Rock Association, Zircon U-Pb Geochronology Of Bainaimiao Group In Sonid Youqi, Inner Mongolia And Its Geological Significance

Study areas, which is to the north of Xuniwusu fault and south to the Wenduermiaosuture zone, is situated in the Xing-Meng orogenic belt. The Bainaimiao group is animportant part of the early Paleozoic strata which distribute extensively with granites inthis region. The research of rock association and geochemical characteristics ofBainaimiao group is a key to reveal the tectonic setting and Early Paleozoic regionaltectonic evolution. In the thesis, taking the volcanic rocks of the typical establishedformation section in the Bainaimiao group as an object of study. Based on the systematicresearch of stratigraphic sequence and rock association, geochemical characteristics andZircon U-Pb geochronology, which is of guiding significance to the research of tectonicregimes and history of evolution for the Xing-Meng orogenic belt.1. Geological Characteristics of the Bainaimiao groupBainaimiao group mostly outcrop in the middle part of Sonid Youqi, Inner Mongolia.According to the exploration of Field Geological and measure of strata profile,Bainaimiao group strata can be divided into2lithologic section: first lithologic section,intermediate-acid volcanic rocks; second lithologic section,intermediate-mafic volcanicrocks.The U-Pb isotopic LA-ICP-MS dating of the biotite leptite （P16b54-1） andchlorite-sericite schist （P16b9-1） shows the ages of499.4±1.8Ma,478.1±1.6Ma。Basedon previous research results, Bainaimiao group are formed in Cambrian-Middle Silurian.2. Geochemical characteristics of the Bainaimiao group The Bainaimiao group volcanic rocks are mainly composed of basalt, rhyolite and afew andesite and rhyodacite. They have a calc-alkaline evolutionary trend in SiO₂vs. K₂Odiagram and show the evolution trend of calc-alkaline series.After corrective Loss on ignition, the contents of SiO₂of greenschist are between44.08%⁵3.07%，TiO₂between0.47%⁰.94%，Al₂O₃between11.25%¹9.76%，TFeObetween9.13%¹4.12%，MgO between4.46%¹3.95%，CaO between7.48%¹8.4%、K₂O between0.29%².83%， Na₂O between1.59%³.51%、 P₂O₅between0.10%⁰.21%；Negative correlations of SiO₂vs. MgO、CaO、TFeO, and positivecorrelations of SiO₂vs. Na₂O and P₂O₅are shown in Harker diagrams. the felsicmetamorphic has SiO₂content of71.89%⁷8.38%，TiO₂of0.13%⁰.38%，Al₂O₃of11.00%¹6.62%， TFeO of2.32%³.87%， MgO of0.71%⁰.78%， CaO of0.47%².21%、K₂O of0.23%³.59%，Na₂O of2.13%⁴.73%、P₂O₅of0.04%⁰.10%；Negative correlations of SiO₂vs. Al₂O₃、K₂O、P₂O₅, and positive correlations of SiO₂vs.Na₂O are shown in Harker diagrams. The metamorphic dike has SiO₂content of51.41%⁶3.92%， TiO₂of0.75%¹.20%， Al₂O₃of14.63%¹8.34%， TFeO of6.97%¹1.93%，MgO of3.09%⁴.82%，CaO of5.81%⁷.87%、K₂O of0.26%⁰.48%，Na₂O of3.72%⁵.66%；Negative correlations of SiO₂vs. Al₂O₃、TFeO、MgO、Na₂O areshown in Harker diagrams.The total rare earth element contents（ΣREE） of the greenschist, metamorphic dikeare between40.54¹68.12ppm.（La/Yb）Nratios and δEu values range from3.08to10.9and from0.77to1.37, respectively. On the chondrite-normalized REE diagram, thegreenschist, metamorphic dike are characterized by enrichment of LREEs, depletion ofHREEs. On the MORB-normalized spider diagram, all the samples are enriched in largeion lithophile elements （e.g., Rb, Ba, and K）, depleted of high field strength elements（HFSEs, e.g., Nb, Ta, Ti）; The total contents of rare earth element （ΣREE） of the felsicmetamorphic are between63.87ppm and133.43ppm. Their （La/Yb）N ratios and δEuvalues range from2.92to9.89and from0.57to1.05, respectively. On the chondritenormalized REE diagram, the rocks are characterized by enrichment of LREEs, depletionof HREEs, and show negative Eu anomalies. On the MORB-normalized spider diagram,all the samples display enrichment of large ion lithophile elements Rb, Ba, and high fieldstrength elements Zr, Hf, depleted of large ion lithophile elements Rb, Ba, and high fieldstrength elements Nb, Ta, Ti.3. Discussion on the genesis of the Bainaimiao group volcanic rocksCompared with the geochemical characteristics of the mantle-derived primary magma, the mafic volcanic rocks are not primary magmas, because they are different tothe mantle-derived inclusions, and Mg#values range from0.59to0.77, Sc、Co and Nivalues range from22.1to50.4μg.g-1、22.9to62.2μg.g-1and17.9to181μg.g-1,respectively.Compared with the petrologic and geochemical characteristics of the activecontinental margin arc which develop in subduction zones, the metamorphic volcanicrocks are considered to be originated from partial melting of the overlying mantle wedgewhich is metasomatized by the fluid which is dehydrated from subducted oceanic crust.geochamistry of the felsic metamorphic shows that the magma derived from partialmelting of the crust.4. Geological significance of Bainaimiao group volcanic rocksThe association and geochemistry characteristics of Bainaimiao group volcanic rocksis similar to those occurred in active continental margins arc，suggesting that they wereformed in active continental margin setting. In the Nb-Zr-Y、Hf-Th-Ta、Th/Yb-Ta/Yb andTh/Hf-Ta/Hf diagram, by contrast with volcanic rocks in typical tectonic setting, not onlythe tectonic setting of active continental margin setting is reflected, but also Bainaimiaogroup volcanic rocks is dehydrated from subducted oceanic crust.The Paleo-Asian Ocean has subducted southward along the north edge of the theNorth China craton since Early Paleozoic, which made the accretionary complexes ofWenduermiao move from north to south and the formation of The frame mechanism ofarchipelagic arc-basin system in the north part of the active continental margin arc ofBainaimiao. Therefore, The formation of large quantities of intrusive rocks are interpretedas subduction-related magmatic rocks emplaced in the active continental margin arcduring the southward subduction of the Paleo-Asian oceanic plate beneath the NCB.