Geological Characteristics And Chronology Of The Archean Granite In The Chentaigou Area Of Anshan

Mass of Archean granitc rocks and metamorphic layered sequence expose inChentaigou area, all kinds of granite formed in Paleoarchean-Neoarchaean （partbelong to eoarchean）. The typical Archean granitic gneiss-mylonite in this area hasprovided clues for a comprehensive understanding of archean crust evolution, therefore,Chentaigou is a typical area for studying Archean crustal formation and evolution.Through the study the Chentaigou Archean rocks from the composition, theoccurrence and the structural features, the Chentaigou granite could be divided intogranitic gneiss-mylonite （porphyritic granitic mylonite and medium–fine grainedgranitic gneiss） and fine-grained trondhjemite. The zircon U-Pb dating study found that,the fine grained trondhjemite was formed in the3.14Ga, is the product of Archeanmagmatic event. Fine-grained granitic gneiss and porphyritic granite should belong topart of the reengineering Paleoarchean crust, mainly containing two groups of zirconage,3.1Ga and3.35Ga, where3.35Ga represents the formation age,3.1Ga belongs tothe Mesoarchean magma intrusion age.The major elements of granitic gneiss-mylonites and fine-grained trondhjemite arecharacterized by high SiO₂、Na₂O、Al₂O₃，and low TiO₂,MgO, CaO, TFe₂O₃, with highgrade of matrity. The similar REE and trace elements patterns suggest that theirprotolithes share common features. Combined with the Hf isotopic study of zircons, theconclusion has been made that the two rocks are formed by partial melting of basalticcrusts with the same geochemistry but different ages, and contanmination of earlycrustal material.Archean crust of Chentaigou area with the complexity of multi-stage evolutioncharacteristics. Fine-grained trondhjemitic mylonite, porphyritic granitic mylonite andmedium-fine grained granitic gneiss have the characteristics of the Paleoarcheangranitic rocks, and underwent ductile deformation of middle-deep crust level.Fine-grained trondhjemite is a product of the Mesoarchean magmatic events, showingimpervious deformation and partial of ductile-brittle deformation fabric. Metamorphiclayered rocks （Anshan Group） mainly record the Neoarchean strong deformation event of middle-shallow crust level. The intrusion of the medium-coarse grained muscovitegranite marked the end of early-phase different structural deformation processes in thelate Neoarchean. On this basis, we discuss the formation and evolution of the Archeancrust in the Chentaigou area.