從中國西南及鄰區(qū)看亞洲形成過程

從中國西南及鄰區(qū)看亞洲形成過程

E-mail: jinxchi@cags.ac.cn.

Southwest China and adjacent areas are formed by suturing continental blocks from Gondwana onto Laurasia. The process, however, is not as simple as some cartoons show that continental slices drifted away successively from the Gondwana margin and collided onto the northern continent.

Permo-Carboniferous glaciation on Gondwana resulted in glacial sediments in depression zones in Gondwana, and glacio-marine deposits on its northern margin. Now the divide between Gondwana-affinity and Cathaysia-affinity sediments and biota in southwest China is marked by the Shuanghu–Lancangjiang zone in Tibet, and the Changning?Menglian belt in western Yunnan. These zones closed folded by the Late Triassic. Suturing of continental blocks in the vast area north of these zones was also completed by that time. Most areas of the Asian continent emerged then, and marine environment was mostly limited to Tibet, western Yunnan and adjacent areas. On the other hand, in the Triassic, especially the Late Triassic,continental rifting took place along the Yarlung Zangbo, Bangong-Nujiang zones and the zones in Gondwanan continent, where the Indian Ocean came into being.

In the east part of the Bangong-Nujiang zone,Late Triassic cherts are reported in the Dengqen area(Geological Survey of Tibet, 2005). Jurassic sequences of silisiclasitics and cherts (the Mugagangri Group)occur in a large part of the zone. Also ultramafic and mafic rocks crop out in the zone, whose age have been mostly inferred based on fossils from neighboring sedimentary rocks, although the contacts between them are often abrupt. Independent isotopic age is expected to provide more precise constraints on them.Upper Jurassic sediments in the middle segment of the zone are found resting unconformably on older rocks(e.g. Chen et al., 2004). Shi (2007) reported SHRIMP U-Pb ages of 162.5±8.6 ? 177.1±1.4 Ma, average 167.0±1.4 Ma of zircons from gabbros of mainly SSZ ophiolite in the Bangong Co area in the west part of the zone. Considering also that late Triassic to Jurassic rocks are uncomformably overlain by Middle Jurassic sediments, the suturing of the Bangong-Nujiang zone seems occur earlier in the east and later in the west.

Late Triassic is also the time when rifting along the Yarlung Zangbo became apparent. The several-thousand-meter thick Late Triassic siliciclastic successions along the Yarlung Zangbo suture are interpreted to be the sedimentary records of a rapid rifting. This process continued in the Jurassic. Since the Cretaceous the once existed oceanic basin started to shrink and disappeared in the Late Cretaceous.

Starting also from the rifting in the late Triassic,the Indian Ocean developed successfully into a big ocean. The Bangong-Nujiang and Yarlung Zangbo rifting zones failed to develop into large oceans,though oceanic crust occurred. They closed in late Jurassic and Cretaceous respectively. The reason may be that the development of the Indian Ocean in the south effectively released the energy/pressure from the interior of the earth, and much reduced the tension along the Bangong-Nujiang and Yarlung Zangbo rifting zones, and gave at the same time a push on the greater India towards north.

CHEN Guo-rong, LIU Hong-fei, JIANG Guang-wu, ZENG Qing-gao, ZHAO Shou-ren, ZHANG Xiang-guo. 2004. Discovery of the Shamuluo Formation in the central segment of the Bangong Co–Nujiang suture zone, Tibet. Geological Bulletin of China, 23(2): 193-193.

Geological Survey of Tibet. 2005. Reports of the Geological Map of Dengen Sheet (1:250,000). 557pp.

SHI Ren-deng. 2007. SHRIMP dating of the Bangong Lake SSZ-type ophiolite: Constraints on the closure time of ocean in the Bangong Lake-Nujiang River, northwestern Tibet. Chinese Science Bulletin, 52(7): 936-941.

Steps of Construction of Asia: A Perspective from Southwest China and Adjacent Areas

JIN Xiao-chi
Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing100037,China

Asia construction; southwest China; Gondwana