Geochemical Characteristics And Genetic Mechanism Analysis Of Mesozoic Acidic Dykes In The Northern Segment Of Taihang Mountains

Magma zone in the northern segment of Taihang Mountains has a long history and its tectonic background is really complicated,which is always the focus to many geologists at home and abroad.The composition and evolution of the crust at different tectonic levels and the mantle-crust interaction are significant information contained in acid dykes.It is a very important role to the study of magmatic process.However,there are not enough researches about acid dykes because of its small scale.Therefore,this paper chooses it as the study target.Based on the field geological observation,microscopic petrography,geochemistry and the zircon U-Pb dating to the acid dykes are studied in detail,and the conclusions are as followings:Acid dykes in this region strike mainly in NE and secondly in NW,which were strictly controlled by the Wulonggou fault.They are mainly distributed in the south of the Wulonggou fault and mostly outcropped in the hanging wall.The dyke and surrounding rocks are mostly zigzag contact,which reflect that they were formed in the extensional environment.Acid dykes in this region include granite porphyry,quartz porphyry and moyite.They are rich in silicon,aluminum and alkali with a higher differentiation,belonging to peraluminous high potassic calc-alkaline series.Characteristic of the acid dykes' trace elements is rich in Rb,Th,Zr,Hf,and depled in Sr,Ba,Ti.What's more,slightly negative anomalies of Eu reveal the fractional crystallization of plagioclase.Distribution pattern of the rare earth elements is characterized by enrichment in LREE and depletion in HREE.According to the data of LA-ICP-MS zircon U-Pb dating,the age of NE trend acid dykes is about 118.32±0.87 Ma and the age of NW trend is about 116.75±0.65 Ma,which indicate that the dykes were developed in the early Cretaceous.Besides,it also shows that acid dykes were emplaced under the extensional tectonics background of this area.The positive correlation between La/Yb and La and the result of ?Nd(t)-?Sr(t),indicate that the substance of acid dykes is derived from ancient crustal materials,and that the Mesozoic acid dykes in the northern segment of Taihang Mountains belt come from the partial melting of the ancient lower crust,and has undergone fractional crystallization during its ascent.The analysis of the environmental tectonic indicates that the acid dykes are A-type granites formed in post-orogeny stretching environment.