西藏南部硅质岩的特征及相关问题探讨

Chert in southern Tibet, China: its field geological features and related scientific problems

西藏南部广泛发育中生代古海洋沉积硅质岩,在空间上集中分布于雅鲁藏布江缝合带和喜马拉雅沉积带,呈东西向条带状分布,断断续续延伸达数千公里。可大致厘定出3条硅质岩带:缝合带蛇绿岩北侧早白垩世硅质岩带、缝合带蛇绿岩南侧三叠纪至白垩纪硅质岩带和喜马拉雅沉积带白垩纪硅质岩带,产出块状、层状、角砾状和结核状4种类型的硅质岩。在沉积建造特征上,缝合带蛇绿岩北侧硅质岩带以蛇绿岩-硅质岩建造为主;缝合带蛇绿岩南侧硅质岩带主要以火山岩-硅质岩建造和改造型硅质岩建造为主;喜马拉雅沉积带硅质岩则以浊积岩-硅质岩建造为特征。藏南硅质岩具有重要的研究意义,其薄弱环节和科学问题主要包括:硅质岩的成因问题、硅质岩的微组构问题、硅质岩中的古海洋沉积事件信息、硅质岩对沉积环境的指相意义等。

As a prominent type of paleoceanic sediment, cherts occur widely in southern Tibet. According to the spatial and temporal distributions, three chert belts are identified in the Yarlung Zangbo suture and Himalaya sedimentation zone： the Early Cretaceous chert belt in the north of ophiolite of the suture （here referred to as the N-chert belt for short）, the Triassic-Cretaceous chert belt in the south of ophiolite （here referred to as the S-chert belt for short） and the Cretaceous chert belt in the Himalaya sedimentation zone （here referred to as the H-chert belt for short）. In these chert belts, various petrological characteristics are discriminated, but four types of chert are more reprehensive： massive chert, bedded chert, breccia chert and nodular chert. In the N-chert belt, ophiolite-chert formations are dominant; the volcanic-chert formation and reworked chert formations predominate in the S-chert belt; the H-chert belt is characterized by turbidite-chert formations. The chert in southern Tibet has great scientific value but weak points and problems as well, such as the petrogenesis and microfabric features of chert, information of paleo-oceanic sedimentary events in chert and facies implications of chert for the sedimentary environment.