Tengchong Volcanoes Research Articles

Reduced post-collisional magmas for Tengchong volcanoes, southeastern Tibetan Plateau

Rapidly cooled and well-preserved magnetite and ilmenite in young volcanic rocks provide good samples for constraining the magmatic redox conditions. We report chemical compositions of magnetite and ilmenite from the youngest Tengchong volcanoes at the southeastern margin of the Tibetan Plateau, to constrain the oxygen fugacity and temperatures for the Tengchong magmas. Oxygen fugacity values (△FMQ) relative to the fayalite-magnetite-quartz (FMQ) buffer are −1.8 ∼ −0.7 (averaging −1.1 ± 0.4) for Dayingshan; −1.3 ∼ −0.7（averaging −1.1 ± 0.3） for Maanshan; and −1.3 for Heikongshan. The temperatures for magnetite-ilmenite pairs of Tengchong lavas range from 789 to 1090 °C. The relatively reducing oxygen fugacity values (△FMQ = −1.8 ∼ −0.7) of the Tengchong magmas are significantly lower than those oxidizing arc magmas (△FMQ = +0.5 ∼ +3.5). The reduced magmas at Tengchong may exemplify other Quaternary post-collisional lavas of the Tibetan Plateau. The crystallization temperatures from mineral-liquid equilibrium are 1160–1192 °C for olivine, 1049–1110 °C for plagioclase, 978–1122 °C for clinopyroxene, 789–1090 °C for Fe-Ti oxides, 743–792 °C for zircon. The high Fe-Ti oxide temperature and Ti-in-zircon temperature indicate warm magmatic conditions (>750 °C) prior to eruptions of Tengchong magmas.

Rayleigh phase velocity and azimuthal anisotropy from ambient noise data in the Sanjiang lateral collision zone in the SE margin of the Tibetan plateau

The Sanjiang lateral collision zone in the SE margin of the Tibetan Plateau is located at the east edge of the junction of the Eurasian and Indian plates. Using the continuous seismic waveforms recorded by 146 temporary and 21 permanent seismic stations in the study area, we obtain Rayleigh wave phase velocity and azimuthal anisotropy for periods 2 s to 40 s from the surface wave direct tomography method. This direct tomography method can obtain finer high-resolution results than the traditional surface wave tomography. Our results show that the low-velocity anomalies are found beneath the Lijiang-Xiaojinhe fault (LXF), Red River fault (RRF), Chuxiong fault and Tengchong volcanoes, the high-velocity anomalies are in the region of Weixi and Panzhihua at periods 5 ~ 8 s. The fast velocity directions mainly align N-S. At periods 10 ~ 15 s, the distributions of low-velocity anomalies are consistent with the strikes of LXF and RRF. At periods 20 ~ 35 s, the high- and low-velocity anomalies are bounded by the RRF, which may imply the fault is divided by the thick crust (indicated by low-velocity anomalies) and the thin crust with shallow mantle (indicated by high-velocity anomalies). The fast velocity directions at the periods 10 ~ 35 s rotate clockwise from north to south of the study area. The intensity of anisotropy in the low-velocity zone is stronger than that in the high-velocity zone, and the intensity in the north of the study area is stronger than that in the south. Results indicate the source of anisotropy may be different in each subzone.

Gravity Anomalies and Lithospheric Flexure in Western Yunnan, China, Deduced From a New Dense Gravimetry

AbstractWe conducted a dense gravity/GNSS survey in western Yunnan, China, and improved the precision of Bouguer gravity anomalies (BGAs) and Free‐air gravity anomalies (FGAs) by about 50 mGal. The BGAs showed that there was no significant BGAs abnormal change over the whole study area, and the density structure of BGAs inversion shows low‐density regions at 20–40 km depth beneath Tengchong and its surrounding areas. Meanwhile, we calculated the loading ratio of the lithosphere in western Yunnan based on merged FGAs using the gravity admittance method. The resulting ratios show that the lithospheric flexure in this region results from a combination of the surface, the internal crust, and the mantle, with the internal crust dominating. Combined with other geophysical results, we speculate that the origin of Tengchong volcanoes is not large‐scale mantle upwelling, but limited mantle material rising along the fracture in the process of material migration in the middle‐lower crust.

Eruption history, petrogeochemistry, and geodynamic background of Tengchong volcanoes in Yunnan Province, SW China

Eruption history, petrogeochemistry, and geodynamic background of Tengchong volcanoes in Yunnan Province, SW China

Eruption history, petrogeochemistry, and geodynamic background of Tengchong volcanoes in Yunnan Province, SW China

Abstract The Tengchong Volcanic Field (TVF) is one of the youngest volcanic areas in China, and is located in the southwestern part of Yunnan Province, China, adjacent to the collision zone between the Indian and Eurasian plates. This paper summarizes the results of previous research on the eruption history, petrochemistry and geodynamic background of the TVF and presents a detailed analysis of the available data. Eruptions took place from the Pliocene to the Holocene and were divided into five stages. The composition of the magma went through two cycles: the N 2 − Q P 1 − Q P 2 volcanic rocks went through the evolutionary process from basalt to dacite and the Q P 3 − Qh magma evolved from trachybasalt to trachyte. The evolution of magma is mainly related to the fractional crystallization of pyroxene and ilmenite. The mantle source area of the TVF volcanic rocks was caused by mixing between the mid-ocean ridge basalt-source mantle and the eastern Indian continental margin sediments to different degrees. Geophysical data also showed that the Indian plate has been subducted under the TVF area. There are also magma chambers in the crust within a depth of 25 km in the TVF that are replenished by mantle magma.

U-series ages of young volcanoes from the Southeastern Tibetan Plateau: Holocene eruptions and magma evolution timescales

The Tengchong volcanic field in southwestern China represents rare Quaternary volcanic eruptions and associated active geothermal activities on the southeastern margin of the Tibetan plateau. The reported eruption ages for the 3 youngest Tengchong volcanoes (Heikongshan, Dayingshan, Maanshan) are highly variable (3.0–482 ka) because of the challenge to accurately and precisely date young volcanic rocks by radiometric methods. Here we use high-resolution 238U-230Th-226Ra disequilibria to constrain the eruption ages and magma evolution timescales. All 3 young volcanoes exhibit whole-rock (226Ra/230Th) disequilibrium ranging from 1.27 to 1.71, indicating eruption ages of <8 ka (5 half-lives of 226Ra). Eruption ages of 2.9–5.1 ka for Heikongshan, 3.3–4.0 ka for Dayingshan, and 2.9–3.6 ka for Maanshan are constrained by (226Ra/230Th) evolution models. This study thus suggests Holocene eruptions for all 3 volcanoes and represents the first Holocene radiometric age for Dayingshan. We also measured 238U230Th ages of zircons from Heikongshan, Dayingshan, and Maanshan. Zircon age populations are 54.6 ± 13 ka and 94.9 ± 4.1 ka for Heikongshan, 59.6 ± 5.8 and 90.1 ± 8.4 for Dayingshan, and 53.8 ± 8.7 ka and 76.5 ± 11 ka for Maanshan. Magma residence times for all 3 volcanoes are ~50 kyr for a shallow stage and 85 kyr for a deep stage in a two-stage magma chamber system below Tengchong. The similar zircon age distributions for the 3 volcanoes from Tengchong indicate simultaneous magmatism at 90 ka and 55 ka and imply interconnected nature of these volcanoes.

The quaternary eruptive sequence of the Tengchong volcanic group, southwestern China

The Tengchong volcanic group in SW China is situated along the NS-oriented tectonic belt in the southeastern margin of the Tibetan plateau. The volcanic field has experienced intense tectonic stress due to the continuous subduction of the Indian plate under the Eurasian plate, and throughout Quaternary has experienced pervasive volcanism. The present volcanic rocks cover the successive domains of basic, intermediate (Group II) and acid (Group I) rocks, belonging to the high-K calc-alkaline series. Correspondingly, the eruption types of the Tengchong volcanoes roughly fall into two categories: explosive eruptions with large amounts of volcaniclastics and lava; and moderately quiet central eruptions with small amounts of scoria and lava overflow.According to systematic geochronological dating methods such as K-Ar, Ar-Ar, 14C AMS, and OSL dating, as well as field work, we separate the eruptive sequence of the Tengchong volcanic group during the Quaternary into four subsequences: the early Pleistocene, middle Pleistocene, late Pleistocene and the Holocene epoch. The early to middle Quaternary eruption sequence consists of a series of explosive eruptions, with almost all the eruptive dacitic volcaniclasts and lava constituting the bottom layer of the Tengchong basin. The Tengchong volcanism reached its climax in the late Pleistocene when it formed numerous cinder cones. A series of moderately quiet central eruptions occurred in the late Quaternary, and basic and intermediate-acid volcanic rocks formed the upper layer of the basin. From this study, three volcanoes are determined to be of Holocene age, and we conclude that the volcanic activity moved from the south and periphery to the north and center, and ended in the Heikongshan eruption ∼ 3600 years ago.

Melt inclusions in phenocrysts track enriched upper mantle source for Cenozoic Tengchong volcanic field, Yunnan Province, SW China

The Tengchong Volcano Field (TVF), located at the southeastern margin of the Tibetan Plateau, is the only Cenozoic volcanic-related geothermal region around the Himalayan geothermal belt that extends from southwestern Tibet to western Yunan along the Indo-Asian suture zone. The volcanic rock suites from this field are important proxies for the India-Eurasia convergence since the Late Cretaceous. Here, we investigate the melt inclusions hosted in the Tengchong volcanoes from different locations including Qingliangshan, Heikongshan and Dayingshan based on petrological and geochemical data with a view to gain insights on the magma sources and petrogenesis. Olivine (Ol), clinopyroxene (Cpx), orthopyroxene (Opx) and plagioclase (Pl) phenocrysts from basalts and andesites of the Tengchong volcanic rocks carry abundant melt inclusions. These melt inclusions are high in silica and alkali contents, and enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) such as Rb, Th, U, K, Pb, but depleted in high field strength elements (HFSEs) including Nb, Ta, Ti, P, with arc-like signature. The geochemical features of the melt inclusions suggest that the TVF volcanic rocks are possibly cogenetic, but with a progressive increase in the degree of magma evolution from Qingliangshan, through Heikongshan to Dayingshan. Melt and trace element modeling indicates that the magmas probably originated through variable degrees of partial melting of LREE-enriched upper mantle within garnet stability field with significant contributions from subducted oceanic crust. The melts were contaminated with continental crustal during ascent, followed by fractional crystallization.The volcanic activity was controlled by major Late Cenozoic dextral strike-slip faults within an extensional setting related to the geodynamic evolution of India plate. Our study provides insights into the tectonics accompanying the India-Eurasia collision.

Age of a late Quaternary (13.6 cal kyr BP) eruption and associated marker tephra of the Tengchong volcanoes, Yunnan Province, southwestern China

The Tengchong volcanoes in western Yunnan Province are some of the youngest volcanoes in China, and determining their eruption ages would further our understanding of their genesis and historical activity. In this study, the age of a late Quaternary eruption of Dayingshan volcano was obtained by dating organic-rich sediments that envelope a 5-cm-thick visible tephra layer found in the Yubi peat section. Compositionally, the purified glass shards extracted from the layer by density separation are calc-alkaline trachyandesite and trachyte/trachydacite, which is consistent with the composition of the Dayingshan lava. An age-depth model of the Yubi section was constructed from accelerator mass spectrometry (AMS) 14C dating of peat samples and Poisson models of deposition (P_sequence) functions, using OxCal with the IntCal 13 database. This yielded an age for the tephra layer of 13,642 ± 84 cal yr BP. A nearly identical tephra layer occurs in the Qinghai Crater Lake sediments. The Yubi tephra is now a reliable age and stratigraphic marker layer, useful for regional paleoclimate and tephrochronology research.

Genesis and open-system evolution of Quaternary magmas beneath southeastern margin of Tibet: Constraints from Sr-Nd-Pb-Hf isotope systematics

Post-collisional volcanic rocks on the Tibetan Plateau and its margins contain valuable information about the geodynamic processes associated with this Cenozoic continent-continent collision. The Quaternary Tengchong volcanic field at the southeastern margin of the Tibetan Plateau formed high-potassium calc-alkaline volcanic rocks. Herein, we present comprehensive Nd-Sr-Pb-Hf isotopic and elemental data for trachybasalts, basaltic trachyandesites, and trachyandesites from four Quaternary Tengchong volcanoes (Maanshan, Dayingshan, Heikongshan, and Laoguipo) in order to understand their magma genesis and evolution as well as tectonic significance. Good correlations between SiO2 content and the ratios 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 177Hf/176Hf for these Quaternary volcanics strongly suggest that the combined assimilation and fractional crystallization (AFC) was an important process in the origin of basaltic trachyandesites and trachyandesites. High Y and Yb contents and low Sr/Y ratios of these basaltic trachyandesites and trachyandesites are uncharacteristic of adakites that formed by partial melting of eclogitic lower crust or partial melting of basaltic oceanic crust with eclogite as a restite. A combined assimilation-fractional crystallization model is proposed for these basaltic trachyandesites and trachyandesites. Nd-Sr-Pb-Hf isotopes for the uncontaminated Tengchong magma (trachybasalts with SiO2<52.5wt.% and MgO>5.5%wt.%) reflect a heterogeneous enriched mantle source. High Th/U, Th/Ta, and Rb/Nb ratios and Nd-Sr-Pb-Hf isotope characteristics of the uncontaminated magmas suggest that the enriched mantle beneath Tengchong formed as a result of subduction of clay-rich sediments, which probably came from the Indian continental plate. Partial melting of the enriched mantle was generated by deep continental subduction coupled with recent regional extension in the Tengchong area.

Crustal and uppermost mantle structure of SE Tibetan plateau from Rayleigh-wave group-velocity measurements

A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity measurements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 interstation paths were measured based on analysis of teleseismic waveform data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10–70 s. The new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones (LVZs) in the crust beneath the northern part of this region. Our imaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper–middle crustal LVZ may be regarded as evidence of a magma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block.

Finite difference tomography of the crustal velocity structure in Tengchong, Yunnan province

This paper applies the finite difference tomography method to invert for the P-wave velocity structure in the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of the Tengchong volcanoes and the deep tectonic genesis of Longling M7.3/M7.4 earthquakes. The results show the mean velocity in the crust with clear heterogeneity is a little lower. The lower velocities in the shallow layers are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high-velocity zone at 5 similar to 15 km depth probably represents the cooled and solidified magmatic intrusions or the less volatile ultramafic remnants. The low-velocity structure in the deep indicates the molten or semi-molten magma. It is probable that the magmatic activity beneath the volcanoes and heat flow intrusions in the deep crust of Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strong lateral heterogeneity is observed in Longling earthquake zone. The east of the Nujiang fault belt and the west of the Long ling fault belt have the characters of high-velocity with the high strength of rock, which are the main bearing body of stress accumulation. However, the low-velocity anomaly between the two faults extends to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Long ling fault and the Nujiang fault. The Long ling M7.3 and M7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons of the strong earthquakes in the Long ling area contain the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic effects.

Finite Difference Tomography of Crustal Velocity Structure Beneath Tengchong, Yunnan Province

Abstract:This work applies the finite difference tomographic method to invert P‐wave velocity structure beneath the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of Tengchong volcanoes and the deep structural genesis of the Longling M7.3/M7.4 earthquakes. The results show the velocity structure of the crust in this area has a pronounced heterogeneity. The lower velocities in the shallow subsurface are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high‐velocity zone at 5∼15 km depth probably represents cooled and solidified magmatic intrusions or less volatile ultramafic remnants. The low‐velocity structure in the deep crust indicates molten or semi‐molten magma. It is probable that the magmatic activity beneath volcanoes and magma intrusions in the deep crust of the Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strongly lateral heterogeneity is observed in the Longling earthquake zone. The east of the Nujiang fault belt and the west of the Longling fault belt are characterized by high velocity, implying large strain strength of rocks, which are the main carriers of stress accumulation. The low‐velocity anomalies between the two faults extend to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Longling fault and the Nujiang fault. The Longling M7.3 and M7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons for the strong earthquakes in the Longling area inlcude the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic actions.

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling–Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian–Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.

Seismological study on the crustal structure of Tengchong volcanic-geothermal area

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the Longling-Ruili fault and Tenchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the Rehai-Retian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to ‘plate boundary’ volcanoes.

K-Ar age and strontium isotopic composition of the tengchong volcanic rocks, West Yunnan province, China

Dating of 47 samples of the Tengchong volcanic rocks gave K-Ar ages ranging between 0.09 and 17.84 My. Thθ 40Ar/ 36Ar vs 40K/ 36Ar plot was made on the basis of those data in which radiogenic 40Ar is more than 2%. Four isochrones at 2.93, 0.81, 0.31 and 0.13 My were then obtained. The eruption time was from the Miocene to Pleistocene, with the climax occurring in the Late Pleistocene. The K-Ar ages were determined by statistical analysis and the probability of eruptions indicates that the Tengchong volcanoes are not to be considered extinguished but dormant volcanoes. Rb and Sr contents and the strontium isotopic compositions were determined on 20 samples of Tengchong volcanic rocks. The initial 87Sr/ 86Sr ratios range from 0.70578 to 0.71437. It would appear that the “parent magma” is of deep origin ( 87Sr/ 86Sr near 0.706) and that it underwent a progressive contamination by crustal material rich in radiogenic 87Sr.