Evolution Characteristics And Dynamic Significance Of Cenozoic Basalt Magma In Fujian Province

The Cenozoic basalts in southeastern China are an important part of the Circum-Pacific volcanic rock belt,and there are still some controversies regarding their petrogenesis,tectonic setting,and magmatic evolution.Fujian Province is an important distribution area for the Cenozoic basalts in Southeastern China.The pyroxene phenocrysts from the basaltic rocks located at Bailin Town,northeastern Fujian Province,exhibit unique reaction rim texture and abundant primary melt inclusions in pyroxene and olivine.Therefore,this study focuses on the pyroxene,olivine,and primary melt inclusions are present in the Cenozoic basalts at Bailin Town,using petrographic,electron probe microanalysis,LA-ICP-MS,and laser Raman spectroscopy and so on to investigate the nature of the mantle source region,magma evolution process,and the dynamics background of the Cenozoic basalts in Fujian Province.The petrographic and major element analysis of pyroxene reveals that it is of magmatic origin and can reach megacrystic size with purple-red reaction rim.There are no inclusions within the reaction rims and the extinction properties of rim are consistent with the phenocryst,which is different from the common white reaction rims around pyroxene megacrysts in Eastern China.Chemical compositional between the cores and rims of the pyroxene,showing decreasing content of SiO₂ and MgO,increasing content of CaO,Al₂O₃,FeO,and TiO₂from the cores to the rims.The pyroxene shows characteristics of being rich in MgO,CaO,and TiO₂ but poor in FeO,and indicating a rift-related intra-plate tectonic setting,which also show a transitional trend from the sub-alkaline series to the alkaline series in terms of rock series.The crystallization temperature of the core ranges from 1270.7 to 857.3℃,with a pressure variation of 24.1 to 10.5 kbar,as for the rim,the crystallization temperature and pressure are vary from 1194.9 to 752.8℃and from 12.1 to 1.7 kbar,respectively,which shows that both decrease from the core to the rim,and the magnitude of pressure decrease is significantly higher than that of temperature,and the Al^vi/Al^iv ratio of pyroxene suggests the rim may experiencing intense decompression melting.The results of laser Raman spectroscopy analysis of melt inclusions in pyroxene and olivine shows that the volatile components are mainly CH₄,CO₂and other hydrocarbons,and a small amount of H₂O,The quantity and volatile content of melt inclusions in olivine that occurs as phenocrysts（Type II olivine）are higher than those in olivine that enclosed in pyroxene（Type I olivine）,and the quantity and volatile content in melt inclusions hosted by pyroxene are much higher than those in olivine,and the volatile content of melt inclusions shows an increasing trend from the core to the mantle.It shows a trend change closely related to mineral types and locations.Therefore,it is inferred that the mineral phenocrysts experienced degassing of volatiles during the crystallization process.The major element analysis of melt inclusions shows that the melt inclusions with high MgO content hosted by olivine and the melt inclusions in pyroxene are distributed in the subalkaline rock series in the TAS diagram,and the melt inclusions with high MgO content in olivine indicate the host rock type is basalt,which is different from the composition characteristics of the whole rock as alkaline basalt,showing a transition trend from subalkaline to alkaline rock series,which is consistent with the distribution trend in the Silico-alkali diagram,showing the characteristics of the transition from subalkaline basaltic magma to alkaline basaltic magma series.This is also consistent with the trend observed in the pyroxene,where the rock series transitioned from subalkaline to alkaline series from the core to the rim.Trace element analysis of pyroxene shows that the rim is richer in LREE than the core,and has a similar trace and REE distribution pattern to the core,indicating that melts of the rim and core are derived from the same or similar source region.The SiO₂ content in the rim of pyroxene is significantly lower than that in the core,and the Zr/Y ratio shows a higher value in the rim than in the core.These characteristics indicate that the degree of partial melting in the rim is lower than that in the core,and the melting depth is greater than that in the core.Meanwhile,the rim of the pyroxene shows depletion of fluid mobile elements such as Rb,Ba,and K compared to the core,suggesting that the rim may have experienced metasomatism.The analysis results of melt inclusions and olivines and whole rocks reveals that the original magma had a MgO content greater than 13.96%,indicating that the primitive magma is high-magnesium tholeiite basalt magma,derived from a mixed source region of carbonated peridotites and garnet pyroxenite,suggesting the possible existence of recycled oceanic crustal components in the source area.The rocks in the source area may have undergone metasomatism by carbon-bearing geological fluids derived from the oceanic crust.Based on the above research results and combined with the regional geological background,it is believed that in the extensional rift tectonic setting,the rollback and retreat of the slab changed the upwelling mode of the asthenospheric mantle,since 45Ma,and carrying the carbonated peridotite and garnet pyroxenite to shallow part where extensive decompression melting occurred,forming the initial tholeiitic basaltic magma.With the increasing degree of fracturing and decreasing degree of partial melting,alkaline basaltic magma was generated.The Bai Lin Town basalt originates from a mixed magma of the same or similar source at different depths and degree of partial melting.The initial melt of the Bailin Town basalts was volatile-poor,with the evolution of the magma system,volatiles were exsolved,and changes in temperature and pressure caused self-metasomatism of pyroxene phenocrysts,forming the rims of the pyroxene,which are products of recrystallization after decompression melting of the pyroxene phenocrysts.