Research In Tectonic Evolutionat Of Neoarchean Dengfeng Group In Songshan Area

Songshan area is located in the southern margin of the North China Craton(NCC), and it is one of the typical areas to reserch the Precambrian geology.DengFeng group of Neo-archaean mainly distributes in the Guojiayao village locates on the northeastern of Junzhao township in Dengfeng.This article selects Lao YangGou formation,JinJia Men formation and Guo JiaYao formation to study the tectonic environment during the formation of DengFeng group through the stratigraphic sequence, petrology, geochemistry and the characteristics of Chronology basing on profile measurement outdoor,and the results as following:(1) Guo JiaYao formation are mainly hornblende schist, amphibolite schist,leptynite, following with leucoleptite and bedded or quartzite output with lens.From the evidence that amygdloidal structure is visible in(Plagioclase) hornblende schist it can be Infered that the original rocks of Guo JiaYao formation are mainly mafic rocks, mafic volcanic lava.JinJia Men formation are mainly Greisen schists which is an important identification with Guo JiaYao formation.According to the apparent Palimpsest layered structure of the rocks and large numbers of Mica minerals and Clay metamorphic minerals appear in the rocks, it can be infered that the original rocks are Semi-clayey sediments from shallow marine.The main rocks in Lao YangGou formation are pebbly quartz sericite schist, pebbly quartz sericite and chlorite schist, sericite and quartz schist,double sericite and quartz schist, Metamorphic conglomerate and so on, and it is a pebbly Semi-clayey sediment that mainly is sedimentary with marble.(2) In DengFeng group, the content of SiO2 in amphibolite rock(average 50.7%) is low, K2O(average 1.11%)is high, rich in Fe2O3(average 5.92%), Al2O3(average of14.68%), MgO(average of 5.82%), low in TiO2(mean 0.56%),(K2O + Na2O)=3.97,alkalinity ? = 2.21, display sub-alkaline basalt features(? <3.3).(3) The Chondrite distribution patterns of REE in DengFeng group appears a negative slope to the right-wing in overall, showing a feature that LREE is enrichment,HREE is deficition and exist an obvious fractionation between the two kinds REE(((La/Yb)N=2.42~20.82).The differentiation of LREE is more obvious than HREE which is with stable differentiation.In summary, the characteristics of REE inDengFeng group are that the amount of total REE is high,enrichment in LREE,deficition in HREE and exists an obvious fractionation between the two kinds REE.(4) Among all the trace elements in DengFeng group, enrichment in large ion lithophile elements Ba, deficition in Rb?Sr Etc., losing high field strength elements Zr,depleted Nb, Ta, Th, Hf, etc., displays an characteristic of island arc environment.(5) Collecting granite gravel in metamorphic of Lao YangGou formation, the age on U-Pb concordia upper intercept is 2573 ± 52 Ma, and surface-weighted age is2580Ma;the results is credible because that the concordia age and the surface-weighted age are very similar. The formation age of granite gravel is 2573 ± 52 Ma, and the strata age of metamorphic conglomerate should be later than 2573 Ma. The characteristics of U-Pb age displays that the formation of Lao YangGou formation should have been affected by the North China Craton tectonic events(convergence) which happened 2.5billion years ago.(6)The formation of DengFeng Group has a process from a back-arc rift to a convergence collision. Specifically from the back-arc basin environmental of the initial Guo JiaYao formation arc rift to the convergence collision of Lao YangGou formation which is the response of the construction process of cratonization in North China Craton2.5 billion years ago.