Research On Late Cretaceous-Paleocene Tuff In The North Centre Part Of The Myanmar Forearc Basin And Geological Significance

In this paper,the Late Cretaceous-Paleocene tuff in the forearc basin(north and middle Myanmar)were selected for study in geochronology,geochemistry,palaeolatitude and petrography.The types and origin of tuff in the forearc basin were analyzed.I will discuss the differences of sedimentary and tectonic structures between the Shigatse forearc basin in Tibet.And referring to other previous research,we have resulted in the tectonic evolution of the study area and its adjacent area since the Late Cretaceous.This paper obtains the following main knowledge:(1)Geochemical and studies have shown that,the fresh section of tuff is gray,tuff and block structure.In the TAS classification map,almost the tuff sites are located in quartz andesite region.The tuff normal distribution pattern of rare earth element chondrite shows a steep,right-leaning pattern.The distribution curve has a larger slope in light rare earth,light rare earth enrichment,heavy rare earth depletion type,consistent with med-acidic rocks.It is speculated that it is syngenetic with Ca in the source area because Eu of YX7 and YX11 showed negative anomaly(8Eu=0.28-0.91,mean 0.58).It is preliminarily inferred that assimilation and crystal differentiation occur during the magmatic outburst,which material exchange occurs with the surrounding rock during the magma upwelling,and the magma condensate minerals are separated from the magma successively according to the crystallization temperature.The tuff composition shows a difference;(2)The geochronological study shows that the rock sample is the tuff.The zircon ages of four rock sample are 70+1.1Ma,65+1.OMa,64.2+1.2Ma and 64+0.8Ma,which belongs to the late Cretaceous to the early Paleocene formation;(3)According to the current research on the Paleo latitude of the study area,the West Myanmar block drifted North instead of rotating.Since the Late Cretaceous,the West Myanmar Block has been drifting northwards along the southwestern margin of the Sunda Block.During the drifting,it was subducted by the Neo-Tethys Ocean.According to statistics of volcanic rock age results from previous studies,the time of subducting should be earlier than 70Ma.;In my opinion,the Neo-Tethys oceanic crust has been subducted towards eastward along the West Myanmar block in the late Cretaceous.The subduction of the Neo-Tethys ocean is the Chile type subduction,which is different from the the Tibet Shigatse forearc basin that was formed by Mariana type subduction.The position of the the West Myanmar block should be in the south of the present position in late Cretaceous.And it is subducted by the Neo-Tethys ocean crust during the northward drifting.The results of subduction and the "soft"?"hard"two collisions of the late India plate caused extensive magmatic activity in the basin.The protype of volcanic magma island arcs formed,and sedimentary volcanic clastic rocks(mainly tuff and tuffaceous clastic rock)in the northern and volcanic magma island arcs of the forearc basin.