内蒙古中部地区镁铁质-超镁铁质岩形成时代及地质意义

LA-ICP-MS Zircon U-Pb Dating of Mafic-ultramafic Intrusions in the Central Inner Mongolia and its Geological Significance

为探讨内蒙古中部地区镁铁质-超镁铁质岩形成时代及地球化学特征,完善区域成岩成矿年代学格架,本文对克布、黄花滩和小南山岩体进行了锆石U-Pb年代学及全岩地球化学分析。LA-ICP-MS锆石U-Pb测年获得克布和黄花滩镁铁质-超镁铁质岩结晶年龄分别为258±2 Ma(MSWD=2.3)和262±1 Ma(MSWD=0.51),属于中-晚二叠世,小南山辉长岩应形成于中-晚二叠世(～273 Ma)。全岩地球化学分析表明,三个岩体样品m/f值介于0.96～3.54之间,主要为镁铁质-超镁铁质岩,其稀土和微量元素曲线基本表现为轻稀土元素(LREE)和大离子亲石元素(LILE)相对富集,高场强元素(HFSE)相对亏损的相似性特征,但也有明显的区别,显示各岩体岩浆演化或分异程度可能不尽相同。综合区域研究资料表明,内蒙古中部地区晚古生代镁铁质-超镁铁质岩浆活动可能至少始于早二叠世(294Ma),持续至中-晚二叠世(273～258Ma),且二叠纪时期中亚造山带南缘较大范围内均发育较为活跃的幔源岩浆活动。克布、黄花滩和小南山岩体应形成于后碰撞伸展环境,其岩浆源区可能混染了俯冲流体交代改造的地幔楔物质,使3个岩体的岩石样品均显示具有富含大离子亲石元素和轻稀土元素,贫高场强元素等部分岛弧岩浆特征的信号。

Geochronology and whole-rock geochemistry of the Kebu,Huanghuatan and Xiaonanshan mafic-ultramafic intrusions are presented to constrain the age of magmatism and regional tectonic evolution.LA-ICP-MS zircon U-Pb dating of the Kebu,Huanghuatan,and Xiaonanshan intrusions yielded mean ages of 258±2 Ma(MSWD=2.3),262±1 Ma(MSWD=0.51),respectively,indicating that they emplaced in the Mid-Late Permain.The Xiaonanshan intrusion should also be formed in the Mid-Late Permain(ca.273 Ma).These intrusions are mainly mafic-ultramafic with the m/f ratios between 0.96 and 3.54.The three intrusions have similar chondrite-normalized REE patterns and primitive mantle-normalized trace element spider diagrams,which are characterized by right-dipping REE patterns,and enrichment of LILEs and depletion in HFSEs.The slight differences in trace element characteristics among the three intrusions may result from the different degrees of magmatic differentiation.Combined with the previous researches,the Late Paleozoic mafic-ultramafic magmatism may begin in the Early Permian(294 Ma)and last until the Mid-Late Permian(273.258 Ma)in the central Inner Mongolia.The age of magmatic activity is close to that of the Cu-Ni sulfide deposits in Xinjiang,which overall shows a widespread mantle-derived magmatism along the south rim of CAOB in Permian period.It is suggested that the three intrusions formed in a post-collisional tectonic setting.The island arc signatures of the three intrusions may imply that the magmas were derived from partial melting of enriched lithospheric mantle or mantle wedge that metasomatized by fluid/melt of subducting slab.