ABSTRACT

The Altyn Orogenic belt is located in the northern margin of the Tibetan Plateau and its geodynamic setting during the early Neoproterozoic is poorly understood. Herein, we present the origin and geodynamic background of a suite of Neoproterozoic granitoids distributed in South Altyn. Representative samples of two-mica, muscovite, and biotite granites in the western region of the South Altyn Terrane, and granodiorite in the middle region were examined. The crystallization age of the two-mica granite is 948 ± 5.3 Ma, while the ages of the muscovite and biotite granites are 944 ± 7.3 Ma and 960 ± 4.5 Ma, respectively, and that of the granodiorite is 944 ± 4.4 Ma. These intrusions are all characterized by enrichment in large ion lithophile elements, as well as depletion in high field strength elements, high (⁸⁷Sr/⁸⁶Sr)_i, (²⁰⁸Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁶Pb/²⁰⁴Pb)_i ratios, negative ε_Nd (t), and significant ε_Hf (t) values ranging from –2.0 to 1.9. The corresponding two-stage model age (T_DM2) values range from 1,673 Ma to 1,928 Ma. These results suggest that these granitoids were sourced from the partial melting of Paleoproterozoic crust, with a negligible contribution of mantle material. During the process of magmatic evolution of the two-mica granite, fractional crystallization of plagioclase and potassium feldspar was dominant, whereas fractional crystallization of amphibole and biotite features in the magmatic evolution of biotite granite, and fractional crystallization of pyroxene, amphibole, plagioclase, and rutile occurred during the magmatic evolution of granodiorite. Moreover, it is shown that the two-mica granite and muscovite granite belong to syn-collisional granite, while granodiorite and biotite granite belong to volcanic arc granite, which are related to an active continental margin. In view of this and previous studies, we favor the conclusion that the west part of the South Altyn Terrane was in a transitional period between pre-collisional and syn-collisional tectonics during the period 944–960 Ma. Nevertheless, the middle region was in the pre-collisional tectonics over a period of 944 Ma, which implies that the convergence in the western part of the South Altyn Terrane occurred earlier than in the middle region.