A Study On The Lithofacies Of Daliuchong Volcano In Tengchong

The Tengchong volcanic field is located in the southeastern region of Yunnan Province,with Myanmar to the west and the Gaoligong Mountains to the east.From the tectonic setting,it is on the the collision boundary between India and Eurasian plate and is situated on the southeast edge of the Tibet Plateau.Though the exposed strata has a long time span,there is serious stratigraphic absence because of the strong tectonic movements occurred in this region.Most of the surface of the region is covered by volcanic rocks from multi-stages and long time volcanic activities.The volcanism from early to middle Pleistocene were dominated by explosive eruptions,with their eruption products,thick-layer tuff and welded tuff distributed pervasively in the region.But most of the pyroclastic rock sections were the remnant of certain volcanic edifice.Siutated in the middle part of the volcanic field,the Daliuchong volcano with the highest elevation and huge body preserved much better volcanic edifice and plenty of lithofacies.Therefore,this thesis focuses on Daliuchong volcanic rocks and combines with other residual tuff and welded tuff sections,try to identify the different lithofacies through observing their microscopic textures and structures,discuss the volcanic signature based on their chemical composition analyzing and crystal fragment distribution study.It is found that the Daliuchong volcanic edifice is rather complete through carefully field investigations,containing volcanic plug,intrusive lava dome,volcanic dyke,volcanic breccia,explosive eruption products and effusive lava.Two connected geological bodies,Xiaoliuchong and Jianshan,are combined into Daliuchong volcanic edifice in this thesis.In addition,according to the calculation from Arcgis,the eruptive area covered by Daliuchong volcanic rocks is about 90 km~2,and the volume is about 32 km~3.Due to the plenty of rock types from the edifice,this thesis aims to research on the lithofacies of the eruptive products and identify the different rock types as effusive,explosive,volcanic conduit and intrusive facies.The effusive facies lava erupted in the early eruptive stage and constituted the base of the Daliuchong volcano.Part of lava were covered by explosive facies in the late eruptive stage.The explosive facies,i.e.breccia,dacitic crystal fragment welded tuff and crystal fragment tuff,are mainly distributed along the W,S and NE flank of the volcanic cone.The volcanic conduit with its diameter more than one hundred meters is located about 100 meters south of the main peak of the Daliuchong volcano.The extrusive facies rock is only exposed near the main peak of the volcano.Therefore,the volcanism of Daliuchong volcano can be speculated as follows:Large-scale lava overflowing occurred in the early eruption period;then explosive eruptions happened;at last,the volcanisms ceased marked with magma extrusion as lava dome and plug.The vertical volcanic plug exposed due to the long-term weathering and denudation.There are other 5 pyroclastic rock sections,Laifengshan,Qujiaying,Yanjiachong,Changposhan and Laoyanghe,are found in the Tengchong volcanic field.Through carefully comparison research work on these rocks with those of Daliuchong,we conclude it is a set of airfall pyroclastic rocks with different welded degree.The pyroclastic rocks in Changposhan and Laoyanghe are stronger welded crystal fragment tuff,those in Yanjiachong are weaker welded crystal fragment tuff,those in Laifengshan and Qujiaying are non-welded crystal fragment tuff.While those in Daliuchong contain both welded and non-welded.The whole rock composition and EPMA results together show that the volcanic rocks in Daliuchong and other five places are dacite and have similar minerals,indicating the similar eruption ages of these rocks.Combined with Ar-Ar dating,the eruption age could be determined as early to mid-Pleistocene.In addition,crystal fragment size distributions of these pyroclastic deposits suggest that Plinian or sub-Plinian eruptions probably happened in Tengchong.Besides,according to the analysis of plagioclase CSD curves from eruptive and extrusive facies rocks in Daliuchong volcano,the magma of Daliuchong volcano probably experienced plagioclase fractional crystallization prior to eruption.The trace elements of Eu,Ba and Sr concentration all show negative anomaly,which also support the speculation.