Geochemical and Geochronological Characteristics of Granitic Gneisses in the Baoban Group, Gong'ai (Hainan) and its Tectonic Implications

Hainan Island geotectonically lies in the junction of Eurasian, India-Australia and Philippine plates. It is a key area for better understanding the Mesoproterozoic tectonic evolution of the South China Block, due to the presence of the Mesoproterozoic igneous rocks. The granitic gneisses from the Baoban Group, Gong'ai area (Hainan Island), consist mainly of monzogranite that underwent significant shearing deformation. Geochemically, they are characterized by high SiO2, K2O, Al2O3, Rb, U and low CaO, MgO, FeOT, TiO2 contents, with A/NK=1.62-1.84 and A/CNK=1.22-1.51, suggestive of the strongly peraluminous S-type granite. In the chondrite-normalized REE patterns and primitive mantle-normalized trace element spider diagram, they show signatures similar to those of the Mesoproterozoic metasedimentary rocks in the Baoban Group. The granitic gneisses have εNd(t) values ranging from -2.02 to -2.38 with Nd model age of 2.2-2.3 Ga, and εHf(t) values from -5.4 to 4.0 with Hf model age of 1.71-2.53 Ga. The zircon U-Pb dating analyses of four representative samples give the mean weighted ages of 1444±15 Ma, 1439±19 Ma, 1433±31 Ma and 1450±23 Ma, respectively, which suggest the original granitic rocks were crystallized in Mesoproterozoic. In combination with other available data, it is inferred that the Gong'ai granitic gneisses, which were previously defined as the crystallize basement were formed at Mesoproterozoic of 1433-1450 Ma, were derived from the continental crustal materials resembling compositionally to the Baoban sedimentary rocks. These rocks might have been generated in the continental marginal setting linked to the marginal accretion of Columbia supercontinent. © 2017, Science Press. All right reserved.