar detrital zircon U–Pb age groups with the exceptions of the Longhai Formation having minor 190–150 Ma zircons and the Longkan Formation containing abundant 190–150 and 140–120 Ma zircons. Provenance studies indicate that the sediments of the Middle Jurassic formations were sourced from the southern Qiangtang, Baoshan and Simao blocks and the Changning–Menglian suture zone. In contrast, the sediments of the Late Jurassic–Early Cretaceous formations were derived mainly from the Tengchong and Baoshan blocks, with an increasing contribution of the Tengchong-derived materials from Late Jurassic to Early Cretaceous. We infer that the absent, minor and significant contributions from Tengchong-derived sources in the provenance of the Baoshan strata were associated with the BNO that separated the Tengchong and Baoshan blocks during the Middle Jurassic, the initiation of the Tengchong-Baoshan collision in the Late Jurassic, and that the Tengchong and Baoshan blocks had been welded together in the Early Cretaceous, respectively. Therefore, the widely spreaded and subduction-related Early Cretaceous magmatic rocks in the Tengchong block were produced by the eastward subduction of the Neo-Tethyan Ocean.

Silurian intermediate–felsic complex in the Xiangtaohu area of central Qiangtang, northern Tibet: Evidence for southward subduction of the Longmuco–Shuanghu Prototethys oceanic plate

Data from ophiolites indicate that the Longmuco–Shuanghu Suture Zone (LSSZ) was connected with the Changning–Menglian Suture Zone (CMSZ) during the Paleozoic. Numerous geological records of late Cambrian–Silurian arc magmatism have been found along the LSSZ and CMSZ. However, all the reported early Paleozoic arc magmatism in the South Qiangtang subterrane (SQT) was based on crust-derived magmatic rocks, which constrained our acknowledge of the complicated geodynamic evolution processes. Here, we present bulk-rock geochemistry, SrNd isotopes, and zircon UPb dating and Hf isotope data for a newly discovered Silurian complex in the Xiangtaohu (XTH) area of the central Qiangtang Terrane, central Qinghai–Tibetan Plateau, to investigate the magmatic and tectonic evolution of the LSSZ and CMSZ. The XTH complex is composed of diorite, monzogranite, and biotite monzogranite. Zircon UPb dating results show that the XTH complex was emplaced at 438–429 Ma. The diorites belong to the calc-alkaline to high-K calc-alkaline series, showing metaluminous to weakly peraluminous features. All of the granites are high K, calc-alkaline, and weakly peraluminous. The diorites have low initial 87Sr/86Sr ratios (0.704526–0.705002) and εNd(t) values (−1.32 to 0.12), with zircon εHf(t) values ranging from −1.2 to 0.4, indicating a metasomatic mantle source with subducted-sediment-derived melts, and low MgO, Mg#, Cr, and Ni values, reflecting crystal fractionation from basaltic magma. The monzogranites and biotite monzogranites show I-type granite geochemical affinity, with slight negative εNd(t) values (−2.03 to −1.67) and low initial 86Sr/86Sr values (0.704056–0.705165), with zircon εHf(t) values ranging from −3.1 to 4.8, suggesting that they were derived mainly from partial melting of juvenile mafic lower crust. The samples are enriched large-ion lithophile elements, depleted high-field-strength elements, and have high U/Yb ratios of zircons, suggesting that the XTH complex was formed in a continental arc setting. The XTH Silurian complex is the youngest magmatic record of the southward subduction of the Longmuco–Shuanghu Prototethys oceanic plate, which represent the melting productions of the mantle wedge and mafic lower crust. It is different from the previously discovered S-type granites in the same area. Combining our new results with comprehensive existed data in the region, we propose that the Changning–Menglian Prototethys oceanic plate subducted beneath the Baoshan block (eastward) and the Yangtze block (westward) during early Paleozoic, while the Longmuco–Shuanghu Prototethys oceanic plate only subducted southward under the southern Qiangtang subterrane.

Sedimentology and detrital zircon geochronology of the Nanpihe Formation in the central zone of the Changning–Menglian Belt in western Yunnan, China: revealing an allochthon emplaced during the closure of Paleo-Tethys

Sedimentology and detrital zircon geochronology of the Nanpihe Formation in the central zone of the Changning–Menglian Belt in western Yunnan, China: revealing an allochthon emplaced during the closure of Paleo-Tethys

Identification of continental-type eclogites in the Paleo-Tethyan Changning–Menglian orogenic belt, southeastern Tibetan Plateau: Implications for the transition from oceanic to continental subduction

As a record of ancient subduction, eclogites preserve critical information for exploring geodynamic processes at convergent plate boundaries. The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is situated between the Gondwana-derived Baoshan–Tengchong Block and the Eurasia-derived Lanping–Simao Block. The belt has long been accepted as a typical cold subduction–accretion belt associated with the closure of the main Paleo-Tethys Ocean, owing to the development of oceanic-type lawsonite-bearing eclogites. However, this viewpoint is challenged by the presence of newly discovered continental-type eclogites hosted by metasedimentary rocks in the CMOB. The continental-type eclogites are depleted in high field-strength elements (e.g., Nb, Ta, and Zr), enriched in Th and U, and preserve highly negative ɛNd(t) values varying from −15.26 to −13.04, indicating derivation from continental mafic protoliths that underwent extensive crustal contamination. Inherited zircons from both the continental-type eclogites and their host schists have characteristic detrital age distributions, with dominant Pan-African age peaks at 630–505 Ma that match well with those in the Baoshan–Tengchong Block. Combined with published age data, our new geochemical and geochronological data provide convincing evidence for the presence of significant volumes of subducted continental fragments derived from the Baoshan–Tengchong Block. Thermodynamic modeling of the continental-type eclogites indicates a clockwise P–T path with a steep prograde path consistent with a geothermal gradient of 5–8 °C/km, which was followed by near-isothermal decompression. Calculations suggest the eclogite-facies assemblage of garnet + omphacite + glaucophane ± talc ± lawsonite ± phengite records peak conditions of 28.5–29.6 kbar and 580–594 °C, equivalent to depths of 85–90 km. U–Pb dating of metamorphic zircon rims from the eclogites gives an age of 234–233 Ma, which is interpreted to reflect the timing of continental subduction. The continental-type eclogites show mineral associations, paragenetic sequences and metamorphic P–T–t evolutions similar to those of low-temperature oceanic-type eclogites in the CMOB. As a result, we propose that rapid, cold subduction of continental crust, and its subsequent rapid exhumation, occurred shortly after the closure of the dense and long-lived Paleo-Tethys oceanic lithosphere. The discovery of continental-type eclogites imparts significant insights into the dynamics of Gondwana–Eurasia convergence in the Paleo-Tethyan domain. The CMOB provides a well-exposed example of the tectonic transformation from final oceanic subduction to the onset of continental subduction and collision in a cold geothermal regime.

Discovery of multi-crustal rejuvenations for the formation of the Lincang granitic batholith, Southwest China: magmatism relating to Changning–Menglian Paleo–Tethyan termination

ABSTRACT The consumption of the Changning–Menglian Palaeo-Tethys ocean in the Triassic formed the Lincang granitic batholith in the southwestern China, once the oceanic supra-subduction zone. The batholith provides us a critical window to understand the nature of the crustal basements and the process of Palaeo-Tethyan termination. Here we report zircon U-Pb ages and Hf isotopes, and whole-rock elements and Sr-Nd isotopes of granitic samples from the central part of the Lincang batholith. The geochemical results reveal two genetic types of granites indicated by low-K and high-K affinities, respectively. The Group 1 low-K granitoids dated at 237–229 Ma are low-K calc-alkaline and metaluminous to peraluminous (A/CNK = 0.75–1.29), with K2O contents of 0.13–1.33 wt%, K2O/Na2O ratios of 0.03–0.30, Rb/Sr ratios < 0.18, and initial 87Sr/86Sr = 0.71693 and εNd(t) = −10.8. In contrast, the Group 2 high-K granitoids dated at 239–200 Ma are calc-alkaline and peraluminous (A/CNK = 1.02–1.20), with higher K2O contents (3.15–4.7 wt%), K2O/Na2O ratios (1.15–2.00) and Rb/Sr ratios (0.77–2.62), and more enriched isotopic compositions (initial 87Sr/86Sr = 0.73129 to 0.74037, εNd(t) = −13.1 to −11.0) as well as highly negative zircon εHf(t) values of −16.8 to −6.7. The formation time of these granitoids is compatible with that of the post-collision of the Palaeo-Tethys, subsequently to the magmatic gap (250–240 Ma) coupling with the syn-collision stage. The magmatic source discriminations suggest that the low-K granitoids were derived from partial melting of the dehydrated low-K amphibolite source and the high-K granitoids were dominantly from the psammitic source with minor mantle-derived magma involvement. Using the geochemical variations of the granitic rocks related to the Palaeo-Tethyan closure, we reveal melting of multiple crustal sources rather than a sole source to construct the Lincang batholith.

Petrogenesis and tectonic significance of mafic–ultramafic rocks from Southwest Yunnan, China

This study focuses on the mafic–ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (T p ) of these picrites is higher than the T p range of mid-ocean ridges and reaches that of mantle plumes. According to the geochemical characteristics, the Type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by ambient depleted mantle, whereas the Type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic–ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.

A New HP–UHP Eclogite Belt Identified in the Southeastern Tibetan Plateau: Tracing the Extension of the Main Palaeo-Tethys Suture Zone

Abstract The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

Rapid cold slab subduction of the Paleo-Tethys: Insights from lawsonite-bearing blueschist in the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock εNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (<10 Ma) between the protolith generation and the high-pressure metamorphic peak further supports a rapid subduction process. Our results suggest that the young oceanic slab was dragged down by the thicker, long-lived, and cold downgoing Paleo-Tethyan Ocean plate, where it experienced blueschist-facies metamorphism. The co-occurrence of lawsonite-bearing epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists and eclogites further points to a cold thermal structure at the convergent plate interface, leading to the interpretation that the CMOB represents a typical oceanic subduction-accretion belt. The results presented in this study provide important insights into the geodynamic evolution of the Paleo-Tethys.

Subduction erosion associated with Paleo-Tethys closure: Deep subduction of sediments and high pressure metamorphism in the SE Tibetan Plateau

Subduction erosion along active convergent plate margins involves arc subduction, tectonic erosion and sediment subduction. Here we investigate the metasedimentary rocks enclosing blueschists and eclogites in the Changning–Menglian orogenic belt (CMOB) of the southeastern Tibetan Plateau which marks the main suture associated with the closure of the Paleo-Tethyan Ocean between the Baoshan–Tengchong and Lanping–Simao blocks. We present an integrated study of petrology, mineral chemistry, whole-rock geochemistry, zircon U–Pb geochronology, and phengite 40Ar/39Ar isotopes. These rocks consist mainly of Ph-Qz schist, Ab-Qz-Ph schist, Grt-Ph schist, St-Ky-bearing Grt-Ms schist, and Cld-Pg-Ph schist. They display compositions similar to arkose and subarkose with trace element patterns overlapping those of the upper continental crust, and suggest immature and weak to moderate chemical weathering. The detrital zircon age spectra are dominated by two major age groups of 600–450 Ma with a peak at 550 Ma, and 1150–850 Ma with a peak at 950 Ma, suggesting that the metasedimentary rocks were sourced from a mixture of Precambrian sediments in the Lanping–Simao Block and Pan-African Orogeny-related magmatic rocks with minor Early Paleozoic Proto-Tethyan subduction- and volcanic rocks related to the onset of oceanic spreading of Paleo-Tethys. Combined with the maximum depositional age of ~428 Ma based on the youngest detrital age peaks, we infer that the Lancang Group could be the Early Paleozoic sedimentary sequence of the Lanping–Simao Block and was deposited in a foreland setting. The metasedimentary rocks display high-pressure (HP) mineral assemblages involving Ph + Ab + Qz, Grt + Ph + Qz, Grt + Ky + Ph, and Cld + Pg + Ph, and record blueschist facies to eclogite facies peak P–T conditions of 18.8–21.6 kbar at 392–553 °C (Grt-Ph schists) and 20–24.2 kbar at 592–654 °C (St-Ky-bearing Grt-Ms schists). Metamorphic zircon U–Pb and phengite 40Ar/39Ar isotopic analyses from these rocks yield consistent peak HP ages of 238–235 and 237–231 Ma, respectively, whereas 40Ar/39Ar dating for the Grt-Ph schist with low Si contents in phengite yields the post-peak age at 226.2 ± 1.3 Ma. We propose that a significant volume of sediments in Lancang Group at the western margin of the Lanping–Simao Block were off scraped by the downgoing oceanic lithosphere associated with the Paleo Tethyan ocean closure, and subducted to depths of ~60–80 km during Middle Triassic. These were subsequently rapidly exhumated at a rate of ~5.7–6.5 km/myr in the Late Triassic. The subduction erosion model proposed in our study has important implications for reconstructing the evolutionary history of the Paleo-Tethys as well as the architecture variation of the paleosubduction zone.

Petrogenesis and tectonic significance of mafic-ultramafic rocks from southwestern Yunnan, China

This study focuses on the mafic-ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (Tp) of these picrites is higher than the Tp range of mid-ocean ridges and reaches that of mantle plumes. According to their geochemical characteristics, type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by the ambient depleted mantle, whereas type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic-ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.