北祁连青山花岗岩体矿物学特征及其对岩石成因的约束
Mineralogical Characteristics of Qingshan Granitic Pluton in North Qilian Orogenic Belt and Their Constraints on Petrogenesis

北祁连造山带经历了洋盆的打开到闭合这一完整的Wilson旋回，并发育有大量与之相关的花岗质岩体.加深对花岗岩的研究可以为重建俯冲-增生/碰撞造山格架提供关键线索.对带内西段的青山二长花岗岩体进行了岩相学以及造岩矿物化学成分的电子探针原位分析，厘定了其矿物形成的物理化学条件，进一步约束了其岩石成因和构造背景.研究表明，青山二长花岗岩中钾长石全部为正长石，斜长石为中酸性的中长石和更长石，黑云母属镁质黑云母，角闪石则为镁角闪石亚类.锆石饱和温度平均为750 ℃，黑云母的结晶温度平均为647 ℃，氧逸度为-15，推测岩体固结压力约为1.85×108 Pa，形成深度约 6.73 km.矿物化学特征显示，青山二长花岗岩为具I型花岗岩特征的低熔线花岗岩，并具有壳幔岩浆混源的特点，可能为含水条件下形成的钙碱性花岗岩.
The North Qilian orogenic belt underwent a complete Wilson Cycle through opening and closing of an ocean basin. Deepening the research of granite within can provide pivotal clues to reconstruct the subductionaccretion/collision orogenic framework. On the basis of systematic petrological and petrographical research, this paper conducts electron microprobe analysis on the main rock-forming minerals, aiming to determine the physicochemical conditions during rock formation and provide further constraints on the petrogenesis and tectonic setting. Results show that Qingshan monzogranite is mainly composed of K-feldspar (orthoclase), plagioclase (andesine-oligoclase), biotite(magnesian biotite), amphibole (magnesiohornblende) and quartz. Zircon saturation thermometer shows that the average temperature of initial magma is 750 ℃. Meanwhile, mineral chemistry analysis reveals that the average crystallization temperature for biotite is 647 ℃ and the oxygen fugacity during the rock formation is -15, corresponding to solidification depth of 6.73 km and solidification pressure of 1.85×108 Pa. The data in this paper indicates that Qingshan monzogranite is I-type subsolvus granite, with the features of crust-mantle mixed source. Thus, it is proposed that Qingshan monzogranite belongs to calc-alkaline granite and produced under water-bearing condition