Subduction Records Of The Paleo-pacific Plate

The formation and upwelling of basaltic magma may be affected by many factors,such as the fractional crystallization of magma,the contamination of surrounding rock,the addition of crustal materials and the diversity of mantle peridotite composition.When traditional methods are used to recognize the source,the influences of temperature,pressure and volatile components are ignored to varying degrees,which leads to lower reliability of the identification results.Pyroxenite and peridotite are easy to have similar trace elements and isotopic characteristics under the long-term mantle metasomatism,which makes it difficult to recognize the source of basalt by traditional methods.Some workers have obtained a discriminating parameter FCKANTMS which can effectively constrain source of basalts by using the log-ratios of major elements through the method of experimental petrology.In this paper,The GEOROC database is taken as the data source,and from the perspective of regional petrology,the major element data of Cenozoic basalts in eastern China were analyzed.Based on the FCKANTMS method and geothermometer,this paper makes a discriminant analysis of the source of basalts,the origin temperature and pressure,the equilibrium temperature and pressure of some mantle xenoliths and the mantle composition of Cenozoic basalts in eastern China.The discrimination results show that the source of Cenozoic basalts is closely related to the subduction of the Paleo-Pacific plate in eastern China.The source of basalts near the continental margin of eastern China is mainly pyroxenites.To the NNW-wards,the source of basalts gradually changes from pyroxenites to transitional source or even depleted mantle peridotites.The origin temperature pressure of basalt in some areas is estimated.The results show that the origin pressure increases gradually to the NNW-wards,which means that the origin depth is gradually deepened(70-130km).The mantle peridotite xenoliths in Niutoushan and Longhai areas of eastern Fujian province are spinel facies,while a large number of garnet facies mantle peridotite xenoliths can be seen in Mingxi area wastern Fujian prevince,which also reflects the characteristics of the gradual deepening of basalt origin depth.There is a transitional boundary in the source of Cenozoic basalts in eastern China.The distribution of the source is segmented.Previous numerical simulation results show that the width of the subduction slab controls the evolution and diversity of the subduction zones.Therefore,we speculate that the front end of the subducted Paleo-Pacific plate was torn to form five lath structures with different subduction angles and widths in eastern China.Finally,the formation of Cenozoic basalts in eastern China is closely related to the subduction of the Paleo-Pacific plate.During the Mesozoic,the Paleo-Pacific plate began to subduct beneath the eastern Eurasia and remained in the mantle transition zone.Under the environment of mantle,the subducted slab or retained slab will dehydrate and decarburize,which will induce partial melting of the subducted slab and form a large amount of Si-rich fluid,which will be strongly metasomatized with the overlying big mantle wedge to form rich pyroxenite mantle metasomatic rocks.In the Cenozoic,the back arc expansion and asthenosphere upwelling were caused by the rollback and retreat of the subduction plate,which once again induced partial melting under reduced pressure of these mantle metasomatic rocks,resulting in the formation of Cenozoic basalts in eastern China.