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Some Stratigraphic and Sedimentologic Constraints on the Features of the Changning-Menglian Tethys, Yunnan, China
Gondwana Research, V 4, No. I, pp. 55-60. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
Some Stratigraphic and Sedimentologic Constraints on the Features of the Changning-Menglian Tethys, Yunnan, China Jin Xiaochil, Xie Guanglianl and Wang Yizhao2 I
Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100 037, P. R. China Geological Survey of Yunnan, 35 Yujiang Road, Yuxi, Yunnan 653 100, P. R. China
(Manuscriptreceived March 20, 2000; accepted September 15, 2000)
Abstract The interpretation of the (Lower Carboniferous) basalt- (Middle Carboniferous to Middle Permian) carbonate succession in the central zone of the Changning-Menglian Belt in western Yunnan, China as a seamount is one of the major arguments supporting a large ocean model of the Changning-Menglian Tethys. Our field investigations and laboratory work lead to the conclusion that the basalt-carbonate succession of the Changning-Menglian Belt is not genetically related to seamount. It is a normal lithological succession that developed on continental crust, and it is underlain and overlain by marine carbonate and/or clastic deposits.
Key words: Changning-Menglian belt, seamount, Yunnan, Tethys, China.
Introduction The Changning-Menglian Belt in western Yunnan, China has been widely accepted as the remnant of the Paleotethys in this region (e.g. Liu et al., 1991, 1993; Jin, 1994,1996,1998; Metcalfe, 1996; Wopfner, 1996; Wang, 1997; Zhong, 1998; Cui et al., 1999; Yao et al., 1999). However, there are different interpretations of features of the Changning-MenglianTethys within the Late Paleozoic. The Changning-Menglian Belt is bounded on the east by the Lanping-Simao block of Cathaysian affinity and on the west by the Baoshan block of Gondwanan affinity (Fig. 1). Based on the characteristics and distribution of lithological successions, the Changning-MenglianBelt can be divided into several north-south trending zones, each of which has a distinctive geological history (Fig. 2). The eastern zone is represented by a succession of siliciclastic rocks intercalated with limestone and bedded cherts, resting on the Middle Proterozoic Huimin Formation (of the Lancang Group). The siliciclastic deposits begin with the Carboniferous Nanduan Formation, which comprises more than 1000 m of quartz sandstones, siltstones and shales. It is overlain by the Laba Group, a conformable contact is seen at a road cut near the village of Ali. The boundary between the Nanduan
Formation and the overlying Laba Group is probably diachronous (Wang, 1997).The Laba Group is composed dominantly of fine siliciclastics and has a purplish red color in several parts. Limestone lenses with Permian fusulinids are intercalated in the middle part. Bedded cherts and siliceous shales of 2-10 m thickness occur in several horizons of the upper part. The lithological change from siliciclastics to cherts is transitional. The central zone of the Changning-Menglian Belt is characterized by a succession of Lower Carboniferous basaltic rocks and an overlying Middle Carboniferous to Middle Permian carbonate sequence. The top of the carbonate sequence is karstified and covered in many places by Lower Triassic clastics. The basaltic rocks rest either on Devonian graptolite-bearing shales and cherts or on karstified Lower Carboniferous limestone. The western zone of the Changning-Menglian Belt is represented by a succession of basalts, cherts, thin-bedded dark limestones, fine clastics and sandstones. A time range of Carboniferous to Middle Triassic is indicated by palynomorph, radiolarians, bivalves and plant fossils from different horizons. The Changning-MenglianBelt is a complicated orogen, and any model of its ancestor, i.e. the Changning-Menglian Tethys, should be based on extensive field observation
56
J. XIAOCHI ET AL.
Fig. 1. Tectonic subdivision of Yunnan (from Jin, 1996). 1.Nujiang fault; 2. Kejie-Nandinghe fault; 3. Lancangjiang fault; 4. Ailaoshan fault; 5. Red River fault; 6. Nanpanjiang fault; 7. Jinshajiang fault; 8. Xiaojinhe fault.
and proper interpretation of geological environments. The seamount interpretation of the (Lower Carboniferous) basalt- (Middle Carboniferous to Middle Permian) carbonate succession in the central zone put forward by Liu et al. (1991, 1993) is one of the major arguments supporting a large ocean model of the ChangningMenglian Tethys. Our field investigations since 1997, however, gathered much evidence that is contrary to the seamount interpretation. As we know, seamounts are submarine mountains rising more than 1000m above the ocean floor. Seamounts are of volcanic origin, and may be either flattopped (guyot) or peaked. On some flat-topped seamounts, carbonate platforms can be formed.
Seamounts may be either discrete, or arranged in a linear or random grouping. For an objective interpretation of the geological existence of an orogen, field observation of the relationships among different lithological units and tracing of geological contacts are essential, although geochemical data and sometimes conceptual modelling can be very useful.
Field Evidence Against Early Carboniferous “Seamount Basalts” Basalts which were thought to represent seamounts in the “Changning-Menglianocean” (Liu et al., 1991,1993) and their sedimentary intercalations are called the Gondwana Research, V. 4, No.1,2001
THE CHANGNING-MENGLIANTETHYS, CHINA
57
Fig. 2. Simplified geological map of the Changning-Menglian belt (modified after Wang, 1997).
Pingzhang Formation (105-620 m thick) in the northern part and Yiliu Formation (200-760 m thick) in the southern part of the Changning-MenglianBelt. These two formations were assigned to the Lower Carboniferous, according t o fossils contained in sedimentary Gondwana Research, V. 4, No. 1,2001
intercalations and the fact that they are conformably overlain by Middle Carboniferous limestones in some places (Geological Survey of Yunnan, 1980,1981,1982). Our field observations reveal that Early Carboniferous basaltic eruptions did not take place on oceanic crust:
J. XIAOCHI ET AL.
58
1) In the Wenquan area, about 20 km south of Changning, the Pingzhang Formation is well exposed along a brook (Fig. 3, section 1).Along the course of the brook, there is a road linking the village of Xingwen to the main road. Here the Pingzhang Formation is overlain by Upper Carboniferous light grey limestones and underlain by the Lower Devonian Wenquan Formation, which is composed of graptolite-bearing shales, siltstones and cherts. The contact between the Pingzhang and Wenquan formations is an unconformity or disconformity. The Pingzhang Formation is composed mainly of basaltic lava sheets, but tuffaceous deposits, conglomerates, fine clastics and carbonates formed during the dormancy periods occur in different horizons. Along the road side, a conglomerate bed more than 2 m thick is seen intercalated in the basalts. Clasts of the
A
Section Location 27
0 l
*
l
54km *
J
4
I
Fig. 3. Index map showing the locations of sections described.
conglomerate range from several millimeters to several centimeters. Observation of polished and thin sections of the conglomerate shows that it is grain-supported, the major components of clasts are gray and dark-colored cherts, and the minor components are mudstone, siltstone, basalt and carbonate. Clasts are randomly arranged, without visible orientation and sorting. At the bottom of the brook, a transition from basalt via tuffaceous deposits to fine clastic intercalations of about 10 m thickness is observed. At the mouth of the brook a 5 m thick intercalation of siltstone-carbonate-siltstone is exposed. In some places near Wenquan, basal conglomerate is found above the Devonian\Carboniferous contact (Wang, 1997). The above mentioned observations indicate that the effusive mass of Early Carboniferous basaltic eruption rests on Devonian clastics and cherts, and that the eruption took place in marine environment on continental crust, in which marine clastic and/or carbonate sediments could be formed during the dormancy periods. 2) The highway linking Lancang and Ximeng goes through the Laochang area, about 40 km northwest of Lancang, another place with a well developed “seamount succession” (Fig. 3, section 2). Here the transition from Lower Carboniferous basalts to limestone via tuffaceous deposits is well exposed on the roadside, purplish tuffaceous limestones of about 1 m thickness are rich in coral fossils. In this area, the Devonian strata are also well exposed. The Lower Devonian sequence comprises mainly of darkcolored shales and bedded cherts with a thickness of about 550 m. The Middle-Upper Devonian sequence is composed of more than 550 m thick yellow, brownish yellow mudstone, siltstone and sandstone. At a roadside section, the Late Devonian plant fossils Leptophloeurn rhombicum Dawson, and Lepidodendropsis cf. hirrneri Lutz have been collected from brownish yellow siltstone which is abundant in plant fragments. Poorly preserved bivalve fossils have also been collected in other horizons. The succession of the Middle-Upper Devonian and the bivalve fossils indicates the Devonian clastics are marine deposits, and the Late Devonian plant fossils show that the sedimentary environment was not far away from the coast. 3) In the southern part of the Changning-Menglian Belt, a section cut by the road connecting Mengsheng and Cangyuan shows that the basalts of the Yiliu Formation are on karstified Lower Carboniferous limestone. The basalts are overlain by Middle Carboniferous limestones (Fig. 3, section 3 ) . 4) Basalts with sedimentary rock intercalations were also found at a section cut by the road heading southward Gondwana Research, V. 4, No. 1,2001
THE CHANGNING-MENGLIAN TETHYS, CHINA
from the village of Manxin in Menglian county to Burma at about 4 km south of the boundary on the Burmese side (Fig. 3, section 4). Here sandstone and fine clastic intercalations are contained in the basalts (lava sheets and tuffaceous deposits). Limestone blocks in the basalts contain the conodonts Zdiognathoides corrugatus, I . Sulcatus, Gnathodus commutatus commutatus, G. girtyi girtyi, and G. bilineatus bilineatus, which indicate an end Visean to Early Namurian age. This means that the age of the basaltic rocks here is not older than Namurian, and that the Changning-MenglianBelt contains basaltic rocks not only of Early Carboniferous age.
Significance of Upper Permian Bauxite and Lower Triassic Coarse Clastics Above the Pingzhang (or Yiliu) Formation, there is a carbonate succession ranging from the Middle Carboniferous to Middle Permian, which was noted as “sea-mount limestones” (Liu et al., 1991, 1993). Field investigation reveals that the top of the carbonate succession is karstified, and the limestones below the karst surface normally have a Maokouan age. Around the village of Papai in Cangyuan county (Fig. 3, section S ) , there is a light-colored bed, consisting mainly of bauxite about 1m thick, immediately above the limestone. X-ray powder diffraction investigation reveals the components of bauxite here are predominantly boehmite, kaolinite and gibbsite. Quartz peaks have not been detected (Fig. 4). The bauxite bed is followed upward by several meters of conglomerate, within which large clasts are mainly cherts of different colors and ages, with minor fine clastics. This is then overlain by pebbly sandstones and siltstones. The bivalve Claraia from the siltstone indicates that it is Lower XI o3 1.00 1
B
4
-
0.70
0.50 -
K
-
0.20-
Conclusion The basalt-carbonate succession of the ChangningMenglian Belt is not genetically related to seamount, but is a normal lithological succession that developed upon the continental crust, and it is underlain and overlain by marine carbonate and/or clastic deposits. B+K
B
0.60-
0.30-
Triassic (Xiao, 1987). Karstified Permian “seamount limestones” covered by Triassic fine clastics are also seen east of Mengsheng near the 28 km-stone of the road from Mengsheng to Lancang (Fig. 3, section 6). Formation of bauxite on karstified carbonates has two prerequisites: 1) supply of detritus which contains high Al,O, (usually more than 15%), such as magmatic and metamorphic rocks, mudstones, shales and sandstones (arkose and greywacke), and 2) relief and a drainage system, which can take bases and SiO, away and is favorable for laterization. On a seamount carbonate platform karstification may take place, if the platform is exposed and there is enough rain water to wash the carbonate. But on a seamount carbonate platform, bauxite cannot be formed, due to the lack of supply of detritus with high aluminum content. In fact, bauxite on karst, which is well known in equatorial regions since the Late Paleozoic, is formed by redepositon of lateritic clastics from higher hinterland onto preexisting coastal karst and subsequent in-situ desilification (Fuchtbauer, 1988). The fact that the bauxite bed is overlain by Lower ’Ikiassic coarse clastic rocks also shows that by the Late Permian, the limestone formations were adjacent to coastal areas, which could supply detritus in the Early Triassic as the limestone formations submerged after karstification and laterization.
B
0.90 0.80-
0.40
59
K
1
K K K G
60
J. XIAOCHI ET AL.
Acknowledgments This research is supported by a grant (No.9501123) of the Ministry of Land and Resources, P. R. China. The authors extend their gratitude to Ji Qiang of Geological Museum of China, and Dong Zhizhong of Geological Survey of Yunnan for identifying conodonts, and to Wang Liben for the help in the work of X-ray powder diffraction.
References Cui Chunlong, Zeng Yunfu, Duan Lilan and Huang Zhixun (1999) Is there a Late Paleozoic ocean in ChangningMenglian Belt, western Yuannan, China? Acta Sedimentologica Sinica, v. 17, pp. 176-182 (in Chinese with English abstract). Fiichtbauer, H. (Ed.) (1988) Sedimente und Sedimentgesteine. Schweizerbart, Stuttgart, 1141p. Geological Survey of Yunnan (1980) Geological Map Baoshan sheet (1:200,000). Geological Survey of Yunnan (1981) Geological Map Fengqing sheet (1:200,000). Geological Survey of Yunnan (1982) Geological Map Menglian sheet (1:200,000). Jin Xiaochi (1994) Sedimentary and paleogeographic significance of Permo-Carboniferous sequences in western Yunnan, China. GeologischesInstitut der Universitat zu Koln Sonderveroffentlichungen, v. 99, 136p. Jin Xiaochi (1996) Tectono-stratigraphicunits in western Yunnan and their counterparts in Southeast Asia. Continental Dynamics, v. 1, pp. 123-133. Jin Xiaochi (1998) A comparison between Permo-Carboniferous
sequences of the Baoshan block and Lhasa block, China. Proceedings of the Royal Society of Victoria, v. 110, pp. 401-401. Liu Benpei, Feng Qinglai and Fang Nianqiao (1991) Tectonic evolution of the Palaeo-Tethys in Changning-Menglian belt and adjacent regions, western Yunnan. J. China Univ. Geosci., V. 2, pp. 18-28. Liu Benpei, Feng Qinglai, Fang Nianqiao, Jia Jinhua and He Fuxiang (1993) Tectonic evolution of Palaeo-Tethys polyisland ocean in Changning-Menglianand Lancangjiangbelts, southwestern Yunnan, China. Earth Science-J. China Univ. Geosci., v. 18, pp. 529-539 (in Chinese with English abstract). Metcalfe, I. (1996) Pre-Cretaceous evolution of SE Asian terranes. In: Hall, R. and Blundell, D. (Eds.), Tectonic evolution of Southeast Asia. Geol. SOC.Sp. Publ., London, V. 106, pp. 97-122. Wang Yizhao (1997) Geological structures and tectonic settings in the Changning-Menglian zone, western Yunnan. Tethyan Geology, v. 21, pp. 31-50 (in Chinese with English abstract). Wopfner, H. (1996) Gondwana origin of the Baoshan and Tengchongterranes of west Yunnan. In: Hall, R. and Blundell, D. (Eds.), Tectonic evolution of Southeast Asia. Geol. SOC. Sp. Publ., London, v. 106, pp. 539-547. Xiao Yinwen (1987) Discovery of Claraia in western Yunnan. Regional Geology of China, v. 6(1), pp. 92 (in Chinese). Yao, A. and Kuwahara, K. (1999) Paleozoic and Mesozoic radiolarians from the Changning-Menglian terrane, western Yunnan, China. In: Yao, A. et al. (Eds.), Biotic and geological development of the Paleo-Tethys in China. Peking University Press, pp. 17-42. Zhong Dalai (Ed.) (1998) Paleotethyan orogen in western Yunnan and western Sichuan. Science Press, Beijing, 231p. (in Chinese).
Gondwana Research, V. 4, No. 1,2001
Some Stratigraphic and Sedimentologic Constraints on the Features of the Changning-Menglian Tethys, Yunnan, China Jin Xiaochil, Xie Guanglianl and Wang Yizhao2 I
Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100 037, P. R. China Geological Survey of Yunnan, 35 Yujiang Road, Yuxi, Yunnan 653 100, P. R. China
(Manuscriptreceived March 20, 2000; accepted September 15, 2000)
Abstract The interpretation of the (Lower Carboniferous) basalt- (Middle Carboniferous to Middle Permian) carbonate succession in the central zone of the Changning-Menglian Belt in western Yunnan, China as a seamount is one of the major arguments supporting a large ocean model of the Changning-Menglian Tethys. Our field investigations and laboratory work lead to the conclusion that the basalt-carbonate succession of the Changning-Menglian Belt is not genetically related to seamount. It is a normal lithological succession that developed on continental crust, and it is underlain and overlain by marine carbonate and/or clastic deposits.
Key words: Changning-Menglian belt, seamount, Yunnan, Tethys, China.
Introduction The Changning-Menglian Belt in western Yunnan, China has been widely accepted as the remnant of the Paleotethys in this region (e.g. Liu et al., 1991, 1993; Jin, 1994,1996,1998; Metcalfe, 1996; Wopfner, 1996; Wang, 1997; Zhong, 1998; Cui et al., 1999; Yao et al., 1999). However, there are different interpretations of features of the Changning-MenglianTethys within the Late Paleozoic. The Changning-Menglian Belt is bounded on the east by the Lanping-Simao block of Cathaysian affinity and on the west by the Baoshan block of Gondwanan affinity (Fig. 1). Based on the characteristics and distribution of lithological successions, the Changning-MenglianBelt can be divided into several north-south trending zones, each of which has a distinctive geological history (Fig. 2). The eastern zone is represented by a succession of siliciclastic rocks intercalated with limestone and bedded cherts, resting on the Middle Proterozoic Huimin Formation (of the Lancang Group). The siliciclastic deposits begin with the Carboniferous Nanduan Formation, which comprises more than 1000 m of quartz sandstones, siltstones and shales. It is overlain by the Laba Group, a conformable contact is seen at a road cut near the village of Ali. The boundary between the Nanduan
Formation and the overlying Laba Group is probably diachronous (Wang, 1997).The Laba Group is composed dominantly of fine siliciclastics and has a purplish red color in several parts. Limestone lenses with Permian fusulinids are intercalated in the middle part. Bedded cherts and siliceous shales of 2-10 m thickness occur in several horizons of the upper part. The lithological change from siliciclastics to cherts is transitional. The central zone of the Changning-Menglian Belt is characterized by a succession of Lower Carboniferous basaltic rocks and an overlying Middle Carboniferous to Middle Permian carbonate sequence. The top of the carbonate sequence is karstified and covered in many places by Lower Triassic clastics. The basaltic rocks rest either on Devonian graptolite-bearing shales and cherts or on karstified Lower Carboniferous limestone. The western zone of the Changning-Menglian Belt is represented by a succession of basalts, cherts, thin-bedded dark limestones, fine clastics and sandstones. A time range of Carboniferous to Middle Triassic is indicated by palynomorph, radiolarians, bivalves and plant fossils from different horizons. The Changning-MenglianBelt is a complicated orogen, and any model of its ancestor, i.e. the Changning-Menglian Tethys, should be based on extensive field observation
56
J. XIAOCHI ET AL.
Fig. 1. Tectonic subdivision of Yunnan (from Jin, 1996). 1.Nujiang fault; 2. Kejie-Nandinghe fault; 3. Lancangjiang fault; 4. Ailaoshan fault; 5. Red River fault; 6. Nanpanjiang fault; 7. Jinshajiang fault; 8. Xiaojinhe fault.
and proper interpretation of geological environments. The seamount interpretation of the (Lower Carboniferous) basalt- (Middle Carboniferous to Middle Permian) carbonate succession in the central zone put forward by Liu et al. (1991, 1993) is one of the major arguments supporting a large ocean model of the ChangningMenglian Tethys. Our field investigations since 1997, however, gathered much evidence that is contrary to the seamount interpretation. As we know, seamounts are submarine mountains rising more than 1000m above the ocean floor. Seamounts are of volcanic origin, and may be either flattopped (guyot) or peaked. On some flat-topped seamounts, carbonate platforms can be formed.
Seamounts may be either discrete, or arranged in a linear or random grouping. For an objective interpretation of the geological existence of an orogen, field observation of the relationships among different lithological units and tracing of geological contacts are essential, although geochemical data and sometimes conceptual modelling can be very useful.
Field Evidence Against Early Carboniferous “Seamount Basalts” Basalts which were thought to represent seamounts in the “Changning-Menglianocean” (Liu et al., 1991,1993) and their sedimentary intercalations are called the Gondwana Research, V. 4, No.1,2001
THE CHANGNING-MENGLIANTETHYS, CHINA
57
Fig. 2. Simplified geological map of the Changning-Menglian belt (modified after Wang, 1997).
Pingzhang Formation (105-620 m thick) in the northern part and Yiliu Formation (200-760 m thick) in the southern part of the Changning-MenglianBelt. These two formations were assigned to the Lower Carboniferous, according t o fossils contained in sedimentary Gondwana Research, V. 4, No. 1,2001
intercalations and the fact that they are conformably overlain by Middle Carboniferous limestones in some places (Geological Survey of Yunnan, 1980,1981,1982). Our field observations reveal that Early Carboniferous basaltic eruptions did not take place on oceanic crust:
J. XIAOCHI ET AL.
58
1) In the Wenquan area, about 20 km south of Changning, the Pingzhang Formation is well exposed along a brook (Fig. 3, section 1).Along the course of the brook, there is a road linking the village of Xingwen to the main road. Here the Pingzhang Formation is overlain by Upper Carboniferous light grey limestones and underlain by the Lower Devonian Wenquan Formation, which is composed of graptolite-bearing shales, siltstones and cherts. The contact between the Pingzhang and Wenquan formations is an unconformity or disconformity. The Pingzhang Formation is composed mainly of basaltic lava sheets, but tuffaceous deposits, conglomerates, fine clastics and carbonates formed during the dormancy periods occur in different horizons. Along the road side, a conglomerate bed more than 2 m thick is seen intercalated in the basalts. Clasts of the
A
Section Location 27
0 l
*
l
54km *
J
4
I
Fig. 3. Index map showing the locations of sections described.
conglomerate range from several millimeters to several centimeters. Observation of polished and thin sections of the conglomerate shows that it is grain-supported, the major components of clasts are gray and dark-colored cherts, and the minor components are mudstone, siltstone, basalt and carbonate. Clasts are randomly arranged, without visible orientation and sorting. At the bottom of the brook, a transition from basalt via tuffaceous deposits to fine clastic intercalations of about 10 m thickness is observed. At the mouth of the brook a 5 m thick intercalation of siltstone-carbonate-siltstone is exposed. In some places near Wenquan, basal conglomerate is found above the Devonian\Carboniferous contact (Wang, 1997). The above mentioned observations indicate that the effusive mass of Early Carboniferous basaltic eruption rests on Devonian clastics and cherts, and that the eruption took place in marine environment on continental crust, in which marine clastic and/or carbonate sediments could be formed during the dormancy periods. 2) The highway linking Lancang and Ximeng goes through the Laochang area, about 40 km northwest of Lancang, another place with a well developed “seamount succession” (Fig. 3, section 2). Here the transition from Lower Carboniferous basalts to limestone via tuffaceous deposits is well exposed on the roadside, purplish tuffaceous limestones of about 1 m thickness are rich in coral fossils. In this area, the Devonian strata are also well exposed. The Lower Devonian sequence comprises mainly of darkcolored shales and bedded cherts with a thickness of about 550 m. The Middle-Upper Devonian sequence is composed of more than 550 m thick yellow, brownish yellow mudstone, siltstone and sandstone. At a roadside section, the Late Devonian plant fossils Leptophloeurn rhombicum Dawson, and Lepidodendropsis cf. hirrneri Lutz have been collected from brownish yellow siltstone which is abundant in plant fragments. Poorly preserved bivalve fossils have also been collected in other horizons. The succession of the Middle-Upper Devonian and the bivalve fossils indicates the Devonian clastics are marine deposits, and the Late Devonian plant fossils show that the sedimentary environment was not far away from the coast. 3) In the southern part of the Changning-Menglian Belt, a section cut by the road connecting Mengsheng and Cangyuan shows that the basalts of the Yiliu Formation are on karstified Lower Carboniferous limestone. The basalts are overlain by Middle Carboniferous limestones (Fig. 3, section 3 ) . 4) Basalts with sedimentary rock intercalations were also found at a section cut by the road heading southward Gondwana Research, V. 4, No. 1,2001
THE CHANGNING-MENGLIAN TETHYS, CHINA
from the village of Manxin in Menglian county to Burma at about 4 km south of the boundary on the Burmese side (Fig. 3, section 4). Here sandstone and fine clastic intercalations are contained in the basalts (lava sheets and tuffaceous deposits). Limestone blocks in the basalts contain the conodonts Zdiognathoides corrugatus, I . Sulcatus, Gnathodus commutatus commutatus, G. girtyi girtyi, and G. bilineatus bilineatus, which indicate an end Visean to Early Namurian age. This means that the age of the basaltic rocks here is not older than Namurian, and that the Changning-MenglianBelt contains basaltic rocks not only of Early Carboniferous age.
Significance of Upper Permian Bauxite and Lower Triassic Coarse Clastics Above the Pingzhang (or Yiliu) Formation, there is a carbonate succession ranging from the Middle Carboniferous to Middle Permian, which was noted as “sea-mount limestones” (Liu et al., 1991, 1993). Field investigation reveals that the top of the carbonate succession is karstified, and the limestones below the karst surface normally have a Maokouan age. Around the village of Papai in Cangyuan county (Fig. 3, section S ) , there is a light-colored bed, consisting mainly of bauxite about 1m thick, immediately above the limestone. X-ray powder diffraction investigation reveals the components of bauxite here are predominantly boehmite, kaolinite and gibbsite. Quartz peaks have not been detected (Fig. 4). The bauxite bed is followed upward by several meters of conglomerate, within which large clasts are mainly cherts of different colors and ages, with minor fine clastics. This is then overlain by pebbly sandstones and siltstones. The bivalve Claraia from the siltstone indicates that it is Lower XI o3 1.00 1
B
4
-
0.70
0.50 -
K
-
0.20-
Conclusion The basalt-carbonate succession of the ChangningMenglian Belt is not genetically related to seamount, but is a normal lithological succession that developed upon the continental crust, and it is underlain and overlain by marine carbonate and/or clastic deposits. B+K
B
0.60-
0.30-
Triassic (Xiao, 1987). Karstified Permian “seamount limestones” covered by Triassic fine clastics are also seen east of Mengsheng near the 28 km-stone of the road from Mengsheng to Lancang (Fig. 3, section 6). Formation of bauxite on karstified carbonates has two prerequisites: 1) supply of detritus which contains high Al,O, (usually more than 15%), such as magmatic and metamorphic rocks, mudstones, shales and sandstones (arkose and greywacke), and 2) relief and a drainage system, which can take bases and SiO, away and is favorable for laterization. On a seamount carbonate platform karstification may take place, if the platform is exposed and there is enough rain water to wash the carbonate. But on a seamount carbonate platform, bauxite cannot be formed, due to the lack of supply of detritus with high aluminum content. In fact, bauxite on karst, which is well known in equatorial regions since the Late Paleozoic, is formed by redepositon of lateritic clastics from higher hinterland onto preexisting coastal karst and subsequent in-situ desilification (Fuchtbauer, 1988). The fact that the bauxite bed is overlain by Lower ’Ikiassic coarse clastic rocks also shows that by the Late Permian, the limestone formations were adjacent to coastal areas, which could supply detritus in the Early Triassic as the limestone formations submerged after karstification and laterization.
B
0.90 0.80-
0.40
59
K
1
K K K G
60
J. XIAOCHI ET AL.
Acknowledgments This research is supported by a grant (No.9501123) of the Ministry of Land and Resources, P. R. China. The authors extend their gratitude to Ji Qiang of Geological Museum of China, and Dong Zhizhong of Geological Survey of Yunnan for identifying conodonts, and to Wang Liben for the help in the work of X-ray powder diffraction.
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