Formation and Evolution of Archean Continental Crust in the Anshan-Benxi Area,North China Craton:A Review
Anshan-Benxi(Anben)is located in the northeastern North China Craton and is well-known for the huge banded iron formation(BIF)and 3.8 Ga rocks.Its importance lies not only in the discovery of 3.8 Ga rocks,but also in the long-term history of Archean crustal formation and evolution from 3.8 to 2.5 Ga.3.8 Ga rocks occur in ancient complexes in the Anshan area.So far,six complexes have been identified.They are mainly composed of trondhjemitic rocks with meta-gabbro-diorite and have similar rock assemblages and zircon age records of 3.8-3.1 Ga,thus considered to be the different remnants of the same one large ancient complex.In addition to the complexes,other important geological bodies include the Paleoarchean Chentaigou supracrustal rocks and the late Neoarchean Anshan Group,the 3.3-3.1 Ga Chentaigou granite,3.1 Ga Lishan trondhjemite,3.0 Ga Donganshan granite,3.0-2.95 Ga Tiejiashan-Gongchangling granite and 2.5 Ga Qidashan granite.Three tectonothermal events have been identified at 3.3 Ga,3.1 Ga and 2.5 Ga.Granitoid magmatism evolved from trondhjemite to K-rich granite with the renewal of time.The total REE amounts and light to heavy REE differentiations of the rocks increased suddenly at 3.3 Ga.The trondhjemitic rocks are considered to be mainly derived from low-K iron-magnesian rocks,with some from TTG rocks,whereas the K-rich granites are from TTG and sedimentary rocks.According to whole-rock Nd and zircon Hf isotopic compositions,both mantle addition and intracrustal recycling played important roles in each magmatic period,with the latter becoming more and more important with time,but the late Neoarchean is also an important period of mantle addition.The following conclusions have been obtained.(1)3.3 Ga is an important period in the continental formation and evolution of the Anben area,with the thickness and scale of continental crust increasing significantly;(2)a long-term crustally-derived magmatism occurred between 3.3 and 2.95 Ga,with the granites increasing with time in magmatic intensity and K2O contents.(3)the Anshan Group was deposited on the＞2.95 Ga ancient continental basement,and there was a"quiet period"of～400 Ma(2.95-2.55 Ga)between the formation of the ancient basement and the deposition of the Anshan Group.(4)The formation and evolution of the Archean basement in the Anben area can be divided into four stages:continental nucleus formation(＞3.3 Ga),ancient block formation(3.3-2.95 Ga),quiet period(2.95-2.55 Ga)and stabilization(2.55-2.5 Ga).
Anshan-BenxiArcheanzircongeochemistryearly continentformation and evolution
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