Genesis Of TTG Gneiss And Its Geological Implication Within Pingquan County,North Hebei Province

The Pingquan area in northern Hebei Province,located in the middle part of the northern margin of the North China Craton.The Shangyi-Pingquan fault zone crosses this area.There are a large number of TTG gneisses in the area.It is mainly distributed in Dayingzi,Wudaohe Township,Liugou,Xiaosigou,Yaogou and Changlianggou of Pingquan.The combination of rock types is dioritic gneiss,quartz dioritic gneiss,and Austrian long granitic gneiss.Among them,dioritic gneisses are widely distributed.The colour of dioritic gneisses,quartz diorite gneisses and olivine granodiorite gneisses change from dark gray to light gray to light red,and the content of dark-colored minerals gradually decreases,showing a process of evolution from neutral to neutral-acidic.This TTG gneisses is rich in sodium and poor in potassium,and its Na₂O content is significantly higher than that K₂O.Trace elements are enriched in large ion lithophile elements such as Sr,Ba,and Rb,and high field strength elements such as Nb,Ta,Ti and P are depleted,consistent with global TTG rock characteristics.The A/CNK is approximately equal to or greater than 1 and is generally expressed as a peraluminous TTG.TTG rocks are generally considered to be produced by the partial melting of basaltic rocks.The TTG rocks in the study area are rich in Sr and lean in Y,which are completely consistent with the characteristics of adakites,suggesting that the formation environment should be the same as that of adakites.Therefore,it is inferred that the TTG rocks in the Pingquan area were subducted by the oceanic crust and the basaltic rocks were partially melted.The U-Pb age test of zircon in dioritic gneiss by SHRIMP method.The magmatic zircon ²⁰⁷Pb/²⁰⁶Pb weighted average age in the nuclear part is 2484±12Ma,which represents the magmatic emplacement age.The weighted average age of edge and metamorphic zircon ²⁰⁷Pb/²⁰⁶Pb was 2480±11 Ma.It shows that the².5Ga magmatic event occurred in the late Neoarchean period in the northern Jibei area,and it also indirectly shows that the².5Ga North China Craton still experienced this continental crust proliferation and did not stabilize.It provides a new basis of magmatic events for the comparison bettwen The early North China Craton and the rest of the world.