内蒙古乌兰沟埃达克岩锆石U-Pb年龄及构造环境

Zircon U-Pb dating and tectonic setting of the Wulan'gou adakite in Inner Mongolia

内蒙古中部贺根山缝合带的梅劳特乌拉蛇绿岩中,新发现乌兰沟埃达克岩,岩性为花岗闪长岩。LA-ICP-MS锆石UPb测年结果表明,乌兰沟埃达克岩的侵位年龄为279.3±1.4Ma,其形成时代为早二叠世。乌兰沟埃达克岩SiO_2含量为65.92%~69.65%,MgO为1.34%~2.16%,Al_2O_3为15.30%~17.33%,Na_2O/K_2O值为3.95~14.09,Sr=359.60×10^(-6)~734.00×10^(-6),Yb=0.83×10^(-6)~2.02×10^(-6),Y=6.65×10^(-6)~12.84×10^(-6);富集K、Rb、Sr等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素;稀土元素总量为32.18×10^(-6)~65.41×10^(-6),明显较低,轻、重稀土元素分馏明显,(La/Yb)N值为2.84~7.56,无明显Eu异常,显示出高硅埃达克岩(HSA)的地球化学特征。该埃达克岩具有岛弧型岩浆岩特征,形成于俯冲带岛弧环境,可能为洋内俯冲洋壳+俯冲深积物部分熔融并与上覆地幔楔橄榄岩反应成因。根据乌兰沟埃达克岩与梅劳特乌拉蛇绿岩的时空分布与演化特征,贺根山缝合带在早二叠世可能存在洋内俯冲作用。

The newly recognized Wulan'gou adakite in the Meilaotewula ophiolite along the Hegenshan suture zone in Inner Mon-golia consists mainly of granodiorites.The LA-ICP-MS zircon U-Pb dating shows that the age of the adakite is 279.3±1.4Ma,sug-gesting a product of Early Permian.The rock has high SiO2(65.92%~69.65%),Al2O3(15.30%~17.33%),Sr(359.60×10^-6~734.00×10^-6),and low MgO(1.34%~2.16%),Yb(0.83×10^-6~2.02×10^-6),Y(6.65×10^-6~12.84×10^-6)content,with Na2O/K2O ratio varying from 3.95 to14.09.The adakiteis enriched in large ion lithophile elements K,Rb and Sr,and depleted in high field strength elements Nb,Ta,Ti and P.The total REE is low(32.18×10-6~65.41×10-6),whereas REE fractionation is clear((La/Yb)N=2.84~7.56),without obvious Eu anomaly.All of the geochemical characteristics indicate that the Wulan'gou pluton belongs to high-SiO2 adakites(HSA).The ada-kiteis characterized by the island-arc magmatite generated in subduction zone,which suggests that it might have been formed in an is-land arc setting in petrology and geochemistry.It might have been derived from the intra-oceanic subducted oceanic crust plus sedi-ment partial melting and subsequently melts reaction with overlying mantle wedge peridotite.According to the temporal and spatial distribution and evolution characteristics of the rock assemblage series of pillow tholeiite,boninite,Nb-enriched basalt,High-Mg di-orite and adakite in the Meilaotewula ophiolite,the authors suggest that there might have been an intra-oceanic subduction in the Hegenshan suture zone during the Early Permian period.