华北板块北缘中段二叠纪的构造属性:来自火山岩锆石U-Pb年代学与地球化学的制约

Permian Tectonic Evolution of the Middle Section of Northern Margin of the North China Plate:Constraints from Zircon U-Pb Geochronology and Geochemistry of the Volcanic Rocks

为了探究苏吉组基性岩岩浆的起源和演化,对华北板块北缘中段土牧尔台地区苏吉组玄武安山岩进行了锆石LA-ICP-MS U-Pb定年和岩石地球化学研究,并用此来制约该地区的构造演化历史。苏吉组玄武安山岩中的锆石形态较好,均为自形—半自形,具有明显的震荡环带,且Th/U值为0.61~1.06,反映其为典型的岩浆成因。测年结果显示,苏吉组玄武安山岩形成于早二叠世((279.0±2.1)Ma)。地球化学特征表明,苏吉组玄武安山岩贫硅(49.0%~55.2%)和HFSEs,富铝(15.70%~17.80%)、镁(Mg#=49.84~67.02)和LILEs,具有较低的稀土元素总量和较平缓的稀土分配方式,显示了岛弧火山岩的地球化学特征,加之本组安山岩具有高镁安山岩的属性,所以该组火山岩源区应为受俯冲板片流体交代的亏损地幔楔。综上,笔者认为苏吉组安山岩形成于俯冲的构造背景下,在早二叠世古亚洲洋未闭合。

The LA - ICP - MS zircon U Pb dating and geochemical data were obtained for the volcanic rocks of the Suzy Formation in the central region of Inner Mongolia, so as to reveal the tectonic nature of the middle section of the northern margin of North China plate in Early Permian. The zircons from the volcanic rocks in the Suzy Formation are euhedral-subhedral in shape, and display fine-scale oscillatory zoning with high Th/U ratios （0.61 - 1.06）, implying their magmatic origin. The LA - ICP -MS zircon U- Pb age data indicate that the andesite from the Suzy Formation was formed in the Early Permian （279.0 ± 2.1 Ma）. The geochemical data indicate that the andesites belong chemically to medium-K calc-alkaline series, poor in SiO2 （49.0%-55.2%） and HFSEs, rich in Al2O3 （15.7%- 17.8%）, Mg （Mg#=49.8- 67.0）, and LILEs, and display low REE abundance and flat REE pattern, similar to those from an island arc. Combined with its characteristics of High-Mg andesite, we consider that its primary magma Were mainly derived from partial melting of the depleted mantle wedge alterated by the subduction of slab-derived fluid. It can be concluded that the andesite from Suzy Formation was formed under a tectonic subduction when the Paleo-Asian Ocean was not closed.