Abstract:Qitianling composite granitic pluton is in the middle south of the southeastern Hunan ore concentration area. The Middle Jurassic Qitianling granite, the main intrusive body of Qitianling pluton, is studied in this paper. The Qitianling granite is composed of hornblende biotite granodiorite, hornblende biotite monzonitic granite and biotite monzonitic granite. SiO2 content increases from early to late granitic units, varying in the range of 65.92%～75.68%. The rocks are potassium-high (K2O=4.10%～5.27%, 4.86% on average) and alkali-rich (Na2O+K2O=7.12%～8.24%, 7.85% on average) ones, and have an average K2O/Na2O value of 1.63, ACNK values of 0.90～1.05 (0.96 on average) and KN/A values of 0.71～0.89 (0.78 on average), thus belonging to metaluminous to weakly peraluminous granitoids of shoshonite series and K_high calc_alkaline series. The rocks have an average ΣREE of 375.6 μg/g and show evident negative Ba,Nb, Sr, P, Ti and Eu anomalies as well as positive U, Th, Nd, Zr, Sm and Y anomalies, suggesting evident fractional crystallization. They have ISrvalues of 0.708?54～0.712?81, εNd (t) values of -5.05～-7.57 and tDM ages of 1.35～1.56 Ga, which is lower than the background age of granitoids in Hunan and Guangxi (1.8～2.4 Ga) and the region_al basement's age (1.7～2.7 Ga), implying participation of mantle materials. Various oxide and trace element diagrams for discrimination of structural environment reveal that the Qitianling granite was formed in a post_orogenic extensionalstructural environment. The authors consider that the early Yanshanian tectonicsetting in southeast Hunan and South China is a post_orogenic one instead of an intracontinental rift.
