西藏南部早第三纪钙质超微化石及东特提斯在西藏境内的封闭时限

EARLY TERTIARY CALCAREOUS NANNOFOSSILS FROM SOUTHERN TIBET AND THE CLOSING TIME OF EAST TETHYS IN TIBET

研究了西藏南部定日地区曲密巴剖面早第三纪遮普惹组上页岩段的钙质超微化石 ,自下而上划分出 3个超微化石组合 :N annotetrina fulgens-Chiasmolithusgigas组合 ;Discoaster bifax-Chiasmolithussolitus组合及 Sphenolithuspseudoradians-Reticulofenestra bisecta组合 ,确定了上述3组合可与 Martini( 1 971 )超微化石分带的 NP1 5、NP1 6及 NP1 7带对比。还确认了第三超微化石组合的层位是西藏地区海相第三纪目前已知的最高层位 ,它的时代应为中始新世晚期 ,即属 Bartonian期。根据超微化石属种的时限分布的分析 ,东特提斯在西藏境内的封闭时间大约在3 8Ma左右 ,即至该时期 ,海水完全退出青藏高原 ,印度板块和欧亚板块完全拼合 ,青藏高原开始大幅度的隆升。

The Tertiary marine strata occur mainly in southern Tibet as well as in the Gandies Mountains of Tibet. The Tertiary strata of southern Tibet can be divided lithologically into three formations: the Jidula Formation, Zongpu Formation and Zhepure Formation. The Zhepure Formation is the highest marine stratum in Qinghai Xizang(Tibet)Plateau.Twelve samples containing calcareous nannofossils were collected for this study from the upper shale member of the Zhepure Formation, in Qumiba section, Dingri County of southern Tibet. According to Martini's Zonation (1970), and Okada and Bukry's Zonation (1980), three nannofossil assemblages are established based on the FAD or LAD of some marker species. The first one is Nannotetrina fulgens Chiasmolithus gigas assemblage corresponding to the NP15 zone of Martini Zonation, the second one is Discoaster bifax Chiasmolithus solithus assemblage corresponding to the NP16 zone, and the third one is Sphenolithus pseudoradians Reticulofenestra bisecta assemblage corresponding to the NP17 zone. The third nannofossil assemblage is dated as late Middle Eocene on the basis of nannofossil characters,and the stratum yielding this assemblage is the highest marine bed in Tibet. This indicates that the sea water did not withdrawn from the Qinghai Xizang (Tibet) Plateau until about 38 Ma according to new data of Berggren et al. (1995) on the last occurrence of Discoaster bifax and the first occurrence of Chiasmolithus oamaruensis. The East Tethys closed in Tibet and the Qinghai Tibet Plateau rose up from the ocean dramatically at the same time.