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Metamorphic and Basic Rocks in Middle Hainan Island: Preliminary Geochemical and Petrological Studies and Tectonic Significance
Gondronna Resenrch, V. 2, No. 4, p p . 643-646.
GR
01999 lnternntional Association for Gondzimnn Research, Inpan. ISSN: 1342-937X
Gorzdwarza Research
Metamorphic and Basic Rocks in Middle Hainan Island: Preliminary Geochemical and Petrological Studies and Tectonic Significance Zhou Zuyi '/Ding Shijiang 2/ Liao Zongting', Xu Changhai' and Lao Qiuyuan' Key Laboratory of Mnrine Geology, Tongji University, Shanghai 20009, China ' Research Institute of Geological Survey of Hainan, Hainan 570005, China
Introduction East and Southeast Asia represent a complex assembly of allochthonous continental blocks, island arcs, accretional complexes and marginal ocean basins (Fig. 1).Stratigraphic, sedimentological, paleobiogeographical and paleomagnetic data suggest that most of these continental blocks were derived directly or indirectly from Gondwana. The evolution of the continental blocks appears to be one of the rifts from Gondwana, northward drift and accretion to form the present day East and Southeast Asia. The northward
drift of these continental blocks were related to the opening and closing of Tethyan oceans. The Middle-Late Triassic collision between the South China Block and the Indochina Block along the Black River (or Song Da) and the Red River or (Song Ma) suture is well documented and recognised. The southeast extension of these two sutures are unclear because of the covering of the sea water in the South China Sea (Fig. 1).Located in the northern shelf of the South China Sea and to the east of the Red Ihver, Hainan Island proves to be one of the key regions for providing the geological records that could be related to
Fig. 1. Distributionof principal continental blocks and sutures of East and SoutheastAsia. HT-Hainan island (from Metcalfe, 1996).
644
the possible continental collisions within, as well as near the island. The similarity in Cambrian sedimentary successions, ore deposits, trilobites and brachiopods between Hainan and Australia (Sun, 1963), as well as the distinctive difference in Early Paleozoic stratigraphy between southern Hainan Island and adjacent South China have led to the postulation that Hainan Island may represent one or more Gondwana-derived allochthonous terranes (e.g., Metcalfe et al., 1994).Yu (1989)reported the discovery of early Permian diamictitesin mid-west Hainan which were discovered in other Gondwana components of nearby Tibet, West Yunan and Indochina. Consequently they suggested that Hainan Island was part of Gondwana in Late Paleozoic. This discovery, however, was thrown into question by later field investigations of Sino-Australian research team (Metcalfe et al., 1994). However, based on the available stratigraphic and paleobiogeographic data, concensus has been reached among different researchers that Hainan Island is composed of at least two tectonostratigraphic blocks (or terranes) which were separated in Early Paleozoic (Chen et al., 1992; Gorur and Sengor, 1992; Metcalfe, 1996; Zhou et al., 1996). However, controversy remains as regard to the
boundaries of the different blocks and the nature and precise timing of the collision events in the island. Zhu (1987) suggested the existence of a pre-Cretaceous active margin along the northern shelf of the South China Sea, which passed through the northern part of Hainan Island and may be connected to the Red River suture. Yang et al. (1989) proposed that the east-west Jiushuo-Lingshui Fault is the boundary between the northern Middle Hainan (Qiong Zhong) and southern Yaxian terranes. Chen et al. (1992), however, interpreted the island as a Mesozoic orogenicbelt created by the collision of the Huanan (South China) and Dongnanya (Southeast Asia) blocks. Gorur and Sengor (1992)explained that Hainan Island was originally a part of Sibumasu terrane and was later separated from other Sibumasu components in the Mesozoic by strike-slip faulting. Metcalfe et al. (1994) put the NE-SW trending Basha Fault (Fig. 2) as the boundary between two different tectonostratigraphic terranes. While most of the previous studies have been mainly concentrated on the sedimentological, paleobiogeographical and petrological aspects of the island and its correlation with other continents of Gondwana origin, few
Fig. 2. Schematic geological map of Hainan island. Dashed lines show the location of suture zones proposed by previous researchers.
Gondwana Research, V. 2, No. 4,1999
645
shidies have been directed toward the metamorphic rocks outcropped across a large area in the central part of the island (Fig. 2). Particularly, the relationship between deformation and metamorphism and the precise absolute dating of the major tectonic events (including regional metamorphism, deformation, magma tic emplacement, etc.) has been rare, which has hindered the understanding of the collision event (or events) that is believed to happen in the island.
Regional Geology of Middle Hainan MesoproterozoicBaoban Group is the oldest strata in the island which is mainly outcropped in the mid-west part of the island. Composed of migmatites (possibly mylonites), gneisses and migmatiticgranites, Baoban Group has yielded Rb/Sr isotopic ages of between 974 Ma and 1750 Ma (Zhang et al., 1990)and U-Pb age of 1400 Ma from zircon crystals in gneissic granites. It is overlain by Neoproterozoic metasedimentary sequences with ironstones (Shilu Group) which has yielded isotope ages of between 850 Ma and 1100 Ma. The carbonaceous slate in the upper part (Layer 6) of the Shilu Group contains Chuaria-Tuzuuia algal fossil assemblage which indicates an age of between 700 Ma and 840 Ma (Vidal et al., 1993).Shilu Group has therefore been interpreted as a Hainan equivalent of the Vindhyan in India and Riphean in Russia. However, well preserved fossils (Shiluites)have also been found in the Shilu Group, which is firmly indicative of Early Paleozoic (Liu, 1986). In addition, there are also reports about younger isotopic ages of Shilu Group ranging from 580 Ma to 200 Ma (Liu, 1986). The age conflicts of Shilu Group made Chen et al. (1992) suggest a melange nature for Shilu Group. On the top of the above groups is the more than 3000m thick Nanbigou Formation which is composed of metasedimentaryrocks and metamorphosed basic volcanic clastics. Interbedded in the Nanbigou Fm. is the chloritized actinolite-schist, actinolite-plagiogneiss and tremolitized komatiite which yield a Sm-Nd age of 458 Ma by our preliminary analysis. No fossil records have been found except for some sporopollen. This formation is now tentatively attributed to Ordovician in consideration that in some locations it has an intrusive contact with the above Late Paleozoic gneissic granites. The Nanbigou Fm. is overlain by carboniferous basal conglomerates followed by meta-shales, quartzites, conglomerates and diamictites which are in turn conformably overlain by Lower Permian limestones with fusulinid foraminifera. Sedimentary rocks of Upper Permian, Triassic and Jurassic age appear to be absent. Lower Cretaceous terrestrial siltstones and mudstones rest unconformablyover the Permian. The Lower Cretaceous is then overlain by red mudstones and carbonaceous shales Gondwnnn Research, V. 2,No. 4,1999
of Middle Eocene age and then by basalt of Neogene and Quaternary ages. In summary,we believe that the precise dating of tectonic events on samples from Baoban Group, Shilu Group and Nanbigou Fm., based on the field and indoor analysis of the relationship between metamorphism and deformation, will contribute key information for a kinematic model of the tectono-thermal evolution of the island. Study on the emplacement mechanisms of the interbedded basic/mafic volcanoes and granites will also facilitate the construction of the model.
The Present Research Firstly, we carried out detailed geological mapping (scale:1:50,000) in areas which are relevant to the study. The principal aim of our work is to obtain precise dating results for major tectono-thermal events from samples of Baoban Group, Shilu Group, Nanbigou Fm. and the interbedded basic volcanic rocks in order to construct a kinematic model for the tectonic evolution of Hainan Island. Field rela tionships between metamorphism and deformation is shidied in order to give better geological constraints for the sampling of rocks for geochronologicaland microstructural analysis and for P-T-t path construction. Chemical analyses including REE, trace element and bulk chemical composition were carried out for the basic rocks which outcrop in the research area. These data, together with information on the basement, structural style and petrology of these rocks, were then analysed by ESCORT system for the discrimination of their tectonic setting. Radiometric dating of U/Pb and Sm/Nd systems on these rocks and metamorphic rocks of Nanbigou Forma tion yield ages ranging between 458 Ma and 616 Ma. Field and petrological investigations on Nanbigou Formation and basic rocks showed that they together represent oceanic rocks which existed in the early Paleozoic ocean that separated northern and southern Hainan. We propose that the zone connectingTunchang and Changjiang regions in the studied area is the suture zone between the northern and southern blocks when the intervening ocean disappeared. Insight into the history of polymetamorphic rock units which have survived a succession of overprinting events is of fundamental importance for the kinematic modeling of the tectonic history of the region. While the techniques of structural mapping, microstructural analysis using optical, automatic X-ray texture goniometer and neutron diffraction provide useful data, special techniques such as combined ca thodo-luminescence and high resolution ion microprobe (SHRIMP), U-Th-Pb dating and fission track analysis to a variety of actinide-enriched minerals would provide
646
important information on the timing of the high and low temperature parts of the tectonic processes.
References Chen Haihong, Sun Shu, Hsu, K.J., Dobson, J. and Yu Zhiye (1992) Tectonics of the Hainan Orogeny Belt: a preliminary study. Mem. Lithosph. Tect. Evol. Res., v.1, pp.43-48. Gorur, N. and Sengor, A.M.C. (1992) Paleogeography and tectonic evolution of the eastern Tethysides: implications for the northwest Australian margin breakup history. In: von Rad, U. and Haq, B.U. (Eds.), Proc. of the Ocean Drilling Program, Scientific Results, v.122, pp.83-106. Liu, Y. (1986) Geology of Hainan Island and Geochemistry of Iron Ore Deposits in Shilu. Science Press, Beijing, p. 376 (in Chinese). Metcalfe, I. (1996) Gondwana dispersion, Asian accretion and evolution of eastern Tethys. Australian J. Earth Sci., v. 43, pp. 605-623. Metcalfe, I., Shergold, J.H. and Li, Z.X. (1994) IGCP 321Gondwana dispersion and Asian accretion: field works on Hainan Island. Episodes, v. 16, pp. 443-447. SunYunchu (1963) On the occurrence of Xystridura fauna from Middle Cambrian of Hainan Island and its significance. Acta Paleontolog. Sinica, v.11, pp. 608-611. (in Chinese with English abstract)
Vidal, G., Moczydlowska, M. a n d Rudavskaya, V.A. (1993) Biostratigraphic implications of a ChuariaTawuia assemblage and associated acritarchs from the Neoproterozoic of Yakutia. Paleontology, v.36, pp. 387-402. Yang Shufeng, Yu Zhiye, Guo Linzhi and Shi Yangsheng (1989) The division and paleomagnetism of the Hainan Island and plate tectonic significance. J. Nanjing Univ. (Earth Sci. Ed.), v.1, pp.38-46 (in Chinese with English abstract). Yu Zhiye (1989) Hainan Island: a terrane from Gondwana. 4th International Symposium on pre-Jurassic evolution of East Asia, IGCP Project 224 Reports and Abstracts, v.1, pp.11-12. Zhang Renjie, Feng Shaonan, Xu Guanghong, Yang Deli, Yan Daoping, Li Zhihong, Jiang Dahai and Wu Wei (1990) Discovery of Chuaria-Tawuia assemblage in Shilu Group, Hainan Island and its significance.Science in China (B), v.33, pp. 211-220 (in Chinese with English abstract). Zhou Zuyi, Lao Qiuyuan, Chen Huanjiang, Ding Shijiang and Liao Zhongting (1996) Early Mesozoic orogeny in Fujian, Southeast China. In: Hall, R. and Blundell, D. (Eds.),Tectonic evolution of Southeast Asia. Geol. SOC.Sp. Publ. No. 106, pp. 549-556. Zhu, X. (1987) On the evolution of Chinese continental margins and basins. Marine Geology and Quaternary Geol., v.7, pp.115-119 (in Chinese).
Gondwana Research, V. 2, No. 4,1999
GR
01999 lnternntional Association for Gondzimnn Research, Inpan. ISSN: 1342-937X
Gorzdwarza Research
Metamorphic and Basic Rocks in Middle Hainan Island: Preliminary Geochemical and Petrological Studies and Tectonic Significance Zhou Zuyi '/Ding Shijiang 2/ Liao Zongting', Xu Changhai' and Lao Qiuyuan' Key Laboratory of Mnrine Geology, Tongji University, Shanghai 20009, China ' Research Institute of Geological Survey of Hainan, Hainan 570005, China
Introduction East and Southeast Asia represent a complex assembly of allochthonous continental blocks, island arcs, accretional complexes and marginal ocean basins (Fig. 1).Stratigraphic, sedimentological, paleobiogeographical and paleomagnetic data suggest that most of these continental blocks were derived directly or indirectly from Gondwana. The evolution of the continental blocks appears to be one of the rifts from Gondwana, northward drift and accretion to form the present day East and Southeast Asia. The northward
drift of these continental blocks were related to the opening and closing of Tethyan oceans. The Middle-Late Triassic collision between the South China Block and the Indochina Block along the Black River (or Song Da) and the Red River or (Song Ma) suture is well documented and recognised. The southeast extension of these two sutures are unclear because of the covering of the sea water in the South China Sea (Fig. 1).Located in the northern shelf of the South China Sea and to the east of the Red Ihver, Hainan Island proves to be one of the key regions for providing the geological records that could be related to
Fig. 1. Distributionof principal continental blocks and sutures of East and SoutheastAsia. HT-Hainan island (from Metcalfe, 1996).
644
the possible continental collisions within, as well as near the island. The similarity in Cambrian sedimentary successions, ore deposits, trilobites and brachiopods between Hainan and Australia (Sun, 1963), as well as the distinctive difference in Early Paleozoic stratigraphy between southern Hainan Island and adjacent South China have led to the postulation that Hainan Island may represent one or more Gondwana-derived allochthonous terranes (e.g., Metcalfe et al., 1994).Yu (1989)reported the discovery of early Permian diamictitesin mid-west Hainan which were discovered in other Gondwana components of nearby Tibet, West Yunan and Indochina. Consequently they suggested that Hainan Island was part of Gondwana in Late Paleozoic. This discovery, however, was thrown into question by later field investigations of Sino-Australian research team (Metcalfe et al., 1994). However, based on the available stratigraphic and paleobiogeographic data, concensus has been reached among different researchers that Hainan Island is composed of at least two tectonostratigraphic blocks (or terranes) which were separated in Early Paleozoic (Chen et al., 1992; Gorur and Sengor, 1992; Metcalfe, 1996; Zhou et al., 1996). However, controversy remains as regard to the
boundaries of the different blocks and the nature and precise timing of the collision events in the island. Zhu (1987) suggested the existence of a pre-Cretaceous active margin along the northern shelf of the South China Sea, which passed through the northern part of Hainan Island and may be connected to the Red River suture. Yang et al. (1989) proposed that the east-west Jiushuo-Lingshui Fault is the boundary between the northern Middle Hainan (Qiong Zhong) and southern Yaxian terranes. Chen et al. (1992), however, interpreted the island as a Mesozoic orogenicbelt created by the collision of the Huanan (South China) and Dongnanya (Southeast Asia) blocks. Gorur and Sengor (1992)explained that Hainan Island was originally a part of Sibumasu terrane and was later separated from other Sibumasu components in the Mesozoic by strike-slip faulting. Metcalfe et al. (1994) put the NE-SW trending Basha Fault (Fig. 2) as the boundary between two different tectonostratigraphic terranes. While most of the previous studies have been mainly concentrated on the sedimentological, paleobiogeographical and petrological aspects of the island and its correlation with other continents of Gondwana origin, few
Fig. 2. Schematic geological map of Hainan island. Dashed lines show the location of suture zones proposed by previous researchers.
Gondwana Research, V. 2, No. 4,1999
645
shidies have been directed toward the metamorphic rocks outcropped across a large area in the central part of the island (Fig. 2). Particularly, the relationship between deformation and metamorphism and the precise absolute dating of the major tectonic events (including regional metamorphism, deformation, magma tic emplacement, etc.) has been rare, which has hindered the understanding of the collision event (or events) that is believed to happen in the island.
Regional Geology of Middle Hainan MesoproterozoicBaoban Group is the oldest strata in the island which is mainly outcropped in the mid-west part of the island. Composed of migmatites (possibly mylonites), gneisses and migmatiticgranites, Baoban Group has yielded Rb/Sr isotopic ages of between 974 Ma and 1750 Ma (Zhang et al., 1990)and U-Pb age of 1400 Ma from zircon crystals in gneissic granites. It is overlain by Neoproterozoic metasedimentary sequences with ironstones (Shilu Group) which has yielded isotope ages of between 850 Ma and 1100 Ma. The carbonaceous slate in the upper part (Layer 6) of the Shilu Group contains Chuaria-Tuzuuia algal fossil assemblage which indicates an age of between 700 Ma and 840 Ma (Vidal et al., 1993).Shilu Group has therefore been interpreted as a Hainan equivalent of the Vindhyan in India and Riphean in Russia. However, well preserved fossils (Shiluites)have also been found in the Shilu Group, which is firmly indicative of Early Paleozoic (Liu, 1986). In addition, there are also reports about younger isotopic ages of Shilu Group ranging from 580 Ma to 200 Ma (Liu, 1986). The age conflicts of Shilu Group made Chen et al. (1992) suggest a melange nature for Shilu Group. On the top of the above groups is the more than 3000m thick Nanbigou Formation which is composed of metasedimentaryrocks and metamorphosed basic volcanic clastics. Interbedded in the Nanbigou Fm. is the chloritized actinolite-schist, actinolite-plagiogneiss and tremolitized komatiite which yield a Sm-Nd age of 458 Ma by our preliminary analysis. No fossil records have been found except for some sporopollen. This formation is now tentatively attributed to Ordovician in consideration that in some locations it has an intrusive contact with the above Late Paleozoic gneissic granites. The Nanbigou Fm. is overlain by carboniferous basal conglomerates followed by meta-shales, quartzites, conglomerates and diamictites which are in turn conformably overlain by Lower Permian limestones with fusulinid foraminifera. Sedimentary rocks of Upper Permian, Triassic and Jurassic age appear to be absent. Lower Cretaceous terrestrial siltstones and mudstones rest unconformablyover the Permian. The Lower Cretaceous is then overlain by red mudstones and carbonaceous shales Gondwnnn Research, V. 2,No. 4,1999
of Middle Eocene age and then by basalt of Neogene and Quaternary ages. In summary,we believe that the precise dating of tectonic events on samples from Baoban Group, Shilu Group and Nanbigou Fm., based on the field and indoor analysis of the relationship between metamorphism and deformation, will contribute key information for a kinematic model of the tectono-thermal evolution of the island. Study on the emplacement mechanisms of the interbedded basic/mafic volcanoes and granites will also facilitate the construction of the model.
The Present Research Firstly, we carried out detailed geological mapping (scale:1:50,000) in areas which are relevant to the study. The principal aim of our work is to obtain precise dating results for major tectono-thermal events from samples of Baoban Group, Shilu Group, Nanbigou Fm. and the interbedded basic volcanic rocks in order to construct a kinematic model for the tectonic evolution of Hainan Island. Field rela tionships between metamorphism and deformation is shidied in order to give better geological constraints for the sampling of rocks for geochronologicaland microstructural analysis and for P-T-t path construction. Chemical analyses including REE, trace element and bulk chemical composition were carried out for the basic rocks which outcrop in the research area. These data, together with information on the basement, structural style and petrology of these rocks, were then analysed by ESCORT system for the discrimination of their tectonic setting. Radiometric dating of U/Pb and Sm/Nd systems on these rocks and metamorphic rocks of Nanbigou Forma tion yield ages ranging between 458 Ma and 616 Ma. Field and petrological investigations on Nanbigou Formation and basic rocks showed that they together represent oceanic rocks which existed in the early Paleozoic ocean that separated northern and southern Hainan. We propose that the zone connectingTunchang and Changjiang regions in the studied area is the suture zone between the northern and southern blocks when the intervening ocean disappeared. Insight into the history of polymetamorphic rock units which have survived a succession of overprinting events is of fundamental importance for the kinematic modeling of the tectonic history of the region. While the techniques of structural mapping, microstructural analysis using optical, automatic X-ray texture goniometer and neutron diffraction provide useful data, special techniques such as combined ca thodo-luminescence and high resolution ion microprobe (SHRIMP), U-Th-Pb dating and fission track analysis to a variety of actinide-enriched minerals would provide
646
important information on the timing of the high and low temperature parts of the tectonic processes.
References Chen Haihong, Sun Shu, Hsu, K.J., Dobson, J. and Yu Zhiye (1992) Tectonics of the Hainan Orogeny Belt: a preliminary study. Mem. Lithosph. Tect. Evol. Res., v.1, pp.43-48. Gorur, N. and Sengor, A.M.C. (1992) Paleogeography and tectonic evolution of the eastern Tethysides: implications for the northwest Australian margin breakup history. In: von Rad, U. and Haq, B.U. (Eds.), Proc. of the Ocean Drilling Program, Scientific Results, v.122, pp.83-106. Liu, Y. (1986) Geology of Hainan Island and Geochemistry of Iron Ore Deposits in Shilu. Science Press, Beijing, p. 376 (in Chinese). Metcalfe, I. (1996) Gondwana dispersion, Asian accretion and evolution of eastern Tethys. Australian J. Earth Sci., v. 43, pp. 605-623. Metcalfe, I., Shergold, J.H. and Li, Z.X. (1994) IGCP 321Gondwana dispersion and Asian accretion: field works on Hainan Island. Episodes, v. 16, pp. 443-447. SunYunchu (1963) On the occurrence of Xystridura fauna from Middle Cambrian of Hainan Island and its significance. Acta Paleontolog. Sinica, v.11, pp. 608-611. (in Chinese with English abstract)
Vidal, G., Moczydlowska, M. a n d Rudavskaya, V.A. (1993) Biostratigraphic implications of a ChuariaTawuia assemblage and associated acritarchs from the Neoproterozoic of Yakutia. Paleontology, v.36, pp. 387-402. Yang Shufeng, Yu Zhiye, Guo Linzhi and Shi Yangsheng (1989) The division and paleomagnetism of the Hainan Island and plate tectonic significance. J. Nanjing Univ. (Earth Sci. Ed.), v.1, pp.38-46 (in Chinese with English abstract). Yu Zhiye (1989) Hainan Island: a terrane from Gondwana. 4th International Symposium on pre-Jurassic evolution of East Asia, IGCP Project 224 Reports and Abstracts, v.1, pp.11-12. Zhang Renjie, Feng Shaonan, Xu Guanghong, Yang Deli, Yan Daoping, Li Zhihong, Jiang Dahai and Wu Wei (1990) Discovery of Chuaria-Tawuia assemblage in Shilu Group, Hainan Island and its significance.Science in China (B), v.33, pp. 211-220 (in Chinese with English abstract). Zhou Zuyi, Lao Qiuyuan, Chen Huanjiang, Ding Shijiang and Liao Zhongting (1996) Early Mesozoic orogeny in Fujian, Southeast China. In: Hall, R. and Blundell, D. (Eds.),Tectonic evolution of Southeast Asia. Geol. SOC.Sp. Publ. No. 106, pp. 549-556. Zhu, X. (1987) On the evolution of Chinese continental margins and basins. Marine Geology and Quaternary Geol., v.7, pp.115-119 (in Chinese).
Gondwana Research, V. 2, No. 4,1999