Origin of mafic intrusions in the Micangshan Massif, Central China: Implications for the Neoproterozoic tectonic evolution of the northwestern Yangtze Block

摘要

The mafic intrusions exposed in the Micangshan Massif preserve important information pertaining to the Neoproterozoic tectonic evolution along the northwestern margin of the Yangtze Block. In this paper, we report new LA-ICP-MS zircon U-Pb ages, Lu-Hf isotopes and whole-rock compositions for mafic sills in the Micangshan Massif to understand their petrogenesis and supplement the regional tectonic evolution. Xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 +/- 4 Ma and 858 +/- 5 Ma, respectively, suggesting the maximum emplaced age of the mafic sills was at ca. 860 Ma. A granitic dyke sample which intruded into the mafic sills, yields a crystallization age of 799 +/- 4 Ma, implying the minimum emplacement of the mafic sills was at ca. 800 Ma. These results indicate that the mafic sills were formed within the span of ca. 860-800 Ma. Geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate FeOtotal and TiO2 contents, as well as moderate to high MgO contents and Mg-# values. They display distinct enriched light rare earth element (LREE) patterns, variable remarkable enrichment in large-ion lithophile elements (LILEs; e.g., Cs and Ba), and evident depletion of high-field strength elements (HFSEs; e.g., Nb, Ta and P), resembling signatures of typical subduction-related basaltic rocks. In addition, these sills exhibit comparatively high Nb/Ta, moderate Nb/Yb, as well as low Nb/La, Th/Yb, and Nb/Y ratios, indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source. These observations, together with regional tectonics, suggest that these mafic sills in the Micangshan Massif might be formed in a subduction-related continental arc setting. Therefore, combining our new data with available results from previous studies in the adjacent regions, our study supports the dominant subduction-related tectonics along the northwestern margin of the Yangtze Block during the Early Neoproterozoic time.