青藏高原西部班公湖岛弧带特提斯洋盆闭合后的地壳伸展作用

青藏高原西部班公湖-怒江缝合带西端的班公湖地区分布着一系列南北向和东西向岩脉: 花岗斑岩和闪长玢岩.通过岩石地球化学系统分析和锆石U-Pb LA-ICP-MS年龄测定, 指出这些岩脉代表了班公湖中特提斯洋盆闭合后的地壳伸展事件.呈南北向产出的花岗斑岩脉13个锆石颗粒测试结果给出了79.59±0.32Ma(MSWD=1.08)的加权平均年龄; 既有南北向也有东西向产出的闪长玢岩脉6个锆石颗粒给出了75.9±1.2Ma(MSWD=2.8)的加权平均年龄.这些结果表明班公湖岛弧带发生在晚白垩世的地壳伸展作用一开始只沿东西向进行, 稍后南北向也开始伸展.岩石地球化学特征表明, 2种脉岩都具有岛弧岩浆特征, 这是由于岩浆源区的印度洋MORB型地幔受到了来自俯冲沉积物熔体的交代.处于较深部位的闪长玢岩源区参与交代的沉积物熔体大体在1%~10%之间, 部分熔融程度约为8%~15%;处于较浅部位的花岗斑岩源区参与交代的沉积物熔体约在10%~15%之间, 部分熔融程度大于15%. 

Crustal Extension of the Bangong Lake Arc Zone,Western Tibetan Plateau,After the Closure of the Tethys Oceanic Basin

A series of NS-and EW-striking dykes occur in Bangong lake region, western segment of the Bangong lake-Nujiang suture in Tibetan plateau, including both granite porphyry and diorite porphyrite. Based on petrochemical analyses and zircon U-Pb LA-ICP-MS dating combined with field investigation, the present authors propose that these dykes represent a crustal extension event of the Bangong lake arc zone after the closure of Bangeng lake middle-Tethys oceanic basin. The granite porphyries occuring only in NS-strike and the diorite porphyrites occuring both in NS- and EW-strike respectively yield a weighted mean age of 79.59±0.32Ma (MSWD=1.08) with 13 zircons and (76.9±1.2)Ma (MSWD=2.8) with 6. These results indicate that the crustal extensional process of the Bangong lake arc zone occurring in Late Cretaceous epoch was initialed only in EW-trending and slightly later also in NS-trending. Petrochemically, the two types of the dykes appear arc magmatic features characterized by enrichment of large iron incompatible elements (Rb, U, Th, K, Pb) and depletion of high field strength elements (Nb and Ti) which were attributed to metasomatism of Indian-MORB mantle by subducted sediment melt. Furthermore, by analyzing geochemical characteristics of the dyke, we come to the conclusion that the granite porphyries were generated at a shallower depth under amphibolite facies conditions, and the diorite porphyrites were probably under eclogite facies or garnet amphibolite conditions. And the amount of sediment melt involved in the mantle metasomatism related to diorite porphyrites varies largely from 1% to 10% with a source partial melting degree ranging from 8% to 15%. Whereas in the granite porphyries, the amount varies from 10% to 15% and their source partial melting degree is up to 15%.