Tengchong volcanic field
The Tengchong Volcanic Field is a Cenozoic volcanic field located in the Southeastern margin of the Tibetan Plateau around 40 km from the Chinese border with Myanmar. The TVF is uniquely the only region affected by Quaternary volcanism that is part of the Himalayan Geothermal Belt caused by the Indo-Asian continent-continent collision. The TVF is characterized by hydrothermal activity and large-scale eruptions last recorded in 1609CE. Although the volcanoes themselves are considered extinct, several geothermal fields geographically linked to the TVF are still highly active. Evidence for geothermal activity can be linked to several prevalent active hot-springs located predominantly within the vicinity of the volcanoes in the TVF. Holocene eruptions occurred predominantly in the three largest volcanoes in the TVF named the Dayingshan, Ma'anshan and Heikongshan, the highest of which reaches 2865 meters above sea level. The volcanoes are distributed in a string-like pattern clustered from North to South in the middle on the Tengchong basin and are characterized by post-collisional high-Potassium calc-alkaline series eruptions. The TVF provides unique geographical and geological knowledge as understanding the geological processes of creation provides insight into aspects such as the history of volcanism during the Quaternary Era in the region and as well as compositional information of its source and crustal assimilates. The TVF can be visited in the Tengchong Volcanic Geothermal National Geological Park.`
Geology
General overview and history
The volcanic field is located geographically in China but geologically more closely related to Southeast Asia's volcano zones. The TVF is a product of one of the most important events that transpired in the Cenozoic Era, the Indo-Asian continent-continent collision which occurred ca. 59 Ma. The result of the collision was the uplift of the Tibetan Plateau along with Asian continent acidification, global climate change, tectonism and volcanic activity associated with the TVF. Prior to continent collision during the Paleozoic, the Tengchong block was located along the Indian margin of the Gondwana. High resolution tomographic data revealed evidence for an eastward under thrusting of the Indian continental lithosphere into the asthenospheric mantle underneath the TVF. This indicates that prior to continent collision, the Tengchong block along with the West Burma block overlaid the subducted Neo-Tethyan oceanic lithosphere. There are a total of 68 volcanoes all of which are pyroclastic cones and around 25 of these still bear recognizable craters and cones. In addition to the volcanoes, there are 58 hot springs in the TVF all derived from the Quaternary Era. The volcanoes in the TVF are typically concentrated in clusters along a string-like north-south pattern. Of the 68 volcanoes, there are three that are still considered active today based on their measured geothermal activity. These volcanoes are Maanshan, Heikonshan, and Dayinshan; Dayingshan being oldest of the three volcanoes and last erupted in CE1609 documented by Chinese geographer Xu Xiake. The crust averages thick in the TVF whereas to the North-South the crust is measured to be on average thick. The lithosphere-asthenosphere boundary is roughly deep in the TVF and deep in the surrounding regions.Petrology and petrogenesis
The TVF is predominantly composed of volcanic rocks that follow a high potassium calc-alkaline suite formed by post-collisional continent subduction of the Indian slab and regional extension. The rocks found in this region are basalt, dacite welded tuff, basaltic trachyandesite and trachyandesite. These rocks where extruded out as lava flows and pyroclastic material. The origin of the rocks present in the TVF can be categorized into three eruption stages deduced by systematic K-Ar Dating. Basalt and olivine basalt forming during the late Miocene to Pliocene Acid rocks forming in the Pleistocene. Basalt and intermediate-acid rocks form during the late Pleistocene to Holocene. The largest of rock distribution in the TVF is characterized by the rocks formed during the Pleistocene indicating that volcanic activity was greatest during this phase.Andesitic lavas comprise the most recent volcanic rocks. The basement rocks are dominantly composed of Precambrian metamorphic rocks underlain by sedimentary and igneous rocks including limestones, sandstones, mudstones, and granitoids followed by volcanic-sedimentary sequences. As eruptions progressed with time, the MgO content in the volcanic rocks decreases whereas the K2O content increases. Chemical variations in HKCA suite exist due to derived petrological variations in the rocks. This includes partial melting of an enriched primitive mantle derived magma source by subducted clay-rich sediments which formed the trachybasalts. Then followed by mantle-derived magmas forming the basaltic trachyandesites and trachyandesites originating from the lower crust by from a process called assimilation-factional crystallization.