Petrogenesis of Middle Devonian Chaoyangdi High Sr/Y Granite and Constraints on the Evolution of Xing-Meng Orogeny

Wang Shuqing; Hu Xiaojia; Zhao Hualei; Liu Wengang; Yang Zeli

Abstract：Xing-Meng orogeny is the accretionary orogeny between Siberia and North China plates with complex multi-stage subduction and accretion, and its tectonic evolution.has remained controversial for a long time.To reveal the arc-continent collision in the end of Early Paleozoic, this paper presents a study on Chaoyangdi Middle Devonian high Sr/Y granite in southern part of Xing-Meng orogeny zircon U-Pb dating results yield a formation age of 386.8±2.9Ma, with the corresponding age of Middle Devonian (D2) .Geochemical data shows that these rocks have high SiO2, Al2O3and Na2O, moderate CaO and K2O, but low FeOtand MgO contents with the Na2O/K2O>1and calc-alkaline series signature.The A/CNK ratios range from 1.02to 1.08, indicating these samples are weak peraluminous series.Trace element analysis results display enrichment of LILEs, such as Rb, Ba, K and Sr, but depletion of HFSE, i.e., Nb, Ta, P and Ti, with high Sr/Y ratios (103-146) , similar with adakitic-affinity.These rocks have strong fractionation between LREE and HREE ((La/Yb) N=9.9-14.6) , no obvious HREE fractionation ((Gd/Yb) N=1.45-1.97) and weak positive Eu anomalies (δEu=1.31-1.80) .All these geochemical characteristics suggest that high Sr/Y signature is inherited from magma source.Sr-NdHf analyses show enriched isotopic signatures-bothεHf (t) andεNd (t) are negative (-38.30to-23.59and-18.6to-17.9) with old two-stage model age (2 825to 3 745Ma for Hf, 2 584to 2 642Ma for Nd) , indicating an old enriched crustal source.Based on petrology, geochemistry and Hf-Sr-Nd isotopic characteristics, we conclude that the Chaoyangdi Middle Devonian intrusive rocks were formed by partial melting of ancient basic lower crust with high Sr/Y ratios and formed by post-collisional extension and heating of asthenosphere upwelling, indicating the regional extension after collision between the Bainaimiao arc and the North China Block.