西藏申扎地区中二叠世类动物群

MIDDLE PERMIAN FUSULINIDS FROM THE XAINZA AREA OF THE LHASA BLOCK, TIBET

结合新采集的材料与前人研究,综合分析拉萨地块申扎永珠乡下拉组中类动物群的构成与时代,并将其与同期华南类动物群进行对比。分析结果表明,申扎下拉组的类动物群共16属(含1亚属,2存疑属),70种(含比较种与未定种),时代为茅口期,大致相当于Roadian阶上部至Wordian阶。与同期华夏区华南的类动物群相比,下拉组的类动物群在属种构成,尤其是优势类群方面差异显著,其主要特征为Neoschwagerinidae与Verbeekinidae两科的种级分异度很低,Chusenella与Nankinella占优势地位,且缺失了约60%华南的常见属。

Over the past two decades, the interpretation regarding the paleogeographic evolution of the Lhasa Block has been rather controversial. Some believed that the Lhasa Block was located in the northern margin of Gondwanaland until Late Triassic （Norian） and then rifted from Gondwana （Metcalfe, 2002）; while others argued that the rift took place in later Early Permian （Ueno, 2003）. Fusulinids have been proved to be a useful fossil tool in Permian paleogeographic reconstruction （Sheng et al. , 1981 ; Ozawa, 1987; Belasky etal., 2002; Ueno, 2003）, thus a detailed study of the Permian fusulinids in the Lhasa Block could provide pivotal paleontological evidence for solving the aforementioned dispute. Based on new collections and previous data, a comprehensive study has been conducted to analyze the composition and age of the fusulinid fauna from the Xiala Formation in the Xainza area of the Lhasa Block, Tibet. The results of study reveal that this fusulinid fauna consists of 70 species belonging to ]5 genera. The age of the fauna is probably Maokouan （Jin, 2002）, approximately late Roadian to Wordian, because of the existence of Neoschwagerina sp. and Verbeekina verbeeki, as well as the absence of Yabeina. A further comparative study has unraveled remarkable differences in composition between the fusulinid fauna in the Xiala Formation and the coeval fusulinid fauna from Cathaysian South China. At generic level, the Maokouan fusulinid fauna in S. China is characterized by the dominance of Neoschwagerinidae and Verbeekinidae, contributing 26% and 37% respectively to the total generic diversity. However, Neos-chwagerinidae is rather impoverished in the Xiala Formation with only one genus Neoschwagerina. Furthermore, about 60% of the Maokouan fusulinid genera in S. China, such as Praesumatrina, Surnatrina, Metadoliolina, Yangchienia etc. , are completely absent from the fauna in the Xiala Formation. As for the specific diversity, an even more remarkable distinction is demonstrated between these two fusulinid faunas. The species of Neoschwagerinidae and Verbeekinidae account for 63% of the total species in the fauna of S. China but only play an insignificant role （8%） in the Xainza area of the Lhasa Block. In contrast, the species of Schwagerinidae, which is subordinate in the fusulinid fauna of S. China, constitute about 53% of the total species in the Xiala Formation. Besides, in the Xiala Formation, Chusenella （24 species） and Nankinella （17 species） are most diversified, while the Neoschwagerina, Parafusulina and Verbeekina which are predominant in S. China each merely yields one species.