| The Dali volcanic swarm, composed of numerous volcanoes of varied sizes, is located on the Dali Nur lava platform between Xilinhot and Chifeng, southeastern Inner Mongolian. In tectonic setting, this volcanic swarm lies on the west side of the Daxing’anling-Taihangshan gravity gradient zone, at the junction of the Xing’an and Songliao blocks, and the composite place of the EW-trending Tianshan-Yinshan faults, the NE-trending Daxing’anling-Taihangshan fault and the NW-trending Abaga-Chifeng fault. In this region, the tectonic pattern and evolution have been changing steadily, creating the conditions for Cenozoic magmatic activity, and keeping a control on the distribution of cones. The volcanic rocks cover about a 3,100 km2 area, with a hundred of volcanic cones, and the lithology dominated by basalt and basanite. The Abaga and Beilike basalt is present in the northwest, and Chifeng basalt in the southeast, both constitutes the volcanic swarm. It formed through multiple episodes of eruption in the Pliocene-early Pleistocene, mainly composed of volcanic cones and lava plateaus, with numerous cones and vast lava flow coverage, but a small amount of new lava flows exposed. This thesis focuses on three issues of this volcanic swarm:volcanic geology and geochemistry, the interpretation of volcanic topography and geomorphology, and the analysis of the cone morphology. The results are as follows:1. Volcanic geology and geochemistry①The characteristics of the volcanic geology:The Dali volcanic swarm contains a great number of volcanic cones and the broad distribution lava plateaus. The majority of volcanic edifice have been leveled off, leaving flat terrain and single surface vegetation. The most volcanoes are of the Strombolian eruption type, with craters and lava outlets. The cones are about 50-200m high. The relatively young volcanoes have residual structures, including scoria cones, craters, collapse pits, lava domes, intrusive dikes and well-preserved fumarolic cones. Volcanic rocks are dominated by lava and cinder. In addition, there are many other volcanic products such as mantle xenoliths, shell inclusions, pyroxene megacrysts, volcanic bombs, ignimbrite and so on, which indicate that magma was originated from deep mantle and underwent strong magmatism.②The-property of rock and mineral:In color, the volcanic rocks are mostly grey and black, occasional reddish brown. Porphyritic texture and massive structure characterize them. The phenocryst is composed of olivine, clinopyroxene and plagioclase. The microscopic view shows that the greater the phenocryst, the more serious the alteration.③Geochemical characters of rocks:Contents of main elements were determined through testing and TAS projections. The results show that the main products of Dali volcanoes are basanites and tholeiites. Analysis reveals negative correlation between the main element SiO2 and the elements TiO2, MgO, P2O5, K2O, and FeO, while positive correlation between SiO2 and Al2O3. Maybe these correlations reflect a certain rule of magma evolution. With reference to previous work on trace elements, this study suggests that basanites is derived from the enriched mantle, and its REE is similar to continental rift type basalt. Meanwhile, tholeiite is the result of the residual mantle’s re-partial melting, of which the REE and spider diagram bear a resemblance to the primitive mantle.2. Interpretation of volcanic topography and geomorphology①Topography:Overall, the Dali volcanic swarm exhibits a step-shaped lava plateau, with the central part being higher than the margins. All the stairs of the plateau exhibit a concentric spread, with extension in both east and west directions. It is divided into four steps according to the elevations, which are 1280m,1360m,1440m and 15000m, respectively. The middle part has large topographic relief, large slope gradients, rough surface, and a dense distribution of volcanic cones, while the rim is flat, low fluctuation of terrain, with less cones. So the Dali volcanic swarm is divided into two parts according to the relief features, and profile analyses are made the two volcanic clusters with different elevations.②Interpretation of lava plateaus:The boundary of the lava plateau was highlighted by multiplying the slope gradient and the undulation, so as to delineate the range of the lava plateau. The lava plateau spreads as an EW-trending belt, with small branches in both south and north directions. Using ArcGIS, the area and volume of the lava plateau are estimated to be about 2,563 km2 and about 615 km3, respectively.③Interpretation of newly formed lava flows:With such means as band selection based on the Envi software, band information enhancement, and visual interpretation, this work extracted the boundary of newly formed lava flows, and interpreted that their overall flow directions are towards the low-lying south. The lava flow areas of Gezishan and Alatantugurige are 54.97 km2, and 17.04 km2, respectively, so the eruption volume of the former is much larger than that of the latter. According to the locations of fumarolic cones and fumarolic dishes, it was inferred that the southern sections of the lava flows were in a wet swamp environment.④Interpretation of faults:By the interpretation of the overall spatial pattern of the Dali volcanic swarm as well as its geomorphological linear elements, the beaded distribution of volcanoes and the major axis azimuths of volcanoes, It is inferred that there are NE-trending and nearly EW-trending faults in the basement of the lava plateau in this area.In combination with the 1:200,000 geological map and previous work, this thesis suggests that the overall distribution of this volcanic swarm is controlled jointly by the EW-trending Tianshan-Yinshan fault and the NE-trending Daxing’anling-Taihangshan fault.3. Cone morphology① Morphological characteristics of cones:There exist a large number of preserved scoria cones in the Dali volcanic swarm, which are of various types. According to the morphological characteristics on the land surface, such as the crater, the slope and the compound degree, these cones can be classified into four types:conical volcano, composite volcano, dome volcano and shield volcano,of which the number is 50,14,28,10 respectively. The conical volcano is truncated circular ring cone, composite volcano is compounded by multiple cones with many craters or ports, dome volcano is of dome shape without crater, shield volcano looks like a gentle peltate. Different types indicate their diverse eruption modes or evolution times.②Forms of cones:This work classified the cones in the study area according to their forms, and statistically analyzed the morphological parameters like cone height, crater diameter, basal diameter, crater depth and cone slope. These parameters are linked to each other, for example cone slope can be expressed by the ratio of height to diameter (H/D). In addition, the slope can be indicated by the degree of weathering to some extent, and the corresponding histogram shows that the ratios of conical volcanoes, composite volcanoes, dome volcanoes and shield volcanoes diminish progressively. The distributions of different types show that composite volcanoes are mainly present in the middle of the platform, while shield volcanoes generaly appear on the edge of the platform. It implies multiple episodes of volcanic eruption in the central plateau, while the formation of volcanoes at the edge is relatively earlier.③Formation and evolution of volcanoes:the physical process of cone’s formation was reconstructed by analyzing cone shape, and the relationship of the evolution between different types of the cones was deduced with reference to numerical simulation of foreign researchers. This study inferred the migration tendency of volcano swam according to the distribution, formation age and geochemical data of different volcano, i.e. the early eruption was of larger scales, covering the entire platform, while since the supply of magma decreased, the later volcanoes contracted to the central part.④Relationhip of new and old cones:The weathering and denudation degrees can be reflected by the cone’s shape to some extent. The time of denudation can be quantified by the variation of slope when denudation rate remains constant, so it is possible to establish the relationship between age and morphology. Through statistical analysis of the cone’s parameters and the extraction of the H/D value which is highly correlated with slope, the relative age of the volcano can be calculated by using evolution law and formula. The results show that the ages of the latest and oldest cones are 0.136Ma and 2.02Ma, respectively, and the average ages of the conical volcano, dome volcano and shield volcano are 0.31 Ma,0.65Ma and 1.53Ma, respectively. The diagram of age projections shows that the volcano in the middle of the platform is relatively new, which is consistent with field observations. |
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