Characteristics of volcanic geomorphology in Dalinor volcanic swarm and petrologic features of the volcanic rocks, Inner Mongolia

The volcanic activities occurred pervasively in northeastern China in Cenozoic era. Four Quaternary volcanic swarms,Nuomin-Kuile River volcanic swarm, Halaha-Chaoer River volcanic swarm, Abaga-Dalinor volcanic swarm and Wulanhada volcanic swarm, locate from north to south along the western part of Daxing ' anling-Taihangshan Gravity Lineament in Inner Mongolia Autonomous Region, extending more than 1000km. There were totally about 400 volcanoes and the area of volcanic rocks was more than 36000km2. The numerous volcanic eruptions suggested the ever strongest intraplate volcanism in China. The Dalinor volcanic swarm, composed of a hundred volcanoes of varied sizes and shapes, is located on the Dalinor lava plateau between Xilinhot and Chifeng in the southeastern region of Inner Mongolia, whereas Abaga basalt and Beilike basalt are found northwest of this volcanic swarm. In the tectonic setting, the Dalinor volcanic swarm lies at the junction of the Xing' an and Songliao blocks, i. e. the composite section of the EW-trending Tianshan-Yinshan deep faults, the NE-trending Daxing' anling-Taihangshan fault, and the NW-trending Abaga-Chifeng fault. Overall, the Dalinor volcanic swarm exhibits a step-shaped plateau, with the central portion at a higher elevation than the margins. The surface of the plateau is relatively flat, with a raised ridge and densely distributed volcanoes. Many newly formed volcanic cones are distributed on the lava ridge in the central area of the plateau, and are roughly arranged in an EW direction. Some of these cones exhibit a beaded distribution pattern striking in the NE direction. The area of the volcanic field is roughly 3100km2 . Most of the Dalinor volcanoes are central eruption or central-crevasse eruption style, leaving cluster of volcanic cones with height of 50 ~130m outstanding in the lava plateau, and the volcanic eruption materials covered and superimposed in the previous erupted lava tableland. The volcanic cones were denuded through the long-term geological evolution, among which the relatively young volcanoes, i. e. Gezishan volcano, kept a whole volcanic apparatus including cinder cone, crater, airfall and spatter deposit, lava flow, fumarole and nvading dike, etc. In order to reveal the overall topographic undulation of the Dalinor volcanic swarm, two profile lines, ' A-B' and ' C-D' across the volcanic field, were drawn to traverse the lava plateau in the study area, with the crossing-point at the Da' aobao volcano. It can be seen both profile lines are relatively flat at the heights of 1280m, 1360m, 1440m and 1500m, exhibiting a clear step topography. The morphology measurement of a volcanic cone is an effective tool to determine the relative age of a volcano. It is well known that the height of a cone, the diameter of a crater, the depth of a crater pit, and the slope gradient of a cone' s outer surface decrease with time, while the width of the crater wall rim and the diameter of the cone bottom consistently increase. The increase rate of the diameter of the cone bottom is much greater than that of the diameter of the cone top; namely, the difference between the diameter of the cone bottom and the width of the crater wall rim also continually increase over time. The ratio of cone height to diameter ( H/D) was obtained by subtracting the cone bottom diameter from the crater diameter, which can reflect the formation time of a cone to a certain extent. Thus, the evolutionary process of a volcanic cone can be indicated in terms of a series of parameters, i. e. the height of a cone, the diameter of a cone bottom, the crater rim, and the slope gradient of a cone. According to their geomorphology, the existing Dalinor volcanic cones could be roughly classified into four types: single truncated conical, composite, shield, and residual. The different denudation degree of the volcanic cones implies the relative eruption sequence of volcanic activities. Though the overall evolutionary process of a cone from new to old can be considered as conical -? shield -? residual, however, the existing cone morphology doesn' t always match the eruption time. A composite volcano, for example, is rebuilt by multiple eruption episodes, its H/D value doesn' t reflect its evolutionary sequence, and instead, its eruption age should be judged on the basis of careful field investigation. As the conical and composite volcanoes located in the central part of the plateau, the shield ones usually located in the margin, and the residual ones widely distributed in the volcanic field, which together indicate the volcanoes in the plateau margin formed earlier than those in the central, i. e. the volcanic eruptions migrated from the margins to the center of the lava plateau, while the even earlier large scale tholeiite overflowed and formed the base of the lava plateau. The lithology of Dalinor volcanic rocks is primarily basanite and alkali basalt, and some is trachybasalt. Though the SiO2 content of the rocks give a wide span ( roughly 41 % ~ 49%) , total alkali vary narrowly within 2% , ploting all above the alkali line in the TAS diagram. The rare phenocrysts, cryptocrystalline or microcrystalline matrix of the volcanic rocks make them quite different from the tableland-forming tholeiite indicated by adjacent Beilike basalt. The spider diagram of trace elements tilts to the right as a whole with LREE enriched and HREE deficit, suggesting the low degree partial melting of the magma. The similar trace element patterns between Dalinor and adjacent volcanic rocks imply they originated from roughly the same magma source, probably the asthenosphere, though Beilike basalt had some exceptions. The lithology of the volcanic rocks and relative eruption sequence instructed by denudation degree of volcanic cones demonstrate the magma evolved from alkali basalt and trachybasalt to basanite, and probably had a gradually deeper source with time.