Paleozoic-Early Mesozoic Magmatic Belts And Tectonic Significance In Siziwangqi Area, Inner Mongolia

Middle part of the Inner Mongolia sited on northern margin of the North China platform and paleozoic Xing-Meng orogenic belt, where the Paleozoic-Early Mesozic igneous rocks are widely distributed in this area, is an important district for crustal growth and recycling research. Those igneous rocks can be divided into the south and north belts by the Solonker suture zone.Based on the geological mapping, petrologic, geochemical and geochronological stuidies were carried out for typical plutons in Siziwangqi area. 13 samples of zircons have been run for U-Pb geochronological dating using LA-ICP-MS techniques: Hadaaobao diorite (508±10 Ma), Huofotan diorite (331±5 Ma, 323.1±3.4 Ma), Xigedan gabrrotic (302±2 Ma), Beijige syenogranite(264±3.4Ma), Amawusu granodiorite (267±9Ma), zircons from hornblende-syenite, monzogranite, syenogranite, quartz vein of Wuertagaolemiao pluton yielded U-Pb ages of 271±18 Ma, 256±3 Ma, 261±2 Ma, 230±12Ma respectively; the Bulong monzogranite and the Geertu muscovite granite formed at 239±4Ma and 238±6Ma (MSWD =0.91), respectively; zircons from muscovite monzogranite vein which intruded into the Geertu pluton, gave a mean 235U/206Pb age of 224±8Ma (MSWD =5.7). Based on these new data and other reliable data Published in recent years, four stages of magmatism are identified: Early Paleozoic(508~420Ma), Carboniferous- Early Permian(340~281 Ma), Middle Permian- Early Trias(281~245 Ma) and Middle–late Trias(239~203 Ma).Samples from Hadaaobao pluton, Bainaimiao volcanic rocks and Xigedan gabrrotic show geochemical features characteristic of island arc, were the products of different stages in the oceanic slab subduction. Geochemistry of the Huofotan diorite show features characteristic of continental alcaline basalt, which belong to back-arc setting. There are abundant mafic microgranular enclaves (MMEs) which generated by magma mixing, occurring in the Beijige pliton and Damiao-Amawusu pluton, indicates an affinity to I-type granite. Geochemistry of the Wuertagaolemiao Hornblende-syenite with high Ga/Al ratio and high abundance of Zn, Y, Nb,indicates an affinity to A-type syenitic magmas. Those plutons correlated with the upwelling of asthenosphere caused by the breaking-off of subduction slab. The Wuertagaolemiao monzogranite with high Sr content(407-626×10-6), low Y and Yb content (Y=2.2-4.8×10-6, Yb=0.27-0.45×10-6), suggest it was partial melts from post-collisional over-thickened lower crust. The Geertu muscovite granites show a spectacular tetrad effect in their REE distribution patterns as well as non-CHARAC (charge-and-radius-controlled) trace element behavior, caused by intense interaction of the residual melts with aqueous hydrothermal fluids. Those characters indicates that the Geertu muscovite granites are not typical S-type granites, belong to post-orogenic setting, represented the ending of orogeny. In combination with the previous studies on regional geology, it can be concluded that the final collision between the Siberian plate and the North China craton happened between280 Ma and 260 Ma. The igneous rock assemblage and geochronological of south and north belts can be contrasted, so Solonker -Xar Moron River as the collisional suture between the Siberian plate and the North China craton is believable.