The Early Jurassic A-Type Granites in Northern Guangxi, China: Petrogenesis and Implications

The Yuanshishan granites have large amounts of mafic enclaves. This paper presents new zircon LA-ICP-MS U-Pb age, and the age analysis shows that the granites were generated in Early Jurassic (179±2 Ma). These granites are characterized by uniform silicon contents, enriched in alkali contents and more enriched in K contents, and enriched in iron and depleted in magnesium contents, with high 10⁴×Ga/A1 values and Zr+Nb+Ce+Y contents. The granites belong to A-type granite. The Yuanshishan granites have homogeneous Sr-Nd isotope compositions (I_Sr＝0.701 7 to 0.710 8, ε_Nd(t)＝-7.77 to - 4.55). Mafic enclaves show lower I_sr(0.705 0 to 0.707 1) and higher ε_Nd(t)(- 4.87 to -2.63) values. In situ zircons Hf isotope compositions from these granites have limited ranges (¹⁷⁶Hf/¹⁷⁷Hf)_i=0.282 62 to 0.282 70), ε_Hf(t) = -1.68 to 1.17, and T_DM2 =1.25 to 1.43 Ga. We suggest that the Yuanshishan granites were likely generated by partial melting of newly underplated basaltic lower crust in extension setting. Since Early Jurassic (~200 Ma), extension has been the major subject of the tectonic background in the interior of the South China. Multistage basaltic magma underplating is direct inducement to the Yanshanian extension. During the Early Jurassic, the South China interior was most likely in post-collisional setting.