Geochemistry, Geochronology And Tectonic Settings Of Archean Gneisses In Lushan, Henan Province

Taihua complex is composed of old crystalline basement rocks and discontinuously exposed along the southwestern margin of the North China Craton, it lies in an northwest-southeast trending belt from Longshan and Lishan to the east, along Xiao Qinling area of Huashan, Lingbao and Xiaoshan, to Xiong'ershan of Luoning country, Beizijie-Malou of Lushan country and Xindian of Yexian country-Tieshanmiao of Wuyang country. Taihua complex is divided into two main rock units, one is quartzofeldspathic gneisses which are composed mainly of TTG suite with mafic and ultramafic inclusion, the other are supracrustal rocks including quartzofelspathic gneisses, sillimanite-garnet gneisses, graphite-gneisses, marble, quartzite, banded iron formation and inbeded mafic metavolcanic rocks. The complex was named Taihua group and divided into two sub-group. There is agreement that the lower group is belong to Neoarchean, but for the upper group, some considered it belongs to Neoarchean while some thought it belongs to Paleoproterozoic. Based on the existing information, this thesis investigates the TTG suite and amphibolites in detail on the geochemistry and geochronology in Taihua complex of Lushan area. The achievements are following:1) Confirming that the grey gneisses which are mainly composed of biotite plagioclase gneisses in Taihua complex of Lushan area are TTG suite. LA-1CPMS zircon U-Pb dating shows that age of the TTG suite is among 2700—2800Ma, which is considered as the time when the main continentalaccretion event happened in the form of formation and evolvement of TTG suite in this area.2)Discovering that the TTG suite, intermediate rocks and amphibolites all formed in the same time among 2700~2800Ma, and the geochemistry character shows these rocks may come into being in the setting similar to modern continental margin volcanic arc environment. So according to rock assemblage, geochemistry and geochronology, Taihua complex originate in the setting similar to modern continental margin volcanic arc in Neoarchean.3)Considering that a protolith of amphibolites is an alkaline basalt dike in Paleoproterozoic (1932±48Ma), indicating that it converts to continental extension setting in the area that time.