LATE CENOZOIC FLOOD BASALT ERUPTION IN DALINOR,INNER MONGOLIA
Fissure eruption refers to volcanic activity where magma is expelled to the Earth's surface through cracks in the crust.During such eruptions,large volumes of low-viscosity lava flow rapidly along these fissures,forming extensive flood-like lava fields known as"flood basalts".Fissure eruptions are characterized by their significant magma scale and devastating impact.Although there is no historical record of volcanic fissure eruptions in China,numerous late Cenozoic lava platforms have been discovered in XilinGol League and Chifeng,Inner Mongolia.These platforms likely represent overflow basalts formed by continental volcanic fissures.In this study,we focus on investigating the lava platform within the Dalinor volcano group located in XilinGol area using tools from volcanic physics and geochemistry.The XilinGol League in Inner Mongolia hosts extensive late Cenozoic lavas,encompassing an area of approximately 10000km2 and representing one of China's largest basaltic provinces.This volcanic field,formed through flood basalt volcanism,has remained active since the late Cenozoic and poses potential risks of future eruptions.To enhance our understanding of its origin and assess eruption hazards associated with intracontinental flood basalt volcanism,this study focuses on the latest lava platforms within the Beilike region of the Dalinor Volcanic Field.In this study,we investigate volcanic activity by examining its eruptive characteristics,including magma temperature,lava viscosity,and lava flow velocity.During the volcanic eruption,a significant portion of the lava engulfed the gently sloping surfaces of fluvial and lacustrine deposits,resulting in the topographic formation of expansive lava plateaus.Throughout this process,surface lava underwent condensation to give rise to pahoehoe lavas.In contrast,the underlying plastic lava continued its effusion along eruptive fissures,causing vertical compression of the solidified crust and subsequent formation of lava ridges.In the vicinity of the eruptive fissure,a series of low ridges measuring 3-8 meters in height and tens to hundreds of meters in length emerged.At a distance over 3km from the eruption fissure,the lava flow exhibits a banded distribution with an elevated central portion and inclined edges,displaying characteristic lava levees.Columnar jointing is commonly observed at the periphery of the plateau profile.The profile exhibits stacked layers of lava,indicating a laminar flow movement of the lava flow.These geological features are consistent with the characteristics typically associated with flood basalt eruptions.The whole rock geochemistry indicates that the lava in Beilike belongs to olivine tholeiitic basalt and alkali olivine basalt.The lava is characterized by phenocrysts primarily composed of olivine and clinopyroxene,particularly augite.The Mg # values of clinopyroxene vary between 58.59 and 80.69.The eruption temperature of Beilike lava is determined by applying clinopyroxene-melts geothermo-barometry inversion,yielding a range of 1123.2-1173.4℃.Additionally,the visco-sity of the erupting lava is obtained using a previously established calculation model,resulting in values ranging from 30 to 187Pa·S.This paper investigates the flow dynamics of"fissure eruption"through an analysis based on principles of lava fluid mechanics while considering relevant physical properties.We assume that high-temperature lava behaves as a Newtonian fluid and consider zero overflow velocity near the vent during fissure eruptions.The influence on lava flow primarily stems from surface slope,gravity,and inherent fluid properties rather than temperature variations throughout the process.We assume that the surface slope is 0.5 degrees.The lava temperature is 1120℃,and the thickness of the lava is 1m.By adopting this approach,we can calculate the maximum attainable flow velocity upon reaching a stable state.It has been calculated that during fissure eruptions in the study area,the velocity of lava flows primarily ranged from 0.4m/s to 1m/s,with occasional instances reaching as high as 2.5m/s.The present study unveils the eruptive characteristics of flood basaltic volcanism in the Belike region.It establishes the physical parameters of the lava flows,thereby providing essential data for formulating strategies to mitigate future volcanic eruption disasters.The key parameters presented here not only contribute to the understanding of volcanism in Inner Mongolia but also hold significant reference value for basaltic volcanism in other continental intraplate environments across China.This research will enhance our understanding of this unique form of volcanism while providing a scientific basis for mitigating volcanic disasters.Moreover,the calculation methods and steps employed to derive these parameters may readily apply to other volcanic fields.
万方数据
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