Characteristics And Structural Setting Of The Pinghe Alkalic Complex In Northerm Sichuan

Late Neoproterozoic magmatism in continental rift and Rodinia supercontinent breakup is a focus of the academic research in recent years.As an important response to the breakup of Rodinia supercontinent, mass magmatic activities of the Late Neoproterozoic in south China have attracted widespread attention inland and abroad geological field. As the most important part of south China,mass Neoproterozoic rock around the Yangtze block is the irreplaceable evidence of the reconstruction of Rodinia supercontinent and research in breakup dynamics of it.Micangshan zone is and there is mass Neoproterozoic magma,which contains mass volcanic rock and a large instrusive rock. So far, age and tectonic attribution of Neoproterozoic magmatic rocks in the northern margin of the Yangtze block has been the researching focus and the considerable progress has been made.But many alkaline complex bodies in this area are not fully used. There are much differences about the tectonic setting of the magmatic activities in this area. Geology workers cannot answer the relation between Micangshan magmatic rocks and breakup of Rodinia supercontinent.Recently,much work has been made in Hannan area of northeast Micangshan.However,there is nearly not work which is made in west Micangshan.Based on the previous work, field investigation, sampling and analysis of information identification,a comprehensive analysis of Neoproterozoic alkaline rock in Pinghe area of Micangshan is made in this essay.Which contains the work in petrology, mineralogy, petrochemistry, REE and trace element geochemistry and stable isotope chronology.After that,linking the breakup of Rodinia supercontinent and global rifting and evolution history of Neoproterozoic magmation in south China,the tectonic environment of alkaline rock in Pinghe area is discussed. Some principal viewpoints are achieved in this essay,as follow:(A) From early late, from the jacupirangite of Chadizi unit and ijolite of Bailinghe unit to urtite of Wangjiaping unit , nepheline particle size and degree of self-shaped with nepheline in the content increases, the particle size increases and increased self-shaped; pyroxene content decreased, the greater the content of the smaller particle size, Ca, Al, poor characteristics of silicon; Vice basically contains apatite minerals; titanium magnetic characterized by the rich iron ore of titanium, as ilmenite, indicates that the magma is of mantle origin. (B) Through petrology and minerals of petrological peculiarities can be seen at unit of Dashuwan and super-unit of Pinghe in the form of time and space are closely related, but the obvious discontinuous mineral composition, alkaline syenite of unit of Dashuwan is not alkaline ultrabasic magma of unit of Pinghe differentiation and evolution of the product directly. (C) The Pinghe alkaline complex body of jacupirangite and ijolite and urtite with a common mineral, rock chemistry similar, showing a clear trend in the evolution and trace element and rare elements have the same or similar partition model, reflecting the causes of which are closely related to rock series. (D) According to geochemical data analysis, Alkaline rock of super-unit of Pinghe tectonic setting are the causes of intraplate alkali basalt. (E) the age of diagenetic of super-unit of Pinghe should be between 780 ~ 700Ma, is a Neoproterozoic Nanhua alkaline magmatism. (F) study of the origin of magma that Pinghe alkaline rocks are upper mantle low-melting generate the CO2-rich alkaline magma, after by the deep part of the different role of crustal contamination, many passive ways to expand on the fracture invasion, at 23 ~ 33km Department in place, the formation of the alkaline condensation compound Igneous rock series.(G)Early on,there is a huge rising mantle plume under the Yangtze block.As the rising mantle plume and its heat erosion,lithosphere will become thin. Ultimately,the lithosphere of the Yangtze block also will become thin along the early suture.As the heat ,which comes from the mantle plume, a widespread crust melting will happen,and many alkaline volcanic rocks,which contain many feldspathoid minerals,will form.Base on the content of feldspathoid minerals and the forming temperature(800～1 200℃) , the alkaline rock in Pinghe area may form in the early stage of the rifting in south China. Along with the continual rising of the mantle plume,the Yangtze block continually becomes thin around its edge,and the rift becomes big. Massive partial melting generates in the bottom and top of Lithosphere.Some materials of high temperature penetrate the crust,which are also contaminated by crustal materials,numerous alkaline basalts and tholeiitic basalts generate.Base on the age(750～780Ma) of the alkaline rocks in Pinghe area, parts of which may generate in the peak season of rifting.Generally, the alkaline rocks in Pinghe area generate when the breakup of Rodinia supercontinent.