| In this study,Lajishan labyrinthine belt is located in the combination of the central and southern Qilian microblocks,and the special geoplast at the convergence plate boundary is often considered as one of the important evidence of plate subduction.Adakite is a new type of rock,which is of great significance to the study of the mixed zone of Lachrymose,and provides new evidence for the subduction and collision process in Lachrymose region.The adakites of Lachinshan is found in Huangyuan area,mainly including and quartz diorite.Its main element contain shows:SiO2=61.9%65.1%,more than 56%;Al2O3=15.85%18.07%,more than 15%;MgO=1.17%1.66%,less than 3%;Na2O=4.03%6.46%,K2O=1.25%3.44%,Na2O/K2O?1,almost all Na2O/K2O?2,showing sodium rich characteristics;The relatively low Mg#?34.0445.15?;LREE is relativelyconcentrated,LREE=99.42128.57×10-6;HREEshowslost,7.7311.30×10-6;A weak negative or unexceptional Eu;Sr=8431371×10-6,?400×10-6;Yb=0.991.39×10-6,<1.9×10-6;Y=7.6111.80×10-6,<18×10-6;Satisfied with the adakites definition.Lajishan mixed belt adakites diagenetic age date by U-Pb shows 463.3±2.8Ma?MSWD?=0.58.Comprehensive graphic analysis shows that the adakites in the mixed belt of Lajishan is the NCA,or C-type adakite.The characteristics of adakites in lachrymose are rich in sodium and low in potassium.It is known that it was formed in partial melting of rock by the characteristics of incompatible elements.Combined with the formation of tectonic environment,it is believed that the adakites of Lachishan is formed by partial melting of basalt from the lower crust.Thus,in the early Ordovician,the former Lajishan Ocean dived northward and collided in the extruded tectonic environment,which led to the thickening of the lower crust of the area and the eventual partial melting of the basalt in the lower crust to form the adakite in the mixed zone of Lajishan. |
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