Zircon SHRIMP U-Pb dating and Lu-Hf isotopic composition for Early Cretaceous plutonic rocks in Tengchong block, southeastern Tibet, and its tectonic implications

The Early Cretaceous granodiorite and quartz monzonite are the main types of igneous rocks in Tengchong block. The characteristics of I-type plutonic rocks are shown by the mineral assemblages, and high Na2O and low K2O contents and A/CNK values. Zircon SHRIMP U-Pb dating show the granodiorite and quartz monzonite emplaced at 122 +/- 1. 3Ma and 125 +/- 1. 3Ma, respectively. The zircon epsilon(Hf)(t) values for granodiorite and quartz monzonite range from -9. 6 to -4. 8 (average -6. 7 +/- 0. 7) and -7. 8 to -4. 7 (average -6. 3 +/- 0. 6), with corresponding single-stage model ages ranging from 979 to 1190Ma and 975 to 1113Ma, respectively. All samples plotted in the region between meteoritic Hf evolution line (CHUR) and the lower crust line in the plot of epsilon(Hf)(t) values vs. U-Pb ages, and high Al2O5 contents and Th/Hf ratios indicate that formed with crust-derived magma. While, the widely range of zircon epsilon(Hf)(t) values which are lower than those of igneous rocks produced by partial melting of crustal material, and high Th/Yb, low Ba/La and Yb/Hf ratios, show that the magmas for granodiorite and quartz monzonite are the mixing of large proportion of crust-derived magma and small proportion of mantle-derived magma. The fact of high Na2O and low K2O contents, and distribution of strongly enrichment of large-ion lithophile elements (K, Rb, Ba and K) relative to those in the primordial mantle, negative anomaly of Ba, Nd, Ta, Sr, P, Ti in the primitive mantle-normalized trace elemental spider diagram, and negative anomaly of Eu in the chondrite-normalized REE patterns, and geochemical tectonic discriminant diagrams, show the granodiorite and quartz monzonite were formed in the tectonic setting related with subduction and collision. The ages, zircon Lu-Hf isotopic compositions and corresponding single-stage model ages, and features of trace element for Early Cretaceous plutonic rocks in Tengchong block, are the same with that in the central section and southeast part of Lhasa block, suggest that the Early Cretaceous magmatic belt in Tengchong block is a part of central Lhasa magmatic belt, and resulted from subduction of middle Tethyan oceanic crust and continental collision between Lhasa-Tengchong and Qiangtang-Baoshan blocks.