内蒙古达里诺尔火山群火山地貌特征及火山岩岩石学特点

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

在内蒙古自治区锡林郭勒盟与赤峰交界处、达里湖的西北部有一群第四纪火山,数目达上百座,多数为中心式或裂隙-中心式喷发,形成相对高度在50~130m之间的火山锥,它们被称为"达里诺尔火山群",也是世界上海拔较高的火山群之一。因锥体形成后受风化、流水侵蚀等外力作用,多数火山锥已受剥蚀,但相对较新的火山还保留着完整的火山机构,包括火山渣锥、火山口、降落和溅落堆积物、塌陷坑、熔岩流、喷气锥等。达里诺尔火山锥体多由火山碎屑物组成,熔岩则流动迭加在早期形成的多级熔岩台地之上,分布总面积约3100km2,与其它火山岩共同组成了广阔的内蒙古高原。火山区整体呈四周低、中间阶梯状隆起的台地地形,按高程可分为四个阶梯,高度分别为1280m、1360m、1440m和1500m。熔岩台地的展布和火山锥的分布均呈东西走向的条带状,火山群的整体分布主要受东西向天山-阴山基底断裂和北东向大兴安岭-太行山断裂的双重影响,与大地构造关系密切。本群火山锥体虽然大小不一、形态各异,但按锥体形态可将火山锥分为截顶单锥形、复合型、盾形和残余型等四类,其剥蚀程度有明显差异,暗示了火山喷发活动时代的先后关系。锥形及复合型火山位于火山群中部,而盾形火山多沿台地边部分布,残余形火山则分布广泛,表明台地边部火山形成时间较早,晚期火山活动集中于台地中部,而更早期造台地的溢流式拉斑玄武岩喷发规模更大,形成本区基底。达里诺尔火山岩的岩性主要为碧玄岩和碱性玄武岩,少量为粗面玄武岩,虽然SiO_2变化范围较大,但全碱含量变化幅度相对较小,均属钾质系列的碱性火山岩范畴。火山岩斑晶数量少,在镜下为斑状结构,基质多为隐晶质,少量微晶质,明显不同于火山岩之下早期熔岩台地火山岩的基质显晶质结构。火山岩的微量元素呈LREE富集、HREE亏损的右倾型,显示了岩浆低度部分熔融的特征,并且本区与邻区火山岩可能有共同或相似的岩浆源。在岩性相近的基础上,结合火山锥体剥蚀和保留程度显示的火山活动相对时序,暗示本区岩浆成分从碱性玄武质和粗面玄武质逐渐演化为碧玄质,岩浆可能来自软流圈,且来源逐渐变深。

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 invading 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.