桂北圆石山早侏罗世A型花岗岩的岩石成因及意义

桂北圆石山花岗岩中发育大量镁铁质包体.LA-ICP-MS锆石U-Pb定年结果显示, 花岗岩形成于早侏罗世(179±2 Ma).花岗岩的地球化学特征表现为硅含量均一, 富碱更富钾、相对富铁而贫镁, 具有高的104×Ga/A1比值和Zr+Nb+Ce+Y含量, 属于A型花岗岩.圆石山花岗岩具有比较均一的Sr、Nd同位素组成(I_Sr＝0.701 7~0.710 8, ε_Nd(t)＝-7.77~-4.55).镁铁质包体则显示了稍低的I_Sr值(0.705 0~0.707 1)和稍高的ε_Nd(t)值(-4.87~-2.63).花岗岩的锆石原位Hf同位素组成为: (176Hf/177Hf)_i=0.282 62~0.282 70, ε_Hf(t)=-1.68~1.17, 相应的Hf同位素两阶段模式年龄T_DM2变化于1.25~1.43 Ga之间.圆石山花岗岩可能是在伸展环境下由低成熟度的下地壳物质部分熔融所形成.自早侏罗世(~200 Ma)以来, 伸展作用是华南内陆构造背景的主体, 多期次的玄武质岩浆底侵作用可能是燕山期伸展作用的直接诱因.华南内陆早侏罗世时期可能仍处于板内“后碰撞”环境. 

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.