Geochronology, Geochemistry and its Tectonic Significance of the Early Paleozoic Magmatic Rocks in Northern Langshan, Inner Mongolia

Zhang Yun; Sun Lixin; Zhang Tianfu; Teng Fei; Zhang Yong; Sun Yiwei; Yang Zeli; Xu Fan

Abstract：The Langshan area in Inner Mongolia is a paramount transitional zone located between the North China Craton (NCC) and the Xingmeng orogenic belt (XOB) , and structurally belongs to the Early Paleozoic marginal accretionary belt in the northern margin of the NCC.The Early Paleozoic magmatic rocks distributed in the Bayinhanggai area, the northern Langshan, have important implications for the reconstruction of regional Paleozoic tectonic setting and orogenic evolution.The dioritic batholith consists mainly of diorite and quartz-diorite, with the zircon 206Pb/238 U ages of 435.8±2.2Ma-437.7±2.2Ma, which represent the crystallization age of the Early Silurian.The rocks are calc-alkaline series, enriched in LREE with the (La/Yb) Nratios 4.30～11.59, depleted in HFSE (especially Nb, Ta and Ti) and enriched in LILE (like Rb, Th, K, Ba and Sr) , and weakly negative Eu anomalies (δEu=0.80～0.96) , indicating that they are formed in island arcs or in active continents margin setting.The Early Silurian diorite batholith shows depleted isotopic compositions, εHf (t) =+5.2-+12.6, (87Sr/86Sr) i=0.704 665-0.706 174, εNd (t) =+1.84-+2.00.Integrated geochronology, geochemical, Sr-Nd-Hf isotopic data and regional geological data suggest that the Early Silurian diorite batholith in northern Langshan was generated in Early Paleozoic and resulted from the southward subduction of Wenduermiao ocean along Wenduermiao-Wude-northern Suolun.The diorites were formed by partial melting of juvenile crust which was modified by subduction slab fluids.The determination of the Silurian arc magmatic rocks makes it clear that the northern Langshan area belongs to the Early Paleozoic active continental margin of the northern NCC, and it is the westward extended part of the Bainaimiao island arc belt.At the same time, it also provides important materials for the Early Paleozoic"trench-arc-basin"system in the west section of the northern NCC.