GEOCHIMICAL COMPARING OF TWO TYPES OF AMPHIBOLITES WITHIN TAIHUA GROUP IN LUSHAN COUNTY,HENAN PROVINCE AND THEIR TECTONIC SETTINGS
Tu Shaoxiong(Yichang Institute of Geology and Mineral Resources, Yichang 443003)
Two types of amphibolitic rocks are distributed over early Precambrian Taihua Group in Lushan County, Henan Province. The first type is normal amphibolites occurring mainly in Dangzehe Formation of lower Taihua Group (Late Archaean Era);the second is sodium-rich amphibolites which was discovered in Xuehuagou Formation of upper Taihua Group (probably Early Proterozoic Era). The two types of amphibolites are similar in many external features and mineral composition each other,but the chemical compositions between them are obviously different. The normal amphibolites have lower contents of Na2O (1.96%-2.86%), K2O (0.24%-0.64%), Ti,Y,Zr, Hf and REE, whereas the sudium-rich amphibolites are characterized by richer Na2O (3.31%-5.50%), K2O (0.47%-1.41%), Ti, Y, Zr, Hf and REE contents. By means of correlation studies of the various geochemical diagrams, it is suggested that the former is of tholeiitic series resembling compositionally the low-K tholeiite discovered in the recent oceanic crust; the latter is mainly sodium-rich basic volcanic rocks, the compositions of which are similar to a spilite evolved in intracontinental rift or geosynclinal area. It is showed that tremendous changes of the structural setting had taken place during the transformation from Late Archaean Era to Early Proterozoic Era in mis region. This conversion process is preliminarily considered as a result of the long-term weathering and denudation caused in the course of rising of the late Archaean crust.