Simao Terrane Research Articles

Norian (Upper Triassic) carbon isotopic perturbations and conodont biostratigraphy from the Simao terrane, eastern Tethys

The Norian “chaotic carbon episode” stands out as a compelling carbon cycle perturbation event within the Late Triassic period. This episode has been documented in marine stratigraphic sections across North America, Italy, Japan, and China. However, limited Norian chronostratigraphy available in the eastern Tethys inhibited the identification of this event in this broad area, and thus the possible global response to this carbon cycle perturbation. To address this issue, we analyzed the conodonts and carbon-isotope of the Norian Sanhedong (SHD) section in the Simao terrane, Southwest China. The identification of the Alaunian Epigondolella spiculata Zone has well documented the base of the Alaunian stage. Moreover, a conodont faunal turnover can be recognized at the base of the Alaunian, with the evolution of Mockina from Ancyrogondolella and Epigondolella, and it is characterized by a gradual decrease in specimen size. The E. spiculata conodont Zone from the SHD section correlates well with the homonymous conodont Zones documented in both the Tethys and the Panthalassa Oceans. Two carbon-isotope negative excursions (CIE1 and CIE2) have also been recorded in the Lacian to Alaunian of the SHD section that can be compared with coeval global sections.

Triassic magmatism along both sides of the Simao terrane, SE Tibetan Plateau: Implications for the evolution of the Main Palaeo‐Tethyan Ocean and the Ailaoshan Ocean

Subduction and closure of the Palaeo‐Tethyan ocean in the south‐eastern (SE) Tibetan Plateau accounted for the amalgamation of the Indochina Block with the Sibumasu and Yangtze blocks during the Triassic, however, the spatial distribution of the Main Palaeo‐Tethyan ocean and the nature of the Ailaoshan ocean in this region remain controversial. This is partially due to the absence of related magmatic records in the middle‐north segment of the Lancangjiang tectonic belt (LTZ) and consequently, the lack of regional comparison with the Ailaoshan tectonic belt (ATZ). In this contribution, we reported for the first time the presence of subduction‐related Triassic dioritic‐granitic rocks from the Chongshan metamorphic belt (CMB) and the Diancangshan metamorphic belt (DMB) along both sides of the Simao terrane. Zircon U–Pb‐Hf isotopic and whole‐rock geochemical data for these dioritic‐granitic rocks and 135 previously published magmatic rocks in the LTZ, ATZ and the Longmu Co‐Shuanghu suture (LSS) are compared and place important constraints on (1) the spatial distribution of the Palaeo‐Tethyan ocean in this region and (2) the nature of the Ailaoshan Ocean. All these dioritic‐granitic samples from the CMB and DMB yielded Triassic crystallization ages from 225 to 251 Ma, representing late‐stage magmatic records for the Palaeo‐Tethyan evolution in the middle‐north segment of both the LTZ and ATZ in this region. These Triassic dioritic‐granitic rocks in the CMB and DMB share similar geochemical features. Dioritic samples display positive εHf(t) values and relatively high MgO, Cr, Ni contents, moderately differentiated rare earth element (REE) patterns and depletion in Nb‐Ta‐Ti, suggesting their generation from partial melts of metasomatized mantle wedge. Granitic rocks are mostly S‐type with low MgO contents but high K2O/Na2O and A/CNK ratios, mainly generated by regional clastic rocks dominant by juvenile contributions, except for granodiorite samples CS1601 and DC1734‐2, exhibiting identical geological features subduction‐derived hybridized magma from regional clastic sediments and basalts. Our discovery of the Triassic dioritic‐granitic rocks in the CMB stands as the first direct evidence for the Main Palaeo‐Tethyan evolution in the middle‐north segment of the LTZ. This discovery bridges 135 contemporaneous reported magmatic records and supports the spatial distribution of the Main Palaeo‐Tethyan ocean from the LTZ to the LSS. Our Triassic dioritic‐granitic rocks in the CMB and DMB recorded comparable late‐stage evolution involving slab break‐off of the Main Palaeo‐Tethyan ocean and the Ailaoshan Ocean, respectively. Together with previous studies, the Ailaoshan ocean should be a wide oceanic Basin that shares similar time span and evolution processes as the Main Palaeo‐Tethyan ocean.

Petrogenesis and geodynamic implications of Early Cretaceous highly fractionated leucogranites in the northern Lanping–Simao terrane, Eastern Tibetan Plateau

A felsic intrusion in the Guaguazi Cu mining district in the northern Lanping–Simao terrane, Eastern Tibetan Plateau was revealed through drill holes and is composed of highly fractionated leucogranites. In this paper, we report zircon U–Pb ages and whole-rock compositions to constrain the petrogenesis and geodynamic setting of this intrusion. Zircon grains separated from two leucogranite samples have identical U–Pb ages of 123.9 ± 2.0 Ma and 123.6 ± 1.9 Ma, indicating that the crystallization age of the intrusion is Early Cretaceous (ca. 124 Ma). Leucogranites from the intrusion have SiO2 contents from 74.3 to 77.6 wt% with differentiation index values of 96.3–97.6, indicative of highly fractionated nature. They are weakly peraluminous (A/CNK = 1.01–1.10) and have light REE enrichment ((La/Yb)N = 5.4–8.5) and negative Eu anomalies (Eu/Eu* = 0.26–0.45). The primitive-mantle-normalized spider diagram indicates strong enrichment in large-ion lithophile elements (Rb, Th, U, K, and Pb) relative to high-field-strength elements (Nb, Ta, P, and Ti). The high initial 87Sr/86Sr ratios (0.7070–0.7094) and negative εNd(t) values (−8.18 to −7.20) and TDM2 ages of 1.58–1.51 Ga of the rocks, suggest that their source was predominantly Mesoproterozoic lower continental crust with insignificant mantle contribution. High initial 207Pb/204Pb (15.74–15.75) and 206Pb/204Pb (18.51–18.63) ratios further confirm their crustal source and highly evolved nature. Considering the petrogenetic similarity of Early Cretaceous granitoids in the Guaguazi area and the Gaoligong tectonic belt, we propose that the eastern Bangong–Nujiang Tethyan Ocean was closed at least no later than the Early Cretaceous, and that the Guaguazi granitic intrusion formed in a crustal thickening setting in response to the Lhasa–Qiangtang collision.

Mesozoic–Cenozoic sedimentary rock records and applications for provenance of sediments and affiliation of the Simao Terrane, SW China

ABSTRACTAs the north part of Simao Terrane, Lanping Basin is located between the Sanjiang Tethys Orogen (STO) and Yangtze Block, also the junction zone between the Gondwanaland and Cathaysian old land (Pan Huaxia mainland), which includes Yangtze and Cathaysian Blocks. The aim of this study is to decipher the provenance of the sedimentary units in the Lanping Basin and affiliation of Simao Terrane by the U-Pb ages, Hf isotope of detrital zircons and whole-rock geochemistry. The whole-rock geochemistry and the mineral composition indicate that most of the Triassic–Paleocene sedimentary rocks are derived from the upper crust and exhibit recycled orogen features. The detrital zircon U-Pb ages from the North Simao Terrane are consistent with the magmatic events during Early Neoproterozoic and Permian in the Western Yangtze Block. And the detrital zircons ages from North Simao Terrane show same distribution features as the Permian–Triassic magmatic rocks, which are distributed in the Simao Terrane and along major sutures. These comparisons suggest that the clastic sediments in Lanping Basin (North Simao Terrane) are derived from Early Neoproterozoic and Permian magmatic rocks from Western Yangtze Block, Permian–Triassic magmatic rocks from Simao Terrane, along Jinshajiang, Garz-Litang and Ailaoshan Sutures. The comparison of the detrital zircon age distributions shows that Simao Terrane and Yangtze Block exhibited similarity tectonic setting in the evolution history, especially during Paleoproterozoic–Late Paleozoic. This suggests that the Simao Terrane is part of Cathaysian old land, although Simao Terrane was separated from Yangtze Block for short period during Early Paleozoic. Besides, the Hf mapping, stratigraphic succession, paleogeography and paleomagnetism in SW China support that Simao Terrane has a Cathaysian old land-affinity, rather than one involving Gondwanaland.

Crustal P wave velocity structure beneath the SE margin of the Tibetan Plateau from Deep Seismic Sounding results

The crust and uppermost mantle beneath the southeastern (SE) margin of Tibetan Plateau record the lateral expansion of Tibet and far-field effects from the ongoing continental collision and convergence. In order to obtain constraints on the deep structure and the eastward expansion of the plateau, we synthesized Deep Seismic Sounding (DSS) results in this region and constructed the P wave velocity models for each terrane. The principal characteristics of the crustal velocity model and tectonic significance are: (1) Crustal thickness decreases from 53 km beneath the Songpan-Ganzi terrane (SGT) to 34 km in the south direction beneath Simao terrane (SMT), and 42 km in the east direction beneath Chengdu area. The crust is dominantly felsic in upper crust, with a small percentage of mafic composition in the lower crust. (2) The crustal structure is very heterogeneous in this region. The Indochina Block is characterized by low seismic velocities in the upper crust, while the west Yangtze terrane (WYZT) and SGT show higher velocities in the upper crust. The lower crust beneath WYZT is relatively thicker than other terranes. (3) We deduce that the high velocity in the upper crust and relatively thick lower crust beneath the SGT and WYZT can be associated with magmatic processes that generated the Emeishan flood basalts which may have been triggered by rifting or mantle plume. The lateral variations of the crustal thickness in SE Tibet may be due to lower crustal flow from the Tibetan Plateau to the SE direction after blocked by the cold and rigid Yangtze craton in the east direction.

Micro-continental blocks in Gondwana assembly: Geological and geochemical evidence of the Indochina block, SE Tibetan Plateau

Tectonic processes from the breakup of Rodinia to Gondwana assembly have been studied on large continents. However, the role of micro-continental blocks in Neoproterozoic supercontinental cycle is barely known. Geochronological and geochemical studies on a combination of 57 (meta-) sedimentary and (meta-) igneous rocks (one igneous rock and four meta-sedimentary samples are from this study) in the Indochina block (including its north part, i.e. the Simao terrane) reveal the paleoposition and tectonic processes during Precambrian supercontinental cycle. A mylonitized monzogranite from the NW part of the Indochina block (the Chongshan area) shows zircon UPb ages of 867 ± 19 Ma. Precambrian and Early Paleozoic strata share similar zircon UPb age peaks at ca. 2.5 Ga and ca. 0.95 Ga. The ca. 1.75 Ga age peak in the Precambrian strata is absent in those Early Paleozoic strata. The newly discovered 867 ± 19 Ma felsic intrusion in the Chongshan Metamorphic Belt and the shift of detrital zircon age patterns from the Neoproterozoic to Early Paleozoic confirm the presence of Precambrian magmatic activities in the Indochina block. Paleoposition and magmatic records show the Indochina block should be part of an Andean-type orogeny along the NW margin of the Rodinia during the Neoproterozoic. During the Neoproterozoic to Early Paleozoic, the role of Indochina as a continental block coherent with the Indian Craton at least during the breakup of Rodinia and Gondwana assembly. After Gondwana assembly, the Indochina block, as a section of an Andean-type continental arc along the eastern margin, blocked the exotic inputs and changed the age distribution patterns dramatically.

Zircon U–Pb dating, geochemistry and Sr–Nd–Hf–O isotopes for the Baimaxueshan granodiorites and mafic microgranulars enclaves in the Sanjiang Orogen: Evidence for westward subduction of Paleo-Tethys

The Sanjiang Orogen in the Southeastern Qinghai–Tibet Plateau is an ideal region to investigate the geodynamic processes related to the evolution of the Paleo-Tethys Ocean. New zircon SHRIMP U–Pb age data, whole-rock major and trace elements, and zircon Hf–O isotopic data of granodiorites and the associated dioritic enclaves from Baimaxueshan pluton in the eastern margin of the Qamdo–Simao Terrane provide key evidence for the subduction products of the Paleo-Tethyan oceanic lithosphere. Zircon U–Pb ages indicate that the dioritic enclaves and their host granodiorites were synchronously emplaced at ~255Ma. The host granodiorites are normal calc-alkaline I-type granitoids, and characterized by uniform zircon εHf(t) values (−6.7 to +1.5), a wide-ranging δ18O values (6.1 to 9.3‰), and negative whole-rock εNd(t) values (−7.3 to −5.8). We suggest that these rocks were derived from the partial melting of ancient mafic lower crust with varying contributions from mantle-derived components. The dioritic enclave samples yield εHf(t) values (−4.7 to +2.1) and δ18O (5.8 to 8.3‰), which are similar to those of the host granitoids. They most likely originated from magma mixing between mantle-derived and crust-derived melts at or close to the Moho. The similar emplacement age but distinct whole-rock εNd(t) values between the Lancangjiang arc-related andesites and granodiorites and Baimaxueshan pluton preclude the possibility that they were generated by the same magmatic source. However, the consistent emplacement ages and whole-rock εNd(t) values of the Jinshajiang subduction-related granites and Baimaxueshan granitoids suggested that they were both generated by the westward subduction of the Jinshajiang PaleoTethyan oceanic lithosphere.

Detrital zircon U–Pb geochronological and sedimentological study of the Simao Basin, Yunnan: Implications for the Early Cenozoic evolution of the Red River

The paleo-Red River is suggested to have been a continental-scale drainage system connecting the Tibetan Plateau to the South China Sea. However, the evolution of the paleo-Red River is still under debate. This study presents new results from sedimentological analyses and detrital zircon U–Pb geochronologic data from fluvial sedimentary rocks of Paleocene to Oligocene age of the Simao Basin to constrain the nature of the paleo-drainage system of the Red River. The detrital zircon U–Pb results reveal multiple age groups at 190–240 Ma, 260–280 Ma, 450–540 Ma, 1700–1900 Ma and 2400–2600 Ma for the Paleocene to late Eocene Denghei Formation (Fm.), but only one conspicuous peak at 220–240 Ma for the late Eocene–Oligocene Mengla Fm. Provenance analyses illustrate that the former likely had source areas that included the Hoh-Xil, Songpan-Ganzi, northern Qiangtang, Yidun and western Yangtze Terranes, which are consistent with the catchments of the Upper and Lower Jinshajiang Segments, whereas the latter mainly transported material from a limited number of sources, such as the Lincang granitic intrusions west of the Simao Basin. Integrated with available detrital zircon U–Pb geochronologic and paleogeographic data, our study suggests the existence of a paleo-Red River during the Paleocene to late Eocene that was truncated and lost its northern sources after approximately 35 Ma, due to left-lateral strike-slip faulting of the Ailao Shan–Red River and clockwise rotation of the Lanping–Simao Terrane.

New insights into the Cenozoic lateral extrusion of crustal blocks on the southeastern edge of Tibetan Plateau: Evidence from paleomagnetic results from Paleogene sedimentary strata of the Baoshan Terrane

AbstractIn order to characterize the lateral extrusion of crustal material during the Cenozoic, we conducted a paleomagnetic study of the Eocene‐Oligocene Zhushan Formation and Paleocene Muguahe Formation in the central part of the Baoshan Terrane (BST), on the southeastern edge of Tibetan Plateau. A primary magnetic component and a remagentized component with the Miocene acquisition age were isolated from the Muguahe Formation and Zhushan Formation, respectively. These data indicate that the BST did not commence clockwise rotation in the period of the Paleocene and the Oligocene and that the BST experienced ~80° clockwise rotation relative to East Asia, since the Miocene. Combining with the documented history of crustal boundary strike‐slip faults on the southeastern edge of the Tibetan Plateau shows that the BST and Tengchong Terrane (TCT) first underwent latitudinal crustal shortening in the Oligocene, and the eastward extension was the main type of motion of the crustal material during this period, which induced the initial strike‐slip movement of their boundary faults. Since the Miocene, the main form of crustal motion of BST, TCT, and Simao Terrane was gradually transformed into clockwise rotation, which possibly indicates that the fold and thrusting fault system‐induced crustal shortening and thickening in the southeastern edge of the Tibetan Plateau have reached extremity, and the southeastern part of the Tibetan Plateau has already been uplifted to the similar elevation with today's elevation in the Miocene.

New paleomagnetic studies of Cretaceous and Miocene rocks from Jinggu, western Yunnan, China: Evidence for internal deformation of the Lanping–Simao Terrane

Cretaceous, Paleogene, and Miocene sandstones were sampled in the Jinggu area to constrain the internal deformation of the Lanping–Simao Terrane of the Indochina Block. The tilt-corrected overall site-mean direction of the middle Cretaceous strata recorded in the Jinggu area is Ds/Is=77.0°/43.0°, with α95=2.9° (N=47), with a positive fold test indicating a primary remanence acquisition. The site-mean direction recorded for the high-temperature component of the Miocene strata is Ds/Is=13.7°/36.0°, with α95=3.3° (n=38). The best-fit linear regressions between regional tectonic lines and rotation degree from each sampled area in the Lanping–Simao Terrane indicate a direct relationship between tectonic rotation and formation of the sinusoidal shape of the Lanping–Simao arcuate structural zone. The large clockwise rotation in the Jinggu area can be subdivided into three periods. During the formation of the sinusoidal shape of the Lanping–Simao Arc, the Jinggu area and the Indochina Block experienced approximately 20° of clockwise rotation. An additional ∼40° of clockwise rotation in the Jinggu area was caused by bending of the Chongshan–Lancang–Chiang Mai belt between 27 and 20Ma. After the early Miocene, a significant small-scale internal rotation (8.2±3.2°) adjustment occurred in the Jinggu area. Quantitative comparison between the paleomagnetically determined clockwise rotation and tectonic–metamorphic events suggests that the Lanping–Simao Arc was formed by the west-to-east compression and southeastward extrusion of the Shan–Thai Terrane since 36Ma in southeast Tibet.

Post-folding magnetization of the Triassic rocks from western Guizhou and southern Yunnan provinces: New evidence for large clockwise rotations in the Simao Terrane

Thermal demagnetization of the redbed samples collected from the Lower Triassic Feixianguan Formation at two localities in western Guizhou Province reveals that the natural remanent magnetization was dominated by a postfolding component of probably recent origin and the prefolding, possibly primary, component was barely preserved. This confirms that the Feixianguan Fm in western Guizhou and northeastern Yunnan provinces was severely remagnetized. Thermal demagnetization also resolves two postfolding components from the limestone samples of the Mid-Triassic Dashuijingshan Formation from southern Yunnan Province: a low temperature component acquired in recent times and a high temperature component probably acquired shortly after the India–Asia collision. The mean in-situ direction for the high temperature component based on eighteen sites is D=95.4°, I=28.4° with α95=6.2°, which indicates a clockwise rotation of 86.3±7.3° with respect to stable Eurasia. This result together with paleomagnetic results previously reported from the Jurassic–Eocene rocks in the same area demonstrates beyond reasonable doubt that the central Simao Terrane has experienced large clockwise rotations due to the India's collision with, and continuing penetration into, Asia.

Contribution of Previously Arc Cumulates to the Post‐Subduction Yangla Skarn Cu Deposit, SW China

The Yangla skarn Cu deposit (150 Mt at 1.03% Cu) is located in the central segment of the Jinshajiang Metallogenic Belt within the Sanjiang (three rivers) region, SW China. The ore associated granodiorite was emplaced at 233.1 ± 1.4 Ma and 231.0 ± 1.6 Ma at 2σ by zircon U-Pb dating (Zhu et al., 2011), coeval to the Cu mineralization (232.0±1.5 Ma; Re-Os dating by molybdenite). Both significantly postdated the Early Triassic collision (246– 247 Ma; Zi et al., 2012) between the Qamdo–Simao terrane and the Zhongza terrane along the Jinshajiang Suture following the closure of the Jinshajiang Paleo-Tethys Ocean. Based on their geochemical compositions, two groups of granodiorite have been defined. Group 1 is characterized by relatively low eNd(t) values (-5.1– -6.7) and high initial 87 Sr/ 86 Sr ratios (0.7078–0.7148), with ancient two stages Nd isotope model ages (TDM2 = 1420– 1551 Ma). In contrast, Group 2 has mantle-like eNd(t) and initial 87 Sr/ 86 Sr ratios (eNd(t) = 1.5–2.9; ( 87 Sr/ 86 Sr)i = 0.7042 –0.7047), with obvious younger TDM2 ages (769–882 Ma). Both of them are enriched in high ion lithophile elements and depleted in high field strength elements, typical subduction signatures. In addition, Group 2 shows higher Sr/Y ratios (45–81) than Group 1 (Sr/Y = 11–37), indicative of a hydrous magma source which would suppress plagioclase fractionation and be in favor of hornblende crystallization (Naney, 1983; Richards and Kerrich, 2007). In combination, we propose Group 2 was probably derived from the melting of previously subduction-modified lithosphere, especially hydrous arc cumulates in the lower crust. However, rocks from Group 1 were derived from a mixing magma from the melting of both residual metasomatic lithosphere and ancient lower crust. The arc cumulates associated with the subduction of the Paleotethys might provide Cu, S, and water for the fertile magmas. For the contribution of the Neoproterozoic slab subduction, further work should be done.

Internal crustal deformation in the northern part of Shan-Thai Block: New evidence from paleomagnetic results of Cretaceous and Paleogene redbeds

In order to describe the internal deformation of the northeast part of the Shan-Thai Block resulting from the penetration of the Indian plate into Asia, a paleomagnetic study was carried out on the Cretaceous and Paleogene strata in the southern part of the Simao Terrane. High-temperature magnetic components with unblocking temperatures around 680 °C were isolated and positive fold test shows its primary nature of magnetization. The tilt corrected site-mean directions of the Lower Cretaceous and Upper Cretaceous from the Menglun area are Ds = 46.2°, Is = 45.9°, k = 11.0, α95 = 10.6° and Ds = 33.2°, Is = 30.9°, k = 68.4, α95 = 8.2°, respectively. The site-mean directions of the Upper Cretaceous and Lower Paleogene for the Mengban area are Ds = 50.5°, Is = 31.0°, k = 207.6, α95 = 6.4° and Ds = 43.5°, Is = 23.0°, k = 26.1, a95 = 13.4°, respectively. The results of the Lower Cretaceous and Upper Paleogene from the Mengla area are Ds = 46.9°, Is = 42.2°, k = 27.3, α95 = 7.7° and Ds = 41.8°, Is = 23.8°, k = 38.9°, a95 = 5.8°, respectively. In comparison with previous paleomagnetic data of the Simao Terrane, the rotational deformation can be divided into four stages. The Cretaceous and Paleocene sediments of the Simao Terrane experienced intensive folding before the early Oligocene. During the early Oligocene to early Miocene, the Shan-Thai and Indochina Blocks experienced southeastward extrusion with ~ 20° clockwise rotation relative to the South China Block. During the middle to late Miocene, the Shan-Thai Blocks experienced more 15° clockwise rotation. Since the late Miocene, the activity of strike-slip faults surrounding the Simao Terrane enhanced the internal differential rotations of the Terrane. These deformation processes finally resulted in a complex internal deformation of the Simao Terrane.

Zircon U–Pb dating and the petrological and geochemical constraints on Lincang granite in Western Yunnan, China: Implications for the closure of the Paleo-Tethys Ocean

Lincang granite is a batholith located in the Sanjiang region and is an important research subject for understanding subduction and collision during the Paleo-Tethyan period. It is widely exposed in the Lincang Terrane and extends south into Burma. Based on various petrological and geochemical investigations performed from south to north across the Lincang granite, a new set of data, which includes zircon chronological and Hf isotopic data, is presented to discuss the origin of the Lincang granite and its tectonic significance. The Lincang granite is a peraluminous, high-K calc-alkaline body with sub-parallel REE patterns and a strong negative Eu anomaly. This anomaly is characteristic of a post-collision peraluminous S-type granitic batholith. The 200–230Ma formation age of the Lincang granite was determined using LA-ICP-MS zircon U–Pb dating. Thus, it has been confirmed that the granite formed during the late Triassic period, and the formation process lasted for approximately 30Ma. Geochemical and isotopic compositions indicate that the primary magma of Lincang granite most likely originated from a crustal source, and possibly underwent an assimilation–fractionation crystallization (AFC) process during its emplacement. The Lincang granite formed during the continental collision between the Baoshan–Gengma Terrane and the Lanping–Simao Terrane after the northeast subduction of the Paleo-Tethyan Oceanic Plate. Therefore, the late Triassic Lincang granite is important evidence for the closure of the Paleo-Tethyan Ocean.

Triassic collision in the Paleo-Tethys Ocean constrained by volcanic activity in SW China

A suite of collision-related Triassic volcanic rocks cropping out within the Jinshajiang–Ailaoshan orogenic belt in SW China offers insights into closure of the Paleo-Tethys and associated terrane/continent collision. Combined geochemical and Sr–Nd isotopic data for samples from the Jinshajiang segment demonstrate that the lower part of the volcanic suite is dominated by high-Si rhyolites (the Pantiange Formation) derived through crustal anatexis; whereas the upper units (the Cuiyibi Formation) are characterized by basalts alternating with intermediate-felsic lavas that share similar Sr–Nd isotopic signatures, and may have originated from partial melting of subduction-enriched lithospheric mantle. SHRIMP U–Pb analyses on zircons yield ages of 247–246Ma for the rhyolites of the Pantiange Formation, and ages ranging from ca. 245Ma to 237Ma for the basaltic and intermediate-felsic samples from the overlying Cuiyibi Formation. These data, integrated with other geologic evidence (e.g., stratigraphic record), suggest an Early-Middle Triassic timing of terrane–continent collision along the Jinshajiang and Ailaoshan sutures with the 247–246Ma Pantiange high-Si rhyolites representing the early magmatic products generated by this event. The development of the 245–237Ma bimodal volcanism was associated with subsidence and deep-marine sedimentation. We interpret this as reflective of an extensional setting within an evolving collisional orogen, probably related to oblique convergence along the collisional zone. Triassic volcanic rocks with similar geochemical signatures and ages have been identified along the >1000km length of the Jinshajiang–Ailaoshan suture zone. The Indochina block to the south of the suture records 250–240Ma tectonothermal activity. Taken together, these relationships suggest that initial collision and amalgamation of the Qamdo–Simao terrane (Indochina) with the Yangtze Block (South China) along the Jinshajiang–Ailaoshan and Song Ma sutures probably took place in the Early Triassic following consumption of the Paleo-Tethys Ocean.

Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (II): Insights from zircon ages of ophiolites, arc/back-arc assemblages and within-plate igneous rocks and generation of the Emeishan CFB province

New SHRIMP zircon ages for ophiolites, arc/back-arc assemblages and within-plate igneous rocks constrain the evolution of the Paleo-Tethys Orogen of southwest China. Ophiolites represent remnants of the Paleo-Tethys ocean, which was divided by the Simao terrane into two tracts, the main ocean in the west (current position) and a branch in the east. The main ocean has been inferred to originate from Gondwana break-up in the Early Devonian, and the heritage is preserved as the Changning-Menglian ophiolite (SSZ-type). The oceanic branch is represented by the Ailaoshan (NMORB-type)–Jinshajinag (EMORB-type) ophiolites. Zircons from a diabase and a plagiogranite of the Ailaoshan ophiolite yielded 206Pb/238U ages of 382.9±3.9 Ma and 375.9±4.2 Ma. The age of a cumulate gabbro from the Jinshajiang ophiolite is 343.5±2.7 Ma. A metagabbro of the Changning-Menglian ophiolite yielded a crystallization age of 267.1±3.1 Ma. We interpret the ages of the Ailaoshan and Jinshajiang ophiolites to represent sea-floor spreading in the branch ocean, whereas the age of the Changning-Menglian ophiolite reflects closure of the main ocean. Arc/back-arc assemblages (zoned mafic–ultramafic complexes and arc volcanic rocks) are records of convergent plate boundary magmatic activity. Rocks of the Gicha complex (back-arc rift) yielded zircon ages of 306.2±4.6 Ma for a tonalite, 301.0±2.9 Ma for a microgabbro, 297.1±2.0 Ma for a plagioclase hornblendite, and 281.3±1.7 Ma for a diabase. A gabbro and a tonalite of the Banpo complex (plutonic arc) yielded identical ages of 285.6±1.7 Ma and 285.8±2.0 Ma. A basalt of the Yaxianqiao volcanic arc was dated at 266.2±2.2 Ma. Within-plate igneous rocks were also dated. Two amphbolite xenoliths in the Jinshajiang ophiolite mélange, derived from low-Ti continental flood basalts, have zircon ages of 439.3±3.5 Ma and 403.7±3.1 Ma, which we interpret to represent the inception of continental rifting prior to the opening of the branch ocean. The small Dalongkai differentiated mafic–ultramafic intrusion reflects post-orogenic extension and is characterized by superimposed arc and within-plate geochemical signatures. Zircons from a plagioclase pyroxenite of this intrusion yielded an age of 245.6±1.4 Ma.These new results, combined with geological and geochemical data, allow the reconstruction of a Devonian to Permian plate tectonic cycle in which generation of the Emeishan CFB province correlates with Paleo-Tethys orogeny in time. This study provides evidence for cyclic continent–ocean–continent development via multiple orogenesis, as well as temporal constraints on the dynamic relationship between plate tectonics and generation of a large igneous province.

Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (I): Geochemistry of ophiolites, arc/back-arc assemblages and within-plate igneous rocks

The Paleo-Tethys Orogen in southwest China is an amalgamation of continental terranes (Sibumasu, Simao and Yangtze), which are bounded by ophiolite belts (Ailaoshan, Jinshajiang and Changning–Menglian) that represent former ocean basins. This study concentrates on ophiolites, arc/back-arc assemblages, and within-plate igneous rocks that occur in this complex orogen. Mainly based on geochemical fingerprinting of basalts, the Paleo-Tethys ophiolites are here classified as MORB and SSZ types. The Ailaoshan ophiolite (NMORB-type; ca. 387–374 Ma) is associated with a non-volcanic segment of the rifted western Yangtze margin. The Jinshajiang ophiolite (EMORB-type; 346–341 Ma), which has ophiolite structure but some samples bear crustal contamination signatures, is associated with a volcanic segment of the rifted Yangtze western margin. These MORB-type ophiolites are interpreted to have formed in the continent-ocean transition zones. The Changning–Menglian ophiolite, which marks the main Paleo-Tethys suture, that is, the boundary between the Gondwana-derived Sibumasu terrane and the Yangtze-derived Simao terrane, has traditionally been thought to have a mid-ocean ridge (MOR) origin. New trace element data of basalts, however, indicate that this ophiolite (ca. 270–264 Ma) was formed at a supra-subduction zone (SSZ). Arc and back-arc assemblages are represented by the Gicha (ca. 311–277 Ma) and Banpo (ca. 288–284 Ma) zoned mafic-ultramafic complexes and the Yaxianqiao arc volcanic rocks (ca. 268–264 Ma). Geochemical data suggest that the Gicha complex originated in a back-arc rift. The Banpo complex makes a plutonic arc that contains low-Ti gabbro and tonalite with volcanic arc granite (VAG) affinity. Within-plate igneous rocks are recognized as xenoliths (amphibolites; ca. 443–401 Ma) in the Jinshajinag ophiolite mélange and the Dalongkai small differentiated mafic-ultramafic intrusion (ca. 247–244 Ma) that intruded the Yaxianqiao Arc. The amphibolite xenoliths are similar to low-Ti continental flood basalts (CFB) in composition. Rocks from the small mafic-ultramafic intrusion have superimposed subduction and within-plate geochemical signatures. These ophiolites, arc/back-arc assemblages and within-plate igneous rocks, are summarized as the magmatic records for the Plaeo-Tethys evolution in the context of a plate tectonic cycle.

The crust and upper mantle structure beneath Yunnan from joint inversion of receiver functions and Rayleigh wave dispersion data

In this study, we apply joint inversion of receiver function and Rayleigh wave dispersion (phase and group) velocities data to estimate a shear wave velocity structure for the Yunnan and its adjacent region. Our joint inversion models indicate strong lateral variations in crustal thickness in this area, which increase gradually from 32 km in SE to 56 km in NW of Yunnan. The gradual change in crustal thickness matches with the gradual topography change over this region. Our models also show a uniform crust–upper mantle S-velocity structure across the region, with a 8–22 km thick upper crust ( V s < 3.4–3.5 km/s), overlying a gradational middle-to-lower crust with S-wave velocities up to 3.5–4.1 km/s at Moho depth. The thickness of middle-to-lower crust is correlated with the Moho depth beneath the seismic stations. There exists low-velocity zone in middle-to-lower crust beneath the Yunnan region, especially in the western Yunnan. It is possible that the thick crust in this study area is mainly caused by a more ductile flow in the middle-to-lower crust. The Sn velocities are slightly higher beneath the southern Yangtze block and Southeast Yunnan fold belt (4.35–4.75 km/s) than the other block (4.15–4.35 km/s). This low Sn velocity beneath western Yunnan and Panxi rift may imply that there has been thermal disturbance to the uppermost mantle away from deep structures in this region. The V s in lower crust and upper mantle are different across the Red-river fault, which imply that Red-river fault is one main boundary fault. Combining results from other geophysical investigations, we conclude that both the Simao terrane and Baoshan–Tengchong terrane tectonically belong to the Gangwana plate.

Paleomagnetic study of Cretaceous rocks from Pu'er, western Yunnan, China: Evidence of internal deformation of the Indochina block

Cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the Jingxing and Nanxin Formation around Pu'er (23.0°N, 101.0°E), western part of Yunnan province, China, in order to examine quantitative amount of tectonic deformation of the Simao Terrane due to collision of India with Asia. After stepwise thermal demagnetization, the high-temperature component magnetization with unblocking temperature of around 680 °C is isolated. Positive field tests show that the high-temperature component magnetization from 25 sites is primary for the Cretaceous formations. A tilt-corrected mean direction is D = 59.9°, I = 45.2° with α 95 = 5.1°, corresponding to a paleopole at 35.8°N, 173.1°E with A 95 = 5.6°. Comparison with Cretaceous paleomagnetic pole of the whole China block indicates that amount of clockwise rotation in the Pu'er area is evaluated to be 46 ± 9° with respect to the whole China block, which is consistent with large amount of rotation in the Mengla and Jinggu areas (47° and 66°) in the central part of the Simao Terrane. Clockwise rotation of the central part is larger by more than 20° compared to the Yunlong and Yongping areas of northern part of the Simao Terrane, suggesting that the Simao Terrane experienced internal deformation. The southward displacement of the Pu'er area expected from inclination data is 7.5 ± 6.6° with respect to the Chuan Dian fragment, which is a similar estimate to other areas in the Simao Terrane and the Indochina block. While the Simao Terrane displaced southward by 800 km together with clockwise rotation of 30° as a whole, further clockwise rotation occurred in the central part.

Permian tectonic framework and palaeogeography of SE Asia

The principal continental lithospheric terranes, now comprising SE Asia and that existed in the Permian, are the South China/Indochina Super Terrane, and the Sibumasu, Simao, West Burma and SW Borneo/Semitau terranes. The Simao Terrane is here regarded a separate terrane, derived from South China by back-arc spreading in the Lower Carboniferous, rather than as an extension of Indochina. The main Palaeo-Tethys ocean in Thailand is identified as being represented by the Chiang Mai and Sra Kaeo sutures, and the eastern boundary of the Sibumasu Terrane is thus interpreted to lie farther west in Thailand than previously inferred. The Nan-Uttaradit Suture is interpreted as representing a marginal back-arc basin, and contiguous with the Ailaoshan and Jinshajiang sutures in SW China. The South China/Indochina Super Terrane, and the Simao Terrane were located within the Palaeo-Tethys in equatorial latitudes during the Permian, where they, together with North China, exhibit Cathaysian floras and faunas. The Sibumasu Terrane was located on the NW Australian margin of Gondwana until the Sakmarian. Major shifts in faunal affinities from Gondwana province faunas to an independent Sibumasu Province and then to assimilation into the Cathaysian Province, records the separation and rapid northward drift of Sibumasu after the Late Sakmarian. The principal Permian palaeogeographic models for the region are assessed for internal consistencies and conflicts.