Origin of Baotoudong syenites in North China Craton:Petrological, mineralogical and geochemical evidence

Baotoudong 正长岩 pluton 被定位到 Baotou 城市的东方，内部蒙古，沿着北方中国 Craton (NCC ) 的北边缘的三叠纪的碱的 magmatic 带的最西的部分。锆石 U-Pb 年龄， petrological，矿物学并且 pluton 的 geochemical 数据在这份报纸被获得，抑制它的起源和披风来源特征。pluton 由 nepheline-clinopyroxene 正长岩和碱长石正长岩组成，与 214.7  ? 的锆石 U-Pb 年龄 ?????????? 潡潴摵湯 ????? 甠瑬慲潰慴獳捩 ????????????????????

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Trias- sic alkaline magmatic belt along the northern margin of the North China Craton （NCC）. Zircon U-Pb age, petrological, miner- alogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteris- tics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside （cores）-aegirine-augite （rims）, biotite, orthoclase and nepheline are the major minerals. The Bao- toudong syenites have high contents of rare earth elements （REE）, and are characterized by enrichment in light rare earth ele- ments （LREE） and large ion lithophile elements （LILE; e.g., Rb, Ba, Sr）, depletion in heavy rare earth elements （HREE） and high field strength elements （HFSE）. They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and eNd（t） values from -9.0 to -11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions （H2O）, and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area （〉80 km）. The occurrence of the Baotoudong sye- nites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.