Zircon U-Pb dating and Hf isotope analysis on the Taihua Complex: Constraints on the formation and evolution of the Trans-North China Orogen

Precise age determination and source tracing of the granitic and pegmatitic gneisses of the Taihua Complex in the Xiaoqinling area are important to address the debates on the timing and processes of the amalgamation of the Western and Eastern Blocks along the Trans-North China Orogen. This paper conducted zircon dating of metamorphic rocks from the Xiaoqinling area. The results demonstrate that the collision between the Western and Eastern Blocks occurred in the Paleoproterozoic. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon uranium-lead (U-Pb) analyses yielded consistent results. The Early Paleoproterozoic (the first period) age of 2346 +/- 28 Ma was determined for granitic gneiss in the Taihua Complex and 2328 +/- 14 Ma for the Xiaohe granite pluton that intruded into the granitic gneiss. The gneissose pegmatitic granite and granitic veins that intruded into the granitic gneiss yielded LA-ICP-MS zircon ages of 1881 +/- 24 and 1866 +/- 19 Ma, respectively. These Paleoproterozoic ages strongly indicate that the ocean between the Eastern and Western Blocks were completely subducted at similar to 1.85 Ga. The epsilon(Hf)(t) values of the zircons from the granitic gneiss can be divided into a positive group (0.44 to 3.77) and a negative group (-3.11 to -1.27). The two-stage hafnium (Hf) model ages (T-DM2(Hf)) corresponding to the positive values are between 2672 and 2887 Ma. The epsilon(Hf)(t) values of the zircons from the Xiaohe granite range from -2.49 to 2.58, with the Hf isotope model ages ranging from 2742 to 3022 Ma. Thus, the Xiaohe granite could have resulted from the partial melting of the Late Archean lower crust caused by subduction and the presence of juvenile mantle material. The primary magma of the granitic gneiss of the Paleoproterozoic Taihua Complex was derived from the differentiation of the depleted Late Archean mantle source with a small portion derived from the remelting of the subducted lower crust. (C) 2012 Elsevier B.V. All rights reserved.