班公湖—怒江缝合带侵入岩地球化学和年代学特征及形成构造背景

Geochemistry and geochronology characteristics,and tectonic setting of the intrusive rocks in the Bangong Co-Nujiang suture,Tibet,China

对西藏纳屋错地区班公湖-怒江缝合带西段的侵入岩进行岩石地球化学和锆石U -Pb年龄测定,并与缝合带以外南面岩浆弧侵入岩进行对比.岩石地球化学分析表明,缝合带内侵入岩样品均属于钙碱性和高钾钙碱性系列,隶属于岛弧型岩浆岩类.稀土元素球粒陨石标准化分布模式均表现出L R E E 富集、H R E E 相对亏损的右倾模式,痕量元素原始地幔标准化蛛网图显示样品总体上富集大离子不相容元素（H F S E ） R b 、K 、B a,亏损高场强元素（L IL E ） N b 、T a、T i、P ,体现了钙碱性系列岩体的特征和演化趋势;年代学分析表明,带内侵入岩的成岩时代集中在早白垩世（104. 5-141. 7 M a B. P. ） ;进一步的构造环境对比分析表明,所有侵入岩样品均属于岛弧型- 大陆碰撞型花岗岩类,但缝合带内岩浆岩属性为同碰撞花岗岩,缝合带具由钙碱性向高钾钙碱性演化的特征和趋势,形成于受逆掩构造或地壳剪切作用的造山时期,处于威尔逊旋回的“陆陆碰撞造山- 山带剥蚀”阶段,认为班- 怒缝合带西段存在双向俯冲的碰撞造山构造背景.

The data of zircon U-Pb LAICPMS dating and lithogeochemical analysis on the west segment of inner Bangong Co-Nujiang suture are compared with igneous island arc outside the suture zone in the south.Lithogeochemical analyses indicate that the intrusive rocks in the suture zone mainly consist of middle to high K calc-alkaline series and belong to arc magmatites.Rare earthelement chondrite-normalized patterns displays right deviation mode with LREE enrichment and HREE deficiency relatively.Trace element distribution spider diagram shows high enrichment of large-ion incompatible element(HFSE)Rb,K,Ba and strong depletion of high field strength elements(LILE)Nb,Ta,Ti,P,and shows characteristic and evolutionary trend of Calcium alkalinity rock series.Chronology analysis indicates that the diagenesis age of intrusive rocks in the suture zone concentrate on early Cretaceous epoch(104.5~141.7 Ma B.P.).Further contrast analysis of tectonic setting indicate that all the intrusive rock samples belong to arc-continental collision granite type,while the magmatic rocks in the suture zone are syn-collisional granite.The magmatites in the suture zone show the evolutionary tendency from middle to high K calc-alkaline,formed in an orogenic period of overthrusting and crust shearing under the environment of erosion after the intracontinent collision.It is considered that the west segment of Bangong Co-Nujiang suture is characterized by bidirectional subduction.