Geochronology,Geochemistry,Sr-Nd-Hf Isotopic Characteristics and Geological Significance of Carboniferous Yuej in Arc Intrusive Rocks of Xilinhot,Inner Mongolia

Wang Shuqing; Hu Xiaojia; Yang Zeli; Zhao Hualei; Zhang Yong; Hao Shuang; He Li

Abstract：The study of magmatism can reveal the structure and tectonic evolution of the orogeny.In this paper,we present zir-con LA-MC-ICP-MS U-Pb dating,Hf-Sr-Nd isotopic compositions and whole rock geochemical data of the Carboniferous Arc intrusive rocks from Yuejin region of Xilinhot,central part of the Xingmeng Orogenic Belt.The rock suit consists of three ma-jor rock types,i.e.,granodiorite,tonalite and quartz-diorite,with zircon U-Pb ages ranging from 310±1 to 330±1 Ma.Geo-chemically,these rocks show moderate SiO2,high and variable Na2O,Al2O3,and CaO contents,but low K2O,FeOtand MgO contents.The Na2O/K2O ratios are all higher than 1(1.05―2.68,average of 1.95),while these rocks are of Calc-Alkaline series.The A/CNK ratios range from 0.86 to 1.17,indicating most of samples are meta-to weak per-aluminous series.The Mg#are moderately high,from 47.5―55.8.The trace elemental data of these rocks display enrichment of LILEs,such as Rb, Ba,Th,U,K,but depletion of HFSE,i.e.,Nb,Ta,P and Ti.The total REE contents are low(3.62×10―6―172.33×10―6, average of 118.33×10―6)with moderate fractionation between LREE and HREE((La/Yb)Nrange from 2.48―9.44),while weak negative Eu abnormity are observed(δEu range from 0.70 to 1.04).The in-situ zircon Hf isotopic analysis show depleted compositions with 176Hf/177Hf ratios varying from 0.282 723 to 0.282 863,εHf(t)values from +2 to +9.9 and Hf two stage model ages(TDMC)from 829 to 1 536 Ma(concentrating from 900 to 1 200 Ma).The whole rock Sr-Nd isotopic analysis of (87 Sr/86Sr)irange from 0.703 7 to 0.704 8,(143 Nd/144Nd)irange from 0.512 320 to 0.512 401,with positiveεNd(t)(+2.1 to+3.6),indicating depleted source,and the Nd two stage model ages(TDM2)are of Neoproterozoic(793―918 Ma).The rock suit,whole rock geochemistry,Hf-Sr-Nd isotopic characteristics indicate that Yuejin Carboniferous intrusive rocks were formed in active continental margin,by the partial melting of basic juvenile crust(formed in Late Mesoproterozoic to Early Neoproterozoic)triggered by subduction fluid and arc basic magma.This tectonic setting demonstrates that there was still sub-duction in Carboniferous in Xingmeng orogeny and the ocean was not closed.