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Evolution of Upper Precambrian volcano-sedimentary sequences in the western part of Jiangnan stratigraphic province, China
Precambrian Research, 29 (1985) 109--119 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
109
EVOLUTION OF UPPER PRECAMBRIAN VOLCANO-SEDIMENTARY SEQUENCES IN THE WESTERN P A R T OF J I A N G N A N STRATIGRAPHIC PROVINCE, CHINA
WANG YANGYENG, XIE JUNBANG, CHEN YULIN, QIN SHOUYONG and ZHU SHUNCAI Regional Geological Survey Team, Geological and Mineral Bureau of Guizhou Province, Huishui Coun ty, Guizhou (People's Republic of China) (Received May 16, 1984; revision accepted September 4, 1984 )
ABSTRACT
Wang, Y., Xie, J., Chen, Y., Qin, S. and Zhu, S., 1985. Evolution of upper Precarnbrian volcano-sedimentary sequences in the western part of Jiangnan stratigraphic province, China. Precambrian Res., 29: 109--119. This paper deals mainly with the stratigraphy of sedimentary and volcanic sequences and tectono-sedimentary evolution. Upper Precambrian rocks are well developed and widely distributed in the study area, which is important for the investigation of Precambrian mobile belts. The stratigraphic succession includes the Jixian, Qinbalkou and Sinian Systems ranging in age from about 1400 Ma to 600 Ma. The late Precambrian evolution of this area can be interpreted in terms of a change from an ocean rise---ocean basin to a continental margin of Andean type.
INTRODUCTION
T h e J i a n g n a n stratigraphic p r o v i n c e c o m p r i s e s the eastern t e r r i t o r y o f China as far s o u t h as the Y a n g t z e River. It runs in a N E - - S W d i r e c t i o n t h r o u g h eastern China and s o u t h ~ e n t r a l China to s o u t h w e s t China. Its western s e c t o r covers west H u n a n , east G u i z h o u and n o r t h G u a n g x i (Fig. 1). In the areas s t u d i e d the u p p e r P r e c a m b r i a n is well d e v e l o p e d and widely d i s t r i b u t e d with a b u n d a n t s e d i m e n t a r y and volcanic rocks. It provides a g o o d e x a m p l e of P r e c a m b r i a n g e o l o g y in a m o b i l e belt and serves as a m o d e l for i n t e r p r e t a t i o n o f crustal e v o l u t i o n . STRATIGRAPHY L a t e P r e c a m b r i a n r o c k s are w i d e s p r e a d in the J i a n g n a n region where t h e y c o n s i s t m a i n l y o f slightly m e t a m o r p h o s e d g r e e n s t o n e s with a thickness o f 2 0 0 0 - - 3 0 0 0 m. Available g e o c h r o n o l o g i c a l d a t a suggest t h a t these r o c k s are b e t w e e n a b o u t 1 4 0 0 a n d 6 0 0 Ma old. In terms o f t e c t o n i c h i s t o r y and
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rock associations, the upper Precambrian can be divided into three systems. In ascending order these are the Jixian (c. 1400--1000 Ma), Qingbaikou (c. 1000--800 Ma) and Sinian (c. 800--600 Ma) Systems (Table I). On the bases of stratigraphy and sedimentary facies the region can be divided into three areas: the Leigong Mt., Xuefeng Mt. and Jiuwan Dashan Mt. (Fig. 1 and Table I). The succession includes the Fanjinl~han Group (or Sibao Group/Lenjiaxi Group) Xiajiang Group (or Danzhou Group) and the lower and upper Series of the Sinian System (Table I). Differences among the stratigraphic areas are listed in Table I. These are mainly differentiated on the basis of the nature of volcanic and pyroclastic rocks since the beginning of the Qingbaikou Period. For example, the volcanics in the Jiuwan Dashan Mountains are characterised by abundant basic volcanics whereas in the Mt. Leigong area, acid pyroclastics predominate. In the Mt. Xuefeng region the volcanic rocks are of intermediate composition. The sedimentary rocks of these areas are also different. For example in the Leigong Mt. area the Lower Sinian is relatively thin and sandstones of the Nantuo Formation are comparatively abundant. In the other two areas the Lower Sinian is represented by a thick succession and ferruginous--siliceous rocks are well developed. Diamictites are predominant in the Nantuo Formation.
111 TABLE I Stratigraphic succession in the study area Strata
Stratigraphic
L~igong Mt.
Xuefeng Mt.
Jiuwan Dashan Mt.
Upper
Liuchapo Fro.
Doushantuo Fro.
Laobao Fro.
Lower
" N a n t u o " Fm. Datangpo Fm. Teisiao Fm. Fulu Fro. Liangjiehe Fro. Changan Fm. IIII IIIIIII Xiajiang Gp. Danzhou Gp.
area
Late Proterozoic
Sinian
800 Ma Qingbaikou 1000 Ma Middle Proterozoic --
Jixian
Fanjingshan Gp.
Lenjiaxi Gp.
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112
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Fig. 3. (a) Bouma sequence (Tb-d) in turbidite in Sibao Group; (b) structureless pebbly sandstone (sediment gravity flow) in the Lower Sinian; (c) olistostromes in Xiajiang Group; (d) outcrop of flysch (turbidites) in Xiajiang Group.
113 SEDIMENTARY AND VOLCANIC ASSOCIATIONS In the western part of the study area the upper Precambrian includes both sedimentary and volcanic rocks. Great thicknesses and a variety of rock associations are the major characteristics of this region (Fig. 2). The typical sedimentary rocks of the Fanjingshan and Sibao Groups are turbidites {Fig. 3a). Red-to-purple mollasic rocks are present at the base of the Xiajiang and Danzhou Groups and pass up into a pre-flysch carbonaceous and siliceous association. The middle and upper parts of both groups are composed of flysch. The thick Xiajiang Group is best developed and most widely distributed. It contains both terrigenous turbidites (Fig. 3d) and pyroclastic turbidites dominated by intermediate--acid tufts as well as wildflysch or olistostromes (Fig. 3c). The Danzhou Group is also composed mainly of terrigenous turbidites. The lower Sinian series is mostly medium~oarse sediments dominated by diamictite that was previously called tillite. Our recent studies indicate that these rocks are marine sediment gravity flows (Fig. 3b) formed in a special depositional setting. The upper Sinian series is rather simple in lithology and consists mainly of dark carbonaceous and siliceous rocks formed in a deeper water environment. The volcanic association is intimately associated with the sedimentary rocks. For example rocks of the pre-orogenic spilite--keratophyre assemblage in the Fanjingshan Group (Fig. 4a,d) underlie flyschoid sedimentary rocks. A similar relationship is seen in the lower part of the Danzhou Group where similar volcanic rocks occur together with pre-orogenic basinal rocks of carbonaceous--argillaceous aspect. Basic volcanic rocks in the lower part of the Xiajiang Group are associated with "pre-quartzites" possibly reflecting a local environment of relative uplift. The pyroclastic fluxo-turbidites and turbidites (Figs. 4b,c) in the middle part of the Xiajiang Group are associated with terrigenous clastic turbidites in a pre-orogenic setting. In these areas basic lavas and spilites reach great thicknesses and are the d o m i n a n t lava types. The predominance of basic lavas may suggest that the western part of the Jiangnan province was mainly underlain by oceanic crust during t h a t period of its geological evolution. GENERAL TECTONO-SEDIMENTARY EVOLUTION The evolution of the study area is considered in the light of plate tectonic theory.
Jixian period (c. 1400--1000 Ma) Early stage of the Jixian: the study area is an integral part of the vast South China oceanic basin; the Fanjingshan area and the Juiwan Dashan areas were possibly located near oceanic rises (see Fig. 8A~). The lower
114
Fig. 4. (a), (d)Pillow structures in spilitesof the Fanjingshan Group; (b), (c)pyroclastic fluxo-turbiditesin the Xiajiang Group.
parts of the Fanjingahan and Sibao Groups are dominated by basic pillow lavas (spRites), together with basic--ultrabasic intrusives in the Fanjingshan area and at the border between Guizhou and Guangxi. These two areas m a y represent the sides of a submarine rift valley. The chemical composition of the sprites is shown in Table II and in Fig. 5. The sprites in the Sibao Group fall into the " O T H " field and those of the Fanjingshan Group in both fields. The average value is in the area of overlap of the two fields and slightly towards the " O T H " side. The spilites belong to the tholeiite series. A variation diagram for the stable trace elements Ti and Cr shows that the spilites of the Fanjingshan Group fall entirely in the O T H field (Fig. 6), possibly indicating an origin in an oceanic basin. At a later stage a deep water flysch developed (see Fig. 8A2). This was followed by further oceanfloor spreading, giving rise to the Fanjing (or Sibao) orogeny at the continent--arc collision zone. This resulted in strong folding, m a g m a intrusion and a change in the m a k e u p of the crust in that region.
115 TABLE II Average chemical composition of volcanic rocks and clastic rocks
SiO: TiO~ AI:O~ Fe:O•, FeO MgO CaO MnO Na:O K:O (FeO) (FeO)/MgO Quartz (%)
A (10)
B (4)
C (5)
D (2)
E (13)
F (12)
50.78 1.47 14.3 1.29 8.34 8.31 4.90 0.12 3.76 0.39 9.50 1.14
52.74 0.63 14.13 1.35 7.71 9.16 8.14 0.14 3.02 0.67 8.93 0.96
51.23 0.79 15.22 1.78 8.25 8.11 6.22 0.18 3.79 0.57 9.83 1.21
50.25 0.82 18.16 1.74 10.4 6.61 0.62 0.22 0.20 2.61 11.87 1.81
68.75 0.63 15.21 2.27 2.55 1.15 0.11 0.06 3.29 2.38
65.58 0.65 13.24 1.27 3.41 1.44 3.24 1.05 2.30 2.10
15--35
15---65
(FeO) = Fe:O 3 x 0.8999 + FeO; (10) No. of samples; A. Spilite of the Danzhou Group; B. Spilite of the Sibao Group; C. Spilite of the Fanjingshan Group; D. Basic volcanic rocks; E. Graywacke of the Xiajiang Group; F. Diamictite of the Lower Sinian.
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116
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Qingbaikou period (c. 1000--800 Ma) After the orogenic movements mentioned above, the region changed from oceanic crust to one of intermediate composition; it remained relatively tectonically active. Initially there were some fold mountains where molassic sediments accumulated. Afterwards, with rapid crustal subsidence, pre-flysch and carbonaceous--argillaceous sediments were deposited. Owing to local crustal expansion or divergence a series of spilite--keratophyre fissure-type eruptions took place in association with the pre-flysch in the Sanmen area. These rocks are rich in Mg (Table II). As shown in Figs. 5 and 6, the series falls entirely in the OTH field, probably representing an oceanic
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117 Oceanic
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Fig. 8. Diagram showing the sedimentary-tectonic evolution: (1) oceanic crust; (2) transitional crust; (3) continental crust; (4) flysch or terrigenous clastic sediments; (5) coarse clastic sediments; (6) fine clastic sediments; (7) carbonaceous-siliceous sediments; (8) carbonate; (9) sediments gravity flow; (10) basic volcanic rock; (11) intermediate--acidic pyroclastic rock.
118
environment. Subsequent faulting led to differential subsidence and elevation of the crust causing the introduction of strong turbidity currents from the northern continental mass. These currents led to deposition of terrigenous and pyroclastic turbidites together with a variety of sediment gravity flows. Study of thin sections shows that the turbidites are mainly greywackes with a moderate quartz c o n t e n t {Fig. 7). As listed in Table II, the chemical composition is characterized by relatively high SiO: and Al203 contents and a K:O/Na20 ratio of < 1. The region is interpreted as an Andean-type continental margin (Fig. 8B). The general characteristics of the turbidites indicate that they were formed on the middle and lower parts of a submarine fan complex. Intensified subduction at the southern margin of the region eventually caused the Jinning orogeny which caused a slight and temporary uplift of the area.
Sinian period (c. 800 600 Ma) Although the region had already been tectonised twice it was not until the Sinian period that it became continental crust. It inherited the sedimentary setting and palaeogeographic framework of the Qingbalkou period. In the early Sinian, intermontane or piedmont sediments were deposited in the northern mountainous area. It is likely that a steep fault scarp developed along the ancient coastline, causing a large amount of terrigenous clastic debris to be deposited on the sloping continental margin to produce thick medium,coarse sediments of diamictite type. The chemical composition of the diarnictite matrix is listed in Table If. The diamictite falls into the greywacke field (Fig. 7). Faulting in the centre of the basin could have resulted in the relative uplift of the region east of Longsheng to form a sedimentary basin that was wide in the west and narrow in the east. A thick succession of sediment gravity flow deposits accumulated in this basin (Fig. 8C:). In the late Sinian the region still remained a relatively deep water environment with little terrigenous influx (Fig. 8C2). CONCLUSIONS
On the basis of the above descriptions the following conclusions can be made (1) The late Precambrian rocks in the western part of Jiangnan province were formed in a mobile belt and are therefore important in the study of Chinese Precambrian geology; (2) The succession in the study area has yielded an age between 1400 and 600 M a and m a y therefore be correlated with the Jixian, Qingbaikou and Sinian systems. (3) Such a thick succession of volcanic and sedimentary rocks provides a good basis for interpretation of tectono-sedimentary evolution of the area.
119 (4) The late P r e c a m b r i a n s e d i m e n t s are i n t e r p r e t e d t o have f o r m e d along a c o n v e r g e n t - t y p e plate margin. T h e available data suggest t h a t the t e c t o n i c setting has been c o m p a r a b l e to an A n d e a n - t y p e margin since the Q i n g b a i k o u period. ACKNOWLEDGEMENTS We t h a n k Chen Yusui for helpful discussions and Wu T a o x u a n for preparing the Figures. We are greatly i n d e b t e d to t h e Regional Geological Survey T e a m o f G u i z h o u for their s u p p o r t o f this w o r k .
REFERENCES Chen Jinbiao, Zhang Huimin, Xing Yusheng and Ma Gougan, 1981. On the Upper Precambrian (Sinian Suberathem) in China. Preeambrian Res., 15: 207--228. Coleman, R.G., 1977. Ophiolites, Ancient Oceanic lithosphere? Springer-Verlag, Berlin, pp. 63-89. Condie, K.C., 1976. Plate tectonic and Crustal Evolution. Pergamon Press Inc., New York, pp. 49--71, 175--188. Dickinson, W.R., 1974. Plate tectonics and Sedimentation. In: W.R. Dickinson (Editor), Tectonics and Sedimentation. Spec. Publ. Soc. Econ. Paleont. Miner., 22, Tulsa, pp. 1--27. Gass, I.G., 1981. Pan-African (Upper Proterozoic) plate tectonics of the Arabian--Nubian Shield, In: A. KrSner (Editor), Precambrian Plate Tectonics. Amsterdam, pp. 387-402. Mitchell, A.H.G. and Reading, H.G., 1978. Sedimentation and tectonics. In: H.G. Reading (Editor), Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford, pp. 439--476. Miyashiro, A., 1974. Volcanic rock series in island arcs and active continental margin. Am. J. Sci., 274: 321--355. Reading, H.G., 1972. Global tectonics and genesis of the flysch successions. 24th Int. Geol. Cong. Proc. Sect., 6, pp. 59--66. Wang Yangeng, Chen Yulin, Wang Ruigang, Chen Xianwei and Wei Xusho, 1980. Stratigraphic types and Characteristics of Sinian System in Hunan, Guizhou and Guangxi. In: Research on Precambrian Geology; Sinian Suberathem in China. Tianjin Science and Technology Press, pp. 147--163 (in Chinese). Wang Yangeng, Xie Junbang and Chen Yulin, 1982. The Late Precambrian Crustal evolution in Guizhou Province. Kexue Tongbao, 27: 533--537.
109
EVOLUTION OF UPPER PRECAMBRIAN VOLCANO-SEDIMENTARY SEQUENCES IN THE WESTERN P A R T OF J I A N G N A N STRATIGRAPHIC PROVINCE, CHINA
WANG YANGYENG, XIE JUNBANG, CHEN YULIN, QIN SHOUYONG and ZHU SHUNCAI Regional Geological Survey Team, Geological and Mineral Bureau of Guizhou Province, Huishui Coun ty, Guizhou (People's Republic of China) (Received May 16, 1984; revision accepted September 4, 1984 )
ABSTRACT
Wang, Y., Xie, J., Chen, Y., Qin, S. and Zhu, S., 1985. Evolution of upper Precarnbrian volcano-sedimentary sequences in the western part of Jiangnan stratigraphic province, China. Precambrian Res., 29: 109--119. This paper deals mainly with the stratigraphy of sedimentary and volcanic sequences and tectono-sedimentary evolution. Upper Precambrian rocks are well developed and widely distributed in the study area, which is important for the investigation of Precambrian mobile belts. The stratigraphic succession includes the Jixian, Qinbalkou and Sinian Systems ranging in age from about 1400 Ma to 600 Ma. The late Precambrian evolution of this area can be interpreted in terms of a change from an ocean rise---ocean basin to a continental margin of Andean type.
INTRODUCTION
T h e J i a n g n a n stratigraphic p r o v i n c e c o m p r i s e s the eastern t e r r i t o r y o f China as far s o u t h as the Y a n g t z e River. It runs in a N E - - S W d i r e c t i o n t h r o u g h eastern China and s o u t h ~ e n t r a l China to s o u t h w e s t China. Its western s e c t o r covers west H u n a n , east G u i z h o u and n o r t h G u a n g x i (Fig. 1). In the areas s t u d i e d the u p p e r P r e c a m b r i a n is well d e v e l o p e d and widely d i s t r i b u t e d with a b u n d a n t s e d i m e n t a r y and volcanic rocks. It provides a g o o d e x a m p l e of P r e c a m b r i a n g e o l o g y in a m o b i l e belt and serves as a m o d e l for i n t e r p r e t a t i o n o f crustal e v o l u t i o n . STRATIGRAPHY L a t e P r e c a m b r i a n r o c k s are w i d e s p r e a d in the J i a n g n a n region where t h e y c o n s i s t m a i n l y o f slightly m e t a m o r p h o s e d g r e e n s t o n e s with a thickness o f 2 0 0 0 - - 3 0 0 0 m. Available g e o c h r o n o l o g i c a l d a t a suggest t h a t these r o c k s are b e t w e e n a b o u t 1 4 0 0 a n d 6 0 0 Ma old. In terms o f t e c t o n i c h i s t o r y and
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
110
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rock associations, the upper Precambrian can be divided into three systems. In ascending order these are the Jixian (c. 1400--1000 Ma), Qingbaikou (c. 1000--800 Ma) and Sinian (c. 800--600 Ma) Systems (Table I). On the bases of stratigraphy and sedimentary facies the region can be divided into three areas: the Leigong Mt., Xuefeng Mt. and Jiuwan Dashan Mt. (Fig. 1 and Table I). The succession includes the Fanjinl~han Group (or Sibao Group/Lenjiaxi Group) Xiajiang Group (or Danzhou Group) and the lower and upper Series of the Sinian System (Table I). Differences among the stratigraphic areas are listed in Table I. These are mainly differentiated on the basis of the nature of volcanic and pyroclastic rocks since the beginning of the Qingbaikou Period. For example, the volcanics in the Jiuwan Dashan Mountains are characterised by abundant basic volcanics whereas in the Mt. Leigong area, acid pyroclastics predominate. In the Mt. Xuefeng region the volcanic rocks are of intermediate composition. The sedimentary rocks of these areas are also different. For example in the Leigong Mt. area the Lower Sinian is relatively thin and sandstones of the Nantuo Formation are comparatively abundant. In the other two areas the Lower Sinian is represented by a thick succession and ferruginous--siliceous rocks are well developed. Diamictites are predominant in the Nantuo Formation.
111 TABLE I Stratigraphic succession in the study area Strata
Stratigraphic
L~igong Mt.
Xuefeng Mt.
Jiuwan Dashan Mt.
Upper
Liuchapo Fro.
Doushantuo Fro.
Laobao Fro.
Lower
" N a n t u o " Fm. Datangpo Fm. Teisiao Fm. Fulu Fro. Liangjiehe Fro. Changan Fm. IIII IIIIIII Xiajiang Gp. Danzhou Gp.
area
Late Proterozoic
Sinian
800 Ma Qingbaikou 1000 Ma Middle Proterozoic --
Jixian
Fanjingshan Gp.
Lenjiaxi Gp.
Diseonformity+-~FUnconformityIIIIIII Gap Gp,=Group
(';lrl)t)t)at,ctH.i
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Fig. 3. (a) Bouma sequence (Tb-d) in turbidite in Sibao Group; (b) structureless pebbly sandstone (sediment gravity flow) in the Lower Sinian; (c) olistostromes in Xiajiang Group; (d) outcrop of flysch (turbidites) in Xiajiang Group.
113 SEDIMENTARY AND VOLCANIC ASSOCIATIONS In the western part of the study area the upper Precambrian includes both sedimentary and volcanic rocks. Great thicknesses and a variety of rock associations are the major characteristics of this region (Fig. 2). The typical sedimentary rocks of the Fanjingshan and Sibao Groups are turbidites {Fig. 3a). Red-to-purple mollasic rocks are present at the base of the Xiajiang and Danzhou Groups and pass up into a pre-flysch carbonaceous and siliceous association. The middle and upper parts of both groups are composed of flysch. The thick Xiajiang Group is best developed and most widely distributed. It contains both terrigenous turbidites (Fig. 3d) and pyroclastic turbidites dominated by intermediate--acid tufts as well as wildflysch or olistostromes (Fig. 3c). The Danzhou Group is also composed mainly of terrigenous turbidites. The lower Sinian series is mostly medium~oarse sediments dominated by diamictite that was previously called tillite. Our recent studies indicate that these rocks are marine sediment gravity flows (Fig. 3b) formed in a special depositional setting. The upper Sinian series is rather simple in lithology and consists mainly of dark carbonaceous and siliceous rocks formed in a deeper water environment. The volcanic association is intimately associated with the sedimentary rocks. For example rocks of the pre-orogenic spilite--keratophyre assemblage in the Fanjingshan Group (Fig. 4a,d) underlie flyschoid sedimentary rocks. A similar relationship is seen in the lower part of the Danzhou Group where similar volcanic rocks occur together with pre-orogenic basinal rocks of carbonaceous--argillaceous aspect. Basic volcanic rocks in the lower part of the Xiajiang Group are associated with "pre-quartzites" possibly reflecting a local environment of relative uplift. The pyroclastic fluxo-turbidites and turbidites (Figs. 4b,c) in the middle part of the Xiajiang Group are associated with terrigenous clastic turbidites in a pre-orogenic setting. In these areas basic lavas and spilites reach great thicknesses and are the d o m i n a n t lava types. The predominance of basic lavas may suggest that the western part of the Jiangnan province was mainly underlain by oceanic crust during t h a t period of its geological evolution. GENERAL TECTONO-SEDIMENTARY EVOLUTION The evolution of the study area is considered in the light of plate tectonic theory.
Jixian period (c. 1400--1000 Ma) Early stage of the Jixian: the study area is an integral part of the vast South China oceanic basin; the Fanjingshan area and the Juiwan Dashan areas were possibly located near oceanic rises (see Fig. 8A~). The lower
114
Fig. 4. (a), (d)Pillow structures in spilitesof the Fanjingshan Group; (b), (c)pyroclastic fluxo-turbiditesin the Xiajiang Group.
parts of the Fanjingahan and Sibao Groups are dominated by basic pillow lavas (spRites), together with basic--ultrabasic intrusives in the Fanjingshan area and at the border between Guizhou and Guangxi. These two areas m a y represent the sides of a submarine rift valley. The chemical composition of the sprites is shown in Table II and in Fig. 5. The sprites in the Sibao Group fall into the " O T H " field and those of the Fanjingshan Group in both fields. The average value is in the area of overlap of the two fields and slightly towards the " O T H " side. The spilites belong to the tholeiite series. A variation diagram for the stable trace elements Ti and Cr shows that the spilites of the Fanjingshan Group fall entirely in the O T H field (Fig. 6), possibly indicating an origin in an oceanic basin. At a later stage a deep water flysch developed (see Fig. 8A2). This was followed by further oceanfloor spreading, giving rise to the Fanjing (or Sibao) orogeny at the continent--arc collision zone. This resulted in strong folding, m a g m a intrusion and a change in the m a k e u p of the crust in that region.
115 TABLE II Average chemical composition of volcanic rocks and clastic rocks
SiO: TiO~ AI:O~ Fe:O•, FeO MgO CaO MnO Na:O K:O (FeO) (FeO)/MgO Quartz (%)
A (10)
B (4)
C (5)
D (2)
E (13)
F (12)
50.78 1.47 14.3 1.29 8.34 8.31 4.90 0.12 3.76 0.39 9.50 1.14
52.74 0.63 14.13 1.35 7.71 9.16 8.14 0.14 3.02 0.67 8.93 0.96
51.23 0.79 15.22 1.78 8.25 8.11 6.22 0.18 3.79 0.57 9.83 1.21
50.25 0.82 18.16 1.74 10.4 6.61 0.62 0.22 0.20 2.61 11.87 1.81
68.75 0.63 15.21 2.27 2.55 1.15 0.11 0.06 3.29 2.38
65.58 0.65 13.24 1.27 3.41 1.44 3.24 1.05 2.30 2.10
15--35
15---65
(FeO) = Fe:O 3 x 0.8999 + FeO; (10) No. of samples; A. Spilite of the Danzhou Group; B. Spilite of the Sibao Group; C. Spilite of the Fanjingshan Group; D. Basic volcanic rocks; E. Graywacke of the Xiajiang Group; F. Diamictite of the Lower Sinian.
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116
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Qingbaikou period (c. 1000--800 Ma) After the orogenic movements mentioned above, the region changed from oceanic crust to one of intermediate composition; it remained relatively tectonically active. Initially there were some fold mountains where molassic sediments accumulated. Afterwards, with rapid crustal subsidence, pre-flysch and carbonaceous--argillaceous sediments were deposited. Owing to local crustal expansion or divergence a series of spilite--keratophyre fissure-type eruptions took place in association with the pre-flysch in the Sanmen area. These rocks are rich in Mg (Table II). As shown in Figs. 5 and 6, the series falls entirely in the OTH field, probably representing an oceanic
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117 Oceanic
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118
environment. Subsequent faulting led to differential subsidence and elevation of the crust causing the introduction of strong turbidity currents from the northern continental mass. These currents led to deposition of terrigenous and pyroclastic turbidites together with a variety of sediment gravity flows. Study of thin sections shows that the turbidites are mainly greywackes with a moderate quartz c o n t e n t {Fig. 7). As listed in Table II, the chemical composition is characterized by relatively high SiO: and Al203 contents and a K:O/Na20 ratio of < 1. The region is interpreted as an Andean-type continental margin (Fig. 8B). The general characteristics of the turbidites indicate that they were formed on the middle and lower parts of a submarine fan complex. Intensified subduction at the southern margin of the region eventually caused the Jinning orogeny which caused a slight and temporary uplift of the area.
Sinian period (c. 800 600 Ma) Although the region had already been tectonised twice it was not until the Sinian period that it became continental crust. It inherited the sedimentary setting and palaeogeographic framework of the Qingbalkou period. In the early Sinian, intermontane or piedmont sediments were deposited in the northern mountainous area. It is likely that a steep fault scarp developed along the ancient coastline, causing a large amount of terrigenous clastic debris to be deposited on the sloping continental margin to produce thick medium,coarse sediments of diamictite type. The chemical composition of the diarnictite matrix is listed in Table If. The diamictite falls into the greywacke field (Fig. 7). Faulting in the centre of the basin could have resulted in the relative uplift of the region east of Longsheng to form a sedimentary basin that was wide in the west and narrow in the east. A thick succession of sediment gravity flow deposits accumulated in this basin (Fig. 8C:). In the late Sinian the region still remained a relatively deep water environment with little terrigenous influx (Fig. 8C2). CONCLUSIONS
On the basis of the above descriptions the following conclusions can be made (1) The late Precambrian rocks in the western part of Jiangnan province were formed in a mobile belt and are therefore important in the study of Chinese Precambrian geology; (2) The succession in the study area has yielded an age between 1400 and 600 M a and m a y therefore be correlated with the Jixian, Qingbaikou and Sinian systems. (3) Such a thick succession of volcanic and sedimentary rocks provides a good basis for interpretation of tectono-sedimentary evolution of the area.
119 (4) The late P r e c a m b r i a n s e d i m e n t s are i n t e r p r e t e d t o have f o r m e d along a c o n v e r g e n t - t y p e plate margin. T h e available data suggest t h a t the t e c t o n i c setting has been c o m p a r a b l e to an A n d e a n - t y p e margin since the Q i n g b a i k o u period. ACKNOWLEDGEMENTS We t h a n k Chen Yusui for helpful discussions and Wu T a o x u a n for preparing the Figures. We are greatly i n d e b t e d to t h e Regional Geological Survey T e a m o f G u i z h o u for their s u p p o r t o f this w o r k .
REFERENCES Chen Jinbiao, Zhang Huimin, Xing Yusheng and Ma Gougan, 1981. On the Upper Precambrian (Sinian Suberathem) in China. Preeambrian Res., 15: 207--228. Coleman, R.G., 1977. Ophiolites, Ancient Oceanic lithosphere? Springer-Verlag, Berlin, pp. 63-89. Condie, K.C., 1976. Plate tectonic and Crustal Evolution. Pergamon Press Inc., New York, pp. 49--71, 175--188. Dickinson, W.R., 1974. Plate tectonics and Sedimentation. In: W.R. Dickinson (Editor), Tectonics and Sedimentation. Spec. Publ. Soc. Econ. Paleont. Miner., 22, Tulsa, pp. 1--27. Gass, I.G., 1981. Pan-African (Upper Proterozoic) plate tectonics of the Arabian--Nubian Shield, In: A. KrSner (Editor), Precambrian Plate Tectonics. Amsterdam, pp. 387-402. Mitchell, A.H.G. and Reading, H.G., 1978. Sedimentation and tectonics. In: H.G. Reading (Editor), Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford, pp. 439--476. Miyashiro, A., 1974. Volcanic rock series in island arcs and active continental margin. Am. J. Sci., 274: 321--355. Reading, H.G., 1972. Global tectonics and genesis of the flysch successions. 24th Int. Geol. Cong. Proc. Sect., 6, pp. 59--66. Wang Yangeng, Chen Yulin, Wang Ruigang, Chen Xianwei and Wei Xusho, 1980. Stratigraphic types and Characteristics of Sinian System in Hunan, Guizhou and Guangxi. In: Research on Precambrian Geology; Sinian Suberathem in China. Tianjin Science and Technology Press, pp. 147--163 (in Chinese). Wang Yangeng, Xie Junbang and Chen Yulin, 1982. The Late Precambrian Crustal evolution in Guizhou Province. Kexue Tongbao, 27: 533--537.