The 2.73 Ga I-type granites in the Lengshui Complex and implications for the Neoarchean tectonic evolution of the Yangtze Craton

摘要

We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 +/- 13 Ma and 2738 +/- 25 Ma, respectively. The more precise age of 2732 +/- 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO2 (73.11-74.01 wt%), K2O (3.93-5.48 wt%), Na2O (3.93-4.86 wt%) and low CaO (0.30-0.69 wt%), MgO (0.17-0.30 wt%), TiO2 (0.14-0.17 wt%), P2O5 (0.01-0.06 wt%), Al2O3 (14.11-14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04-1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2-34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their epsilon(Hf)(t) values (-2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive epsilon(Nd)(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67-2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.