Study On The Phreatomagmatic Eruption In Datong Volcano

Phreatomagmatic eruption is a kind of special eruption,which usually occurs when hot magma rises and contacts ground water.The water-melt interactions produce explosive eruption and base surge deposits,which result in maars.It's key to research phreatomagmatic eruption,as it would bring great damage to people's life and environment due to its special transportation process.Base surge deposits were discovered in the Datong volcanic field.They are mainly distributed in the Daxinzhuang village,Wulong Valley and north bank of Cetian reservoir along the Sanggan River crossing the field.The lacustrine sediment of the famous ‘Nihewan Formation' is located just beneath the base surge and later the overflowing tholeiite covered them.The base surge deposits from the proximal phase is as thick as up to35 m,whereas those from the distal phase is as thin as ~ 1m.It is inferred that the base surge at Daxinzhuang is near the crater according to the occurrence and size of clastic particles,while it is difficult to determine the crater locations for other two base surges.Some indicative sedimentary structures,such as crossing bedding,parallel bedding and accretionary lapilli,would usually form in the base surge deposits due to their special genetic mechanism.The color,composition,and sedimentary structure of the base surge deposits can distinguish them from lacustrine and fluvial sediments.The base surge deposits in the Datong volcanic field are mainly composed of basaltic pyroclastic particles,wall rock fragments and lacustrine sediments etc.We've also identified three key intergradational types of ash aggregates(coated pellets,cored pellets,accretionary lapilli),and each of them has a distinct internal microstratigraphy.The main and trace elements,as well as Sr-Nd-Pb isotopic characteristics of the pyroclastic particles in the Daxinzhuang base surge deposit are similar to those of the alkaline basalts in the western part of the field.The trace element normalized curves show relatively richer light rare earth elements abundances,and the primitive mantle-normalized spider patterns show the enriched large ion lithophile element compositions such as Ba,Sr,K,slightly enrich in Nb and Ta,similar to those of ocean island basalt(OIB).Sr-Nd-Pb isotopic characteristics suggest the basaltic fragments entrained by the base surges are a binary mixture of PREMA and EMI.Combined with the occurrence of the deposits,it is inferred that both the alkaline magma eruption and phreatomagmatic explosion occurred earlier than tholeiitic magma eruption.The similar geochemical characteristics propose that the basaltic fragments entrained by the base surges share the similar petrogenesis with the alkaline basalt erupted latest in the western part of the field.The particle size analyses found that the size of the coarse particles are concentrated within the range of 0.21-1.19 mm,and that of the fine particles are concentrated between 0.04-0.20 mm.Among them,the particle size of the proximal phase is significantly larger than that of the distal phase.Frequency distribution histograms of all samples show the single peak graphs,reflecting the single genesis of deposit accumulation.Most of the samples plotted in the histogram of median particle size and sorting coefficient fall in the range of base surge deposits,which is consistent with the field observations.The detailed microstructures of pyroclastic particles under SEM analysis show morphologies of blocky,long columnar,ellipsoidal,moss-like,elongated,etc.Some small spherical particles or aggregates adhere to the surface of the pyroclastic particles.Different types of particles have different morphological parameters."Passive" particles with a rounded shape and a smooth contour have a circularity value close to 1.Particles with elongated deformation have higher elongation,and particles with regular contours and right-angled edges have rectangularity close to 1.While particles with angular shapes and irregular contours have a higher rectangular value and a relatively lower compactness.The Datong-Yangyuan fossil lake occupied the Datong area during the Pleistocene.The OSL analysis for the upper baked soil,above the base surge deposit at Wulong Valley,yields 256.2±17.2 ka.Pollen analysis for the lacustrine sediments beneath the deposit indicated the ancient climate evolved from cold and wet to cold and dry before phreatomagmatic eruption.Besides,the lacustrine sedimentary records also showed the evidence of lake regression period.In summary,the phreatomagmatic eruption in Datong occurred in mid-Pleistocene.The Datong-Yangyuan lake finally receded to extinction,while the Sanggan River fault cut down to cause tholeiitic magma eruption and covered some base surge deposits.