雅鲁藏布结合带中段混杂岩带内玄武岩的地球化学特征

Geochemical characteristics of basalt in a mélange zone in the central segment of the Yarlung Zangbo juncture.

将雅鲁藏布结合带中段混杂岩内的玄武岩分为北带玄武岩和南带玄武岩。北带白垩纪玄武岩的SiO2含量多为44.86%～50.96%,TiO2均在3%以上,Na2O+K2O为3.78%～6.10%,Na2O>K2O;南带中晚三叠世玄武岩的SiO2含量为46.94%～49.58%,TiO2为1.17%～2.04%,Na2O+K2O为3.79%～7.15%,Na2O>K2O。北带玄武岩属碱性玄武岩系列,南带玄武岩属拉斑玄武岩系列;北带玄武岩富集Rb、Th、Ta、Nb,与洋岛碱性玄武岩稀土曲线一致;南带玄武岩除Y、Yb、Sc、Cr外,其他均富集,显示与大洋拉斑玄武岩曲线一致。构造背景判别和分析显示,北带碱性玄武岩形成于大洋板内洋岛环境,南带拉斑玄武岩形成于洋盆海山环境。

On the basis of 1∶250000 regional geological survey, the authors have studied basalt in a mélange zone in the central segment of the Yarlung Zangbo juncture and divided it into basalt in the north zone and basalt in the south zone. The Cretaceous basalt in the north zone has the following chemical composition: SiO2 mostly 44.86-50.96%, TiO2 >3% and Na2O+K2O 3.78-6.10% with Na2O>K2O; and the Middle-Late Triassic basalt in the south zone has the following chemical composition: SiO2 46.94-49.58%, TiO2 1.17-2.04% and Na2O+K2O 3.79-7.15% with Na2O>K2O. The basalt in the north zone belongs to the alkaline basalt series, enriched in Rb, Th, Ta and Nb, showing that its characteristics are consistent with those of oceanic island basalt. The basalt in the south zone belongs to tholeiite series, enriched in all the trace elements and REE except Y, Yb, Sc and Cr, showing its characteristics are consistent with those of oceanic tholeiite. Discrimination and analysis of its tectonic setting made by using the results of the geochemical analysis show that the alkaline basalt in the north zone formed in an ocean island environment within the oceanic plate, while the tholeiite in the south zone formed in an oceanic basin seamount environment.