西藏拉萨地块的乐平统及其古地理意义

西藏拉萨地块乐平统以海相与陆相沉积并存为特征。海相乐平统含复体珊瑚Waagenophyllum、■类Reichelina和有孔虫Colaniella。陆相乐平统含两类植物群,一类发育于措勤盆地中北部地区,为舌羊齿植物群分子Glossopteris、Noeggerathiopsis、Phyllotheca和华夏植物群分子Pecopteris、Sphenopteris共生的混生植物群;另一类主要发现于改则县下岗江和昂仁县贡久布地区,以Lepidophylloides、Paracalamites、Pecopteris等华夏植物群分子为主。根据沉积层序分析,拉萨地块乐平世时表现为整体上升。藏南喜马拉雅特提斯带的乐平统以产有冈瓦纳型Glossopteris植物群、冷水型单体珊瑚Cyathaxonia和腕足动物群为特征,并且从曲布组到曲布日嘎组表现为海进的沉积序列。拉萨地块和喜马拉雅特提斯带乐平世在植物群、动物群以及沉积层序方面存在着本质的差别,因此,拉萨地块在晚二叠世已基本脱离位于冈瓦纳大陆北缘的喜马拉雅特提斯带。

THE LOPINGIAN SERIES IN THE LHASA BLOCK, TIBET AND ITS PALAEOGEOGRAPHICAL IMPLICATIONS

Latest investigation indicates that the Lopingian Series including both terrestrial and marine deposits are developed in the Lhasa Block. The marine Lopinigian Series in the Lhasa Block contains the compound coral Waagenophyllum, fusulinid Reichelina, and foraminifer Colaniella faunas, and the terrestrial Lopingian Series is characterized by both Cathaysian floras and mixed floras consisting of Gondwanan elements such as Glossopteris, Noeggerathiopsis, Phyllotheca and Cathaysian elements such as Pecopteris, Sphenopteris. An anlaysis of the Lopingian sequences in the Lhasa Block reveals that it experienced a regression stage from Guadalupian to Lopingian. By contrast, the Himalayan Tethys Zone south to the Lhasa Block is characterized by typical Gondwanan Glossopteris flora, cold-water brachiopod and solitary coral faunas. In addition, the Lopingian sequence in the Himalayan Tethys Zone reflects a transgressive process from the terrestrial Qubu Formation to the shallow marine Qubuerga Formation. Therefore, the Lhasa Block shows significant differences in both biota and depositional features from the Himalayan Tethys Zone during the Lopingian, which implies that the Lhasa Block had rifted from the northern peri-Gondwanan margin before the Lopingian.