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Geochronology and Geochemistry of Haergentoukoubu Granites in the Beishan Area,Gansu,China and Their Geological Significance

WANG Lishe1,2),YANG Jianguo2),XIE Chunlin2),LEI Yongxiao2),WANG Yuxi2),CAO Hongxia1),YANG Pengfei2),QI Yalin3),JIN Suanlian3)1) State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi'an,Shaanxi,710069;2) Xi'an Institute of Geology and Mineral Resources,Xi'an,Shannxi,710054;3) Research Institute of Changqing Oil Field,Xi'an,Shannxi,710021  
LA-ICP-MS U-Pb isotopic dating for zircon separated from the Haergentoukoubu granites yields two ages: 387.6±8.2Ma and 1172±7Ma.These granites were emplaced in the Changcheng System,Jixian System and Silurain System in Huoshishan,Beishan,Gausu.The former probably represents intrusive age(early-middle Devonian)of the granites and the latter probably represents the age of late Jixian System inherited zircon inside the granites.Composed mainly of moyites,monozogranite,granodiorite,the granites are characterized by high SiO2(70.28%~79.4%),and high Na2O + K2O(6.52%~9.11%),low Al2O3(8.82%~14.05%),and low Sr(40~150×10-6,averaging 101.5×10-6).Trace elements analysis shows that the average ΣREE is 168.52×10-6,the ratios of LREE/HREE and LaN/YbNare 2.89~16.4,1.82~25.18 respectively.The REE distribution patterns show that the granites are enriched in Rb,Th Ce and Sm,and depleted in Eu,TZ,Nb,Zr and Hf,with LREE/HREE=2.89~16.4,LaN/YbN= 1.82~25.18.The comprehensive analysis also suggests that the granites are metaluminous to slightly peraluminous calcicolous alkaline series rocks,which formed in the tectonic setting of transformation from collision to extension in earlier-middle Devonian.
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