Tengchong Terrane Research Articles

Tectonic and climate forcing of exhumation in the SE Tibetan Plateau over the past 7 Ma: Insights from the deltaic-submarine fan system in the Andaman Sea, northeastern Indian Ocean

The Tibetan Plateau and its adjacent areas are a typical example for studying the relationship between plateau uplift, climate change, and surface erosion. It has been argued that the South Asian monsoon (SAM) strengthened significantly at 10–8 Ma, whereas others suggested that the SAM weakened at that time. Here we report a large deltaic-submarine fan system in the Andaman Sea with a high sediment accumulation rate during the Pliocene. We propose that the formation of the deltaic-submarine fan was a result of climate and tectonics over the past 7 Myr. Our findings are based on multi-method provenance analyses (heavy minerals, bulk-rock geochemistry, detrital zircon UPb ages, and seismic data) of the Late Miocene-Pleistocene deltaic-submarine fan system. The analyses, in combination with those of the modern river sediments and potential source regions, suggest that the Upper Miocene sediments were mainly derived from the eastern Lhasa Terrane and northern West Burma Terrane (Upper Salween and Upper Irrawaddy, respectively), whereas the Pliocene-Pleistocene deposits were primarily derived from the eastern Lhasa Terrane, northern West Burma Terrane, and Tengchong Terrane (Lower Salween River), with small inputs from the Sibumasu and Baoshan terranes. We propose a modified scenario in which the Salween initiated in the early Late Miocene (10 Ma) and the Irrawaddy developed a modern geometrical morphology after the Pliocene. These two large rivers transported eroded materials from the SE Tibetan Plateau into the Andaman Sea and formed the deltaic-submarine fan. The high sediment accumulation rate (>530 m/Myr) in the Pliocene is consistent with the strengthened SAM and heterogeneous crustal deformation of the SE Tibetan Plateau during the same interval. We conclude that both the climate and tectonics have direct effects on exhumation of the SE Tibetan Plateau over the past Late Miocene-Quaternary.

Petrogenesis of the Early Jurassic Longtang and Menglong Peraluminous Granites in Tengchong Terrane, and their Tectonic Implication

AbstractA comprehensive study of zircon U‐Pb geochronology, in situ Hf isotopes, whole‐rock major and trace element geochemistry, and Nd isotopes was carried out for two Early Jurassic two‐mica granites (Longtang and Menglong) in the southern part of the Tengchong terrane, which is in the northern part of the larger Sibumasu terrane. We assess the origin of the granites and explore their possible genetic relationship to the Paleo‐Tethyan regime. LA‐ICP‐MS zircon U‐Pb dating shows that they were simultaneously emplaced in the Early Jurassic (ca. 199 Ma). They have SiO2 contents of 69.7–75.1 wt% and are mainly strongly peraluminous with alumina saturation index (ASI) values ranging from 1.06 to 1.46. They show similar Mg# (0.29–0.42) to experimental partial melts of metasedimentary rocks under continental pressure‐temperature (P‐T) conditions. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with moderately negative Eu anomalies and flat HREEs patterns. They show negative εNd(t) values (–9.0 to –12.4) and εHf(t) values (–8.0 to –9.1). Elemental and isotopic data suggest that they most likely to formed by muscovite‐dehydration melting of a metapelitic source at lower temperatures in the range of 700°C to 750°C. The granites might represent a post‐collisional tectonic setting response to Paleo‐Tethyan regime.

Evolution of the Tethyan Bangong-Nujiang Ocean and its SE Asian connection: Perspective from the Early Cretaceous high-Mg granitoids in SW China

High-Mg diorites (HMDs) have important implications on crust-mantle interactions and subduction processes, and are widely used to delineate ancient convergent plate margins. In this study, we described a newly-identified high-Mg diorite-granodiorite suite in the Tengchong terrane (SE Tibetan Plateau), and presented its petrography, whole-rock geochemistry, SrNd isotopes, and zircon U-Pb-Hf-O isotopic compositions. High-precision SIMS zircon UPb dating yielded consistent Early Cretaceous ages (~120 Ma) for all lithologies in this high-Mg suite. The high-Mg diorite is characterized by high MgO (4.7–9.2 wt%), Cr (169–323 ppm) and Ni (32–138 ppm) contents, together with high (87Sr/86Sr)i (0.70867–0.71060) and negative εNd(t) (−4.07 to −6.80) values. Their zircon εHf(t) vary from −5.64 to +4.12, and their δ18O (6.0 to 6.9‰) are slightly higher than the average mantle value. The granodiorite has higher SiO2 and lower MgO contents, and varying zircon εHf(t) (−12.19 to −2.71) and δ18O (6.3 to 8.1‰). The rocks are characterized by enrichments in large ion lithophile elements (LILEs: e.g., Rb, Th, U, K) and light rare earth elements (LREEs), but depleted in high field strength elements (HFSEs), and have also high (87Sr/86Sr)i (0.71057–0.711860) and negative εNd(t) (−8.42 to −9.14). Mafic microgranular enclaves (MMEs) in the granodiorite have intermediate SiO2, MgO, and trace elements and Sr-Nd-Hf-O isotope compositions between the high-Mg diorite and the granodiorite. Based on these geochemical and isotopic features, we interpreted that the high-Mg diorite was generated from the interaction of subducted sediment-derived melt with the overlying mantle wedge, whilst the granodiorite was sourced from the ancient continental crust with minor mantle-derived input. Mixing between the mantle-derived high-Mg dioritic melt and the crustal-derived granodioritic melt may have formed the MMEs. Our new geochemical (incl. isotopic) data, combined with published ones across the region, suggest that these Early Cretaceous plutons were formed by asthenospheric upwelling, which was probably led by slab rollback of the west-dipping subduction of the Luxi-Ruili Ocean between the Baoshan and Tengchong terranes. This implies that the southeastern continuation of the Tethyan Bangong-Nujiang Ocean was located between the Baoshan and Tengchong terranes, instead of between the Tengchong and West Burma terranes as previously suggested.

Early-Cretaceous highly fractionated granites from the Tengchong terrane: Petrogenesis and tectonic implication

Early-Cretaceous highly fractionated granites from the Tengchong terrane: Petrogenesis and tectonic implication

Cenozoic retrogression and exhumation of the amphibolites in the eastern Gangdese Belt, SW China

Abstract The collision between India and Asia is marked by the Yarlung Zangbo suture zone, which can be traced from Tibet into northwest Yunnan. The suture zone provides a record of the subduction of Neotethyan oceanic lithosphere beneath the Lhasa Block, creating the Gangdese magmatic belt, prior to the India–Asia collision. This paper reports petrological, zircon U–Pb, and whole-rock elemental and isotope data for amphibolite from the extension of the Gangdese belt in northwestern Yunnan. Two metamorphic stages are identified in relict amphibolite-facies rocks preserved in granitic gneisses from the belt. They are early amphibolite-facies metamorphism (M1) and late amphibolite-facies retrograde metamorphism (M2). The assemblage of M1 comprises garnet, amphibole, and plagioclase that formed at 0.89–0.97 GPa and 658–700 °C. Dating of syn-metamorphic zircons assigns this event to the middle Eocene (44–38 Ma). Symplectites (corona) of amphibole, plagioclase, and quartz surrounding garnet formed during retrograde stage M2, at 30–29 Ma, under P–T conditions of 0.41–0.68 GPa and 549–716 °C. The amphibolites display geochemical characteristics similar to those of mid-ocean ridge basalt, with eNd(t) values of +1.8 to +8.9, and are inferred to have formed in a back-arc-basin setting, suggesting that they constitute part of the lithospheric mantle of the Tengchong terrane that lies to the east. The amphibolite-facies metamorphism was related to underplating of the Indian plate during the India–Asia collision, which led to clockwise rotation of eastern Tibet during the middle Eocene, generating amphibolite recrystallization and, subsequently, with ongoing convergence of the two plates, exhumation of the region during the middle Oligocene.

Combined Zircon, Molybdenite, and Cassiterite Geochronology and Cassiterite Geochemistry of the Kuntabin Tin-Tungsten Deposit in Myanmar

AbstractThe Kuntabin Sn-W deposit, located in southern Myanmar, is characterized by abundant greisen-type and quartz vein-type cassiterite and wolframite mineralization. We have conducted multiple geochronological methods and isotope and trace element analyses to reveal the age and evolution of the Kuntabin magmatichydrothermal system.Zircon U-Pb dating of the two-mica granite yielded a weighted mean 206Pb/238U age of 90.1 ± 0.7 Ma. Cassiterite U-Pb dating provided a lower intercept age of 88.1 ± 1.9 Ma in the Tera-Wasserburg U-Pb concordia diagram. Molybdenite Re-Os dating returned a weighted mean model age of 87.7 ± 0.5 Ma and an isochron age of 88.7 ± 2.7 Ma. These ages indicate a genetic relationship between granite and Sn-W mineralization in the Kuntabin deposit and record the earliest magmatism and Sn-W mineralization in the Sibumasu and Tengchong terranes related to subduction of the Neo-Tethys oceanic slab. Three generations of cassiterite have been identified with distinctive cathodoluminescence textures and trace element patterns, indicating the episodic input of ore-forming fluids and distinctive changes in the physical-chemical conditions of the Kuntabin magmatichydrothermal system. Sudden changes of fluid pressure, temperature, pH, etc., may have facilitated the deposition of Sn and W. Rhenium contents of molybdenite from the Kuntabin deposit and many other Sn-W deposits in Myanmar are characteristically low compared to porphyry Cu-Mo-(Au) deposits worldwide. In combination with zircon Hf isotope signatures, we infer that granites associated with Sn-W deposits in Myanmar were predominantly derived by melting of ancient continental crust and contain minimal mantle contribution.Subduction of the Neo-Tethys oceanic slab from west of the West Burma terrane reached beneath the Sibumasu terrane and led to magmatism and Sn-W mineralization at ~90 Ma when the Kuntabin deposit was formed. The Paleoproterozoic Sibumasu crust was activated during the subduction-related magmatism to form predominantly crust derived melts. After a high degree of fractional crystallization and fluid exsolution, physical-chemical changes of the hydrothermal fluid resulted in Sn and W precipitation to form the Kuntabin Sn-W deposit.

Compositional changes of granitoids from the Menglian Batholith in SW China at ca. 122 Ma: Implications for the origin of decoupled Nd-Hf isotopic compositions and crust generation in collision zones

Determining the origin and related mantle dynamics responsible for the generation of giant granitic batholiths in convergent margins is critical to unravel the formation mechanism of the silica–rich continental crust. This paper reports the whole–rock major element, trace element, Sr-Nd isotopic, and zircon U-Pb age and Hf isotope data of samples from the Menglian Batholith in the eastern Tengchong Terrane of southwest Yunnan. These samples consist of two groups, including the Group I rocks that are mostly granitic with zircon U-Pb ages of 132–122 Ma and the Group II rocks that display varying compositions from gabbroic to granitic with zircon U-Pb ages of 122–112 Ma. The Group I samples have zircon εHf(t) of −11.7 to −4.2 and whole–rock εNd(t) of −10.1 to −8.9, whereas the Group II samples show elevated zircon εHf(t) of −7.8 to +7.1 and whole–rock εNd(t) of −9.0 to −6.9 and εHf(t) of −3.1 to −1.0. The Nd-Hf isotopic compositions of the Group I rocks are coupled, likely resulting from the partial melting of an ancient lower crust. However, the Nd-Hf isotopic compositions of the Group II rocks are decoupled, likely inheriting from the mantle–derived material with decoupled Nd-Hf isotopic compositions. Regional geological correlation and compositional transition documented by the Group I and Group II samples collectively indicate that the slab rollback and subsequent breakoff of the subducting Bangong–Nujiang ocean lithosphere during the Tengchong–Baoshan collision can be invoked to explain the generation of the Menglian Batholith. The similarity of average compositions between the Group II samples and the bulk continental crust suggests that the addition of mantle–derived material to the ancient crust–derived melts seems to be an efficient process for crustal growth during continental collisions.

Petrogenesis and tectonic setting of the Early Cretaceous granitoids in the eastern Tengchong terrane, SW China: Constraint on the evolution of Meso-Tethys

Abstract As a result of the evolution of Meso-Tethys, Early Cretaceous granitoids are widespread in the eastern Tengchong terrane, SW China, which is considered as the southern extension of the Tibetan Plateau. These igneous rocks are therefore very important for understanding the tectonic setting of Meso-Tethys and the formation of the Tibetan Plateau. In this paper, we present new zircon U-Pb dating, whole-rock elemental, and Nd isotopic data of granitoids obtained from the eastern Tengchong terrane. Our results show that these granitoids are composed of monzogranites and granodiorites and formed at ca. 124 Ma in the Early Cretaceous. Mineralogically and geochemically, these granitoids display metaluminous nature and affinity to I-type granites, which are derived from preexisting intracrustal igneous source rocks. The predominantly negative whole-rock εNd(t) values (−10.86 to −8.64) for all samples indicate that they are derived mainly from the partial melting of the Mesoproterozoic metabasic rocks in the lower crust. Integrating previous studies with the data presented in this contribution, we propose that the Early Cretaceous granitic rocks (135–110 Ma) also belong to I-type granites with minor high fractionation. Furthermore, in discriminant diagrams for source, granitoids are mainly derived from the partial melting of metaigneous rocks with minor sediments in the lower crust. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar and mafic enclaves indicate that these Cretaceous igneous rocks were the products of the tectonic evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab rollback. Moreover, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane.

Late Cretaceous–Eocene magmatism induced by slab rollback and breakoff in the Tengchong terrane, SW China

ABSTRACT Late Cretaceous–Eocene granites are widespread in the Tengchong terrane and related to subduction and closure of the Neo–Tethyan Ocean. However, the reason of their compositional and spatial variations is still under explored. This study reports zircon U–Pb geochronological, whole–rock and Nd isotopic data for Guyong, Houqiao and Menglong granites in the Tengchong terrane. Our data show that the three plutons crystallized at 70.4 ± 0.3Ma, 64.4 ± 0.3Ma and 47.6 ± 0.2Ma. Guyong monzogranites show an I–type affinity and may have experienced fractional crystallization of plagioclase, biotite and sphene. Negative εNd(t) values (−9.19 to −11.01) with two-stage model ages (T2DM = 1.59–1.78 Ga) indicate that Guyong monzogranites are derived from a predominantly Paleoproterozoic crustal source with little involvement of mantle–derived materials in an extensional setting. Lower εNd(t) (−9.33 to −11.63) and negative correlations of HREE, LREE, Th, and Y vs. SiO2 indicate that Houqiao plutons are S–type granites derived from a pure crustal source during crustal thickening. Low Zr/Hf and Nb/Ta ratios, zircon saturation temperature (TZr = 750°C–786°C) and negative δEu (0.11–0.40) of Menglong biotite granites indicate that they have experienced fractional crystallization of plagioclase, biotite and sphene. Relatively high εNd(t) (−6.81 to −7.07) values indicate that they are products of felsic magma mixed with mafic magma, which is also supported by numerous coeval mafic–intermediate rocks (Eocene) such as metagabbros, diorites and MMEs (mafic microgranular enclaves) in the western Tengchong terrane. A process of slab rollback and slab breakoff may have triggered the late Cretaceous–Eocene magmatism and the compositional variation of associated granitoids in the Tengchong terrane.

Geochronology, geochemistry and Sr–Nd–Hf isotopic compositions of Late Cretaceous–Eocene granites in southern Myanmar: Petrogenetic, tectonic and metallogenic implications

The geochronology, petrogenesis and tectonic setting of granites associated with tin–tungsten mineralization in southern Myanmar remain unclear. This work presents whole-rock geochemical, Sr–Nd isotopic, and zircon U–Pb and Hf isotopic data for felsic intrusions in the Sibusima terrane. Zircon U–Pb dating indicates formation in the Late Cretaceous–Early Eocene (~84–48 Ma) and suggests that these intrusions likely represent a southward extension of the coeval magmatic belt in the Tengchong terrane. Primarily classified as high-K calc-alkaline, the granites show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, P, and Ti, and negative Ba and Sr anomalies on primitive mantle-normalized diagrams. P2O5, CaO, Al2O3, MgO, TiO2 contents, and Nb/Ta ratios decrease with increasing SiO2 contents, possibly consistent with evolution trends in fractionated I-type granitic magmas. In addition, these intrusions display a wide range of negative εNd(t) (−14.6–−5.5) and zircon εHf(t) values (–22.7–5.7), suggesting a dominant Sibusimacrustal source with a minor mantle contribution. Notably, the Eocene and a few Paleocene granites show more contribution of mantle material based on higher εNd(t) (−8.5–−5.5) and zircon εHf(t) (−11.0–−3.2) values. Large variations in zircon Hf isotopic compositions within an individual Eocene granitic intrusion (e.g., −15.2–5.7) and corresponding biotite-rich enclave (−17.8–0.2) likely indicate magma mixing. Considering the spatial-temporal distribution of magmatism in the West Burma and Sibusima terranes, the studied Late Cretaceous–Eocene felsic magmas likely formed in a continental arc setting during normal subduction (~100–60 Ma) and subsequent roll-back of the Neo-Tethyan oceanic lithosphere (~60–50 Ma). Additionally, most of granites have the elevated Sn with increasing Rb/Sr ratios, and decreasing TiO2 contents and Nb/Ta ratios, suggesting Sn enrichment is primarily controlled by magmatic fractionation. Meanwhile, Sn and Li depletions in some granites deviate from the magmatic evolution trend, likely as a result of fluid exsolution. The exsolved Sn-rich fluids could have made a genetic contribution to hydrothermal Sn mineralization in southern Myanmar.

Origin and tectonic implications of Early Cretaceous high- and low-Mg series rocks and mafic enclaves in the Bomi–Chayu Fold Belt, SE Tibet

Early Cretaceous granitoids are widespread on the Tibetan Plateau and record the tectonic evolution of several Tethyan oceans. We describe Early Cretaceous high-Mg (Mg# > 50) and low-Mg (Mg# < 50) granitoids, minor diorites, and mafic enclaves from the Bomi–Chayu Fold Belt, SE Tibet. Zircon U-Pb dating reveals two episodes of granitoid emplacement at ~122 and ~117 Ma. High-Mg series rocks have higher Mg# and compatible element concentrations (e.g., Cr and Ni) than low-Mg series samples. Their Mg# and compatible element abundances are also elevated relative to an experimental melt generated under the conditions of a thickened lower crustal. Enriched whole-rock Nd and zircon Hf isotopic signatures, coupled with low Y (<18 ppm) and Yb (<1.9 ppm) concentrations in some samples, indicate that the high-Mg series was derived from thickened lower crust and underwent minor mixing with cumulate pyroxenite in the lower crust. In contrast, low-Mg series rocks record a purely crustal origin, although differences are noted between Na-rich (subgroup 1) and K-rich (subgroup 2) compositions. Mafic enclaves hosted in the granitoids yield identical formation ages and isotopic compositions to their host rocks, suggesting a common origin. The spatial–temporal distribution of Early Cretaceous granitoids in the Bomi–Chayu Fold Belt indicates that magmatism occurred in an analogous tectonic regime to that of similar rocks in the Tengchong Terrane. The Bomi–Chayu granitoids are therefore distinct from those in the northern and central Lhasa subterranes and are related to the evolution of the Myitkyina Tethys Ocean in eastern Myanmar.

Origin of the Mo-bearing Xiaoshuijing Syenogranite in the Tengchong Terrane, SW China

The Xiaoshuijing Syenogranite is the host of Mo mineralisation recently discovered in the Tengchong Terrane of southwestern China. Here, we present new zircon U-Pb ages and molybdenite Re-Os ages, whole-rock geochemical data, and zircon Hf isotopic data for the Xiaoshuijing Syenogranite hosting disseminated Mo mineralisation. The syenogranite’s mineralogical and geochemical characteristics indicate it is a high-K, alkaline, and peraluminous granite. LA-ICP-MS analysis of zircon yields an age of 65 ± 1 Ma (n = 14, MSWD = 2.1), and yields negative zircon εHf(t) values ranging from −18.23 to −12.92, interpreted to have a crustal source. The Re-Os Mo age of 64 ± 2 Ma (n = 4, MSWD = 2.2) for the mineralisation is the same as the syenogranite’s age, within error. The geochemistry also indicates that the syenogranite is related to a thickened crust resulting from the eastward subduction of the Neo-Tethyan oceanic lithosphere shortly before the collision of the Indian and Eurasian continental crusts.

Mesozoic-Cenozoic tectonic evolution and metallogeny in Myanmar: Evidence from zircon/cassiterite U–Pb and molybdenite Re–Os geochronology

It is well known that there are many Sn–W–Mo and Cu–Au metal resources in Myanmar. However, the absence of precise mineralization ages for these deposits significantly hinders to understand the close genetic relationship among metallogeny, ore-bearing intrusions, and tectonic evolution. In this study, two groups of consistent molybdenite Re–Os and zircon U–Pb ages indicate that the Dapingba Mo–W deposit in northern Myanmar (Tengchong terrane) formed at Early Cretaceous (∼119 Ma and ∼114 Ma). The Dapingba ore-bearing granitic magmas were mainly derived from melting of ancient Tengchong crust on the basis of zircon Hf isotopic compositions (εHf(t) = −9.3 to 2.5). Moreover, the first reported cassiterite U–Pb ages of ∼60 Ma for the Bawapin and Kalonta Sn–W deposits in southern Myanmar (western Sibumasu terrane) suggest that the two deposits formed at Paleocene. Whereas the precise molybdenite Re–Os age for the Shangalon porphyry Cu–Au deposit in the West Burma terrane indicates that this deposit formed at Eocene (∼39 Ma). Importantly, combined with previous studies on tectonic evolution and magmatic petrogenesis in Myanmar and southwest China, a comprehensive Mesozoic-Cenozoic tectono-magmatic and metallogenic model are proposed. Two periods of Early Cretaceous (∼120–114 Ma) and Late Cretaceous (∼75–70 Ma) crust-derived felsic melts and related Mo–Sn–W deposits from the Tengchong and western Sibumasu terranes likely formed in a continental arc during subduction of the Meso- and Neo-Tethys oceanic lithosphere, respectively. Subsequently, Paleocene-Eocene (∼60–50 Ma) Sn–W deposits and coeval ore-bearing granites in the western Sibumasu terrane possibly formed during roll-back of the Neo-Tethys oceanic slab. After ∼50 Ma India–Asia collision, Eocene (∼41–39 Ma) porphyry Cu–Au (West Burma terrane) and Sn–W deposits (western Sibumasu terrane) likely formed during the Neo-Tethys oceanic slab tear and break-off in a collisional setting. Whereas Miocene (∼19–14 Ma) high sulfidation epithermal Cu deposit in the West Burma terrane possibly formed in an arc setting during oblique subduction of the Indian oceanic lithosphere.

Petrogenesis of the Early Cretaceous granitoids and its mafic enclaves in the Northern Tengchong Terrane, southern margin of the Tibetan Plateau and its tectonic implications

Cretaceous granites are widely exposed in the Tengchong Terrane that is the southern extension of the Tibetan Plateau, resulting from the evolution of the Tethys. They, therefore, play a critical role in deciphering the Tethys evolution and the growth of the Tibetan Plateau. In this paper, we present new zircon UPb dating and Hf isotopic results, along with whole-rock elemental and SrNd isotopic data of the granitoids from the Lushui-Pianma (LP) batholith and its mafic enclaves and the strongly deformed granites from the Gaoligong shear zone in the northern Tengchong Terrane. The dating results show that both the LP granitic batholith and the strongly deformed granites in the Gaoligong shear zone formed in the Early Cretaceous of 122–110 Ma. They are composed of granodiorite, quartz monzodiorite and monzogranite with minor gabbroic enclaves. Mineralogically and geochemically, the granitoids in the LP batholith, particularly for the less evolved ones, display a metaluminous nature and an affinity to I-type granite. All the granitoids are characterized by similar rare and trace patterns and whole-rock Nd and zircon Hf isotopic compositions, indicating a common origin. The sodium-rich feature for the less evolved dioritic rocks and the predominantly negative zircon εHf(t) and whole-rock εNd(t) values for all the granitoids demonstrate that they were derived mainly from the partial melting of the Mesoproterozoic metabasic rocks (likely the basement rocks of the Gaoligong Group) in the lower crust of the Tengchong Terrane. In the case of the mafic enclaves, they share similar mineralogical assemblages and NdHf isotopic compositions to its host granodiorite, indicating a cognate origin with the latter.In combination with previous data for the granitoids in the Tengchong Terrane, our study further illustrates that the Early Cretaceous granitic rocks are dominated by metaluminous I-type granites with minor highly fractionated peraluminous I-type granites, rather than the dominant S-type granites as assumed before. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar, together with regional Early Cretaceous magmatic and sedimentation patterns, indicate that these Early Cretaceous magmatic rocks were the products of the evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab breakoff.

Geochemistry, zircon U–Pb geochronology and Hf isotopes of Jurassic-Cretaceous granites in the Tengchong terrane, SW China: implications for the Mesozoic tectono-magmatic evolution of the Eastern Tethyan Tectonic Domain

ABSTRACTRecently identified Early Jurassic, Early Cretaceous, and Late Cretaceous granites of the Tengchong terrane, SW China, help to refine our understanding of the Mesozoic tectonic-magmatic evolutionary history of the region. We present new zircon U–Pb geochronological, Lu–Hf isotopic and geochemical data on these rocks. The zircon LA-ICP-MS U–Pb ages of the Mangzhangxiang, Laochangpo, and Guyong granites, and Guyong granodioritic microgranular enclaves are 185.6, 120.7, 72.9, and 72.7 Ma, respectively. Geochemical and Hf isotopic characteristics suggest the Mangzhangxiang and Laochangpo S-type granites were derived from partial melting of felsic crust and that the Guyong I-type granite and associated MMEs were generated through magma mixing/mingling. Mesozoic magmatism in the Tengchong terrane can be divided into three episodes: (1) the Triassic syn- and post-collisional magmatic event was related to the closure of the Palaeo-Tethyan Ocean, as represented by the Changning-Menglian suture zone; (2) the Jurassic to Early Cretaceous magmatism was related to the subduction of the Meso-Tethyan oceanic crust, as represented by the Myitkyina ophiolite belt; and (3) the Late Cretaceous magmatism was related to the subduction of the Neo-Tethyan oceanic crust, as represented by the Kalaymyo ophiolite belt.

Paleomagnetism of Eocene red-beds in the eastern part of the Qiangtang Terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the Tibetan Plateau

A primary magnetic component with the acquisition time of 56.0–43.2 Ma was isolated between 580 °C and 685 °C from the Eocene Gonjo and Ranmugou Formations in the eastern part of the Qiangtang Terrane, Tibetan Plateau. The tilt corrected site-mean direction is Ds=35.5°, Is=29.3°, k=45.9, α95=3.2°. The site-mean inclination increased from 29.3° to 41.6° after multiple inclination shallowing corrections, giving a paleopole of 57.9°N/192.1°E, A95=2.9°. Comparison of the Paleogene paleomagnetic data for the Qiangtang Terrane and Lhasa Terrane reveals that both terranes experienced latitudinal crustal shortening before 54–43 Ma, indicating the uplift of southern and central Tibet in the early Eocene. Subsequently, since 35.4 ± 2.4 Ma, the north of the Qiangtang Terrane experienced ∼1300 ± 410 km of crustal shortening, indicating the uplift of the northern Tibetan Plateau. The Lhasa Terrane and Qiangtang Terrane have not experienced further crustal shortening since the late Eocene, and the southeastern part of Tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the Tibetan Plateau. The crust of the Tengchong Terrane and Shan–Thai Block did not experience significant southward extrusion since the late Oligocene–Early Miocene. The Indochina Block was situated in the north of the Qiangtang Terrane before the Oligocene, and since the early Oligocene, the Indochina Block began to experience southward extrusion from the north of the Qiangtang Terrane, which absorbed part of the crustal shortening in the north of the Qiangtang Terrane.

Petrogenesis of an early Eocene gabbro–granite complex in Kachang (SW Yunnan) and its implications for Eocene magmatism in the Tengchong terrane of SW China

ABSTRACTMajor and trace element, zircon U–Pb and Hf-isotope data are reported for mafic intrusions and host granite from the Kachang pluton (North Yingjiang of SW Yunnan) in the Tengchong Terrane, in order to investigate their sources, petrogenesis and tectonic implications. The zircon U–Pb age of the mafic rocks (~55 Ma) is identical to that of the host granite (56.7 ± 0.6 Ma). The mafic rocks have high MgO concentrations (up to 13.43 wt.%) at low SiO2 contents (low to 42.73 wt.%) and slight negative to positive εHf(t) values (−2.26 to +0.59). They are enriched in LILE and LREEs and depleted in HFSEs, which can be explained as melts derived from a enriched mantle, with some crustal contamination. The host granite have high SiO2 contents (69.18–72.65 wt.%), highly negative εHf(t) values (−9.08 to −5.14), suggesting mainly derived from an ancient crustal source. Field observations, geochronology, geochemistry and zircon Hf isotopic compositions point to a complex petrogenesis, where enriched mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic magmas, result of mafic magma upwelling triggered by the subduction rollback of the Neotethyan slab. Our new data, along with the data reported (especially zircon U–Pb dating and Lu–Hf isotope data) in the Tengchong Terrane, indicate that the spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern and central Tibet.

Mineralogy, zircon U–Pb–Hf isotopes, and whole‐rock geochemistry of Late Cretaceous–Eocene granites from the Tengchong terrane, western Yunnan, China: Record of the closure of the Neo‐Tethyan Ocean

New data from mineralogy, geochemistry, zircon U–Pb dating, and Hf isotopes have revealed Late Cretaceous to Eocene magmatic intrusions in the Tengchong terrane and constrained the origin, tectonic setting, and characteristics of tin‐bearing and barren granitoids. We divide the studied granitoids into two groups: (a) the Late Cretaceous Zhinaxiang (70 Ma) and Early Paleocene Dazhupeng (65 Ma) S‐type barren granites and (b) the Eocene Lailishan A2‐type tin‐bearing granite (50 Ma). All these granitoids display Si‐ and K‐rich and calc‐alkaline characteristics and have similar chondrite‐normalized rare earth element patterns. However, geochemical differences do exist between the two groups of granites. The Zhinaxiang and Dazhupeng S‐type granites are slightly peraluminous, enriched in large‐ion lithophile elements (e.g., Rb and K), and depleted in high‐field‐strength elements (e.g., Nb, Ta, Zr, and Hf), whereas the Lailishan A2‐type granite has higher TFe2O3, high‐field‐strength element content, and Ga/Al ratios. The geochemical and Hf isotopic data indicate that the granites in this study were generated by partial melting of Paleoproterozoic metasedimentary rocks. Due to the subduction of the Neo‐Tethyan Ocean beneath the Eurasian plate, the Zhinaxiang and Dazhupeng S‐type granites were formed in a thickened‐crustal environment, whereas the Lailishan A2‐type granite was emplaced in a post‐collisional extensional setting attributed to slab break‐off. By comparison between tin‐bearing and barren granites, we propose that the Sn mineralization could be related to relatively high‐temperature and low‐pressure crystallization conditions.

Early Cretaceous I-type granites in the Tengchong terrane: New constraints on the late Mesozoic tectonic evolution of southwestern China

The Early Cretaceous granitoids that are widespread in the Tengchong terrane of Southwest China play a critical role in understanding the tectonic framework associated with the Tethyan oceans. In this study, we present a detailed description of zircon U–Pb ages, whole-rock geochemistry and Hf isotopes for the Laoxiangkeng pluton in the eastern Tengchong terrane and elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating of the Laoxiangkeng pluton yields ages of 114 ± 1 Ma and 115 ± 1 Ma, which imply an Early Cretaceous magmatic event. The Laoxiangkeng pluton enriched in Si and Na, is calc-alkaline and metaluminous, and has the characteristics of highly fractionated I-type granites. Zircons from the pluton have calculated εHf(t) values of −12.7 to −3.7 and two-stage model ages of 1327–1974 Ma, respectively, indicating a mixed source of partial melting of Paleo-Neoproterozoic crust-derived compositions with some inputs of mantle-derived magmas. By integrating all available data for the regional tectonic evolution of the eastern Tethys tectonic domain, we conclude that the Early Cretaceous magmatism in the Tengchong terrane was produced by the northeastward subduction of the Meso-Tethyan Bangong–Nujiang Ocean.

Intra-continental transpression and gneiss doming in an obliquely convergent regime in SE Asia

The Tengchong terrane comprises a sequence of linear dome-like zones, cored by granite and migmatitic layers. These cores are mantled by predominantly gneiss and subordinate schist sequences, decreasing in deformation intensity outward from extensive mylonitization to weak mylonitization. The pre-doming deformation was characterized by the formation of large-scale top-to-the-east shearing (D1) in the gneiss terrane, locally preserved flat-lying foliation (S1), weak folding (F1) and emplacement of the Mangbang granite during the Cretaceous (114-104 Ma). The second stage of deformation (D2) consisted of map-scale east-verging folds (F2, dome amplification) and minor lateral strike-slip shear zones between the anticlines in the gneiss and migmatitic sequences. Extensive partial melting and emplacement of 67-30 Ma synkinematic granitoid bodies/veins occurred, leading to the emplacement of wedges of granite into the easterly directed F2 fold cores. These wedges formed kilometer-scale granitoid domes. The post-doming D3 deformation with transpression recorded strain partitioning with simple shear-dominated high-strain zones along the Gaoligong and Nabang dextral lateral strike-slip shear zones (active during 30-11 Ma). Late transtensional deformation (D4) during cooling of the entire terrane involved the localized low-temperature Gaoligong west and east detachment faults that controlled the late exhumation of the Gaoligong metamorphic zone (since 10 Ma). Our structural observations, combined with previous studies, suggest that this style of doming is a representative type of intra-continental deformation in the Cenozoic during the oblique India-Asia collision. The actual dome shapes reflect formation of antiforms during compression-dominated transpression, prior to localized strike-slip shearing, in the accommodation belt around the Eastern Himalayan Syntaxis. Vertical exhumation of crustal material by contractional doming played an important role in absorbing the vast majority of the internal deformation of crustal fragments during oblique collision.