- Email: [email protected]
Precambrian Crustal Evolution in the Central Northern Margin of the Yangtze Block
515
Rangin,C. (1990) Proceedings of the Ocean Drilling Program, Init. Repts. ODP, College Station, TX. p.906. Ridd, M.F. (1990) Possible Palaeozoic drift of SE Asia and Triassic collision with China. J. Geol. SOC.London, v.137, pp.635-640. Silver, E.A. (1991) Leg 124 tectonic synthesis, In: Rangin, C. (Ed.) Proceedings of the Ocean Drilling Program, Scientific Results, v.124, pp.3-9. Taylor, B. and Hayes, D.E. (1980)The tectonic evolution of the South China Basin. In: Hayes, D.E. (Ed.) The tectonic and
geologic evolution of Southeast Asian Seas and Islands, American Geophys. Union Monograph, v. 23, p p . 89-104. Taylor, 8. and Hayes, D.E. (1983) Origin and history of the South China Basin. In: Hayes,D.E. (Ed.)The tectonic and geologic Evolution of Southeast Asian Seas and Islands, Part 2, American Geophys. Union Monograph, v. 27, pp.23-56. Williams, H.H. (1992) Geochemistry of Palawan Oils, Phi 1i p p ines : source imp 1ic a t i o ns . Pro c . 9t'' Offshore Southeast Asia Conf., Singapore, pp.1-4.
Gondzuana Research, V. 2, No. 4, pp. 515-518. 01999 International Association for Gondzuana Rescnrch, japan. ISSN: 1342-937X
GR
Goizdwarza Research
Precambrian Crustal Evolution in the Central Northern Margin of the Yangtze Block Xie Caifu, Fu Jianming, Xiong Chengyun, Zhang Yeming, Hu Ning and Huang Zhaoxian Yichang Institute of Geology and Mineral Resources, CAGS, PO. Box 502, Yichang, Hzibei 443003, China
Meso-and Neoarchean Initial Crust Archean basement rocks in the studied area are found only in the cores of the Huangling Arch of the Yangtze Gorges area, Hubei and in the Zhaochuan dome fold, Shanxi. They are separately called as the Kongling Complex (lower part) and the Zhuanlugou Complex (Fig.1). The original rocks of these comprise a suite of bimodal volcanics dominated by tholeiite and minor dacite, intercalated with small number of siliceous rocks. The metamorphic grade is high amphibolite to partly granulite facies, with strong nuginatization and a number of TTG plutons. The formation age of these rocks was dated between ca. 2600-ca.3460 Ma (FLIet al., 1993; Zhang W. J. et al., 1996; Ma et al., 1997). Among them the age of TTG intrusive rocks varies between 2800-2900 Ma (Ma et al., 1997).This suite is different from the Archean of North China block in age and rock assemblage. The latter is characterized by the presence of continental crustal rocks of 3500- 3800 Ma and a lot of Neoarchean khondalite series and siliceous iron formations.
Moreover, Pb and Nd isotopic compositions and peak values of Nd model ages reflect the basements in the North China block tobe notably different from those of the Yangtze block (Zhang B. R. et al., 1996; Lu et al., 1996). It is inferred, therefore, that the North China block and Yangtze block might be two landmasses, independently grown during the Archean.
Palaeoproterozoic Continental Crust Accretion Palaeoproterozoic continental crustal rocks, exposed in the Huangling Arch, Dadouling-Xiaodouling, ShangzhouZhengping and Xiaomoling areas, are called the Kongling Complex (upper part), Douling "Lithogroup", Qinling "Lithogroup" and Xiaomoling Complex respectively. They were probably formed in the setting similar to active continental margin following weathering and erosion of the initial Archean salic continental crust and composed of a suite of terrigenous clastic (volcanic) formation, dominated by terrigenous clastic rocks interbedded with
516
minor basic and acidic volcanic rocks, carbonates and ironbearmg siliceous rocks. The grade oi metamorphism attains high amphibolite facies with mignia tiza tion and intrusion of potassic granitic plutons. Their isotopic ages are between 1820 and 2430 Ma (Zhanget al., 1994; Jiang, 1986; Ma et al., 1997),of which the age of K-granite is ca. 1840 Ma (Fu et al., 1993). The Qinling "Lithogroup" was usually recognized as an accretionary crust in the southern margin of North China block (Zhang Z. Q. et al., 1996). It seems to the present authors that, however, the northern margin of Yangtze block and the boundary between the North China block and the Yangtze block should be placed at the Shangzhou- Xiaguan fault, and not at the Shangzhou-Danfeng fault, which was always regarded as the boundary between these two blocks (Zhang G. W. et al., 1995).The reasons for this argument are as follows: (1)the Qinling "lithogroup" is very similar to the Douling "lithogroup", Xiaomoling Complex and Kongling Complex (upper part) in rock assemblages, initial rock formations, metamorphic intensity, isotopic ages etc. They are characterized by a suite of khondalite series and C-bearing rocks at their lower parts, which obviously differ from thc Palaeoprotcrozoic Songshan Group, Hongan Group, Shusong Group and Fengyang Group exposed in
North China block
oXi'an o Luonan
..
I
Danfefl
4
I ?
0
100
0 Yichang
200 km
u
6 4 3
Fig.1. Sketch map showing the outcrop of the I'rccambrian in the central northern margin of the Yangtze block. F,. LuanchuanHeigou fault. F,. Shangzhou-Xiaguan (Shangxia) fault. F,. Shangzhou-Danfeng (Shangdaii) fault. F,,. Xiangfan-Fangxian fault. 1. Kongling Complex, 2. Zliuaiilugou Complex, 3. Douling "lithogroup", 4. Xiaomoling Complex. 5. Qinliiig "Litliogroup", 6 . Sheruiongjia Group, 7. Wudang "Lithogroup" and Yaolinglie Group, 8. Xiahe "Lithogyoup", 9. Kuanping "Lithogroup", 10. Erlangping "Litliogrokip", 11. Danfeng "Litliogroup", 12. Macaoyuan Group, 13. Lower Sinian and Upper Sinian, 14. Upper Sinian
the southern margin of the North China block. The latter belongs to a suite of (volcanic) sedimentary series me tamorphosed in the mid- to low grade, consisting of P-, Fe-, Mn- a n d Y-bearing marble, quartzite, phylli te, hornblende schist and schernikite plagiogneiss (Zhang et al., 1993). (2) Pb and Nd isotopic compositions of the Q d i n g "lithogroup" show a close affinity with those of equivalents from the South Qinling and Yangtze block and differ from those of the North China block (Zhang B.R.et al., 1996). (3)The Shangxia fault is much stronger in control of mineralization than the Shangdan fault. The Hg, Sb deposits in China chiefly occur in the Yangtze block to the southern side of the Shangxia fault. A lot of ore deposits are present on both sides of the fault, but the meso-katathermal type deposits are mainly distributed in the northern side and the meso-epithermal type deposits in the southern side. In addition, the suture zone feature of Shangxia fault has been gradually recognized in recent years (discussed below). Considering that there commonly existed an intense metamorphic and intrusive granitic event at ca.1850 Ma in both the Yangtze block and the southern margin of the North China block, it is inferred that the two blocks were assembled and formed a united continent in the late Palaeopro terozoic.
Mesoproterozoic Continental Crust Breakup It is still controversial as to whether the studied area belongs to the continental rift associated with micro-oceanic basin (Zhang et al., 1995),or the island arc (Ha0 et al., 1996) during the Mesoproterozoic time. The Shennongjia Group, Wudang "Lithogroup', Danfeng "Lithogroup", Xiahe "Lithogroup" a n d Kuangping "Lithogroup" etc. are successively distributed from south toward north in the studied area. Their isotopic ages range between 1600-1000 Ma (Zhang et al., 1994). It is thought that they were formed in different tectonic settings caused by rifts of different degree. The Shennongjia Group, composed mainly of slightly metamorphosed carbonate rock (65%),clastic rock (31%)and a little alkali basaltic rock (4%), was formed in a tensional setting of the inner side of the continental margin. The Wudang "lithogroup" is composed of middle to low grade metamorphosed sedimentary rocks (50%) and acidic and basic volcanic rocks (50%), of which volcanic rocks display "bimodal" assemblage and belong to spilite- keratophyre formation. It could have chiefly formed in a rift setting of the outer side of the continental margin. The Danfeng "Lithogroup" and the Kuanping "Lithogroup", of which the original rocks are tholeiite, clastic rock and carbonate rock, were formed under a transitional setting from a continental rift to micro-ocean basins (termed here as the Kuanping and Danfeng microGondwana Research, V. 2, No. 4, 1999
517
ocean basins separately). This indicates that the North Qinling separating the two micro-ocean basins was a microblock (called as North Qinling micro-block) between them during that time. The primitive rocks of the Xiahe "Lithogroup" consisting of terrigenous clastic rocks interbedded with basic volcanics, were formed in an aulacogen setting. The study of rock type, magmatic series, rock assemblage and some geochemical characteristics suggests that the Mesoproterozoic volcanic rocks in the studied area might be continental rift volcanics, though part of these rocks can be plotted in ocean ridge, ocean island, and island arc fields in geochemical discrimination diagrams. In the logar-log-1 diagram, these rocks are mainly located in the active tectonic field (orogenic field). The main factor leading to the above mentioned complex conditions might be related with the thinner continental crust formed in this area during Archean-Palaeoproterozoic, so that the Mesoproterozoic crust still possessed a transitional feature although it was already under a rifting environment
Late Mesoproterozoic-Neoproterozoic Collision and Breakup Events The two Mesoproterozoic micro-oceans began to subduct along the Shangxia belt and the Shangdan belt at ca.1200 Ma, and then a strong collisional orogeny termed earlier as Jinning movement or Shennong (Sibao) movement took place a t ca. 1000 Ma. This collisional event led to the unification of the Yangtze block, North Qinling micro-block and North China block. The evidence for this is as follows. (1) There are angular unconformities between the Neoproterozoic Macaoyuan Group and the Mesoproterozoic Shennongjia Group, between the Yaolinghe Group and the Wu d a n g "Lit h o g r o up ", be tween the Er 1an g p ing "Li thogroup" and the Kuanping "Lithogroup", separately. (2)There are ca. 1000 Ma ophiolite suites (Zhang, G. W. et al., 1995;Jiang et al., 1998),hgh-pressure metamorphic zone (Hu et al., 1995), subduction and collision-type granitoid rock belts (Wang et al., 1998; Hao et al., 1996) and tectonic melange belts found separately along the ShangzhouDanfeng and Shangzhou-Xiaguan fault zones. These reveal that the two fault zones might have been suture zones at ca. 1000 Ma. (3)The existence of a peak at ca. 1000 Ma in isotopic ages from this area (Zhang Z. Q. et al., 1996). (4) A strong crust-forming period during ca. 1000-1200 Ma in North Qinling indicated by isotopic studies (Zhang H. F. et al., 1995; Zhang et al., 1994; Zhang B. R. et al., 1996). At ca. 950 Ma, the breakup began to occur along the original assembly belt, so that the Erlangping and Danfeng micro-oceans and North Qinling micro-block formed again during Neoproterozoic. The intensity of breakup generally Gondzuana Research, V 2, No. 4, 1999
decreased from north toward south. In Wudang area, the formation of Yaolinghe Group caused by this rifting event is dominated by tholeiitic rocks along with a great volume of basic dyke swarms intruding the basement rock series. These basic volcanic rocks and dyke swarm possess typical geochemical characteristics of wi thin-plate-basalt (Zhang W. J. et al., 1996), implying the formation of a thicker and rigid crust in the studied area during the Early Jinningstage. Meanwhile, the basic dyke swarm and the Macaoyuan Group consisting of molasse formation with the intermediate-basic volcanic materials formed in the Shennongjia area in association with the pull-apart action. C or r e sp ond in g 1y, the Sh an d ui h e s u pe r 1a r g e- sca l e epithermal silic deposit was formed in the tensional faults during that time. The collision orogeny of ca. 800 Ma called the late Jinning movement or Jinning movement was recorded in this area. In the southern part of the studied area this event led to the assembly and collision of the Jiangnan island arc with the Yangtze block along the Youyang-Yichang fault and the emplacement of a lot of granitic bodies and formation of some magma tic hydrothermal gold deposits in Huangling area. Meanwhile, strong tectonic action occurred along the Xiangfan-Fangxian-Lieyang zone in the central part of the studied area, along with the occurrence of a lot of granitic bodies in the Hanzhong area. In the northern part, the Danfeng micro-ocean was closed. The North Qinling microblock was assembled with the Yangtze block again. It caused the emplacement of the intermediate-acidic bodies on the two sides of Danfeng fault and the formation of rocks with ca. 800 Ma peak isotopic ages (Zhang Z. Q. et al., 1996).The Erlangping ocean might have had a larger extent during that time and was not closed till the early Palaeozoic. The Yangtze-type Doushantuo Formation and Dengyin Formation of the Upper Sinian are widely spread in the South Qinling and Yangtze platform to the south of the Shangdan fault, but not found in the North Qinling to the north of this fault. These facts indicate that the South Qinling and the North Qinling were separated from each other and that the North Qinling micro-block and Danfeng ocean were present again due to the breakup event along the Shangdan fault during the early Late Sinian. Meanwhile, the extension occurred along the Xiangfan-Fangxian-Lieyang zone and caused emplacement of the alkalic rock bodies, such as Micangshan alkalic rock body with an isotopic age of 687 Ma (Qiu, 1993). Summing up the above, the assembly and subsequent breakup which occurred at ca.1000 Ma between Yangtze block and North China block can be correlated in time with the Rodinia supercontinental forma tion and breakup (Dalziel, 1991; Hoffman, 1991; Moores, 1991).An additional
518
collisional event at ca. 800 Ma has also been recorded in the studied area, after the Grenvillian assembly.
References Dalziel, I. W. D. (1991) Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, V. 19, pp. 598-601. Fu, G. Q.,Yuan, H. H. and Li, S. L. (1993) Discovery of archean granite-greenstone terrain of the northern Huangling block, Western Hubei province, China. J. Mineral. and Petrol., v. 13, pp. 5-13 (in Chinese). Hao, J., Li, Y. J., Liu, X. H. and Zhu, X. R. (1996) The Douling paleo-island arc and Wudang paleo-backarc basin in East Qinling and their geological significance.Regional Geology of China, Part-1, pp. 44-50 (in Chinese). Hu, N. G., Wang, T., Yang, G. X. and Zhao, D.L. (1995) HighPressure eclogite metamorphic zones and proterozoic collision in the Qinling orogen. Regional Geology of China, Part-2, pp. 142-148 (in Chinese). Hoffman, P. F. (1991) Did the breakout of Laurentia turn Gondwanaland inside-out? Science, v. 252, pp. 1409- 1412. Jiang, C. Y., Su, S. R., Zhao, T. P. and Yang, Z. H. (1998) The intrusive rock belt and Jinning movement in North Qinling. Beijing: Geological Publ. House, pp. 1-97 (in Chinese). Jiang, J. S. (1986) Isotopic geochronology and crustal evolution of Huangling metamorphic terrain. J. Changchun College of Geol., v. 3, pp.1-11 (in Chinese). Lu, S. N., Yang, C. L., Jiang, M. M., Li, H. K. and Li, H. M. (1996) The traces of evolution of the Precambrian continental crusts. Geological Publ. House, Beijing, pp. 1-156 (in Chinese). Ma, D. Q., Li, Z. C. and Xiao, Z. F. (1997) The constitution, geochronology and geologic evolution of the Kongling Complex, Western Hubei. Acta Geoscientia Sinica, v. 18, pp. 233-241 (in Chinese).
Moores, E. M. (1991) Southwest U.S. East Antarctic (SWEAT) connection: a hypothesis. Geology, v. 19, pp. 425-428. Qiu, J. X. (1993) Alkali rocks in Qinling- Dabashan area. Geological Publishing IHouse, Beijing, pp. 1-379 (in Chinese). Wang, T., Li, W. P. and Wang, X. X. (1998) Zircon U-Pb age of the Niujiaoshan granitoid gneisses in the Qinling complex of the Qinling orogenic belt-with a discussion of its geological significance. Regional Geology of China, v. 17, pp.262-265 (in Chinese). Zhang, B. R., Zhang, H. F., Zhao, Z. D. and Lin, W. L. (1996) The geochemical provinces and their evolutions of the crust and mantle of East Qinling and adjacent regions: implications for tectonics. Science in China (Series D), v. 26, pp. 201-208 (in Chinese). Zhang, E. I?, Niu, D. Y., HLIO,Y. G., Zhang, L. H. and Li,Y. G. (1993) Geologic-tectonic features of Qinling- Dabashan mountains and adjacent regions. Geological Publ. HOLIS~, Beijing, pp. 1-291 (in Chinese). Zhang, G. W., Zhang, Z. Q. and Dong, Y. P.(1995) Nature of main tectono-lithostratigraphic units of the Qinling orogen: implications for the tectonic evolution. Acta Petrologica Sinica, v.11, pp. 101-114 (in Chinese). Zhang, H. F., Zhao, Z. D., Luo, T. C. and Zhang,B. R. (1995) Crustal growth and lower crust nature of North Qinling: Study of Sm-Nd isotopic model ages. Acta Petrologica Sinica, v. 11, pp.160-170 (in Chinese). Zhang, W. J., Wang, Q. Q. and Hu, N. G. (1996) Research of the eastern Qinling geological corridor. Shanxi Sci. and Tech. Press, Xi’an, pp. 1-222 (in Chinese). Zhang, Z. Q., Liu, D. Y. and Fu, G. M. (1994) A study on isotopic chronology of the metamorphic strata in North Qinling. Geological Publ. House. Beijing (in Chinese). Zhang, Z. Q., Zhang, G. W., Fu, G. M., Tang, S. H. and Song, B. (1996) Ages of the metamorphic strata in Qinling: implications for the tectonic evolution. Science in China (Ser. D), v. 26, pp.216-222 (in Chinese).
Gondeuana Xcscarch, V. 2,No. 4, 1999
Rangin,C. (1990) Proceedings of the Ocean Drilling Program, Init. Repts. ODP, College Station, TX. p.906. Ridd, M.F. (1990) Possible Palaeozoic drift of SE Asia and Triassic collision with China. J. Geol. SOC.London, v.137, pp.635-640. Silver, E.A. (1991) Leg 124 tectonic synthesis, In: Rangin, C. (Ed.) Proceedings of the Ocean Drilling Program, Scientific Results, v.124, pp.3-9. Taylor, B. and Hayes, D.E. (1980)The tectonic evolution of the South China Basin. In: Hayes, D.E. (Ed.) The tectonic and
geologic evolution of Southeast Asian Seas and Islands, American Geophys. Union Monograph, v. 23, p p . 89-104. Taylor, 8. and Hayes, D.E. (1983) Origin and history of the South China Basin. In: Hayes,D.E. (Ed.)The tectonic and geologic Evolution of Southeast Asian Seas and Islands, Part 2, American Geophys. Union Monograph, v. 27, pp.23-56. Williams, H.H. (1992) Geochemistry of Palawan Oils, Phi 1i p p ines : source imp 1ic a t i o ns . Pro c . 9t'' Offshore Southeast Asia Conf., Singapore, pp.1-4.
Gondzuana Research, V. 2, No. 4, pp. 515-518. 01999 International Association for Gondzuana Rescnrch, japan. ISSN: 1342-937X
GR
Goizdwarza Research
Precambrian Crustal Evolution in the Central Northern Margin of the Yangtze Block Xie Caifu, Fu Jianming, Xiong Chengyun, Zhang Yeming, Hu Ning and Huang Zhaoxian Yichang Institute of Geology and Mineral Resources, CAGS, PO. Box 502, Yichang, Hzibei 443003, China
Meso-and Neoarchean Initial Crust Archean basement rocks in the studied area are found only in the cores of the Huangling Arch of the Yangtze Gorges area, Hubei and in the Zhaochuan dome fold, Shanxi. They are separately called as the Kongling Complex (lower part) and the Zhuanlugou Complex (Fig.1). The original rocks of these comprise a suite of bimodal volcanics dominated by tholeiite and minor dacite, intercalated with small number of siliceous rocks. The metamorphic grade is high amphibolite to partly granulite facies, with strong nuginatization and a number of TTG plutons. The formation age of these rocks was dated between ca. 2600-ca.3460 Ma (FLIet al., 1993; Zhang W. J. et al., 1996; Ma et al., 1997). Among them the age of TTG intrusive rocks varies between 2800-2900 Ma (Ma et al., 1997).This suite is different from the Archean of North China block in age and rock assemblage. The latter is characterized by the presence of continental crustal rocks of 3500- 3800 Ma and a lot of Neoarchean khondalite series and siliceous iron formations.
Moreover, Pb and Nd isotopic compositions and peak values of Nd model ages reflect the basements in the North China block tobe notably different from those of the Yangtze block (Zhang B. R. et al., 1996; Lu et al., 1996). It is inferred, therefore, that the North China block and Yangtze block might be two landmasses, independently grown during the Archean.
Palaeoproterozoic Continental Crust Accretion Palaeoproterozoic continental crustal rocks, exposed in the Huangling Arch, Dadouling-Xiaodouling, ShangzhouZhengping and Xiaomoling areas, are called the Kongling Complex (upper part), Douling "Lithogroup", Qinling "Lithogroup" and Xiaomoling Complex respectively. They were probably formed in the setting similar to active continental margin following weathering and erosion of the initial Archean salic continental crust and composed of a suite of terrigenous clastic (volcanic) formation, dominated by terrigenous clastic rocks interbedded with
516
minor basic and acidic volcanic rocks, carbonates and ironbearmg siliceous rocks. The grade oi metamorphism attains high amphibolite facies with mignia tiza tion and intrusion of potassic granitic plutons. Their isotopic ages are between 1820 and 2430 Ma (Zhanget al., 1994; Jiang, 1986; Ma et al., 1997),of which the age of K-granite is ca. 1840 Ma (Fu et al., 1993). The Qinling "Lithogroup" was usually recognized as an accretionary crust in the southern margin of North China block (Zhang Z. Q. et al., 1996). It seems to the present authors that, however, the northern margin of Yangtze block and the boundary between the North China block and the Yangtze block should be placed at the Shangzhou- Xiaguan fault, and not at the Shangzhou-Danfeng fault, which was always regarded as the boundary between these two blocks (Zhang G. W. et al., 1995).The reasons for this argument are as follows: (1)the Qinling "lithogroup" is very similar to the Douling "lithogroup", Xiaomoling Complex and Kongling Complex (upper part) in rock assemblages, initial rock formations, metamorphic intensity, isotopic ages etc. They are characterized by a suite of khondalite series and C-bearing rocks at their lower parts, which obviously differ from thc Palaeoprotcrozoic Songshan Group, Hongan Group, Shusong Group and Fengyang Group exposed in
North China block
oXi'an o Luonan
..
I
Danfefl
4
I ?
0
100
0 Yichang
200 km
u
6 4 3
Fig.1. Sketch map showing the outcrop of the I'rccambrian in the central northern margin of the Yangtze block. F,. LuanchuanHeigou fault. F,. Shangzhou-Xiaguan (Shangxia) fault. F,. Shangzhou-Danfeng (Shangdaii) fault. F,,. Xiangfan-Fangxian fault. 1. Kongling Complex, 2. Zliuaiilugou Complex, 3. Douling "lithogroup", 4. Xiaomoling Complex. 5. Qinliiig "Litliogroup", 6 . Sheruiongjia Group, 7. Wudang "Lithogroup" and Yaolinglie Group, 8. Xiahe "Lithogyoup", 9. Kuanping "Lithogroup", 10. Erlangping "Litliogrokip", 11. Danfeng "Litliogroup", 12. Macaoyuan Group, 13. Lower Sinian and Upper Sinian, 14. Upper Sinian
the southern margin of the North China block. The latter belongs to a suite of (volcanic) sedimentary series me tamorphosed in the mid- to low grade, consisting of P-, Fe-, Mn- a n d Y-bearing marble, quartzite, phylli te, hornblende schist and schernikite plagiogneiss (Zhang et al., 1993). (2) Pb and Nd isotopic compositions of the Q d i n g "lithogroup" show a close affinity with those of equivalents from the South Qinling and Yangtze block and differ from those of the North China block (Zhang B.R.et al., 1996). (3)The Shangxia fault is much stronger in control of mineralization than the Shangdan fault. The Hg, Sb deposits in China chiefly occur in the Yangtze block to the southern side of the Shangxia fault. A lot of ore deposits are present on both sides of the fault, but the meso-katathermal type deposits are mainly distributed in the northern side and the meso-epithermal type deposits in the southern side. In addition, the suture zone feature of Shangxia fault has been gradually recognized in recent years (discussed below). Considering that there commonly existed an intense metamorphic and intrusive granitic event at ca.1850 Ma in both the Yangtze block and the southern margin of the North China block, it is inferred that the two blocks were assembled and formed a united continent in the late Palaeopro terozoic.
Mesoproterozoic Continental Crust Breakup It is still controversial as to whether the studied area belongs to the continental rift associated with micro-oceanic basin (Zhang et al., 1995),or the island arc (Ha0 et al., 1996) during the Mesoproterozoic time. The Shennongjia Group, Wudang "Lithogroup', Danfeng "Lithogroup", Xiahe "Lithogroup" a n d Kuangping "Lithogroup" etc. are successively distributed from south toward north in the studied area. Their isotopic ages range between 1600-1000 Ma (Zhang et al., 1994). It is thought that they were formed in different tectonic settings caused by rifts of different degree. The Shennongjia Group, composed mainly of slightly metamorphosed carbonate rock (65%),clastic rock (31%)and a little alkali basaltic rock (4%), was formed in a tensional setting of the inner side of the continental margin. The Wudang "lithogroup" is composed of middle to low grade metamorphosed sedimentary rocks (50%) and acidic and basic volcanic rocks (50%), of which volcanic rocks display "bimodal" assemblage and belong to spilite- keratophyre formation. It could have chiefly formed in a rift setting of the outer side of the continental margin. The Danfeng "Lithogroup" and the Kuanping "Lithogroup", of which the original rocks are tholeiite, clastic rock and carbonate rock, were formed under a transitional setting from a continental rift to micro-ocean basins (termed here as the Kuanping and Danfeng microGondwana Research, V. 2, No. 4, 1999
517
ocean basins separately). This indicates that the North Qinling separating the two micro-ocean basins was a microblock (called as North Qinling micro-block) between them during that time. The primitive rocks of the Xiahe "Lithogroup" consisting of terrigenous clastic rocks interbedded with basic volcanics, were formed in an aulacogen setting. The study of rock type, magmatic series, rock assemblage and some geochemical characteristics suggests that the Mesoproterozoic volcanic rocks in the studied area might be continental rift volcanics, though part of these rocks can be plotted in ocean ridge, ocean island, and island arc fields in geochemical discrimination diagrams. In the logar-log-1 diagram, these rocks are mainly located in the active tectonic field (orogenic field). The main factor leading to the above mentioned complex conditions might be related with the thinner continental crust formed in this area during Archean-Palaeoproterozoic, so that the Mesoproterozoic crust still possessed a transitional feature although it was already under a rifting environment
Late Mesoproterozoic-Neoproterozoic Collision and Breakup Events The two Mesoproterozoic micro-oceans began to subduct along the Shangxia belt and the Shangdan belt at ca.1200 Ma, and then a strong collisional orogeny termed earlier as Jinning movement or Shennong (Sibao) movement took place a t ca. 1000 Ma. This collisional event led to the unification of the Yangtze block, North Qinling micro-block and North China block. The evidence for this is as follows. (1) There are angular unconformities between the Neoproterozoic Macaoyuan Group and the Mesoproterozoic Shennongjia Group, between the Yaolinghe Group and the Wu d a n g "Lit h o g r o up ", be tween the Er 1an g p ing "Li thogroup" and the Kuanping "Lithogroup", separately. (2)There are ca. 1000 Ma ophiolite suites (Zhang, G. W. et al., 1995;Jiang et al., 1998),hgh-pressure metamorphic zone (Hu et al., 1995), subduction and collision-type granitoid rock belts (Wang et al., 1998; Hao et al., 1996) and tectonic melange belts found separately along the ShangzhouDanfeng and Shangzhou-Xiaguan fault zones. These reveal that the two fault zones might have been suture zones at ca. 1000 Ma. (3)The existence of a peak at ca. 1000 Ma in isotopic ages from this area (Zhang Z. Q. et al., 1996). (4) A strong crust-forming period during ca. 1000-1200 Ma in North Qinling indicated by isotopic studies (Zhang H. F. et al., 1995; Zhang et al., 1994; Zhang B. R. et al., 1996). At ca. 950 Ma, the breakup began to occur along the original assembly belt, so that the Erlangping and Danfeng micro-oceans and North Qinling micro-block formed again during Neoproterozoic. The intensity of breakup generally Gondzuana Research, V 2, No. 4, 1999
decreased from north toward south. In Wudang area, the formation of Yaolinghe Group caused by this rifting event is dominated by tholeiitic rocks along with a great volume of basic dyke swarms intruding the basement rock series. These basic volcanic rocks and dyke swarm possess typical geochemical characteristics of wi thin-plate-basalt (Zhang W. J. et al., 1996), implying the formation of a thicker and rigid crust in the studied area during the Early Jinningstage. Meanwhile, the basic dyke swarm and the Macaoyuan Group consisting of molasse formation with the intermediate-basic volcanic materials formed in the Shennongjia area in association with the pull-apart action. C or r e sp ond in g 1y, the Sh an d ui h e s u pe r 1a r g e- sca l e epithermal silic deposit was formed in the tensional faults during that time. The collision orogeny of ca. 800 Ma called the late Jinning movement or Jinning movement was recorded in this area. In the southern part of the studied area this event led to the assembly and collision of the Jiangnan island arc with the Yangtze block along the Youyang-Yichang fault and the emplacement of a lot of granitic bodies and formation of some magma tic hydrothermal gold deposits in Huangling area. Meanwhile, strong tectonic action occurred along the Xiangfan-Fangxian-Lieyang zone in the central part of the studied area, along with the occurrence of a lot of granitic bodies in the Hanzhong area. In the northern part, the Danfeng micro-ocean was closed. The North Qinling microblock was assembled with the Yangtze block again. It caused the emplacement of the intermediate-acidic bodies on the two sides of Danfeng fault and the formation of rocks with ca. 800 Ma peak isotopic ages (Zhang Z. Q. et al., 1996).The Erlangping ocean might have had a larger extent during that time and was not closed till the early Palaeozoic. The Yangtze-type Doushantuo Formation and Dengyin Formation of the Upper Sinian are widely spread in the South Qinling and Yangtze platform to the south of the Shangdan fault, but not found in the North Qinling to the north of this fault. These facts indicate that the South Qinling and the North Qinling were separated from each other and that the North Qinling micro-block and Danfeng ocean were present again due to the breakup event along the Shangdan fault during the early Late Sinian. Meanwhile, the extension occurred along the Xiangfan-Fangxian-Lieyang zone and caused emplacement of the alkalic rock bodies, such as Micangshan alkalic rock body with an isotopic age of 687 Ma (Qiu, 1993). Summing up the above, the assembly and subsequent breakup which occurred at ca.1000 Ma between Yangtze block and North China block can be correlated in time with the Rodinia supercontinental forma tion and breakup (Dalziel, 1991; Hoffman, 1991; Moores, 1991).An additional
518
collisional event at ca. 800 Ma has also been recorded in the studied area, after the Grenvillian assembly.
References Dalziel, I. W. D. (1991) Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, V. 19, pp. 598-601. Fu, G. Q.,Yuan, H. H. and Li, S. L. (1993) Discovery of archean granite-greenstone terrain of the northern Huangling block, Western Hubei province, China. J. Mineral. and Petrol., v. 13, pp. 5-13 (in Chinese). Hao, J., Li, Y. J., Liu, X. H. and Zhu, X. R. (1996) The Douling paleo-island arc and Wudang paleo-backarc basin in East Qinling and their geological significance.Regional Geology of China, Part-1, pp. 44-50 (in Chinese). Hu, N. G., Wang, T., Yang, G. X. and Zhao, D.L. (1995) HighPressure eclogite metamorphic zones and proterozoic collision in the Qinling orogen. Regional Geology of China, Part-2, pp. 142-148 (in Chinese). Hoffman, P. F. (1991) Did the breakout of Laurentia turn Gondwanaland inside-out? Science, v. 252, pp. 1409- 1412. Jiang, C. Y., Su, S. R., Zhao, T. P. and Yang, Z. H. (1998) The intrusive rock belt and Jinning movement in North Qinling. Beijing: Geological Publ. House, pp. 1-97 (in Chinese). Jiang, J. S. (1986) Isotopic geochronology and crustal evolution of Huangling metamorphic terrain. J. Changchun College of Geol., v. 3, pp.1-11 (in Chinese). Lu, S. N., Yang, C. L., Jiang, M. M., Li, H. K. and Li, H. M. (1996) The traces of evolution of the Precambrian continental crusts. Geological Publ. House, Beijing, pp. 1-156 (in Chinese). Ma, D. Q., Li, Z. C. and Xiao, Z. F. (1997) The constitution, geochronology and geologic evolution of the Kongling Complex, Western Hubei. Acta Geoscientia Sinica, v. 18, pp. 233-241 (in Chinese).
Moores, E. M. (1991) Southwest U.S. East Antarctic (SWEAT) connection: a hypothesis. Geology, v. 19, pp. 425-428. Qiu, J. X. (1993) Alkali rocks in Qinling- Dabashan area. Geological Publishing IHouse, Beijing, pp. 1-379 (in Chinese). Wang, T., Li, W. P. and Wang, X. X. (1998) Zircon U-Pb age of the Niujiaoshan granitoid gneisses in the Qinling complex of the Qinling orogenic belt-with a discussion of its geological significance. Regional Geology of China, v. 17, pp.262-265 (in Chinese). Zhang, B. R., Zhang, H. F., Zhao, Z. D. and Lin, W. L. (1996) The geochemical provinces and their evolutions of the crust and mantle of East Qinling and adjacent regions: implications for tectonics. Science in China (Series D), v. 26, pp. 201-208 (in Chinese). Zhang, E. I?, Niu, D. Y., HLIO,Y. G., Zhang, L. H. and Li,Y. G. (1993) Geologic-tectonic features of Qinling- Dabashan mountains and adjacent regions. Geological Publ. HOLIS~, Beijing, pp. 1-291 (in Chinese). Zhang, G. W., Zhang, Z. Q. and Dong, Y. P.(1995) Nature of main tectono-lithostratigraphic units of the Qinling orogen: implications for the tectonic evolution. Acta Petrologica Sinica, v.11, pp. 101-114 (in Chinese). Zhang, H. F., Zhao, Z. D., Luo, T. C. and Zhang,B. R. (1995) Crustal growth and lower crust nature of North Qinling: Study of Sm-Nd isotopic model ages. Acta Petrologica Sinica, v. 11, pp.160-170 (in Chinese). Zhang, W. J., Wang, Q. Q. and Hu, N. G. (1996) Research of the eastern Qinling geological corridor. Shanxi Sci. and Tech. Press, Xi’an, pp. 1-222 (in Chinese). Zhang, Z. Q., Liu, D. Y. and Fu, G. M. (1994) A study on isotopic chronology of the metamorphic strata in North Qinling. Geological Publ. House. Beijing (in Chinese). Zhang, Z. Q., Zhang, G. W., Fu, G. M., Tang, S. H. and Song, B. (1996) Ages of the metamorphic strata in Qinling: implications for the tectonic evolution. Science in China (Ser. D), v. 26, pp.216-222 (in Chinese).
Gondeuana Xcscarch, V. 2,No. 4, 1999