Chronology And Geochemistry Of Late Mesozoic Basic-intermediate Dike Swarms From The Coastland Of Fujian Province

The coastland of Fujian Province is located in the active continent margin of SE China.The magmatism in the Mesozoic Era was very intense and this area was an important tectonic-magmatic activity belt of SE China.In the Late Mesozoic granite and volcanic rocks,there appears basic-intermediate dike swarms universally.They originated deep earth(mantle or lower crust) and emplaced different crustal layer, implying a lot of magmatic activity,crust-mantle mutual action information.Taking the Late Mesozoic basic-intermediate dike swarms from this area as the main object, we have studied their Petrology,Geochemistry and Isotopic Chronology,and discussed the property of their magmatic source,tectonic setting and origin as well. The significant conclusions acquired are as follows:1,The Late Mesozoic dike from the coastland of Fujian Province appears as clusters,intruding into the Late Mesozoic granite or volcanic rocks,and the lithologies are diabase,hornblende diabase and quartz-diorite etc,mainly diabase. The mode of their occurrence is mainly precipitous and they tend towards NE, approximately EW or NNE.The dike swarms did not suffer metamorphose or distortion,holding the stress state as the dike formed.2,The zircon SHRIMP U-Pb isotopic analytical results of the representative samples indicate that their forming ages are all comparatively adjacent,ranging from 87Ma to 96Ma,which illustrates they emplaced approximately the same time and they are all the products of the magmatic activities in the Late Cretaceous,Late Mesozoic Era.3,The SiO₂ content of the dike swarms varies from 50.22%to 61.10%and they are basic-intermediate rock series,mainly basic.The Al₂O₃ content is generally higher than 15%and they are high aluminum basaltic rocks.The lower magnesium index(Mg^#=42.77～62.28) and TiO₂ content(0.97～2.41%) illuminate that they su圷?ed magnesium-iron minerals' fractional crystallization and that they did not crystallize from original basaltic magma.The values of Na₂O/K₂O are all above 1, indicating lower K character.Chemically discriminated,these dike swarms are sub-alkaline rock series.4,The dike swarms are enriched in light rare earth element and the rare earth element assemblaging curves are relatively smooth,bending rightward in general and showing no Eu abnormity.They are enriched in LILE and Ce(Coming from the plate subduction) and depleted in HFSE in different degree.The Sr-Nd isotopes are rich in Sr but poor in Nd and the original ratio of Sr(⁸⁷Sr/⁸⁶Sr)_i is in good negative correlation withε_Nd(t).All of these indicate that they are in typical island-arc character and the basaltic magma generated from the lithosphere mantle which is metasomatic by subduction belt liquid and contaminated by the lithosphere.5,The dike swarms formed in back-arc extension setting and the magma generated from the mantle wedge peridotite' waterish partial melt.From the late stage of the Early Cretaceous Era to the Late Cretaceous Era(about 90Ma),the coastland of Fujian Province changed from extension setting to crack or rift valley stage.In the extension tectonic setting,the basaltic magma that suffered bottom emplacement,detention and extensive fractional crystallization emplaced upwards rapidly along the joint of the Late Yanshan Period granite and volcanic rocks,and then finally formed the basic-intermediate dike swarms.The Late Mesozoic basic-intermediate dike swarms from the coastland of Fujian Province were the last products of the Late Mesozoic magmatic activity and their formation was an important tectonic-magmatic events of the active continental margin of the coastland in Zhejiang and Fujian Province.The discussion on the composition and origin of the basic-intermediate dike swarms from the coastland of Fujian Province would help to get a profound comprehension of the Late Mesozoic magmatic activity rule of the coastland in SE China.