Geochemistry And Tectonic Setting Of The Volcanic Rocks Of Early Permian Dashizhai Formation In Dashizhai Area, Inner Mongolia

IntroductionThe early Permian volcanic rocks in the Dashizhai Formation are widely distributed in the middle part of Inner Mongolia. The research of lithologic characteristics and spacial distribution of volcanic rocks is a key to reveal the tectonic setting and late Paleozoic regional tectonic evolution. In the thesis, taking the volcanic rocks of the typical established formation section in the Dashizhai formation as an object of study. Based on the systematic research of petrology, geochemistry and component charge of regional volcanic rocks, the formation age of the volcanic rocks are determined and the magma sources and its properties are studied. The research also elucidates the tectonic setting of volcanic rocks.1. Spacial and temporal distribution of the volcanic rocks of Dashizhai FormationGeochemical component of the volcanic rocks of Dashizhai formation in Inner Mongolia shows apparent polarity characteristics. According to the component polarity, the volcanic rocks can be divided into south and north zones. The south volcanic zone extends from Mandoula via Linxi to Aluqi area,where the volcanic rocks is mainly composed of low-K tholeiitic series and a small portion of calc-alkaline series. The north volcanic zone extends from Sunitezuoqi–Xiwuqi to Dashizhai area, where calcalkaline series is dominant. The spacial distribution of the Dashizhai volcanic rocks is related with the suturing of Xar Moron river tectonic belt.We think that the age of Dashizhai formation is Artinskian in early Permian according to the evidences of stratigraphy, paleontology, geochronology of regional Permian volcanic rocks and granite, and detrital zircons geochronology of the late Permian stratum of Linxi formation.2. Rock associations and geochemical characteristics of the Dashizhai volcanic rocksThe Dashizhai volcanic rocks are mainly composed of basalt, basaltic andesite, andesite and rhyolite .They have a calc-alkaline evolutionary trend in SiO₂ vs. K2O diagram.The contents of SiO₂ of basalt, basaltic andesite, andesite are between 47.60%～57.99%, Al₂O₃ between 16.45%～18.3%. They are characterized by low content of MgO (Mg#=33³8),K₂O (0.30%～0.99%) and relatively high content of Na₂O (1.96%～3.16%) . Negative correlations of SiO₂ vs. FeOT,MgO,TiO₂,CaO and P₂O₅, and positive correlations of SiO₂ vs. Al₃O₂,Na₂O,and K₂O are shown in Harker Diagrams. The rhyolite has SiO₂ content of 75.77%～81.06%, Al₂O₃ of 9.86%～12.91%, and is characterized by lower MgO content, Mg#=15～20, and higher contents of Na2O (3.4%～4.28%) and K2O( 3.44%～5.11%).The total rare earth element contents (ΣREE) of the basalt, basaltic andesite, andesite are between 81.72¹80.37ppm. Their (La/Yb)N ratios andδEu values range from2.43 to 5.87 and from 0.91 to 1.03, respectively. On the chondrite-normalized REE diagram, the Dashizhai volcanic rocks are characterized by enrichment of LREEs, depletion of HREEs, and minor negative Eu anomalies or no Eu anomalies. On the MORB-normalized spider diagram, all the samples are enriched in large ion lithophile elements (LILEs, e.g., Rb, Ba, Th, and K) and LREEs, depleted of high field strength elements (HFSEs, e.g., Nb, Ta, Ti), which is similar to the volcanic rocks of Andean arc. The total contents of rare earth element (ΣREE) of rhyolite are between 63.02ppm and 121.74ppm. Their (La/Yb)N ratios andδEu values range from 2.45 to 7.73 and from 0.35 to 0.50, respectively. On the chondrite-normalized REE diagram, the rocks are characterized by enrichment of LREEs, depletion of HREEs, and show negative Eu anomalies. On the MORB-normalized spider diagram, all the samples display enrichment of large ion lithophile elements (LILE e.g.,Rb,Ba,K), and LREEs and depletion of high field strength elements (HFSEs, e.g., Nb, Ti).3. The nature of magmatic source and the Magmatic evolution characteristics of the Dashizhai volcanic rocksCompared with the geochemical characteristics of the mantle-derived primary magma, the early Permian volcanic rocks in Dashizhai area are not primary magmas, because they are lack of the mantle-derived inclusions, and have low Mg# values(<65) and low contents of Sc (20-32μg.g-1), Co (17³533-42μg.g-1) and Ni (88-120μg.g-1).The existence of phenocrysts, such as pyroxene and plagioclase, in the volcanic rocks suggests the magma experiences fractional crystallization of these minerals during evolution, which is also proved by the linear correlations between SiO₂ and other major oxides and trace elements.Geochemical characteristics of major elements and trace elements suggest that depletion of HFSEs, such as Nb and Ta, can not be explained by simple residue of minerals, which are enriched in HFSEs in magmatic source, or by contamination of crustal materials. The characteristics of component polarity of the vocanic rocks indicate that the parental magma could be derived from partial melting of the mantle wedge metasomatized by subduction-related fluids. Geochamistry of rhyolite shows that the magma derived from partial melting of the crust.4. The tectonic setting of Dashizhai volcanic rocksThe association and geochemistry characteristics of early Permian volcanic rocks in Dashizhai is similar to those occurred in active continental margins (e.g., Andes), suggesting that they were formed in active continental margin setting.In the M/Y-Nb/Y(M=Zr,Nd,La,Th,Rb,Ba)diagram, by contrast with volcanic rocks in typical tectonic setting, not only the component polarity of the Dashizhai volcanic rocks is showed, but also the tectonic setting of active continental margin setting is reflected.The Subduction of Paleo-Asian Ocean plate is bidirectional in Early Permian. From middle Permian the active continental margin of the Jiamusi-Mongolia block turn to passive continental margin. The North China plate and Jiamusi-Mongolia block sutured along Suolun-Xar Moron river-Changchun in late Permian ultimately.