Institute of Geology, Chinese Academy of Sciences, Beijing 100029)

Kuang Guodun

(Guangxi Geological Survey and Research Institute, Nanning 530023)

Guangxi is located in the south of China, bordering the South China Sea in the south and Viet Nam in the southwest. Traditionally, it is considered as the interaction model between platform and basin in late Paleozoic. In recent years, we have found that siliceous rocks and siliceous limestone interact with thick pillow or massive basalt in western Guangxi. Conodont fossils indicate that it was from late Devonian to late Permian. The obvious geochemical characteristics of siliceous rocks, such as negative cerium anomaly, indicate that they were formed in deep-water marine environment with hot water activity. The geochemical composition of basalt also shows the tectonic background of ocean plate. At the same time, the late Devonian to late Permian 1 1 radiolarian assemblage and 10 conodonts were found in a set of continuous siliceous rock series in southern Guangxi. Radiolarian combinations can be compared well with those in western Yunnan and Japan. These findings indicate that Guangxi may have a foreign-dominated environment in the late Paleozoic. Late Paleozoic platform carbonate rocks with weak deformation are exposed in some parts of Guangxi, but the lack of terrigenous debris and the negative cerium anomaly of siliceous interlayer also point out its marine background. They may gather in underwater highlands. These underwater highlands are sometimes exposed to the sea, forming an island-sea environment between the Yangtze block and the Indo-China Peninsula block.

Deep-water paleotethys; Guangxi; Tectonic background

1 Introduction

Guangxi is located in the south of China, bordering on the South China Sea. Traditionally, it is considered as a typical area of Caledonian movement. After Caledonian movement, it merged with Yangtze Craton and became a part of South China block. Guangxi borders Vietnam in the southwest. It is generally believed that the boundary between South China block and Indo-China Peninsula block is in the Red River belt in northern Vietnam, but the collision time between them has always been different. There are obvious negative cerium anomalies in many late Paleozoic siliceous rocks in western Guangxi. Late Paleozoic carbonate platform deposits are mainly exposed in Liuzhou area of central Guangxi. However, in Tancun, southwest of Liuzhou City 14km, the Upper Carboniferous is dark gray-black siliceous limestone mixed with siliceous rocks, and the Middle-Upper Permian along Liujiang River in northwest of Liuzhou City is interbedded with dark gray-black siliceous limestone, which has the sedimentary structure of remote turbidite and contains volcanic debris in the upper part. The siliceous rocks in these two places also have obvious negative cerium anomalies. At the same time, the platform carbonate rocks here also lack terrigenous detritus [14]. Therefore, Liuzhou area may also be in a marine environment in the late Paleozoic. However, in the Bancheng siliceous belt in southern Guangxi, only the upper Carboniferous-lower Permian siliceous rocks show weak negative cerium anomalies, while the upper Devonian-lower Carboniferous and middle-upper Permian siliceous rocks have no negative cerium anomalies [15]. This may be due to the influence of terrigenous materials in late Paleozoic Qinzhou-Yunkai terrane. On the other hand, this situation also supports that the negative cerium anomaly of siliceous rocks in western and central Guangxi can be used as evidence of offshore sedimentary background.

In addition, the recent paleomagnetic research conducted by Dr. Zhao Xixi of the University of California, Santa Cruz and us shows that the paleolatitude of the early Carboniferous Qinzhou-Yunkai terrane was 20.6 S, while that of the Baise area was 4. 1 N in the same period (the sampling position is as shown in figure 1). This also means that there were thousands of kilometers of Guang Hai between the two regions at that time.

5 tectonic significance

The above data show that the deep-water marine environment with extensive basalt eruption appeared in western Guangxi at least from the late Devonian to the Carboniferous and Permian. Central Guangxi is also in a marine environment, which is influenced by volcanic activities at some time. The Upper Devonian-Upper Permian siliceous rocks in southern Guangxi contain well-preserved radiolarian fossil assemblages [2 1]. It can be well compared with the radiolarian fauna of western Yunnan, northwestern Thailand and Japan at the same time [20,12,4]. The Permian fauna shows the characteristics of the deep-water radiolarian fauna around the Pacific Ocean [1]. Therefore, at that time, most of Guangxi was occupied by deep-water marine environment, which constituted an important sea passage between Yangtze block and Indo-China Peninsula block (Figure 3). The carbonate platforms in Liuzhou, Jingxi and Daming Mountain are just small underwater highlands, which does not mean that Guangxi has developed into a stable platform in Paleozoic tomorrow night.

Fig. 3 Schematic diagram of islands and oceans in Southeast Asia in Late Paleozoic.

DM- Daming Mountain Platform; JX- Jingxi platform; North Vietnam block; YK- Qinzhou-Yunkai block

In addition, the Devonian strata in western Guangxi are directly covered with Cambrian (lower Ordovician in a few places) rocks [17], and the stratigraphic sequence is similar to the adjacent North Vietnam block, but different from the Yangtze block. These platforms or small terranes, including Jingxi platform, Liuzhou platform, Beiyue terrane, Damingshan terrane and Qinzhou-Yunkai terrane, and the deep-water facies deposits between them, constitute a broad and complex transition zone between Yangtze block and Indosinian block.

Thank you. This research project is supported by the National Natural Science Foundation of China. Thank you.

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