Geochronology and Geochemistry of Granite Pluton in the North of Qiaowan,Beishan Mountain,Gansu Province,China,and Its Geological Significance

To deepen the understanding on tectonic evolution of Beishan Moutain,granite pluton which lies in the north of Qiaowan,northern margin of Dunhuang block(south belt of Beishan Mountain),Gansu province,is studied in geochronology and geochemistry.The hornblende granite contains a LA-ICP MS zircon U-Pb aged at 303.7±2.4 Ma.Its eHf(t) value varies from 1.2 to 5.8,eNd(t) value is high(0.40 to 0.06),and(87Sr/86Sr)i(0.704524 to 0.705062) is relatively low.The pluton apparently belongs to sub-alkline and high-K cal-alkaline series(K2O is 4.09% to 5.58%,K2O/Na2O is 1.34 to 2.07) and shows as I-type granite with metaluminous-peraluminous characteristics for A/CNK is between 0.92 and 1.04.The hornblende granite pluton has LILE enrichment,especially rich in Pb,Rb,Th,and U,depleted in HFSE(Nb,Ta) obviously in spidergram.It also has a rich LREE and flat HREE in REE patterns with no or extremely weak Eu anomalies(δEu = 0.67 to 1.17).From the eHf(t),eNd(t) values and geochemical characteristics,it can be concluded that the pluton is a mixture of crustal and mantle derived magma.The mantle derived magma may be directly from anatexis magma in deep mantle and then underplate below the continental crust.Based on regional geology and the geochemical characteristics,the hornblende granite pluton in the north of Qiaowan was formed during post-collision period,which suggests the collision of Beishan Mountain is already finished in late Carboniforous.