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Early Devonian Tectono-Magmatic Events in the Middle Beishan,Gansu Province:Evidence from Chronology and Geochemistry of Gongpoquan Granite

ZHENG Rongguo,WU Tairan,ZHANG Wen,XU Cao,MENG Qingpeng The Key Laboratory of Orogenic Belts and Crustal Evolution(MOE),School of Earth and Space Sciences,Peking University,Beijing 100871  
To deepen the understanding on tectonic evolution of Beishan Moutain,Gongpoquan granite pluton in middle Beishan,Gansu Province,is studied in geochronology and geochemistry.The Gongpoquan granite is located in the west part of the middle Beishan belt between the Mingshui-Xiaohuanshan and Hongliuhe-NiujuanziXichangjing ophiplitic belts.Based on mineralogy and geochemistry,the Gongpoquan goanite can be divided into two groups.1) The leucogranites which are characterized by garnet,primary muscovite and tourmaline,exhibit slightly enrichments in LREE with(La/Yb)N=1.38–1.93,and negative Eu anomalies(δEu =0.19–0.28).They also exhibit enrichments in Cs,Rb,Th,U,K,P and Hf,and depletions in Ba,Nb,Nd,and Ti.2) The monzogranites show features of peraluminous S-type granites,with A/CNK1.1(on average).The monzogranites also have weak fractionations between LREE and HREE((La/Yb)N=2.74–22.78),but unobvious Eu anomalies(δEu =0.83–1.07).They display enrichments in Cs,Rb,Ba,Th,U,K,and depletions in Nb,Ta,P and Ti.According to the mineral assemblage and geochemical characteristics,it can be concluded that the Gongpoquan granite was formed in the post-collisional setting.The LA-ICP MS U-Pb zircon age of Gongpoquan granite is 402.2±3.0 Ma,which represents its crystallization age,and the εHf(t) values of zircons vary from 0.1 to 2.9,indicating the additions of mantle-derived materials.The Gongpoquan post-collisional granite,together with adjacent grantiods,confirms that the paleo-ocean basin closed before Early Devonian in the west part of Beishan,meanwhile the whole area was in the setting of post-collisional environment.
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