Genetic Relationship Among The Cenozoic Tectonic Evolution, Volcanic Magmatism And High Geothermal Anomaly Activities In Tengchong, Western Yunnan Province And Its Tectonic Implications

Tengchong area, located in the southeastern margin of the Tibetan Plateau, is an integrated region composed of volcanoes, geothermal field (hot spring) and earthquake with characteristics of intense tectonic movement, frequent magmatic activity and active geothermal fields in Chinese mainland. Considered with geotectonic view, Tengchong block is located at the south of Eastern Himalayan Syntaxis of eastern Tethys tectonic domain, bounded between the Meso-Tethys Nujiang suture to the east and Neo-Tethys myitkyina suture to the west, which is the northern part of Sibumasu block and southern part of Lhasa block. This paper focuses on the study of the geological characteristics of Mesozoic-Cenozoic tectono-magmatic-volcanic activity and high geothermal anomaly areas in Tengchong area, and of the the genetic relationship among them for analyzing and discussing the unique regional geological structure features and tectonic evolution process since Cenozoic by the methods of multidisciplinary information integration and comparative geology.In this research, the author re-analyzed the collected data of geological, mineral resources and geophysical fields which are related to Tengchong and adjacent area, and then carried out the detailed field geological survey and scientific revision of geological map on some typical high geothermal anomaly area. On the basis of integration and summarization of previous data, this paper focuses on the following study aspects:a, This paper selects the late Cretaceous-Eocene granites from the Langpu-Rehai-Maanshan high geothermal anomaly area and Wuhe-Puchuan-Tuantian high geothermal anomaly area, and then studies of deformation, metamorphism, petro-geochemistry, zircon U-Pb geochronology and Hf isotope of them, and then explaines their tectonic implication and the relationship with the high geothermal anomaly areas; b, Studies of 40Ar-39Ar geochronology and geochemistry of the late Cenozoic trachyandesite from the Mangbang Formation and its tectonic implication; c, Studies of Petrography, mineralogy, equilibrium temperature and pressure and micro-fabric characteristics of deep source xenoliths and its genetic significance; d, The distribution and activity of the Cenozoic magma chamber in Tengchong volcano area deduced from the MODIS multi-temporal monthly night LST data; e, Studies of kinematics and geochronology of active fault since Cenozoic and the characteristics of fault basin, and then discusses the time of tectonic deformation and analysis of tectonic stress field, and their tectonic implication; f, Analyzing and evaluating the crustal stability characteristics at the present time by the technical method of ArcGis spatial analysis ability based on the comprehensive data of the seismic, geothermal and active fault data of Tengchong area; g, Discussing the genetic relationship among the tectonics, volcanic magmatism and high geothermal anomaly activities of the high geothermal anomaly area and its tectonic implications, and then building conceptual model for the Cenozoic tectonic evolution in Tengchong block. The main results are as follows:(1) Proto-mylonitic biotite monzonitic granite, ultramylonitized biotite monzonitic granite and silicified fractured syenogranite within the high geothermal anomaly areas in the Tengchong block are exposed in Qingshui Zuosuoying, Xinhua Heishihe, and Rehai Liuhuangtang, respectively. Studies of deformation, metamorphism, petro-geochemistry and geochronology suggest that the late Cretaceous (73 Ma) proto-mylonitic biotite monzonitic granite is a high temperature shoshonitic and strongly peraluminous granite formed in a transition setting between the active continental margin volcanic arc and the post-collisional tectonic environment. The granite has a nearly horizontal (<30°) stretching shear mylonitic foliation in early stage, and locally developed high-angle dextral strike-slip extrusion mylonitic foliation in late stage. Eocene (48-46 Ma) ultra-mylonitized biotite monzonitic granite and silicified fractured syenogranite are classified into medium-high temperature shoshonitic and strongly peraluminous A-type granites with characteristics of high-angle (70°-87°) dextral strike-slip shear mylonitic foliations in late stage. The geochemical features indicate that the granites formed in a post-collisional and within plate tectonic settings. Therefore, in general, the late Cretaceous-Eocene shoshonitic and strongly peraluminous granitic magmatism have a tightly genetic relationships with basaltic magma underplating and partial melting of the crust generated by upwelling mantle, which resulted from extensional collapse, delamination, or slab break-off following the subduction-collision orogenic uplift.(2) Pleistocene trachyandesites from the Mangbang Formation in Tengchong yielded a high-precise 40Ar-39Ar plateau ages of 2.1±0.1 Ma and 2.0±0.1 Ma. They show high-potassium calc-alkaline features, and are enriched in both light rare earth elements and large ion lithophile elements, but depleted in heavy rare earth elements and high field strength elements, which show dual characteristics of the arc and intraplate volcanic rock formed possibly in continental margin post-orogenic environment. Hence, southeast-ward escape of the Indosinian Block and rejuvenation of Tengchong arcuate strike-slip fault zone (e.g., Longchuanjiang fault belt and Dayingjiang fault belt) triggered the eruption and emplacement of the volcanic rocks in the late Cenozoic.(3) Research on the Cenozoic volcanic rocks of the Mangbang formation in the Mangbang area, reveals many granulite xenoliths of felsic granulites and two-pyroxene granulite in trachyandesite. Equilibrium temperature and pressure estimates for the felsic granulite xenoliths and two-pyroxene granulite xenoliths are 869-969 ℃,0.94-1 GPa and 841-972 ℃, respectively, indicating that they are high temperature granulites. The crystallization temperature and pressure of trachyandesite phenocryst is:1008-1059 ℃,1.26-1.33 GPa, suggesting that they were formed and originated in the depth of 21-36 km between the lower crust and the top of the upper mantle. The discovery of high temperature granulite in volcanic rocks suggests that the Tengchong Cenozoic magmatic activity may be related with volcanic magamatism and high temperature metamorphism in the lower crustal caused by post-collisional extensional collapse, delamination and slab break-off after subduction and collision of the Indian Plate and the Tengchong block.(4) In order to study the spatial distribution and activity of the underground magma chamber, MODIS LST (Land Surface Temperature) data were used to identify the geothermal anomalies that may be created by the heating from the underground magma chamber. In the light of the previous studies in this area, it can be deduced that there are three magma chambers beneath the three thermal anomalies. The first one is located along Wuhe-Xinhua-Puchuan-Tuantian with the most significant geothermal anomaly. The second one is situated in the Langpu-Rehai-Maanshan area with a significant geothermal anomaly. The third one is located between Mazhan and Qushi with an obvious geothermal anomaly. The geothermal anomaly in Langpu-Rehai-Ma’anshan area shows the greatest variations among the three locations, which suggests the active convective exchange of heat between the underground magma chamber and the surface water, and can be inferred that the beneath this magma chamber is the most active among the three. The findings of this study agree well with the results from seismology, GPS-based deformation detection, He isotopic emission and relative geothermal gradient measurements. It demonstrates the effectiveness and potential of thermal infrared remote sensing in geothermal studies.(5) Tengchong area mainly exist two important tectonic deformation events since Cenozoic. The early stage is characterized by nearly NS-trending dextral strike-slip shear extrusion movement in Gaoligongshan tectonic zone, and the late stage is characterized by NE-trending sinistral brittle strike-slip fault in the south part of the arc-shapes fault zone. Tengchong block was bounded by Gaoligongshan dextral strike-slip fault zone to the east and Myanmar Sagaing dextral strike-slip fault zone to the west, forming a series of nearly NS-trending to NE-trending arc-shaped strike-slip and pull-part fault that controls the distribution of the fault basin, igneous and volcanic activities, and geothermal activities.(6) Eocene-Quaternary volcanic-magma activity was spatially associated with high geothermal anomaly areas. The tectonic evolution during late Neocene to Quaternary is represented by the formation of the strike-slip and extensional normal faults associated with extensional fault basin. Accordingly, the boundary faults of fault basins as well as deep volcanic magmatism are the predominant reasons for the concentrated distribution of the medium-high temperature geothermal springs along the strike-slip extensional fault basins.(7) Volcanic field of the Tengchong area could be divided into three regions:Ⅰ-relatively stable region, Ⅱ-relatively unstable region, Ⅲ-unstable region basis on the regional crustal stability analysis and evaluation. Relatively stable area is mainly located in the northern area with characteristics of unconspicuous geothermal activity, developing NS-trending faults and M<3 earthquake. Relatively unstable area is mainly located in the southwestern area, with obvious geothermal activity, developing NE-trending faults and M<3 earthquake. Unstable area is mainly located in the southeastern area with strong seismic activity and M>3 earthquake (even be up to M>5). The unstable area also is the tectonic junction among the NS-trending faults, NE-trending faults and NW-trending faults.(8) This paper is proposed a model for the Cenozoic tectonic evolution of the Tengchong area, which can be divided into three stages as follows:the early stage is dominated by compressional thrust, and then switched to the strike-slip shearing movement, and the late stage is characterized by tensile-shear structure. The modern tectono-magmatic-volcanic activity and geothermal activity in Tengchong area are mainly affected by late stage tectonic movement. The tensile-shear faults formed in the hinge zone of arc-shaped fault zone provide the upwelling magma channels for the Pliocene to Holocene volcanic continuous eruption, and control the distribution of the hot springs and geothermal anomaly area.