班公湖-怒江洋打开时间的地层古生物约束

西藏班公湖-怒江洋的打开时间是争议性较大的科学问题。班公湖-怒江洋两侧的拉萨地块和南羌塘地块的古生物地理和地层层序的对比可以约束班公湖-怒江洋的形成时间。从地层层序上来看,拉萨地块在晚古生代大冰期结束之后是一由碎屑岩到碳酸盐转变的稳定地层序列;而南羌塘地块从早二叠世晚期开始东西向存在较大的相变,西部下二叠统吞龙共巴组之上存在间断面,不整合于上二叠统吉普日阿群之下;而东部下二叠统到中二叠统都是玄武岩和灰岩组成的鲁谷组。古生物地理上,南羌塘地块晚二叠世含有典型暖水的类Palaeofusulina动物群,与拉萨地块形成显著差别;南羌塘地块中二叠世主要的Eopolydiexodina类动物群也与拉萨地块的Nankinella-Chusenella类动物群产生明显差异;再者,南羌塘地块早二叠世晚期的类、珊瑚类和腕足类都呈现明显暖水的特征,但这些动物群在拉萨至今未有报道。综合南羌塘和拉萨地块地层层序、古生物地理特征上的差异,班公湖-怒江洋至少从中二叠世(～269Ma)就已经形成一定的规模。因此,班公湖-怒江洋在中二叠世以前和冰期结束之后的时间段内打开。

Stratigraphic and paleontological constraints on the opening time of the Bangong-Nujiang Ocean

The opening time of the Bangong-Nujiang Ocean has been a contentious issue for a long time. The correlations of sequences and paleobiogeography can constrain the opening time of the Bangong-Nujiang Ocean. From the viewpoint of the depositional sequences, the whole Lhasa Block has evidenced a stable transition from clastic rocks to carbonates after the Late Paleozoic glaciation. By contrast, the South Qiangtang Block varies a lot in depositional sequences. The western South Qiangtang Block has uplifted after the deposition of the Lower Permian Tunlonggongba Formation that results in an unconformity between the Tunlonggongba Formation and the Upper Permian Jipuria Group. The eastern South Qiantang Block, however, is characterized by a sequence of basalts and carbonates during the Middle Permian. In paleobiogeography, the South Qiangtang Block yields the fusuline Palaeofusulina fauna during the Late Permian. This fauna is absent in the whole Lhasa Block. During the Middle Permian, the fusuline Eopolydiexodina fauna in the South Qiangtang Block contrasts with the dominant Nankinella-Chusenella fauna in the Lhasa Block. In addition, the South Qiangtang Block is dominated by warm-water fusulines, brachiopods and corals during the late Early Permian. But, these warm-water taxa have not been documented from the Lhasa Block so far. The Bangong-Nujiang Ocean is considered to have formed a considerable width during the Middle Permian based on their contrast differences in depositional sequences and paleobiogeography between the South Qiangtang Block and the Lhasa Block. Consequently, the Bangong-Nujiang Ocean is considered to have opened before the Middle Permian but after the Early Permian deglaciation event.