Characteristic and tectonic significance of zircon SHRIMP U-Pb geochronology and Hf-O isotopes from～2.9Ga metamorphic basic volcanics in Qixia area,North China Craton
Jiaodong area is one of the important Early Precambrian research areas in the North China Craton.There have been few studies on the formation age,petrogenesis and tectonic environment of the Tangjiazhuang Group in the Middle Archaean.Based on the systematic geological investigation of metamorphic basic volcanics of the Tangjiazhuang Group in Qixia area,the authors have carried out whole-rock geochemistry analysis of plagioclase amphibolite,zircon SHRIMP U-Pb chronology and Hf-O isotopes.The results show that the zircon age of the metamorphic basic volcanics in the Tangjiazhuang Group is 2907.9±6.3Ma,which represents that rock assemblages of the Tangjiazhuang Group initially formed in the Mid-Archaean and underwent the early and late Neo-Archaean metamorphism.This is the first report of～2.9Ga metamorphic basic volcanic rocks of Tangjiazhuang Group in Jiaodong area.The geochemical characteristics of the rocks are relatively enriched in large ion lithophile elements K,Rb,Sr,etc.,and depleted in high field strength elements such as Th,Nb,Zr,Hf,etc.,and REE pattern displays medium-degree depletion.The magmatic zircons have similar Hf and O isotopic compositions with εHf(t)values ranging from-0.68 to+4.21 and δ18 O values ranging from 1.38％o to 6.73‰.Based on the previous work,this paper proposes main conclusions as follows:(1)Jiaodong area experienced～2.9Ga magmatic event,and some parts still retain metamorphosed supracrustal rocks of the same period;(2)The Tangjiazhuang Group is a set of volcano-sedimentary formations formed in the initial ocean basin environment of the Mid-Archaean;(3)Accretionary neo-crust derived from depleted mantle of the North China Craton occurred mainly from the late Middle Archaean Period,and partially recycled terrigenous materials;(4)The North China Craton experienced extensional tectonic in the late Neo-Archaean Period,and magma floor invasion caused strong metamorphism of rocks.
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