Genesis And Tectonic Significance Of The Mesozoic Granitoids In The Tengchong-Baoshan Block, Sanjiang Area

Based on the previous research work, this paper aiming to studying on the petrology, geochemistry, U-Pb ages of zircons, petrogenesis, and the geodynamic settings of the magmatic rocks related with the multi-metal deposits in the Tengchong tin belt. Samples for this study were collected from Tieniushan, Guyong, Caojian and Kejie plutons of the Tengchong-baoshan block.From the data of the whole-rock major and trace elements of the granites in Tengchong-baoshan block, zircon dating and Sr-Nd-Pb-Hf isotope geochemistry, we identified two epochs of the granites related with the Tengchong tin belt in Cretaceous, which is corresponding with the regional granite dating periods. 1) Early Cretaceous granite(~120 Ma), e.g., Tieniushan granite in the eastern part of Tengchong Block. 2) Late Cretaceous granite(~74Ma), e.g., Guyong granite in the middle part of Tengchong Block, Caojian and Kejie granite in Baoshan Block.The Cretaceous granite in the Tengchong Block are I-type granite and characterized by high silica, alkali-rich, and peraluminous. The Cretaceous granite in the Baoshan Block are S-type granite, characterized by high silica alkali and shoshonitic.The Cretaceous granite in Tengchong and Baoshan Block have concentrated negative ?Hf(t) values(-5.0 to-3.5),with initial 87Sr/86 Sr bigger than 0.706 and εNd(t) less than-5. These geochemical characteristics suggest that granites derived from crust with less mantle contribution. Hf isotopic crust model ages from Baoshan and Tengchong Block show evident differences which may indicate different crustal basement materials.Comprehensive comparison of granites in Baoshan and Tengchong Block, three stages of magmatism have been identified, e.g., Early Cretaceous granite(139-113 Ma), Late Cretaceous(85-65Ma), and Paleogene granite(65-46Ma). The adding amount of the mantle materials begin to increase from Early Cretaceous to Paleogene, indicating different tectonic settings.The Early Cretaceous granite may associate with the closure of Meso-Tethy Ocean which lead to the collision between Tengchong and Baoshan Block, and caused the crustal thickening. The Late Cretaceous granites could be produced by decompression melting of the thickened crust of Baoshan block, which was probably related with the northward subduction of the Neo-Tethys.