内蒙古梅劳特乌拉蛇绿岩中早二叠世高镁闪长岩的发现及洋内俯冲作用

The discovery of the Early Permian high-Mg diorite in Meilaotewula SSZ ophiolite of Inner Mongolia and and its Intra-oceanic Subduction

内蒙古梅劳特乌拉SSZ型蛇绿岩中,新发现了早二叠世巴嘎哈尔高镁闪长岩。该闪长岩位于贺根山缝合带内。锆石LA-ICP-MS U-Pb测年表明,巴嘎哈尔闪长岩的侵位年龄为(282±2)Ma,其形成时代为早二叠世。该岩石高镁,MgO含量5.00%~10.94%,Mg#值54~74;SiO_2含量53.52%~58.64%,Al_2O_3含量13.48%~14.98%,Na_2O含量1.36%~3.59%;贫钾,K_2O含量0.48%~1.61%;贫TiO_2(0.28%~0.76%)和P_2O_5(0.080%~0.160%);富Cr(75.00×10^(-6)~555.90×10^(-6))和Ni(26.20×10^(-6)~228.40×10^(-6));富集K、Rb和Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti和P等高场强元素;稀土元素总量较低(25.16×10^(-6)~83.57×10^(-6)),稀土曲线为略右倾的平坦型。该岩石具有高镁闪长岩的地球化学特征,与赞岐岩相类似,属于俯冲带岛弧型岩浆岩。结合其与梅劳特乌拉SSZ型蛇绿岩中玻安岩和埃达克岩的岩石组合时空分布特征,表明古亚洲洋贺根山洋盆在早二叠世可能仍然处于大洋俯冲消减状态。该高镁闪长岩可能为俯冲洋壳+俯冲深积物来源的硅质熔体交代上覆地幔楔使其部分熔融的产物。

This paper reports a new discovery of the Early Permian Bagahaer high-Mg diorite in the Meilaotewula SSZ ophiolite in Xi Ujimqin Banner of Inner Mongolia.The Bagahaer diorite occurs along the Hegenshan suture zone.The zircon U-Pb LA-ICPMS dating shows that the age of the diorite is 282±2Ma,suggesting Early Permian.The rock has high MgO(5.00%-10.94%,Mg#54-74),Cr(75.00×10-6-555.90×10-6)and Ni(26.20×10-6-228.40×10-6)values and low Al2O3(13.48%-14.98%),K2O(0.48%-1.61%),TiO2(0.28-0.76%)and P2O5(0.080%-0.160%)values.The diorite is enriched in K,Rb and Sr large ion lithophile elements,and depleted in Nb,Ta,Zr,Ti and P high field strength elements.The total REE is low(25.16×10-6-83.57×10-6).Rare earth curve is of flat slightly right pattern.All of the geochemical characteristics indicate that the Bagahaer pluton belongs to high-Mg dioritoids and island-arc magmatite generated along a subduction zone,similar to features of sanukitoid.Combined with the temporal and spatial distribution characteristics of the boninite and adakite within the Meilaotewula SSZ ophiolite,it is suggested that,in Early Permian,the Paleo-Asian Ocean was not closed in the study area but was at its subduction stage.It is inferred that the high-Mg diorite might have been derived from partial melting of the mantle wedge metasomatised by subducted oceanic crust+sedimentderived siliceous melts during the Paleo-Asian Ocean subduction.