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包头东正长岩位于内蒙古包头市东,阴山南麓,是华北克拉通北缘三叠纪碱性岩带最西端的岩体.本文对此岩体进行了锆石U-Pb年代学、矿物化学、岩石地球化学和Sr-Nd同位素的研究,讨论了其成因及源区地幔性质.岩体由霞石辉石正长岩和碱性长石正长岩组成,锆石LA-ICPMS U-Pb年龄为(214.7±1.1)Ma,为晚三叠世岩浆活动的产物.单斜辉石(从核部到边部,成分由透辉石向霓石-普通辉石演化),黑云母,正长石和霞石是岩体主要组成矿物.岩体稀土总量较高,稀土配分模式为右倾,无明显铕异常;相对富集轻稀土元素和大离子亲石元素(如Rb、Ba、Sr和Pb),亏损重稀土元素和高场强元素(如Nb、Ta和Ti);具有富集的Sr-Nd同位素组成,初始~(87)Sr/~(86)Sr为0.7061~0.7067,εNd(t)值为–9.0~–11.2.矿物学和地球化学数据揭示包头东正长岩属于硅不饱和钾质-超钾质碱性岩岩浆系列,高CaO、Fe_2O_3、K_2O、Na_2O含量,低SiO_2含量,富水,高温,高氧逸度;岩体起源于位于石榴石稳定区富含金云母的古老,富集岩石圈地幔.包括包头东正长岩在内的华北克拉通北缘三叠纪钾质碱性岩带的发育,揭示华北克拉通北缘下覆岩石圈地幔受到了来自古亚洲洋俯冲板片(及上覆沉积物)流体/熔体的交代改造;晚三叠世,华北克拉通北缘已经进入了被破坏大陆边缘的强烈伸展演化阶段.
The eastern part of Baotou is located in the east and the foot of Yinshan in Baotou City, Inner Mongolia, and is the rock mass at the western end of the Triassic alkaline rocks in the northern margin of the North China Craton. In this paper, zircon U-Pb chronology, Chemistry, geochemistry and Sr-Nd isotopes, and discuss the genesis and mantle properties of the source area.The rock mass consists of nepheline syenite and alkaline feldspar syenite, zircon LA-ICPMS U- The Pb age is (214.7 ± 1.1) Ma, which is the product of Late Triassic magmatic activity.Prinic pyroxenes (from diopside to niger-pluton), biotite, The orthoclase and nepheline are the main components of the rock mass.The total amount of rare earth in the rock mass is relatively high and the REE pattern is rightward dipole with no obvious europium anomaly.It is relatively enriched in both light rare earth elements and large ion lithophile elements such as Rb, Ba and Sr And Pb), depleted in heavy rare earth elements and high field strength elements (such as Nb, Ta and Ti), and enriched Sr-Nd isotopes with initial 87 Sr / 86 Sr of 0.7061-0.7067 and εNd (t) values ranged from -9.0 to -11.2. The mineralogical and geochemical data reveal that the Baotou eastern syenite belongs to the Si-unsaturated potassic-ultra-potassic alkaline magmatic series with high contents of CaO, Fe_2O_3, K_2O and Na_2O , Low SiO 2 content, rich water, high temperature and high oxygen fugacity. The rock mass originated from the ancient and enriched lithospheric mantle rich in phlogopite in the garnet stability area. The North China Craton, including the Baotou East Syenite The development of the margin Triassic potassic alkaline zone reveals that the mantle beneath the northern margin of the North China Craton has been replaced by fluids / melts from the subducted slab (and overlying sediments) of the Paleo-Asian Ocean; In the stratiform, the northern margin of North China Craton has entered the stage of intense extension and evolution of the marginalized continental margin.