Datebase Construction Of Ancient Craton Metamorphic Mafic Rocks And Research On The Origin Of Primitive Geochemical Heterogeneity

The inhomogeneity of the mantle is one of the major frontier issues of earth science research,and researches on origin of primitive inhomogeneity have contributed to the development of basic theory of the origin of the earth.In this paper,based on the establishment of the database of metamorphic mafic rocks in world's major ancient craton,combined with the zircon U-Pb ages and related geochemical data of samples from Zhongtiao Mountain and Lvliang Mountain fields.In order to providing more evidence for the two-stage heterogeneous planetary accumulation model of the primitive earth established by Ouyang Ziyuan's team,some appropriate ancient cratons are selected for comparative study,and then we discuss the primitive geochemical heterogeneity and its origin.LA-ICP-MS zircon U-Pb data shows that the formation ages of the Lengkou and Tongao formations of Jiangxian group,and Bizigou formation of Zhongtiao group are2516±26Ma,?2213Ma and 2077 ± 29 Ma,respectively.The rock forming age of amphibolite in Jiehekou group is 1998 ± 23Ma;The metabasalt in Jinzhouyu formation forms at 2152±52Ma,and its metamorphic peak age is 1776 ± 17Ma;the plagioclase amphibolite in Bailongshan formation of Yejishan group forms at 2053 ± 42 Ma.Studies on the Zhongtiaoshan and Lvliangshan meta-mafic rocks reveal that in the Zhongtiaoshan region,the Paleoproterozoic Zhongtiao Group and the underlying Jiangxian Group meta-mafic rocks have similar geochemical characteristics,whilst the Jiangxian Group is also geochemically similar to the Neoarchean-Paleoproterozoic basement meta-volcanics.Similarly in the Lvliangshan region,the Lvliang Group and Yejishan Group meta-mafic volcanics share also similar geochemical characteristics,but are geochemically distinct from the coeval Jiangxian and Zhongtiao Group mafic volcanics in the Zhongtiaoshan region,as demonstrated in immobile element ratio discrimination diagrams such as La/Yb vs.Nb/Yb and Ba/La vs.Sr/NdThe geochemical characteristics of metamorphic mafic rocks from Eoarchean to Paleoproterozoic indicate that primitive geochemical heterogeneity is widespread in major ancient cratons in the world.For example,the metamorphic mafic rocks in the eastern and western Pilbara terranes display trace element geochemical inhomogeneities.The La/Yb and Nb/Yb ratios of metamorphic mafic rocks of the Western Pilbara terrane are relatively high,whereas the La/Yb and Nb/Yb ratios of metamorphic mafic rocks from the Eastern Pilbara terrane are relatively low.The Sr/Nd ratios of metamorphic basic rocks of the Eastern Pilbara terrane have a wide range,but the Sr/Nd ratio of metamorphic mafic rocks of the Western Pilbara terrane is low.The Y/Tb value of metamorphic mafic rocks from the southern terrane in the Isua greenstone belt in southwest Greenland is lower than that of the intra-arc metamorphic mafic rocks in the 3.7 Ga northern terrane;The La/Yb and Nb/Yb ratios of metamorphic basic rocks in the southern part of 3.8 Ga are relatively high,while the La/Yb and Nb/Yb ratios of the center-arc metamorphic basic rocks in the northern part of 3.7 Ga are relatively low;The Sr/Nd and Ba/La ratios of metamorphic mafic rocks from the southern terrane are low,while the Sr/Nd ratios of northern terrane intra-arc metamorphic mafic rocks have a wide range.By comparing the REE and HFSE of metamorphic mafic volcanic rocks with different ages and basement in the Lvliang Mountain,Zhongtiao Mountain and the East Pilbara terrane,it indicates that rocks formed in the later period have inherited the geochemical characteristics of the ancient rocks.The spatial geochemical inhomogeneity of rock formation is the inheritance of the earlier inhomogeneity.For example,the normalized REE patterns of chondrites of the Sushui Complex,the overlying Jiangxian group and Zhongtiao group in the Zhongtiao Mountain area are all right dipping distribution patterns with slightly enriched light rare earth elements and flat heavy rare earth elements;The normalized trace element maps of the primitive mantle are all depleted of high field strength elements such as Nb,Ta and Ti.