- Email: [email protected]
Tectonics of the Tongbai-Dabie fold belt
Journal of Southeast Asian Earth Sciences, Vol. 3, Nos 1-4, pp. 77-85, 1989
0743-9547/89 $3.00 + 0.00 Maxwell Pergamon Macmillan pie
Printed in Great Britain
Tectonics of the Tongbai-Dabie fold belt* M A WENPU Department of Geology, Peking University, Beijing, People's Republic of China Almraet--The Qinling fold belt is the northernmost branch of the Paleozoic and Mesozoic Tethys in eastern Asia and contains all the inter-reacting records of the North China Block (NCB) and the South China Block (SCB). The Tongbai-Dabie fold belt is its eastern section. The Qinling belt was a collage of two orogens. The north Qinling is a late Paleozoic collision orogen, which started with Proterozoic rifting, transforming into active continental margin during the middle Ordovician. The intermittent ophiolite fragments indicate the existence of an oceanic crust at that time. The collision that occurred in the Devonian was an east-west diachronous process. The main suture zone is marked by ophiolitic melange extending nearly 500 km. The late Mesozoic molasse, volcano-plutonic activity and huge imbricate thrust system all respond to the further shortening of the crust and adjustment of the intracontinental strain. The south Qinling east of the Nanyang Basin is located to the south of Tongbai mountain. A pile of thick Sinian-Silurian volcanic and marine elastic deposits imply another marine incursion. Its history in south Shaanxi could continue to the Mesozoic. The flysch trough represented by Xinyang group may be the key area for revealing the relationship between north and south Qinling. The discussion in the paper is mainly focused on the nature and evolution of the Tongbai-Dabie fold belt.
INTRODUCTION THE QINLINGfold belt is a continental collision orogen, marking the position of the Paleozoic Tethys which records the interaction of the adjacent North and South China Blocks during pre-, main and post-collision periods. The Tongbai-Dabie belt with a length of more than 400 km is the eastern section of the Qinling belt and extends from the Nanyang Basin to Hefei where it has been cut by the Tan-Lu fault zone. Based on field geological mapping during the period 1984-87, the paper deals with the geometry of the belt, the position of the main suture and their implications for the tectonic evolution of the eastern China continent.
TECTONIC ZONATION OF THE BELT The north part of Tongbai-Dabie fold belt mostly has been buried under the Huai River plain, good exposures are present only to the west of the Beijing-Guangzhou railroad. The following petrotectonic zones can be identified there from north to south (inset of Fig. 1): (I) The south edge of the North China Block (NCB) is composed of unmetamorphosed Mid-Upper Proterozoic terrestrial lavas, shallow marine quartzose sandstones containing abundant cross-bedding and ripple marks (Fig. 3) and lower Paleozoic carbonates. The content of quartz in most of the sandstones is more than 90% and their exposed width reaches 120 kin, reflecting a highly deplanate gentle south-dipping topography. The coastline during the middle Proterozic stretched along the Linru-Xipin line, north of the study area. Queshan (Fig. 1) was situated in the continental shelf at that time. * This project was supported by the National Natural Science Foundation of China
77
A border plutonic-migmatite complex on the south side which separates the above platform type sequences from the more metamorphosed geosynclinal type sequences represents a mobilized basement uplift. Zhang (1981) considered it as a low pressure-high temperature zone of paired metamorphic belts, related to an early Paleozoic subduction event. (II) The Maoji group, a suite of fine-grained elastics and muddy carbonates, containing some meta-mafic subvolcanic rocks in the lower part, has been metamorphosed to amphibolite grade. The slump features in banded marbles (Fig. 4) and the remnant of rhythmic sediments in the green schists indicate that a considerable part of the Maoji group is of mass-gravity flow deposits, representing continental slope environments. (III) The arc system, separated from the above zone by the Huanggang dioritic pluton complex, can be divided into three parts. The northern part (IIIa), i.e. the Zhuzhuang group, consists of felsie granulites, amphibolites and marble lenses. The granulites and amphibolites contain numerous coarse pyroclastic rocks, polymictic breccias and meta-basic lavas, respectively and are cut by a dyke swarm. The petrochemistry of mafic lavas indicates that they are calc-alkali-volcanic series, and the REE pattern is consistent with that of island are volcanic rocks. The lithozones in general seem to be symmetrical in distribution, the marbles appearing only on the northern and southern sides, the core being occupied by a number of various plutons both in size and compositions. The lithofacies is unstable and changes rapidly. The middle part (IIIb) is of spilite-keratophyres and is composed of meta-pillow lavas (Fig. 5), silicalites and diabase sills, but rare terrigenous elasts are present. The space between pillows is filled with red carbonate and umber. Radiolaria and sponge spicule have been collected from the same zone west of the Nanyang Basin (Zhang and Tang 1983). The di S34 of ores in Dahe copper-zinc mine ranges from + 2.3%0 to + 6.1%0, rain-
78
MA WENPU
\
0 i I
lOkm I
I
[~'-I
1
[--~
12
[ ~
]8
M
Ptz
Zhugou
E-Q Biyang
\
\
\
\'\ f
/ Mingang
E-Q 40
"iV
C
IS]
)
Tongbai
\~
'K-E
v
..
+
i
)
\
*
\
< \
J'~
~-~
Fig. 1. Geological map of Tongbai-Queshan district. 1. Paleogene to Quaternary; 2. Paleogene; 3. Cretaceous; 4. Xinyang group; 5. Lower Paleozoic Zhuzhuang group; 6. Platform type Cambrian~)rdovician system; 7. Platform type Sinian-Cambrian; 8. Geosyncline type Sinian-Cambrian; 9. Mid to Upper Protcrozoic Ruyang group; 10. Mid to Upper Proterozoic Maoji group; 11. Upper Proterozoic Shujiahe group; 12. Lower Proterozoic system in Queshan area; 13. Lower Proterozoic Tongbai group; 14. MiBmatite and plutonic rocks; 15. Spilite-keratophyre complex; 16. Olistostrome; 17. Ultramafic and mafic intrusion; 18. Carbonates; 19. Quartzose sandstone; 20. Tillites; 21. Mesozoic granite; 22. Upper Paleozoic granite; 23. Lower Paleozoic granite; 24. Lower Paleozoic diorite; 25. Regional foliation pattern; 26. Railroad. For symbols in inset see text.
eralization temperature being 96°C (Geng 1985), indicating a sea-floor hot spring origin for the ores, similar to that of the Troodos type sulfide mineral deposits (Rona 1986). The diabase sills show obvious rhythmic cumulate texture (Fig. 6). These features indicate that the rocks are of pelagic origin and a part of a dismembered ophiolite suite. The southern part (IIIc) is composed of chaotic masses of rocks, which can be further divided into two subzones. The northern subzone is of ophiolitic melange with enormous ultramafic blocks being arranged zonally within amphibolite schist and talc schist matrices. Large numbers of drillholes bored for chromite exploration in the 1970s have proven that these bodies are rootless and are in tectonic contact with the country rocks (Yu et al.
1984). The southern subzone is of olistostrome. Banded carbonaceous marbles, graphitic marbles and granulites occur as blocks of up to several cubic kilometers embedded in greenschist matrices (Fig. 7). The blocks themselves also contain various debris and pebbles and are associated with extremely complicated syndepositional folds. Since it has characteristic texture and is easily identified in the field, the olistostrome subzone has become a marked horizon for mapping. It has been ascertained that the subzone may extend from western Henan to the Jinzai area of Anhui, with a length attaining up to 500 km (Shi et al. 1982, Chou et al. 1983), thus delineating the position of the main suture in Tongbai-Dabie fold belt. All three parts constitute a south facing arc system
Tectonics of Tongbai-Dabie fold belt I k"x.~pt~-3 a [°Nanyan ~ f i g - 1
N
C
B
79
-z12
I'F'm s ,
I
L
~-'-..\~\J
5- S
,.
~Z--o
i Ph'~
efei ]
~.._~
o
V-z--is f- q6 V' 7 IT'll s
f :;q 9
12
l-x-:~ 13 I-
0 - , - ,
1,Okm " ~ P % ~
S
C
B
[-'~
14
i-"~--] 15
Fig. 2. Sketch map of east Qinling fold belt. 1. Paleozoic arc system; 2. Melange zone; 3. Flysch trough; 4. Carboniferous coal series; 5. Cambrian; 6. Sinian-Triassic; 7. Sinian-Silurian; 8. Sinian-Ordovician; 9. Mid to Upper Proterozoic Ruyang group; 10. Mid to Upper Proterozoic Maoji group; 11. Mid Proterozic; 12. Lower Proterozoic; 13. Archean to Early Proterozoic; 14. Archean; 15. Migrnatite.
embracing a forearc subduction complex, a trench-arc gap remnant oceanic crust and a volcanic arc from south to north. The Huanggang diorite complex seems to be related to some kind of back arc processes. Important metallic ore deposits are located in this zone. Among these deposits, the large gold and silver mines are concentrated on the forearc basin side, skarn iron ore deposits tend to occur on the back arc side and the massive sulfide ore deposits are related with the oceanic crust. (IV) The flysch trough, represented by the Liuling group (Shaanxi), the Xinyang group (Henan) and the Foziling group (Anhui), extends along strike, for about 600 km, but the exposed width in general is no more than 20 kin. This trough is composed of metamorphosed argillo-arenaceous and argillo-siliceous rocks. The lithofacies is monotonous, and thin rhythmic units can be seen in many places, probably representing distal turbidites with some contourites. The metamorphic grade in this zone is lower than that on the northern and southern sides, but it has undergone intense ductile shear deformation, with mylonite occurring frequently, and its contact with the adjacent zones on both sides along large faults. This trough possibly represents the remnant basin of the eastern Qinling fold belt east of Nanyang basin, and separates the northern and southern continental margins. (V) Lower Proterozoic high grade metamorphic complex of the Tongbai-Dabie main ridge. There are widespread schistoscd gabbroite and eclogite blocks within the gncisses in its northern part. Based on their work in the Xinxian area (Fig. 2), Yc and Li (1980) pointed out that the complex belongs to C-type eclogite and all the b0 values of phengite in schists arc more than 9.050 A. Hence it is a typical high pressure low temperature metamorphic zone, implying a pre-Mesozoic subduction event.
GEOMETRIC PATTERN All the above-mentioned zones are now arranged in a northwest direction and roughly parallel to each other, but are separated either by ductile faults or by intrusion and the original stratigraphic relationship has been destroyed. Field geological mapping shows evidently that the structure style, the number of deformational episodes and the lineament direction of different petrotectonic zones are different, and there is intersection between the zones, as can be seen in the map (Fig. 1). Therefore we can conclude that the present geometric pattern only reflects the result of deformation during and after the main collision period of the fold belt, which has undergone a long and complex evolutionary history before collision. It is possible that these zones had originally no spatial association with each other. The same conclusion can be drawn from the extension of the petrotectonic zones. One of the important discoveries made during the mapping is that the exposure width of the zones north of the flysch trough is gradually tapered along the strike towards the east and they disappear one after another under the Xinyang group. For instance, the width of the Maoji group is maximal between Queshan and Tongbai districts, but towards the southeast along the strike, the exposures on the east bank of Zhugan River are of numerous shallow marine quartzose sandstones (Liu et al. 1983) which should appear more to the north. This shows that the Maoji group has disappeared before the east bank of Zhugan River (Fig. 2). More persuasive evidence for this diminution is from the melange zone, which is composed of three marble ridges to the west of Tongbai basin and has a total width of 7 km, but has narrowed to 3 km west of Xinyang although the continuity of the three marble bands can still be recognized. Further towards the east, two marbles occur as fragments embedded in the north-
80
MA WENPU
S
?"---:::..~ 2
•
/
rr-~,, 4
J2
with the partial melting resulting from intracontinental subduction during the post-collision period. If so, the evolutional process of the volcanic belt would be very important for explaining the final history of the Dabie fold belt.
CHRONOLOGY
Before the 1980s all the metamorphosed strata in Qinling fold belt had been assigned to the Precambrian [ l 96.46m (Jin 1979). Since they have undergone greenschist or t more regional metamorphism, there is little possibility of Fig. 8. Profile section showing metamorphosed strata thrusted over Jurassic red beds in eastern Henau. 1. Conglomerate; 2. Brecciated preservation of fossils; the replacement of bedding by quartzite; 3. Mica schist; 4. Dolomitic marble; 5. Carbonaceous foliation and the dislocation between petrotectonic phosphatic schist (redrawn after the Tenth Geological Team of Henan, zones have further destroyed the original stratigraphic 1976). uncomformity and the sequence relationship. In this situation a knowledge of the paleographic evolution would be very useful for inferring the stratigraphic side arc plutonic complex, the distance from which to the pattern of the fold belt. quartzose sandstone of NCBs edge in the Xinxing area Zhang et al. (1979, Zhang 1984) reported that the is only 12 km, but the corresponding distance in Fig. 1 bottom of the early Cambrian on the NCB's southern exceeds 100km. The significance of these facts is as edge contains Hsuaspis (Madianaspis), hence it is the follows. lowermost horizon of the NCB's Cambrian, linked First, the absence of certain stratigraphic horizons in westward with the Helanshan and Qilianshan trough. the NCB's southern margin, a phenomena which is more Based on the spatial distribution and thickness of early evident on moving towards the east, is synchronous with Cambrian phosphate rock group, Ye and Zhao (1983) the intensity of deformation increasing towards the east. confirmed that the transgression at the time came from In the Henan-Anhui boundary area there are large scale the south and gradually overlapped northward (Fig. 9). imbricated nappe structures consisting of a series of They pointed out: "The Cambrian deposition model south-dipping listric thrusts, thrust melange, phosphate from NCB to Qinling geosyncline is characterized by a sequence and Carboniferous system emplaced north- transition from continental shelf sediments to slope and ward over the Jurassic red molasse deposits (Fig. 8). The deep sea sediments" (1983, p. 63). This framework in eastward increase of crustal shortening seems to be which the open sea was situated in the south came into connected with the shape of the NCB's margin, whose existence in the middle Proterozoic, since its lower nearest crystalline basement is situated at Wuyang in volcanic sequence (1700 Ma) and upper clastic sequence the west and at Huoqiu in the east, indicating that the (1300-1000 Ma) all have similar facies changes; that is, boundary of the NCB projected southward. Conse- land in the north and sea in the south (Sun et al. 1981). quently, collision with a continental block from the This framework continued at least to the early Ordosouth should have happened first in its eastern section. vician, since the Sanshanzi dolomite has been proven to Second, the fact that the Xinyang group flysch trough be a diachronous unit whose age became younger on cuts petrotectonic zones to the north of it implies it moving to the north (Fig. 10) (Zhang 1980, An 1984). should belong to another tectonic regime. Shortly afterwards An (1985) recognized upper OrdoThird, extensive acid volcano-plutonic activity oc- vician sequences north of Xian, NCB, Yang (1986) curred along the northern foot of the Dabie mountain found Radiolaria and allochthonous Cephalopoda in during the late Jurassic to early Cretaceous period. The these rocks, indicating that they contained semi- and volcanic basins are aligned in a N.W. direction, with deep sea deposits. The data mentioned above suggest their axes in general parallel to the fold belt, and the that numerous lower Paleozoic rocks should exist or exposures decrease in size from east to west and until it have existed in the Qinling fold belt. reaches the Tongbai area. Petrochemically the volcanic Recently this inference has been confirmed by the rocks and associated small porphyry bodies have high following findings: Zhang and Tang (1983) collected silica and rich alkali, and the REE pattern is very Liosphaeridae gen. et sp. indet, Stylosphaeridae, gen. et different from rocks of mantle origin (Lu 1987). Because sp. indet in a silicalite interlayer of pillow lavas in this volcanic belt is located in the interior of the con- western Henan, which corresponds to our spilite--keratotinent, its direction is nearly perpendicular to that of phyre subzone. We discovered Spumellina spp. in the synchronous volcanic belt on the southeast China con- limestone block of olistostrome south of Dahe area in tinental margin and it has different geochemistry charac- 1984. Xu (1985) found polychaeta, Zygodiscus spp. and teristics. The upper Mesozoic magmatism in the Dabie Bivalvia mould in the same olistostrome horizon west of fold belt should not be genetically related to the sub- Xinyang. Therefore the Zhuzhuang group arc system duction of the Kula oceanic plate, but may be connected should be early Paleozoic in age, but it differs from the
Fig. 3. Cross bedding in quartzose sandstone of Ruyang group, north of Queshan, Henan. Fig. 4. Slumping feature in banded marble of the Maoji group, north of Maoji. Fig. 5. Deformed meta-pillow lava in Dahe area. Fig. 6. Cumulate texture in meta-diabase sills, Dahe area. Phenoblasts altered to hornblende. Fig. 7. Exposure of marble olistostrome, west of Tongbai.
81
Tectonics of Tongbai-Dabie fold belt ~ ' X
Ii
83
Shi ji a z hua ng _ _ _ _ _ . . . . _ _ ~ Taiyuan
I~" °~L~
~-"'"P';"'~"7"~"
..°a~J~.
.
[~'r~__~"~
I I " T -4+./_~
, /,[ i I, [((7]'+ , l/ . . . .~
p~ ~NJ Z h e n g z h o u - - ~
-'~,,xz,
x,,,x.~.. Pingdingshat~'
I
L/ r
" ~
~'Io
.
IIlllll
//
i
e f o r m a t on
~ ~#
/
..J...K~.]]
'
~" of NCB
Fig. 9. Sketch map of LowerCambrian phosphate sequenceoverlappingpattern in NCB (after Ye and Zhao 1983).
counterpart of the NCB in that the sedimentation here belongs to eugeosyncline type. The Maoji group with the platform-type Ruyang group on its north side constitutes two limbs of the same anticlinorium, its bottom starting with quartzites containing ripple marks and the age of the lower part of Maoji group is middle Proterozoic. However, the possibility that its upper part includes late Proterozoic, even the beginning of Phenerzoic couldn't be ruled out. Owing to the discovery of Devonian fossils from the Liuling group in Shaanxi, many geologists believe that the age of Xinyang flysch is also middle Paleozoic. But Yang (1982) insists that its age could be as young as Carboniferous-Permian, based on his own collection. Seng6r (1985a) suggests that the Devonian macrofossils of the Liuling group "come from exotic limestone blocks found in the turbidites. Hsii points out that Triassic ammonites have been recovered from similar rocks in the same zone farther west, thus making at least some parts of the turbidite sequence... Triassic." The important point is that all these workers think that the Xinyang group is the youngest among the metamorphic rocks of Tongbai-Dabie fold belt. Its accurate age range and change along strike would be the key factor for revealing the last step in the history of the eastern China continent collage east of the Nanyang Basin.
Mattauer et al. (1985) and Seng/Sr (1985a) have expounded their own view on this problem. Combining with specific material of the region, the following tectonic model can be proposed: The Tongbai-Dabie fold belt occurred in the middle Proterozoic as a rift under extension conditions. The north-south trending Xiong-er group projecting into the interior of the NCB represented a failed arm of a triple junction and the Maoji group resulted from this spreading event (Sun 1981), thus the NCB's southern margin was a passive continental margin in its early stage. The middle to late Proterozoic mature quartzose sandstone sheet of the Ruyang group marks the position of the continental margin at the rifting stage, and the large scale transgression taking place during the early Cambrian is considered to be the beginning of a drifting stage. Judging from the ophiolitic fragments dispersed from southern Shaanxi to southern Henan, at least a small ocean basin should have existed at that time. The transition from passive margin to active margin or from sea floor spreading to subduction might have occurred in the middle Ordovician, because there seems to be extensive uplift in the NCB at that time. The increasing hiatus from the north to the south on the top of the Sanshanzi dolomite (Fig. 10) implies that this uplifting should originate within the Qinling belt. All of the early Paleozoic volcanic rock groups which have reliable ages in the Qinling belt are Ordovician, such as the Caotangou group in Feng Country, Zaqu formation in Neixian and the meta-pillow lavas on the both sides of Qinling group continental crust fragment. The Zhuzhuang group arc system in the area is possibly of this age. These volcanic groups roughly indicate the position of a subduction zone. Yang (1986) considered
TECTONIC EVOLUTION The records for the development and evolution of the eastern China continent are preserved in the Qialing fold belt, hence it has become one of the most interesting regions in the world. Wang (1982), Klimetz (1983), Linru I = ~ = =. - - -
Jiawar4 a l e
- -~
.....
Fengfeng !. . . . . . .
Quyang I
Beijing I
LiansjlashanaFlllrr.+,,r,JPrrrJ++rmrrlli~.r.u.~q-....-:; Yell
ale
+ till
-
p+~l~
s] J ~
[
l i l l e + * ii * n I
Femml~n are i ~ i . e • ' l * II T-" ' "..~lns~+n -L" +-" ~ -
ChanMhan Guslffo" .---
~abe
~
j
• n'Y
. I~
Z h a n g x i a ale i
.
• ~
~n~
..,
.
,
... r
OO.lO.m!te~-- l __
I
i
i
. i
II ' . ~
I i i |
I
I
i
11 ~ II .11 M I. " ~ , L I I I u * • *
• U+ " + "~ha' i~. i
rigs i
0 i
l~n
~'
I
lOrlllit i i
i
I
i
,-;
.
|11' i 1+ .
l
I
',
~.~ ~-:~.'~ . . . . l
I
Lienlljiasha~ I '
fornwtion '
] -- * .' '+ ' " Yell formation renEsnan tormatior." , ' ~ •
. ,
o11 i
'
I
Pingquan I
i " I I
I
I
I
I I
I
+ l ' I ' i i
I
I
I
I
l I l
,
i I + i
I
I
140kin I
Fig, I0. Diachronousregularityof Sanshangzidolomiteand change of its top hiatus from NCB's southern margin to its
interior (after An 1984).
84
MA WENPU
the Mid Ordovician deep water facies north of Xian as a back-arc basin implying that Qinling began its subduction under NCB at that time. The ocean closure should have occurred prior to the end of Devonian, since the Carboniferous system in the area of Henan-Anhui boundary is represented by a coal series intercalated with marine shales (Wang 1982), and is quite different from the underlying metamorphosed rocks. The upper Paleozoic plutons in the fold belt are elongated synorogenic granites, whereas isometric postorogenic granites did not appear until the late Mesozoic. The main suture is situated on the south side of the melange zone, which is consistent with the conclusion from Shaanxi where the suture of the Qinling belt extends along the Shangdan fault (Zhang et al. 1987), and the 4°Ar/39Ar age of pure metamorphic mica on the north side of suture is 328-348 Ma (Mattauer 1985). Paleogeographic analysis from the Devonian south of the suture zone indicates the clastic material is supplied from the north, implying there was land at that time (Wang 1982). The differences between these two regions are that the ocean closure east of Nanyang basin resulted from collision between the NCB and Dabie Block, but west of Nanyang basin only a subduction event took place and the ocean closure and continental collision were postponed to the Triassic. The movement direction of the plate being not perpendicular to the continental margin, which gave in most cases an oblique convergence, plus the irregularity of continental margins on both sides, caused the collision to be a diachronous process, starting from projection points of continental block where the deformation was most intense, and then spreading laterally to concave parts. The NCB's southern margin trended N.W. to the east of Nanyang basin, so its collision with the block drifting from the south might have started from the easten section and then spread to the west. This agrees with the fact that the strata have become younger towards the west along strike in the Qinling belt. Triassic flysch exists mainly in Shaanxi and Gansu provinces and intensity of deformation in general increases eastward. An effect of the oblique collision is displayed by a huge strike-slip movement or tectonic escape, making two petrotectonic zones which were unrelated to each other in evolution become juxtaposed together. The relationship between the Carboniferous system and Xinyang group on its south side is a very persuasive example: the exposure of the Carboniferous on the Henan-Anhui boundary extends intermittently for almost 200 km, being in contact with Xinyang group by a ductile fault zone. There are extensive conglomerates along the south limb of the Carboniferous, their multifarious gravels including quartz, black chert, sandstone, limestone etc., but not the schists of the Xinyang group. Lu et al. (1987) collected many fossils from limestone gravels in the basal conglomerates of the Carboniferous, such as Heliolites cf. anhuiensis, ostracods, conodont, etc., indicating that vast areas were covered with Silurian shallow marine limestones while the conglomerates were being deposited. It was very different from the present
geological situation. Therefore, it is possible that the emplacement of the Xinyang group flysch may be later than the Carboniferous period. The calculation in Peltzer's (1985) paper that the total strike-slip offset of the Shangdan fault could be of the order of 150 km during the Tertiary can serve as an example. The red molasse in this district does not appear until the Mid Jurassic along the northern foot of Dabie mountain. It is the first time the gneiss gravel of the Dabie group has been found, indicating that the Dabie mountain did not emerge as a topographic mountain until then. This uplift could be an episodic process, since the Paleogene is composed of red conglomerates whose boulders can reach more than 0.5m in diameter in Tongbai and Zhugou Basin, becoming typical pediment deposits. The molasse deposits and associated upper Mesozoic volcano-plutonic rocks all reflect a series arising from intracontinental strain adjustments responding to the post-collisional continuous convergence and the crustal shortening, so it can be inferred that they are a part of an intracontinental high strain belt. The Tongbai-Dabie belt north of the flysch trough is only the northern half of the Qinling fold belt. There is another even more complicated half located in Suixian area, south of Tongbai mountain (Fig. 2), where the thick pile of marine volcanic and clastic sediments from Sinian to Silurian implies the existence of another sea area during the early Phenerozoic in the south of the Tongbai-Dabie continental block. The same sea area in western Henan to southern Shaanxi continued until the Triassic. It is the northernmost branch of the Paleotethys in eastern Asia (Sengrr 1985b), but little is known about its relationship with the north Qinling belt. Research on the tectonics of the flysch trough and its adjacent areas would be a key factor for revealing the collage process of NCB and SCB. Acknowledgements--This paper was written with the help and support
of my postgraduates and graduate students. Special thanks are given to Xu Qian, Kang Bokai and Zheng Chaoyang for their collaboration.
REFERENCES
An, T. 1984. Recent progress in Ordovician system of N. China stratigraphical province. In: The Records of Geological Research, pp. 44-58. Dept. Geol., Peking University. An, T. 1985. Ordovician conodonts from Yaoxian and Fuping, Shaanxi and their stratigraphic significance. Acta Geol. Sin. 59, 97-108. Chou, G., Chen, Q. and Liu, J. 1983. On melange in the area of Xinyang-Tongbai. J. Changchun Coll. Geol. 2, 47-56. Geng, Q. 1985. Discussion on geologicalcharacteristicof copper-zinc ore deposit and its origin in Liushanyanarea. Nonferr. Metal Geol. Henan 1, 33-40 (in Chinese). Jin, S. 1979.The view on Pre-Cambrian stratigraphyof Qinling-Dabie Mountain, Henan. Henan Geol. 3, 1-11: 4, 31-40 (in Chinese). Klimetz, M. P. 1983. Speculations on the Mesozoic plate tectonic evolution of Eastern China. Tectonics 2, 139-166. Liu, H., Lao, Q. and Chi S. 1983. Lithofacies and paleography of middle Proterozoic Ruyang group in Wuyang-Huoqiu district. In: Scientific Papers on pre-Cambrian Geology, pp. 83-90 (in Chinese). Geol. Publ. House, Beijing. Lu X. 1987. Tectonic-magmatic activity and Mesozoic porphyry mineral deposits belt in Dabie mountain, Hcnan. In: Symposium on Geological Conference of Dabieshan District, pp. 56-66 (in Chinese).
Tectonics of Tongbai-Dabie fold belt Lu, G., Jin, F. and Wang, D. 1987. Discovery of Eopaleozoic fossils from Yangshan formation in Gushi of Henan and its significance. J. Hefei Polytech. Univ. 9, 13-19. Mattauer, M. 1985. Tectonics of the Qinling Belt: build-up and evolution of eastern Asia. Nature 317, 496-500. Peltzer, G., Tapponnier, P., Zhang, Z. and Xu, Z. 1985. Neogene and Quaternary faulting in and along the Qinling Shan. Nature 317, 500-505. Rona, P. A. 1986. Mineral deposits from sea-floor hot springs. Scient. Am. 254, 66-75. Seng6r, A. M. C. 1985a. East Asia tectonic collage. Nature 318, 167-17. SengGr, A. M. C. 1985b. The story of Tethys. Episodes 8, 3-12. Sift, Q., Mou, Y. and Zhang, E. 1982. Xinyang subduction complex, Henan. Seism. Geol. 4, ll-17. Sun, S., Cong, B., Li, J. 1981. Evolution of the Henan-Shaanxi sedimentary basin of the middle and late Proterozoic age. Scient. Geol. Sin. 4, 314-322. Wang, H., Xu, C. and Zho, Z. 1982. Tectonic development of the continental margins on both sides of the palaeo-Qinling margin realm. Acta Geol. Sin. 56, 270-279. Wang, D. 1982. Study of the Miaochong Formation of Gushi, Henan. Acta Stratig. Sin. 6~ 60-63. Xu, H. 1985. Discovery of fossils in Waimiao formation, northern Dabieshan. Geol. China 6, 33 (in Chinese). Yang, S. 1986. Turbidite flysch trace fossils from China and their paleoecology and palaeoenvironments. In: Selected Papers from the 13th and 14th Annual Conventions of Paleontological Society of China, pp. 143-166.
85
Yang, Z. 1982. The characteristics of the geological and tectonic evolution of the Tongbai-Dabie Mountains. Acta Geol. Sin. 56, 123-135. Ye, D. and Li, D. 1980. On the 3T-phengite and C-type ¢clogite in the Xinyang metamorphic belt and their tectonic significance. In: Formation and Development of the North China Fault Block Region, pp. 122-132. Science Press, Beijing. Ye, L. and Zhao, D. 1983. Sedimentary Associations of the North China Platform, pp. 32-48. Science Press, Beijing (in Chinese). Yu, G., Tian, G. and Lian, J. 1984. Ophiolites in Tongbai area. Henan Geol. 2, 56-63 (in Chinese). Zhang, G., Mei, Z., Zhou, D., Sun, Y. and Yu, Z. 1987. The evolving Qinling tectonic belt. In: Selected Papers from the 45th Anniversary of N W China University, pp. 281-298 (in Chinese). Zhang, S. and Tang, S. 1983. Discovery of early Paleozoic radiolarian chert and plate tectonics in North Qinling. Geol. Shaanxi 2, 1-19. Zhang, S. X. 1980. On some diacronous lithostratigraphic units in the North China Fault Block region. In: Formation and Development of the North China Fault Block Region, pp. 109-121. Science Press, Beijing. Zhang, W. 1984. Recent advances in the study of the Cambrian in N. China. In: The Records of Geological Research, pp. 33-43. Dept. of Geology, Peking University, Beijing. Zhang, W., 1979. Boundary between Cambrian and latest preCambrian in southern and southwestern parts of N. China. Acta Stratig. Sin. 3, 51-56. Zhang, Z. 1981. The Tongbai-Dabie Shan paired metamorphic rock belt in Henan and Hubei provinces. Seism. Geol. 3, 27-37.
0743-9547/89 $3.00 + 0.00 Maxwell Pergamon Macmillan pie
Printed in Great Britain
Tectonics of the Tongbai-Dabie fold belt* M A WENPU Department of Geology, Peking University, Beijing, People's Republic of China Almraet--The Qinling fold belt is the northernmost branch of the Paleozoic and Mesozoic Tethys in eastern Asia and contains all the inter-reacting records of the North China Block (NCB) and the South China Block (SCB). The Tongbai-Dabie fold belt is its eastern section. The Qinling belt was a collage of two orogens. The north Qinling is a late Paleozoic collision orogen, which started with Proterozoic rifting, transforming into active continental margin during the middle Ordovician. The intermittent ophiolite fragments indicate the existence of an oceanic crust at that time. The collision that occurred in the Devonian was an east-west diachronous process. The main suture zone is marked by ophiolitic melange extending nearly 500 km. The late Mesozoic molasse, volcano-plutonic activity and huge imbricate thrust system all respond to the further shortening of the crust and adjustment of the intracontinental strain. The south Qinling east of the Nanyang Basin is located to the south of Tongbai mountain. A pile of thick Sinian-Silurian volcanic and marine elastic deposits imply another marine incursion. Its history in south Shaanxi could continue to the Mesozoic. The flysch trough represented by Xinyang group may be the key area for revealing the relationship between north and south Qinling. The discussion in the paper is mainly focused on the nature and evolution of the Tongbai-Dabie fold belt.
INTRODUCTION THE QINLINGfold belt is a continental collision orogen, marking the position of the Paleozoic Tethys which records the interaction of the adjacent North and South China Blocks during pre-, main and post-collision periods. The Tongbai-Dabie belt with a length of more than 400 km is the eastern section of the Qinling belt and extends from the Nanyang Basin to Hefei where it has been cut by the Tan-Lu fault zone. Based on field geological mapping during the period 1984-87, the paper deals with the geometry of the belt, the position of the main suture and their implications for the tectonic evolution of the eastern China continent.
TECTONIC ZONATION OF THE BELT The north part of Tongbai-Dabie fold belt mostly has been buried under the Huai River plain, good exposures are present only to the west of the Beijing-Guangzhou railroad. The following petrotectonic zones can be identified there from north to south (inset of Fig. 1): (I) The south edge of the North China Block (NCB) is composed of unmetamorphosed Mid-Upper Proterozoic terrestrial lavas, shallow marine quartzose sandstones containing abundant cross-bedding and ripple marks (Fig. 3) and lower Paleozoic carbonates. The content of quartz in most of the sandstones is more than 90% and their exposed width reaches 120 kin, reflecting a highly deplanate gentle south-dipping topography. The coastline during the middle Proterozic stretched along the Linru-Xipin line, north of the study area. Queshan (Fig. 1) was situated in the continental shelf at that time. * This project was supported by the National Natural Science Foundation of China
77
A border plutonic-migmatite complex on the south side which separates the above platform type sequences from the more metamorphosed geosynclinal type sequences represents a mobilized basement uplift. Zhang (1981) considered it as a low pressure-high temperature zone of paired metamorphic belts, related to an early Paleozoic subduction event. (II) The Maoji group, a suite of fine-grained elastics and muddy carbonates, containing some meta-mafic subvolcanic rocks in the lower part, has been metamorphosed to amphibolite grade. The slump features in banded marbles (Fig. 4) and the remnant of rhythmic sediments in the green schists indicate that a considerable part of the Maoji group is of mass-gravity flow deposits, representing continental slope environments. (III) The arc system, separated from the above zone by the Huanggang dioritic pluton complex, can be divided into three parts. The northern part (IIIa), i.e. the Zhuzhuang group, consists of felsie granulites, amphibolites and marble lenses. The granulites and amphibolites contain numerous coarse pyroclastic rocks, polymictic breccias and meta-basic lavas, respectively and are cut by a dyke swarm. The petrochemistry of mafic lavas indicates that they are calc-alkali-volcanic series, and the REE pattern is consistent with that of island are volcanic rocks. The lithozones in general seem to be symmetrical in distribution, the marbles appearing only on the northern and southern sides, the core being occupied by a number of various plutons both in size and compositions. The lithofacies is unstable and changes rapidly. The middle part (IIIb) is of spilite-keratophyres and is composed of meta-pillow lavas (Fig. 5), silicalites and diabase sills, but rare terrigenous elasts are present. The space between pillows is filled with red carbonate and umber. Radiolaria and sponge spicule have been collected from the same zone west of the Nanyang Basin (Zhang and Tang 1983). The di S34 of ores in Dahe copper-zinc mine ranges from + 2.3%0 to + 6.1%0, rain-
78
MA WENPU
\
0 i I
lOkm I
I
[~'-I
1
[--~
12
[ ~
]8
M
Ptz
Zhugou
E-Q Biyang
\
\
\
\'\ f
/ Mingang
E-Q 40
"iV
C
IS]
)
Tongbai
\~
'K-E
v
..
+
i
)
\
*
\
< \
J'~
~-~
Fig. 1. Geological map of Tongbai-Queshan district. 1. Paleogene to Quaternary; 2. Paleogene; 3. Cretaceous; 4. Xinyang group; 5. Lower Paleozoic Zhuzhuang group; 6. Platform type Cambrian~)rdovician system; 7. Platform type Sinian-Cambrian; 8. Geosyncline type Sinian-Cambrian; 9. Mid to Upper Protcrozoic Ruyang group; 10. Mid to Upper Proterozoic Maoji group; 11. Upper Proterozoic Shujiahe group; 12. Lower Proterozoic system in Queshan area; 13. Lower Proterozoic Tongbai group; 14. MiBmatite and plutonic rocks; 15. Spilite-keratophyre complex; 16. Olistostrome; 17. Ultramafic and mafic intrusion; 18. Carbonates; 19. Quartzose sandstone; 20. Tillites; 21. Mesozoic granite; 22. Upper Paleozoic granite; 23. Lower Paleozoic granite; 24. Lower Paleozoic diorite; 25. Regional foliation pattern; 26. Railroad. For symbols in inset see text.
eralization temperature being 96°C (Geng 1985), indicating a sea-floor hot spring origin for the ores, similar to that of the Troodos type sulfide mineral deposits (Rona 1986). The diabase sills show obvious rhythmic cumulate texture (Fig. 6). These features indicate that the rocks are of pelagic origin and a part of a dismembered ophiolite suite. The southern part (IIIc) is composed of chaotic masses of rocks, which can be further divided into two subzones. The northern subzone is of ophiolitic melange with enormous ultramafic blocks being arranged zonally within amphibolite schist and talc schist matrices. Large numbers of drillholes bored for chromite exploration in the 1970s have proven that these bodies are rootless and are in tectonic contact with the country rocks (Yu et al.
1984). The southern subzone is of olistostrome. Banded carbonaceous marbles, graphitic marbles and granulites occur as blocks of up to several cubic kilometers embedded in greenschist matrices (Fig. 7). The blocks themselves also contain various debris and pebbles and are associated with extremely complicated syndepositional folds. Since it has characteristic texture and is easily identified in the field, the olistostrome subzone has become a marked horizon for mapping. It has been ascertained that the subzone may extend from western Henan to the Jinzai area of Anhui, with a length attaining up to 500 km (Shi et al. 1982, Chou et al. 1983), thus delineating the position of the main suture in Tongbai-Dabie fold belt. All three parts constitute a south facing arc system
Tectonics of Tongbai-Dabie fold belt I k"x.~pt~-3 a [°Nanyan ~ f i g - 1
N
C
B
79
-z12
I'F'm s ,
I
L
~-'-..\~\J
5- S
,.
~Z--o
i Ph'~
efei ]
~.._~
o
V-z--is f- q6 V' 7 IT'll s
f :;q 9
12
l-x-:~ 13 I-
0 - , - ,
1,Okm " ~ P % ~
S
C
B
[-'~
14
i-"~--] 15
Fig. 2. Sketch map of east Qinling fold belt. 1. Paleozoic arc system; 2. Melange zone; 3. Flysch trough; 4. Carboniferous coal series; 5. Cambrian; 6. Sinian-Triassic; 7. Sinian-Silurian; 8. Sinian-Ordovician; 9. Mid to Upper Proterozoic Ruyang group; 10. Mid to Upper Proterozoic Maoji group; 11. Mid Proterozic; 12. Lower Proterozoic; 13. Archean to Early Proterozoic; 14. Archean; 15. Migrnatite.
embracing a forearc subduction complex, a trench-arc gap remnant oceanic crust and a volcanic arc from south to north. The Huanggang diorite complex seems to be related to some kind of back arc processes. Important metallic ore deposits are located in this zone. Among these deposits, the large gold and silver mines are concentrated on the forearc basin side, skarn iron ore deposits tend to occur on the back arc side and the massive sulfide ore deposits are related with the oceanic crust. (IV) The flysch trough, represented by the Liuling group (Shaanxi), the Xinyang group (Henan) and the Foziling group (Anhui), extends along strike, for about 600 km, but the exposed width in general is no more than 20 kin. This trough is composed of metamorphosed argillo-arenaceous and argillo-siliceous rocks. The lithofacies is monotonous, and thin rhythmic units can be seen in many places, probably representing distal turbidites with some contourites. The metamorphic grade in this zone is lower than that on the northern and southern sides, but it has undergone intense ductile shear deformation, with mylonite occurring frequently, and its contact with the adjacent zones on both sides along large faults. This trough possibly represents the remnant basin of the eastern Qinling fold belt east of Nanyang basin, and separates the northern and southern continental margins. (V) Lower Proterozoic high grade metamorphic complex of the Tongbai-Dabie main ridge. There are widespread schistoscd gabbroite and eclogite blocks within the gncisses in its northern part. Based on their work in the Xinxian area (Fig. 2), Yc and Li (1980) pointed out that the complex belongs to C-type eclogite and all the b0 values of phengite in schists arc more than 9.050 A. Hence it is a typical high pressure low temperature metamorphic zone, implying a pre-Mesozoic subduction event.
GEOMETRIC PATTERN All the above-mentioned zones are now arranged in a northwest direction and roughly parallel to each other, but are separated either by ductile faults or by intrusion and the original stratigraphic relationship has been destroyed. Field geological mapping shows evidently that the structure style, the number of deformational episodes and the lineament direction of different petrotectonic zones are different, and there is intersection between the zones, as can be seen in the map (Fig. 1). Therefore we can conclude that the present geometric pattern only reflects the result of deformation during and after the main collision period of the fold belt, which has undergone a long and complex evolutionary history before collision. It is possible that these zones had originally no spatial association with each other. The same conclusion can be drawn from the extension of the petrotectonic zones. One of the important discoveries made during the mapping is that the exposure width of the zones north of the flysch trough is gradually tapered along the strike towards the east and they disappear one after another under the Xinyang group. For instance, the width of the Maoji group is maximal between Queshan and Tongbai districts, but towards the southeast along the strike, the exposures on the east bank of Zhugan River are of numerous shallow marine quartzose sandstones (Liu et al. 1983) which should appear more to the north. This shows that the Maoji group has disappeared before the east bank of Zhugan River (Fig. 2). More persuasive evidence for this diminution is from the melange zone, which is composed of three marble ridges to the west of Tongbai basin and has a total width of 7 km, but has narrowed to 3 km west of Xinyang although the continuity of the three marble bands can still be recognized. Further towards the east, two marbles occur as fragments embedded in the north-
80
MA WENPU
S
?"---:::..~ 2
•
/
rr-~,, 4
J2
with the partial melting resulting from intracontinental subduction during the post-collision period. If so, the evolutional process of the volcanic belt would be very important for explaining the final history of the Dabie fold belt.
CHRONOLOGY
Before the 1980s all the metamorphosed strata in Qinling fold belt had been assigned to the Precambrian [ l 96.46m (Jin 1979). Since they have undergone greenschist or t more regional metamorphism, there is little possibility of Fig. 8. Profile section showing metamorphosed strata thrusted over Jurassic red beds in eastern Henau. 1. Conglomerate; 2. Brecciated preservation of fossils; the replacement of bedding by quartzite; 3. Mica schist; 4. Dolomitic marble; 5. Carbonaceous foliation and the dislocation between petrotectonic phosphatic schist (redrawn after the Tenth Geological Team of Henan, zones have further destroyed the original stratigraphic 1976). uncomformity and the sequence relationship. In this situation a knowledge of the paleographic evolution would be very useful for inferring the stratigraphic side arc plutonic complex, the distance from which to the pattern of the fold belt. quartzose sandstone of NCBs edge in the Xinxing area Zhang et al. (1979, Zhang 1984) reported that the is only 12 km, but the corresponding distance in Fig. 1 bottom of the early Cambrian on the NCB's southern exceeds 100km. The significance of these facts is as edge contains Hsuaspis (Madianaspis), hence it is the follows. lowermost horizon of the NCB's Cambrian, linked First, the absence of certain stratigraphic horizons in westward with the Helanshan and Qilianshan trough. the NCB's southern margin, a phenomena which is more Based on the spatial distribution and thickness of early evident on moving towards the east, is synchronous with Cambrian phosphate rock group, Ye and Zhao (1983) the intensity of deformation increasing towards the east. confirmed that the transgression at the time came from In the Henan-Anhui boundary area there are large scale the south and gradually overlapped northward (Fig. 9). imbricated nappe structures consisting of a series of They pointed out: "The Cambrian deposition model south-dipping listric thrusts, thrust melange, phosphate from NCB to Qinling geosyncline is characterized by a sequence and Carboniferous system emplaced north- transition from continental shelf sediments to slope and ward over the Jurassic red molasse deposits (Fig. 8). The deep sea sediments" (1983, p. 63). This framework in eastward increase of crustal shortening seems to be which the open sea was situated in the south came into connected with the shape of the NCB's margin, whose existence in the middle Proterozoic, since its lower nearest crystalline basement is situated at Wuyang in volcanic sequence (1700 Ma) and upper clastic sequence the west and at Huoqiu in the east, indicating that the (1300-1000 Ma) all have similar facies changes; that is, boundary of the NCB projected southward. Conse- land in the north and sea in the south (Sun et al. 1981). quently, collision with a continental block from the This framework continued at least to the early Ordosouth should have happened first in its eastern section. vician, since the Sanshanzi dolomite has been proven to Second, the fact that the Xinyang group flysch trough be a diachronous unit whose age became younger on cuts petrotectonic zones to the north of it implies it moving to the north (Fig. 10) (Zhang 1980, An 1984). should belong to another tectonic regime. Shortly afterwards An (1985) recognized upper OrdoThird, extensive acid volcano-plutonic activity oc- vician sequences north of Xian, NCB, Yang (1986) curred along the northern foot of the Dabie mountain found Radiolaria and allochthonous Cephalopoda in during the late Jurassic to early Cretaceous period. The these rocks, indicating that they contained semi- and volcanic basins are aligned in a N.W. direction, with deep sea deposits. The data mentioned above suggest their axes in general parallel to the fold belt, and the that numerous lower Paleozoic rocks should exist or exposures decrease in size from east to west and until it have existed in the Qinling fold belt. reaches the Tongbai area. Petrochemically the volcanic Recently this inference has been confirmed by the rocks and associated small porphyry bodies have high following findings: Zhang and Tang (1983) collected silica and rich alkali, and the REE pattern is very Liosphaeridae gen. et sp. indet, Stylosphaeridae, gen. et different from rocks of mantle origin (Lu 1987). Because sp. indet in a silicalite interlayer of pillow lavas in this volcanic belt is located in the interior of the con- western Henan, which corresponds to our spilite--keratotinent, its direction is nearly perpendicular to that of phyre subzone. We discovered Spumellina spp. in the synchronous volcanic belt on the southeast China con- limestone block of olistostrome south of Dahe area in tinental margin and it has different geochemistry charac- 1984. Xu (1985) found polychaeta, Zygodiscus spp. and teristics. The upper Mesozoic magmatism in the Dabie Bivalvia mould in the same olistostrome horizon west of fold belt should not be genetically related to the sub- Xinyang. Therefore the Zhuzhuang group arc system duction of the Kula oceanic plate, but may be connected should be early Paleozoic in age, but it differs from the
Fig. 3. Cross bedding in quartzose sandstone of Ruyang group, north of Queshan, Henan. Fig. 4. Slumping feature in banded marble of the Maoji group, north of Maoji. Fig. 5. Deformed meta-pillow lava in Dahe area. Fig. 6. Cumulate texture in meta-diabase sills, Dahe area. Phenoblasts altered to hornblende. Fig. 7. Exposure of marble olistostrome, west of Tongbai.
81
Tectonics of Tongbai-Dabie fold belt ~ ' X
Ii
83
Shi ji a z hua ng _ _ _ _ _ . . . . _ _ ~ Taiyuan
I~" °~L~
~-"'"P';"'~"7"~"
..°a~J~.
.
[~'r~__~"~
I I " T -4+./_~
, /,[ i I, [((7]'+ , l/ . . . .~
p~ ~NJ Z h e n g z h o u - - ~
-'~,,xz,
x,,,x.~.. Pingdingshat~'
I
L/ r
" ~
~'Io
.
IIlllll
//
i
e f o r m a t on
~ ~#
/
..J...K~.]]
'
~" of NCB
Fig. 9. Sketch map of LowerCambrian phosphate sequenceoverlappingpattern in NCB (after Ye and Zhao 1983).
counterpart of the NCB in that the sedimentation here belongs to eugeosyncline type. The Maoji group with the platform-type Ruyang group on its north side constitutes two limbs of the same anticlinorium, its bottom starting with quartzites containing ripple marks and the age of the lower part of Maoji group is middle Proterozoic. However, the possibility that its upper part includes late Proterozoic, even the beginning of Phenerzoic couldn't be ruled out. Owing to the discovery of Devonian fossils from the Liuling group in Shaanxi, many geologists believe that the age of Xinyang flysch is also middle Paleozoic. But Yang (1982) insists that its age could be as young as Carboniferous-Permian, based on his own collection. Seng6r (1985a) suggests that the Devonian macrofossils of the Liuling group "come from exotic limestone blocks found in the turbidites. Hsii points out that Triassic ammonites have been recovered from similar rocks in the same zone farther west, thus making at least some parts of the turbidite sequence... Triassic." The important point is that all these workers think that the Xinyang group is the youngest among the metamorphic rocks of Tongbai-Dabie fold belt. Its accurate age range and change along strike would be the key factor for revealing the last step in the history of the eastern China continent collage east of the Nanyang Basin.
Mattauer et al. (1985) and Seng/Sr (1985a) have expounded their own view on this problem. Combining with specific material of the region, the following tectonic model can be proposed: The Tongbai-Dabie fold belt occurred in the middle Proterozoic as a rift under extension conditions. The north-south trending Xiong-er group projecting into the interior of the NCB represented a failed arm of a triple junction and the Maoji group resulted from this spreading event (Sun 1981), thus the NCB's southern margin was a passive continental margin in its early stage. The middle to late Proterozoic mature quartzose sandstone sheet of the Ruyang group marks the position of the continental margin at the rifting stage, and the large scale transgression taking place during the early Cambrian is considered to be the beginning of a drifting stage. Judging from the ophiolitic fragments dispersed from southern Shaanxi to southern Henan, at least a small ocean basin should have existed at that time. The transition from passive margin to active margin or from sea floor spreading to subduction might have occurred in the middle Ordovician, because there seems to be extensive uplift in the NCB at that time. The increasing hiatus from the north to the south on the top of the Sanshanzi dolomite (Fig. 10) implies that this uplifting should originate within the Qinling belt. All of the early Paleozoic volcanic rock groups which have reliable ages in the Qinling belt are Ordovician, such as the Caotangou group in Feng Country, Zaqu formation in Neixian and the meta-pillow lavas on the both sides of Qinling group continental crust fragment. The Zhuzhuang group arc system in the area is possibly of this age. These volcanic groups roughly indicate the position of a subduction zone. Yang (1986) considered
TECTONIC EVOLUTION The records for the development and evolution of the eastern China continent are preserved in the Qialing fold belt, hence it has become one of the most interesting regions in the world. Wang (1982), Klimetz (1983), Linru I = ~ = =. - - -
Jiawar4 a l e
- -~
.....
Fengfeng !. . . . . . .
Quyang I
Beijing I
LiansjlashanaFlllrr.+,,r,JPrrrJ++rmrrlli~.r.u.~q-....-:; Yell
ale
+ till
-
p+~l~
s] J ~
[
l i l l e + * ii * n I
Femml~n are i ~ i . e • ' l * II T-" ' "..~lns~+n -L" +-" ~ -
ChanMhan Guslffo" .---
~abe
~
j
• n'Y
. I~
Z h a n g x i a ale i
.
• ~
~n~
..,
.
,
... r
OO.lO.m!te~-- l __
I
i
i
. i
II ' . ~
I i i |
I
I
i
11 ~ II .11 M I. " ~ , L I I I u * • *
• U+ " + "~ha' i~. i
rigs i
0 i
l~n
~'
I
lOrlllit i i
i
I
i
,-;
.
|11' i 1+ .
l
I
',
~.~ ~-:~.'~ . . . . l
I
Lienlljiasha~ I '
fornwtion '
] -- * .' '+ ' " Yell formation renEsnan tormatior." , ' ~ •
. ,
o11 i
'
I
Pingquan I
i " I I
I
I
I
I I
I
+ l ' I ' i i
I
I
I
I
l I l
,
i I + i
I
I
140kin I
Fig, I0. Diachronousregularityof Sanshangzidolomiteand change of its top hiatus from NCB's southern margin to its
interior (after An 1984).
84
MA WENPU
the Mid Ordovician deep water facies north of Xian as a back-arc basin implying that Qinling began its subduction under NCB at that time. The ocean closure should have occurred prior to the end of Devonian, since the Carboniferous system in the area of Henan-Anhui boundary is represented by a coal series intercalated with marine shales (Wang 1982), and is quite different from the underlying metamorphosed rocks. The upper Paleozoic plutons in the fold belt are elongated synorogenic granites, whereas isometric postorogenic granites did not appear until the late Mesozoic. The main suture is situated on the south side of the melange zone, which is consistent with the conclusion from Shaanxi where the suture of the Qinling belt extends along the Shangdan fault (Zhang et al. 1987), and the 4°Ar/39Ar age of pure metamorphic mica on the north side of suture is 328-348 Ma (Mattauer 1985). Paleogeographic analysis from the Devonian south of the suture zone indicates the clastic material is supplied from the north, implying there was land at that time (Wang 1982). The differences between these two regions are that the ocean closure east of Nanyang basin resulted from collision between the NCB and Dabie Block, but west of Nanyang basin only a subduction event took place and the ocean closure and continental collision were postponed to the Triassic. The movement direction of the plate being not perpendicular to the continental margin, which gave in most cases an oblique convergence, plus the irregularity of continental margins on both sides, caused the collision to be a diachronous process, starting from projection points of continental block where the deformation was most intense, and then spreading laterally to concave parts. The NCB's southern margin trended N.W. to the east of Nanyang basin, so its collision with the block drifting from the south might have started from the easten section and then spread to the west. This agrees with the fact that the strata have become younger towards the west along strike in the Qinling belt. Triassic flysch exists mainly in Shaanxi and Gansu provinces and intensity of deformation in general increases eastward. An effect of the oblique collision is displayed by a huge strike-slip movement or tectonic escape, making two petrotectonic zones which were unrelated to each other in evolution become juxtaposed together. The relationship between the Carboniferous system and Xinyang group on its south side is a very persuasive example: the exposure of the Carboniferous on the Henan-Anhui boundary extends intermittently for almost 200 km, being in contact with Xinyang group by a ductile fault zone. There are extensive conglomerates along the south limb of the Carboniferous, their multifarious gravels including quartz, black chert, sandstone, limestone etc., but not the schists of the Xinyang group. Lu et al. (1987) collected many fossils from limestone gravels in the basal conglomerates of the Carboniferous, such as Heliolites cf. anhuiensis, ostracods, conodont, etc., indicating that vast areas were covered with Silurian shallow marine limestones while the conglomerates were being deposited. It was very different from the present
geological situation. Therefore, it is possible that the emplacement of the Xinyang group flysch may be later than the Carboniferous period. The calculation in Peltzer's (1985) paper that the total strike-slip offset of the Shangdan fault could be of the order of 150 km during the Tertiary can serve as an example. The red molasse in this district does not appear until the Mid Jurassic along the northern foot of Dabie mountain. It is the first time the gneiss gravel of the Dabie group has been found, indicating that the Dabie mountain did not emerge as a topographic mountain until then. This uplift could be an episodic process, since the Paleogene is composed of red conglomerates whose boulders can reach more than 0.5m in diameter in Tongbai and Zhugou Basin, becoming typical pediment deposits. The molasse deposits and associated upper Mesozoic volcano-plutonic rocks all reflect a series arising from intracontinental strain adjustments responding to the post-collisional continuous convergence and the crustal shortening, so it can be inferred that they are a part of an intracontinental high strain belt. The Tongbai-Dabie belt north of the flysch trough is only the northern half of the Qinling fold belt. There is another even more complicated half located in Suixian area, south of Tongbai mountain (Fig. 2), where the thick pile of marine volcanic and clastic sediments from Sinian to Silurian implies the existence of another sea area during the early Phenerozoic in the south of the Tongbai-Dabie continental block. The same sea area in western Henan to southern Shaanxi continued until the Triassic. It is the northernmost branch of the Paleotethys in eastern Asia (Sengrr 1985b), but little is known about its relationship with the north Qinling belt. Research on the tectonics of the flysch trough and its adjacent areas would be a key factor for revealing the collage process of NCB and SCB. Acknowledgements--This paper was written with the help and support
of my postgraduates and graduate students. Special thanks are given to Xu Qian, Kang Bokai and Zheng Chaoyang for their collaboration.
REFERENCES
An, T. 1984. Recent progress in Ordovician system of N. China stratigraphical province. In: The Records of Geological Research, pp. 44-58. Dept. Geol., Peking University. An, T. 1985. Ordovician conodonts from Yaoxian and Fuping, Shaanxi and their stratigraphic significance. Acta Geol. Sin. 59, 97-108. Chou, G., Chen, Q. and Liu, J. 1983. On melange in the area of Xinyang-Tongbai. J. Changchun Coll. Geol. 2, 47-56. Geng, Q. 1985. Discussion on geologicalcharacteristicof copper-zinc ore deposit and its origin in Liushanyanarea. Nonferr. Metal Geol. Henan 1, 33-40 (in Chinese). Jin, S. 1979.The view on Pre-Cambrian stratigraphyof Qinling-Dabie Mountain, Henan. Henan Geol. 3, 1-11: 4, 31-40 (in Chinese). Klimetz, M. P. 1983. Speculations on the Mesozoic plate tectonic evolution of Eastern China. Tectonics 2, 139-166. Liu, H., Lao, Q. and Chi S. 1983. Lithofacies and paleography of middle Proterozoic Ruyang group in Wuyang-Huoqiu district. In: Scientific Papers on pre-Cambrian Geology, pp. 83-90 (in Chinese). Geol. Publ. House, Beijing. Lu X. 1987. Tectonic-magmatic activity and Mesozoic porphyry mineral deposits belt in Dabie mountain, Hcnan. In: Symposium on Geological Conference of Dabieshan District, pp. 56-66 (in Chinese).
Tectonics of Tongbai-Dabie fold belt Lu, G., Jin, F. and Wang, D. 1987. Discovery of Eopaleozoic fossils from Yangshan formation in Gushi of Henan and its significance. J. Hefei Polytech. Univ. 9, 13-19. Mattauer, M. 1985. Tectonics of the Qinling Belt: build-up and evolution of eastern Asia. Nature 317, 496-500. Peltzer, G., Tapponnier, P., Zhang, Z. and Xu, Z. 1985. Neogene and Quaternary faulting in and along the Qinling Shan. Nature 317, 500-505. Rona, P. A. 1986. Mineral deposits from sea-floor hot springs. Scient. Am. 254, 66-75. Seng6r, A. M. C. 1985a. East Asia tectonic collage. Nature 318, 167-17. SengGr, A. M. C. 1985b. The story of Tethys. Episodes 8, 3-12. Sift, Q., Mou, Y. and Zhang, E. 1982. Xinyang subduction complex, Henan. Seism. Geol. 4, ll-17. Sun, S., Cong, B., Li, J. 1981. Evolution of the Henan-Shaanxi sedimentary basin of the middle and late Proterozoic age. Scient. Geol. Sin. 4, 314-322. Wang, H., Xu, C. and Zho, Z. 1982. Tectonic development of the continental margins on both sides of the palaeo-Qinling margin realm. Acta Geol. Sin. 56, 270-279. Wang, D. 1982. Study of the Miaochong Formation of Gushi, Henan. Acta Stratig. Sin. 6~ 60-63. Xu, H. 1985. Discovery of fossils in Waimiao formation, northern Dabieshan. Geol. China 6, 33 (in Chinese). Yang, S. 1986. Turbidite flysch trace fossils from China and their paleoecology and palaeoenvironments. In: Selected Papers from the 13th and 14th Annual Conventions of Paleontological Society of China, pp. 143-166.
85
Yang, Z. 1982. The characteristics of the geological and tectonic evolution of the Tongbai-Dabie Mountains. Acta Geol. Sin. 56, 123-135. Ye, D. and Li, D. 1980. On the 3T-phengite and C-type ¢clogite in the Xinyang metamorphic belt and their tectonic significance. In: Formation and Development of the North China Fault Block Region, pp. 122-132. Science Press, Beijing. Ye, L. and Zhao, D. 1983. Sedimentary Associations of the North China Platform, pp. 32-48. Science Press, Beijing (in Chinese). Yu, G., Tian, G. and Lian, J. 1984. Ophiolites in Tongbai area. Henan Geol. 2, 56-63 (in Chinese). Zhang, G., Mei, Z., Zhou, D., Sun, Y. and Yu, Z. 1987. The evolving Qinling tectonic belt. In: Selected Papers from the 45th Anniversary of N W China University, pp. 281-298 (in Chinese). Zhang, S. and Tang, S. 1983. Discovery of early Paleozoic radiolarian chert and plate tectonics in North Qinling. Geol. Shaanxi 2, 1-19. Zhang, S. X. 1980. On some diacronous lithostratigraphic units in the North China Fault Block region. In: Formation and Development of the North China Fault Block Region, pp. 109-121. Science Press, Beijing. Zhang, W. 1984. Recent advances in the study of the Cambrian in N. China. In: The Records of Geological Research, pp. 33-43. Dept. of Geology, Peking University, Beijing. Zhang, W., 1979. Boundary between Cambrian and latest preCambrian in southern and southwestern parts of N. China. Acta Stratig. Sin. 3, 51-56. Zhang, Z. 1981. The Tongbai-Dabie Shan paired metamorphic rock belt in Henan and Hubei provinces. Seism. Geol. 3, 27-37.