The Formation And Evolution Of Granite-greenstone Belt In The Qixingtai Area, Western Shandong Province

The Archean granite-greenstone belt of Western Shandong Province (WSP) is located in the eastern North China Craton and is mainly composed of the Taishan Group and surrounding granitoid association. In early studies, the Taishan group, formed at～2.7 Ga, was subdivided into the Yanglingguan, Shancaoyu and Liuhang formations, which are considered to be comfortable to each other and constitute a monoclinal strata. Qixingtai is one of the typical areas of the WSP granite-greenstone belt, where the Taishan Group and different kinds of granitoids of Archean are well exposed. Based on geological mapping, this paper carried out a series of research, including SHRIMP zircon U-Pb chronology and geochemistry, in the Qixingtai Area And arrived some important conclusions as follows.(1) "The Liuhang Formation" was mainly composed of massive meta-basalt and pillow meta-basalt intercalated with some thin tuff and sedimentary layers. The meta-basalts are same in geochemistry as MORB, derived form depleted mantle source at 2.75-2.71Ga.(2) "The Shancaoyu Formation" was composed of leptite and fine-grained gneiss, with the latter being formed by anatectic reconstruction of the former. It shows a tectonic contact with "the Liuhang Formation". All the meta-sedimentary rocks of "the Shancaoyu Formation" show the geochemical charactaristics of magmatic rocks formed in island arc environmnet. Nd isotope compositions suggest a short crustal residence time in their source region. The meta-sediments contain detrital zircons of 2.56-2.53 Ga,-2.7 Ga and 2.53-2.52, limiting the deposition age of "the Shancaoyu Formation".(3) Three suites of TTG rocks, formed at 2.71Ga,2.62Ga and 2.52Ga respectively, have been identified in the area.(4) An anatectic belt has been identified. It can be further divided from west to east into four sub belts, namely the belts of lepite, leptite+felsic or granitic veins, fine-grained gneiss+felsic or granitic veins and migmatite. It is probably the anatexis at～2.51 Ga that resulted in the formation of crustally-derived granites which are widely distributed in the east of the anatectic belt.(5) The WSP granite-greenstone belt is considered to undergo two evolution periods. First period (2.75-2.6 Ga), "the Liuhang Formation" was formed in oceanic plateau environment, and the TTG of～2.7 Ga was formed as a result of intra-ocean subduction. Second period (2.6-2.5Ga), "the Shancaoyu Formation" was formed in island arc environment. The large-scale crustally-derived granites were produced at～2.5 Ga, remarking the cratonization of the WSP Block.