东乌旗晚石炭世辉石橄榄岩锆石U-Pb年龄及其构造意义

Zircon U-Pb Dating and Geochemistry of Late Carboniferous Pyroxene Peridotite in Dong Ujimqi Inner Mongolia and Its Tectonic Significance

目前关于内蒙古东乌旗晚古生代花岗岩中辉石橄榄包体的精确年代学及其构造意义不清,直接制约了该区晚古生代地幔性质及构造演化的探讨.对东乌旗新发现的辉石橄榄岩进行了岩相学、全岩地球化学和锆石U-Pb定年研究.结果表明,辉石橄榄岩主要由橄榄石、角闪石、斜方辉石及少量斜长石、黑云母、单斜辉石组成;锆石U-Pb年龄为317.8±1.6 Ma,属晚石炭世.地球化学数据显示,岩石SiO2含量为40.28%~44.50%、MgO含量为23.42%~29.44%、Na2O+K2O含量为1.00%~2.12%(小于3.5%),具有低m/f比值(3.13~3.86)和高FeO^T含量(11.18%~14.70%)、高Mg^#值(75.60~79.26),属铁质超镁铁岩和拉斑玄武岩系列.岩石稀土总量较高(ΣREE=31.98×10^-6~72.60×10^-6),轻稀土(LREE)相对于重稀土(HREE)富集,(La/Yb)N=3.56~7.72,Eu异常不明显(δEu=0.79~1.65),球粒陨石标准化稀土元素配分模式表现为右倾型.岩石富集大离子亲石元素Rb、Sr、K等,相对亏损高场强元素Nb、Ta,具明显的Nb、Ta、Ti负异常;其形成于受俯冲流体改造的岩石圈地幔的减薄作用,并且岩浆在演化过程中遭受了地壳物质的同化混染作用.

The precise chronology and tectonic significance of pyroxene peridotite in Late Paleozoic granites in Dong Ujimqi,Inner Mongolia are still unclear,which will directly restrict the discussion of the properties and tectonic evolution in the study area.In this paper,petrography analysis,geochemistry and zircon U-Pb dating for the newly discovered pyroxene peridotite were studied.The analysis results show that main rock forming minerals are olivine,hornblende,orthopyroxene,and small amounts of plagioclase,biotite and clinopyroxene.The weighted average age obtained is 317.8±1.6 Ma,indicating that the intrusion was formed in the Late Carboniferous.Geochemical data show that,samples have SiO2 content of 40.28%-44.50%,MgO content of 23.42%-29.44%and Na2O+K2O content of 1.00%-2.12%,have low m/f ratio(3.13-3.86),high FeO^T content(11.18%-14.70%)and Mg^#(75.60-79.26),and belong to the series of ferrous ultramafic rocks and tholeiites.These rocks have high REE(31.98×10^-6-72.60×10^-6),enrichment in LREE and relatively depletion in HREE,(La/Yb)N=3.56-7.72,slightly Eu anomalies(δEu=0.79-1.65).The standardized distribution pattern of REE chondrites is right-leaning.The rocks are enriched in LILE(Rb,Sr,K)and depleted in HFSE(Nb,Ta),and there are obvious negative anomalies of Nb,Ta and Ti.Thus,we consider that the pyroxene peridotite had been formed in extensional tectonics from partial melting of the Sp-peridotite,which was modified by the subducted slab-derived fluid.