Advanced Search

Indexed by SCI、CA、РЖ、PA、CSA、ZR、etc .

Volume 31 Issue 1
Jan.  2020
Article Contents
Turn off MathJax

Citation:

Geochemistry and Geochronology of Diorite in Pengshan Area of Jiangxi Province: Implications for Magmatic Source and Tectonic Evolution of Jiangnan Orogenic Belt

  • Magmatic activities associated with tectonic events play a significant role in understanding the evolution of an orogenic belt. The Jiangnan orogenic belt has been regarded as the collisional suture zone between the Yangtze Block and the Cathaysia Block. Although the magmatic activities during the period of intra-plate extension after the collision have been well studied in recent years,some remaining issues,including source nature and geodynamic mechanism,need to be further addressed. In this paper,based on a detailed field geological,petrological,geochemical and geochronological study,we focus our work on diorites in the Pengshan area located at the northwestern margin of the Jiangnan orogenic belt. The mineral assemblages are mainly composed of plagioclase (55 vol.%-65 vol.%) and hornblende (35 vol.%-45 vol.%). One diorite sample yields zircon 206Pb/238U mean age of 768±8 Ma (MSWD=0.29). The diorites have enriched large ion lithophile elements (Ba,K and Rb) and incompatible elements (Th and U),and are depleted in high field-strength elements including Ta,Ti and Nb. Diorites in this study have relatively high MgO content (6.56 wt.%-7.58 wt.%,7.07 wt.% on average) and Mg number values (65-67,65.8 on average). The diorites are metaluminous,high K calc-alkaline series rocks with high contents of K2O (1.59 wt.%-1.97 wt.%) and total alkali (Na2O+K2O=5.56 wt.%-6.05 wt.%). The Nd/Th ratio (4.34-5.27) is higher than that of crust-derived rocks and lower than mantle-derived rocks. The Rb/Sr ratio (0.19-0.22) is slightly lower than crust,but significantly higher than upper mantle. Based on the above geochemical and geochronological analyses,we suggest that the diorites in the Pengshan area were mainly derived from crustal materials with a small amount of mantle-originated materials involved,and possibly produced from an extensional tectonic setting after the collision between the Yangtze Block and Cathaysia Block.
  • 加载中
  • Arnaud, N. O., Vidal, P., Tapponnier, P., et al., 1992. The High K2O Volcanism of Northwestern Tibet: Geochemistry and Tectonic Implications. Earth and Planetary Science Letters, 111(2/3/4): 351-367. https://doi.org/10.1016/0012-821x(92)90189-3
    Bea, F., Arzamastsev, A., Montero, P., et al., 2001. Anomalous Alkaline Rocks of Soustov, Kola: Evidence of Mantle-Derived Metasomatic Fluids Affecting Crustal Materials. Contributions to Mineralogy and Petrology, 140(5): 554-566. https://doi.org/10.1007/s004100000211
    Bi, H., 1999. The Pengshan Source Structure and Its Control over Mineralization. Geology and Exploration, 9: 12-16 (in Chinese with English Abstract)
    Cheng, H., 1991. The Late Proterozoic Collision Orgen in Northwestern Zhejiang Province. Geological Review, 3: 203-213 (in Chinese with English Abstract)
    Deng, G. H., Luo, F., Song, Z. R., et al., 2003. Mapping of the Low-Grade Terrains in the Northeast of Jiangxi, the South of Anhui, Tectono-Rock Block-Strata Method. Journal of East China Geological Institute, 1: 32-37 (in Chinese with English Abstract)
    Deng, Q., Wang, Z. J., Wang, J., et al., 2016. 800-780 Ma Continental Rift Magmatism in the Eastern Part of the Jiangnan Orogen: Implications from ~790 Ma Aluminous A-Type Granites in Zhejiang-Anhui-Jiangxi Border Area. Geological Bulletin of China, 35(11): 1855-1868 (in Chinese with English Abstract)
    Dong, S. W., Zhang, Y. Q., Gao, R., et al., 2015. A Possible Buried Paleoproterozoic Collisional Orogen beneath Central South China: Evidence from Seismic-Reflection Profiling. Precambrian Research, 264: 1-10. https://doi.org/10.1016/j.precamres.2015.04.003
    Ge, W. C., Li, X. H., Li, Z. X., et al., 2001. Mafic Intrusions in Longsheng Area: Age and Its Geological Implications. Chin. J. Geol., 36: 112-118 (in Chinese with English Abstract)
    King, P. L., Chappell, B. W., Allen, C. M., et al., 2001. Are A-Type Granites the High-Temperature Felsic Granites? Evidence from Fractionated Granites of the Wangrah Suite. Australian Journal of Earth Sciences, 48(4): 501-514. https://doi.org/10.1046/j.1440-0952.2001.00881.x
    Li, X. H., 2000. Cretaceous Magmatism and Lithospheric Extension in Southeast China. Journal of Asian Earth Sciences, 18(3): 293-305. https://doi.org/10.1016/s1367-9120(99)00060-7
    Li, X. H., Li, W. X., Li, Z. X., et al., 2008. 850-790 Ma Bimodal Volcanic and Intrusive Rocks in Northern Zhejiang, South China: A Major Episode of Continental Rift Magmatism during the Breakup of Rodinia. Lithos, 102(1/2): 341-357. https://doi.org/10.1016/j.lithos.2007.04.007
    Li, X. H., Li, Z. X., Li, W. X., 2014. Detrital Zircon U-Pb Age and Hf Isotope Constrains on the Generation and Reworking of Precambrian Continental Crust in the Cathaysia Block, South China: A Synthesis. Gondwana Research, 25(3): 1202-1215. https://doi.org/10.1016/j.gr.2014.01.003
    Li, Z. X., Li, X. H., Kinny, P. D., et al., 2003. Geochronology of Neoproterozoic Syn-Rift Magmatism in the Yangtze Craton, South China and Correlations with Other Continents: Evidence for a Mantle Superplume that Broke up Rodinia. Precambrian Research, 122(1/2/3/4): 85-109. https://doi.org/10.1016/s0301-9268(02)00208-5
    Lassiter, J. C., DePaolo, D. J., 1997. Plume/Lithosphere Interaction in the Generation of Continental and Oceanic Flood Basalts: Chemical and Isotope Constraints. In: Mahoney. J., ed., Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monography 100, American Geophysical Union, Washionton DC. 335-355
    Liu, Y. S., Gao, S., Hu, Z. C., et al., 2010. Continental and Oceanic Crust Recycling-Induced Melt-Peridotite Interactions in the Trans-North China Orogen: U-Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths. Journal of Petrology, 51(1/2): 537-571. https://doi.org/10.1093/petrology/egp082
    Lou, F. S., Huang, Z. Z., Song, Z. R., et al., 2003. Geotectonic Evolution Modal of the Middle-New Proterozoic Orogenic Belt in the Central Part of South China. Geological Survey and Research, 26(4): 200-206 (in Chinese with English Abstract)
    Ma, C. X., 1989. A High-Volatile Diapiric Granite Dome in the Pengshan Area and Its Ore-Controlling Role. Geological Review, 35(2): 127-135 (in Chinese with English Abstract)
    Patiño Douce, A. E., 1997. Generation of Metaluminous A-Type Granites by Low-Pressure Melting of Calc-Alkaline Granitoids. Geology, 25(8): 743 https://doi.org/10.1130/0091-7613(1997)025 < 0743:gomatg > 2.3.co; 2 doi: 10.1130/0091-7613(1997)025<0743:gomatg>2.3.co;2
    Pearce, J. A., Cann, J. R., 1973. Tectonic Setting of Basic Volcanic Rocks Determined Using Trace Element Analyses. Earth and Planetary Science Letters, 19(2): 290-300. https://doi.org/10.1016/0012-821x(73)90129-5
    Pearce, J.A., 1996. Sources and Settings of Granitic Rocks. Episodes, 19(4): 120-125. https://doi.org/10.18814/epiiugs/1996/v19i4/005
    Polat, A., Hofmann, A. W., 2003. Alteration and Geochemical Patterns in the 3.7-3.8 Ga Isua Greenstone Belt, West Greenland. Precambrian Research, 126(3/4): 197-218. https://doi.org/10.1016/s0301-9268(03)00095-0
    Rudnick, R. L., Gao, S., 2003. Composition of the Continental Crust. Treatise on Geochemistry (Second Edition), 4: 1-51
    Shu, L. S., 2012. An Analysis of Principal Features of Tectonic Evolution in South China Block. Geological Bulletin of China, 31(7): 1035-1053 (in Chinese with English Abstract)
    Shu, L. S., Shi, Y. S., Guo, L. Z., et al., 1995. Plate Tectonic Evolution and the Kinematics of Collisional Orogeny in the Middle Jiangnan, Eastern China. Nanjing University Press, Nanjing (in Chinese with English Abstract)
    Sun, S. S., McDonough, W. F., 1989. Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes. Geological Society, London, Special Publications, 42(1): 313-345. https://doi.org/10.1144 /gsl.sp.1989.042.01.19. doi: 10.1144/gsl.sp.1989.042.01.19
    Taylor, S. R., McLennan, S. M., 1995. The Geochemical Evolution of the Continental Crust. Reviews of Geophysics, 33(2): 241. https://doi.org/10.1029/95rg00262
    Thompson, A. B., 1996. Fertility of Crustal Rocks during Anatexis. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 87(1/2): 1-10. https://doi.org/10.1017/s0263593300006428
    Wang, J. P., Kusky, T. M., Wang, L., et al., 2017. Petrogenesis and Geochemistry of Circa 2.5 Ga Granitoids in the Zanhuang Massif: Implications for Magmatic Source and Neoarchean Metamorphism of the North China Craton. Lithos, 268-271: 149-162. https://doi.org/10.1016/j.lithos.2016.10.028
    Wang, J. P., Li, X. W., Ning, W. B., et al., 2019. Geology of a Neoarchean Suture: Evidence from the Zunhua Ophiolitic Mélange of the Eastern Hebei Province, North China Craton. GSA Bulletin. https://doi.org/10.1130/b35138.1
    Wang, Q., Wyman, D. A., Li, Z. X., et al., 2010. Petrology, Geochronology and Geochemistry of ca. 780 Ma A-Type Granites in South China: Petrogenesis and Implications for Crustal Growth during the Breakup of the Supercontinent Rodinia. Precambrian Research, 178(1/2/3/4): 185-208. https://doi.org/10.1016/j.precamres.2010.02.004
    Wang, S. J., Schertl, H. P., Pang, Y. M., 2019a. Geochemistry, Geochronology and Sr-Nd-Hf Isotopes of Two Types of Early Cretaceous Granite Porphyry Dykes in the Sulu Orogenic Belt, Eastern China. Canadian Journal of Earth Sciences. https://doi.org/10.1139/cjes-2019-0003
    Wang, S. J., Li, X. P., Schertl, H. P., et al., 2019b. Petrogenesis of Early Cretaceous Andesite Dykes in the Sulu Orogenic Belt, Eastern China. Mineralogy and Petrology, 113(1): 77-97. https://doi.org/10.1007/s00710-018-0636-1
    Wang, W., Zhao, J. H., Zhou, M. F., et al., 2014. Neoproterozoic Mafic- Ultramafic Intrusions from the Fanjingshan Region, South China: Implications for Subduction-Related Magmatism in the Jiangnan Fold Belt. The Journal of Geology, 122(4): 455-473. https://doi.org/10.1086/676596
    Wang, W., Zhou, M. F., Yan, D. P., et al., 2013. Detrital Zircon Record of Neoproterozoic Active-Margin Sedimentation in the Eastern Jiangnan Orogen, South China. Precambrian Research, 235: 1-19. https://doi.org/10.1016/j.precamres.2013.05.013
    Wang, X. L., Shu, X. J., Xing, G. F., et al., 2012. LA-ICP-MS Zircon U-Pb Ages of the Shijiao-Huangshan Intrusive Rocks in Zhuji Area, Zhejiang Province: Implications for the Petrogenesis of the Ultramafic Orbicular Rocks. Geological Bulletin of China, 31(1): 75-81 (in Chinese with English Abstract)
    Wang, X. L., Zhou, J. C., Chen, X., et al., 2017. Formation and Evolution of the Jiangnan Orogen. Bulletin of Mineralogy. Petrology and Geochemistry, 5: 714-735 (in Chinese with English Abstract)
    Wang, X. L., Zhou, J. C., Qiu, J. S., et al., 2008. Geochronology and Geochemistry of Neoproterozoic Mafic Rocks from Western Hunan, South China: Implications for Petrogenesis and Post-Orogenic Extension. Geological Magazine, 145(2): 215-233. https://doi.org/10.1017/s0016756807004025
    Wang, X. X., Wang, T., Qi, Q. J., et al., 2011. Temporal Spatial Variations, Origin and Their Tectonic Significance of the Late Mesozoic Granites in the Qinling, Central China. Acta Prtrologica Sinica, 27(6): 1573-1593 (in Chinese with English Abstract)
    Wang, Y. Y., Song, C. Z., Li, J. H., et al., 2019. Deformational Characteristics and LA-ICP-MS Zircon U-Pb Ages of Granites at Shiershan in the Jiangnan Orogen and Their Geological Significance. Geological Review, 65(1): 85-102 (in Chinese with English Abstract)
    Wang, Z. J., Wang, J., Duan, T. Z., et al., 2010. Geochronology of Middle Neoproterozoic Volcanic Deposits in Yangtze Craton Interior of South China and Its Implications to Tectonic Settings. Science China Earth Science, 53: 1307-1315 (in Chinese with English Abstract) doi: 10.1007/s11430-010-4012-1
    Wu, F. Y., Li, X. H., Yang, J. H., et al., 2007. Discussions on the Petrogensis Granites. Acta Petrologica Sinica, 23(6): 1217-1238 (in Chinese with English Abstract)
    Wu, R. X., Zheng, Y. F, Wu, Y. B., 2005. Zircon U-Pb Age, Element and Oxygen Isotope Geochemisty of Neoproterozoic Granites at Shiershan in South Anhui Province. Geological Journal of China Universities, 11(3): 364-382 (in Chinese with English Abstract)
    Wu, R. X., Zheng, Y. F., Wu, Y. B., et al., 2006. Reworking of Juvenile Crust: Element and Isotope Evidence from Neoproterozoic Granodiorite in South China. Precambrian Research, 146(3/4): 179-212. https://doi.org/10.1016/j.precamres.2006.01.012
    Wu, W. G., Xie W. H., 2005. The features of Pengshan Dome Strata Bound the Dominating Magma and Mine of the Pengshan Area, Dean Country, Jiangxi. Beijing Geology, 17(1): 7-11 (in Chinese with English Abstract)
    Wu, Y. B., Zheng, Y. F., 2004. Genesis of Zircon and Its Constraints on Interpretation of U-Pb Age. Chinese Science Bulletin, 49(15): 1554-1569. https://doi.org/10.1007/bf03184122
    Xia, Y., Xu, X. S., Niu, Y. L., et al., 2018. Neoproterozoic Amalgamation between Yangtze and Cathaysia Blocks: The Magmatism in Various Tectonic Settings and Continent-Arc-Continent Collision. Precambrian Research, 309: 56-87. https://doi.org/10.1016/j.precamres.2017.02.020
    Xin, Y. J., Li, J. H., Dong, S. W., et al., 2017. Neoproterozoic Post-Collisional Extension of the Central Jiangnan Orogen: Geochemical, Geochronological, and Lu-Hf Isotopic Constraints from the Ca. 820-800 Ma Magmatic Rocks. Precambrian Research, 294: 91-110. https://doi.org/10.1016/j.precamres.2017.03.018
    Yang, F., Song, C. Z., Ren, S. L., et al., 2015. Metamorphism and Deformation of the Lushan Metamorphic Core Complex and Their Tectonic Significance. Geological Review, 61(4): 752-766 (in Chinese with English Abstract)
    Yao, J. L., Shu, L. S., Santosh, M., 2014. Neoproterozoic Arc-Trench System and Breakup of the South China Craton: Constraints from N-MORB Type and Arc-Related Mafic Rocks, and Anorogenic Granite in the Jiangnan Orogenic Belt. Precambrian Research, 247: 187-207. https://doi.org/10.1016/j.precamres.2014.04.008
    Yin, G. S., Xie, G. G., 1996. Extensional Structure and the Xingzi Metamorphic Core Complex in the Lushan Area, Jiangxi. Regional Geology of China, 1: 7-26 (in Chinese with English Abstract)
    Zhang, G. W., Guo, A. L., Wang, Y. J., et al., 2013. Tectonics of South China Continent and Its Implications. Science China Earth Sciences, 56(11): 1804-1828. https://doi.org/10.1007/s11430-013-4679-1
    Zheng, Y. F., Wu, R. X., Wu, Y. B., et al., 2008. Rift Melting of Juvenile Arc-Derived Crust: Geochemical Evidence from Neoproterozoic Volcanic and Granitic Rocks in the Jiangnan Orogen, South China. Precambrian Research, 163(3/4): 351-383. https://doi.org/10.1016/j.precamres.2008.01.004
    Zhou, J. B., Li, X. H., Ge, W. C., et al., 2007. Age and Origin of Middle Neoproterozoic Mafic Magmatism in Southern Yangtze Block and Relevance to the Break-Up of Rodinia. Gondwana Research, 12(1/2): 184-197. https://doi.org/10.1016/j.gr.2006.10.011
    Zhou, J. C., Wang, X. L., Qiu, J. S., 2009. Geochronology of Neoproterozoic Mafic Rocks and Sandstones from Northeastern Guizhou, South China: Coeval Arc Magmatism and Sedimentation. Precambrian Research, 170(1/2): 27-42. https://doi.org/10.1016/j.precamres.2008.11.002
    Zhou, J. C., Wang, X. L., Qiu, J. S., et al., 2004. Geochemistry of Meso- and Neoproterozoic Mafic-Ultramafic Rocks from Northern Guangxi, China: Arc or Plume Magmatism. Geochemical Journal, 38(2): 139-152. https://doi.org/10.2343/geochemj.38.139 (in Chinese with English Abstract)
  • 加载中
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Figures(9) / Tables(4)

Article Metrics

Article views(115) PDF downloads(37) Cited by()

Related
Proportional views

Geochemistry and Geochronology of Diorite in Pengshan Area of Jiangxi Province: Implications for Magmatic Source and Tectonic Evolution of Jiangnan Orogenic Belt

    Corresponding author: Junpeng Wang, wangjp@cug.edu.cn
  • 1. Nanjing Geological Survey Center, China Geological Survey, Nanjing 210016, China
  • 2. State Key Laboratory of Geological Processes and Mineral Resources, Hubei Key Laboratory of Critical Zone Evolution, Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
  • 3. Institute of Natural Resources and Environment Audit, Nanjing Audit University, Nanjing 211815, China
  • 4. Hebei Bureau of Geology and Mineral Resources Exploration, Shijiazhuang 050081, China

Abstract: Magmatic activities associated with tectonic events play a significant role in understanding the evolution of an orogenic belt. The Jiangnan orogenic belt has been regarded as the collisional suture zone between the Yangtze Block and the Cathaysia Block. Although the magmatic activities during the period of intra-plate extension after the collision have been well studied in recent years,some remaining issues,including source nature and geodynamic mechanism,need to be further addressed. In this paper,based on a detailed field geological,petrological,geochemical and geochronological study,we focus our work on diorites in the Pengshan area located at the northwestern margin of the Jiangnan orogenic belt. The mineral assemblages are mainly composed of plagioclase (55 vol.%-65 vol.%) and hornblende (35 vol.%-45 vol.%). One diorite sample yields zircon 206Pb/238U mean age of 768±8 Ma (MSWD=0.29). The diorites have enriched large ion lithophile elements (Ba,K and Rb) and incompatible elements (Th and U),and are depleted in high field-strength elements including Ta,Ti and Nb. Diorites in this study have relatively high MgO content (6.56 wt.%-7.58 wt.%,7.07 wt.% on average) and Mg number values (65-67,65.8 on average). The diorites are metaluminous,high K calc-alkaline series rocks with high contents of K2O (1.59 wt.%-1.97 wt.%) and total alkali (Na2O+K2O=5.56 wt.%-6.05 wt.%). The Nd/Th ratio (4.34-5.27) is higher than that of crust-derived rocks and lower than mantle-derived rocks. The Rb/Sr ratio (0.19-0.22) is slightly lower than crust,but significantly higher than upper mantle. Based on the above geochemical and geochronological analyses,we suggest that the diorites in the Pengshan area were mainly derived from crustal materials with a small amount of mantle-originated materials involved,and possibly produced from an extensional tectonic setting after the collision between the Yangtze Block and Cathaysia Block.

0.   INTRODUCTION
  • The Neoproterozoic tectono-magmatic activities in South China have been well studied, and has received much attention in the Jiangnan orogenic belt which is regarded as the collisional suture zone between the Yangtze Block and the Cathaysia Block (Fig. 1; Shu, 2012; Zhou et al., 2009, 2004; Deng et al., 2003; Lou et al., 2003; Shu et al., 1995; Cheng, 1991; Ma et al., 1989). The Neoproterozoic magmatic activities are represented by widely distributed granitic rocks and simultaneous litho- tectonic units in the study area (Fig. 1; Wang J P et al., 2019; Xin et al., 2017; Li et al., 2014; Yao et al., 2014; Shu et al., 2012; Wang X L et al., 2012; Wang Z J et al., 2010; Zheng et al., 2008). In addition, the petrogenesis and formation environment causing the Neoproterozoic tectono- magmatic events among the Jiangnan orogenic belt have been well studied. However, due to rare exposures, the tectono- magmatic events in the Pengshan area of Jiangxi Province haven't been well studied. Geochemical and geochronological analyses of granitic plutons may put significant constraints on the petrogenesis, source nature and geodynamic mechanism for different stages of tectonic evolution of the Jiangnan orogenic belt. This study presents detailed data of petrographic, geochemical and LA-ICP-MS zircon U-Pb dating of the Pengshan diorites located at the northern margin of the Jiangnan orogenic belt. The results will provide critical constraints on the formation of the magma, and also give key information for the tectonic evolution of the Jiangnan orogenic belt.

    Figure 1.  Tectonic subdivision of South China and surrounding orogenic belt, showing the major blocks including the Yangtze Block, Cathaysia Block, Jiangnan orogenic belt, Songpan-Ganzi belt, Qinling-Dabie belt and North China. The Neoproterzoic granitic plutons and age-similar litho tectonic units are shown in this figure. Sample locations are shown in this figure. Map modified after Wang et al. (2014).

1.   GEOLOGICAL BACKGROUND
  • The South China consists of the Yangtze Block, the Cathaysia Block and the Jiangnan orogenic belt (Fig. 1; Zhang et al., 2013). The Jiangnan orogenic belt is considered as the collisional suture zone between the Yangtze Block and the Cathaysia Block (Xia et al., 2018; Wang et al., 2013; Wu et al., 2006). The Neoproterozoic Banxi Group and granitic rocks, and Sinian and Quaternary sediments are distributed in the Pengshan area in the Jiangxi Province (Fig. 2; Wang Q et al., 2010). The Precambrian lithological units are mainly composed of Neoproterozoic granitic rocks, low-grade metamorphic rocks and few mafic and ultramafic rocks (Deng et al., 2016; Dong et al., 2015). The Pengshan area is located at the northern boundary of the Jiangnan orogenic belt (Figs. 1 and 2). Therefore, this area is of importance to study the collisional orogenesis of the Jiangnan orogenic belt between the Cathaysia Block and the Yangtze Block.

    Figure 2.  Regional geological map of the Pengshan area.

    The Pengshan area in the Jiangxi Province was bounded by Gushi-De'an fault and Wangyinfu-Tuolin fault in the south, Changjiang fault in the north, Guangji-Xiushui fault in the west and Jing'an-Jiujiang fault in the east (Figs. 1 and 2). All the litho-tectonic units in the Pengshan area were folded and faulted (Fig. 2). The Pengshan area has been regarded as a dome structure due to magma diapir (Yang et al., 2015; Li, 2000; Yin and Xie, 1996; Bi, 1990). Concealed granites are located under the Pengshan dome (Wu and Xie, 2005; Ma, 1989). The Pengshan diorites have been exposed in the northwestern part of the dome (outcrop location shown in Fig. 2). However, the field relationships between the dome and diorite haven't been studied in this area. In addition, further studies on the magma source and formation age of the Pengshan diorite in the Jiangnan orogenic belt should be investigated.

2.   FIELD AND PETROGRAPHIC CHARACTERISTICS OF THE DIORITE
  • The Pengshan diorite experienced weak weathering, and are exposed as dykes (Fig. 3a). The exposed diorite dyke is ca. 10 m wide and 50 m long. The layers of sandstone dip to the northwest with a dip angle of 40°, and have structural contact relationships by a fault with the Pengshan diorite (Fig. 3a). The diorite is overall texturally homogeneous. However, cumulates of hornblende (Fig. 3b) and calcite crystals (Fig. 3c) are observed in some local places within the diorite.

    Figure 3.  Field relationships between diorite and sandstone (a) showing structural contact relationship, field photographs of diorite showing the cumulates of hornblende (b) and calcite crystals (c).

    The diorites are dark gray in the field and have experienced minor alteration. The major minerals include plagioclase (55 vol.%-65 vol.%), hornblende (35 vol.%-45 vol.%) and a small amount of chlorite, apatite, and magnetite (Fig. 4a). The plagioclase, which is generally less than 0.5 mm across, mostly presents irregular to hypidiomorphic columnar grains. Some of the plagioclase has visible polysynthetic twinning between crossed polarizers (Fig. 4a). Hornblende occurs as small (0.1-0.2 mm) and irregular grains, which generally shows pleochroism from dark green to light yellow-green. Hornblende grains with hexagonal cross-sections exhibit two sets of cleavages, whose angles are about 56° and 124° (Fig. 4b).

    Figure 4.  Photomicrographs under microscope of the diorite. (a) Diorite containing Hornblende (Hbl) and Plagioclase (Pl) (cross-polarized light); (b) hornblende showing two sets of cleavages (cross-polarized light). All the photomicrographs are taken from probe thin sections (0.04 mm).

3.   SAMPLING AND ANALYTICAL METHODS
  • One sample of diorite (D2043-H1) and six samples of diorites (D2043-H1, D2043-H2, D2043-H3, D2043-H4, D2043-H5, D2043-H6) were sampled for zircon U-Pb dating and whole-rock major and trace element analyses, respectively. All the analysed samples were taken from the least weathered outcrops in different places of the study area. All the samples were collected around the GPS location of N29°28′20″, E115°36′13″ (locations of sample are shown in Fig. 2).

    We have conducted a series of analyses including zircon separation, cathodoluminescence imaging of zircon, zircon U-Pb dating and whole-rock major and trace element analyses. Zircon separation from one sample was completed at the Rock and Mineral Sorting Technical Services Company in Langfang City of Hebei Province. Cathodoluminescence (CL) photomicrographs of zircons were taken by Gatan MonoCL4+ in the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences, Wuhan. The U-Pb dating and trace element analyses of zircons were performed using the laser-ablation-inductively coupled plasma- mass spectrometer (LA-ICP-MS) at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei Province, China with specific test conditions presented in Liu et al. (2010). ICPMSDataCal and Isoplot/Ex_ver3 were used for the offline selection and integration of background and analytical signals, and time-drift correction and quantitative calibration. Whole- rock major and trace element analyses were carried out in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences by scanning X-ray fluorescence spectrometry (Shimadzu XRF- 1800) and ICP-MS (Agilent 7500a), respectively. All the analytical methods are detailedly described as the same as the ones in Wang J P et al.(2019, 2017) and Wang S J et al. (2019a).

4.   RESULTS
  • The zircons from the diorite are circa 50-100 um long and circa 50-80 um wide, ratio of long axis to short axis is from 1 : 3 to 1 : 1. Zircon grains have short-to-long prismatic shape and show internal oscillatory zoning and recrystallization structures as shown from the CL images suggesting that the zircons have a magmatic origin (Fig. 5; Wu and Zheng, 2004).

    Figure 5.  Cathodoluminescence (CL) images of zircons from the diorite in the Pengshan area.

    Table 1 shows the zircon U-Pb dating results including twelve analyzed spots from the diorite. The upper intercept age is yielded to be 771±10 Ma (Fig. 6a). The weight mean age of 206Pb/238U is 768±8 Ma (Fig. 6b). The data-point error symbols are 1σ. The rare earth element contents of zircons were analyzed during the process of dating (Table 2).

    Sample Pb(ppm) Th(ppm) U(ppm) Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ
    D2043-H1-1 12.5 58.5 59.3 0.99 0.062 6 0.003 0 1.106 5 0.058 5 0.125 6 0.002 7 0.037 9 0.001 7 694 102 756 28 763 15
    D2043-H1-2 11.1 58.9 60.2 0.98 0.066 1 0.003 3 1.136 2 0.046 6 0.125 7 0.003 0 0.039 5 0.001 4 809 104 771 22 763 17
    D2043-H1-3 30.3 372.6 380.1 0.98 0.080 4 0.002 7 1.415 3 0.051 6 0.126 4 0.002 4 0.037 1 0.000 8 1 207 66 895 22 767 14
    D2043-H1-4 32.4 161.6 168.2 0.96 0.070 9 0.002 1 1.240 2 0.036 3 0.126 5 0.001 9 0.038 4 0.001 0 967 62 819 16 768 11
    D2043-H1-5 11.6 46.8 48.6 0.96 0.073 9 0.004 7 1.299 4 0.091 9 0.125 0 0.002 5 0.042 7 0.001 9 1 039 127 845 41 759 15
    D2043-H1-6 32.6 179.9 183.8 0.98 0.068 0 0.003 0 1.188 7 0.051 5 0.126 4 0.002 8 0.037 4 0.001 3 878 97 795 24 767 16
    D2043-H1-7 33.9 163.3 213.6 0.76 0.063 4 0.001 9 1.117 3 0.034 9 0.126 6 0.002 3 0.033 6 0.001 0 720 63 762 17 768 13
    D2043-H1-8 24.1 210.4 119.1 1.77 0.066 7 0.002 0 1.179 5 0.034 2 0.127 6 0.001 9 0.036 3 0.000 9 829 68 791 16 774 11
    D2043-H1-9 63.0 599.3 292.5 2.05 0.078 8 0.002 6 1.386 7 0.044 2 0.126 3 0.002 0 0.037 7 0.001 0 1 169 67 883 19 767 12
    D2043-H1-10 151.5 327.0 1 003.7 0.33 0.073 6 0.001 9 1.273 2 0.038 0 0.124 1 0.002 0 0.037 4 0.001 4 1 029 54 834 17 754 12
    D2043-H1-11 52.1 137.0 333.8 0.41 0.064 1 0.002 1 1.135 1 0.034 5 0.127 8 0.003 0 0.038 5 0.001 5 746 69 770 16 775 17
    D2043-H1-12 108.3 336.8 687.5 0.49 0.065 6 0.001 5 1.170 8 0.027 8 0.128 3 0.001 9 0.038 2 0.001 1 794 46 787 13 778 11

    Table 1.  LA-ICP-MS zircon U-Pb data for the Pengshan diorite

    Figure 6.  Concordia of zircons (a), weighted mean 206Pb/238U age (b) and Chondrite-normalized zircon REE patterns (c) from the diorite in the Pengshan area. Normalization values from Sun and McDonough (1989).

    Sample La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
    D2043-H1-1 0.037 11.053 0.045 0.889 2.049 0.548 11.863 3.855 52.95 23.168 119.757 28.562 299.538 69.731
    D2043-H1-2 0.036 16.285 0.071 1.471 2.379 0.970 13.968 5.001 63.577 26.348 126.319 28.762 280.112 60.366
    D2043-H1-3 3.978 112.772 1.63 11.909 11.185 4.84 51.847 16.995 196.486 73.032 318.054 66.768 624.238 126.104
    D2043-H1-4 1.086 21.237 0.565 8.884 10.778 2.581 49.693 15.525 181.392 66.037 282.194 55.036 487.104 95.536
    D2043-H1-5 0 12.678 0.047 0.444 1.303 0.483 8.586 3.544 47.864 21.362 108.338 27.343 284.076 67.133
    D2043-H1-6 7.908 44.97 2.501 14.871 9.002 1.039 41.585 12.824 148.662 54.684 242.924 48.997 449.089 88.928
    D2043-H1-7 1.11 15.457 0.407 3.276 3.958 1.137 24.098 8.697 114.625 48.578 232.474 51.864 498.612 103.832
    D2043-H1-8 0.516 68.114 0.402 6.079 9.496 4.458 43.375 13.273 158.043 59.641 265.676 54.416 495.481 102.616
    D2043-H1-9 1.699 59.237 1.002 6.157 6.651 2.642 28.866 8.583 102.964 39.434 181.249 42.100 422.938 93.822
    D2043-H1-10 0.868 8.019 0.214 2.187 4.626 0.585 25.888 9.599 130.25 51.528 247.949 51.414 495.941 95.998
    D2043-H1-11 0.072 2.351 0.236 3.987 9.721 0.622 52.614 12.946 103.051 28.594 115.02 22.634 194.099 36.285
    D2043-H1-12 3.707 18.919 1.485 8.578 5.914 0.199 29.837 11.599 149.034 58.634 269.08 57.202 506.506 97.479

    Table 2.  LA-ICP-MS trace element analyses for zircons from the Pengshan diorite

  • The Pengshan diorites have a relatively narrow range of geochemical values of major elements (Table 3), including values of Al2O3 (14.90 wt.%-15.86 wt.%), SiO2 (52.20 wt.%- 54.78 wt.%), CaO (5.06 wt.%-6.49 wt.%), MgO (6.56 wt.%- 7.58 wt.%), and Fe2O3T (8.30 wt.%-8.84 wt.%). All the analyzed samples contain alkalis of Na2O (3.71 wt.%-4.14 wt.%), K2O (1.59 wt.%-1.97 wt.%), and total alkalis (ALK=K2O+ Na2O; 5.56 wt.%-6.05 wt.%) (Table 3). However, the samples have low TiO2 content of 0.93 wt.% to 1.04 wt.%. The values of aluminum saturation index (A/CNK=0.78-0.88), showing metaluminous features (Fig. 7c; Table 3). The K2O content is high which is consistent with the high K calc-alkaline series (Fig. 7b). The Mg number (Mg#=100×Mg2+/(Mg2++Fe2+)) is between 65 and 67 with an average value of 65.8.

    Sample D2043-H1 D2043-H2 D2043-H3 D2043-H4 D2043-H5 D2043-H6
    Rock Type Diorite Diorite Diorite Diorite Diorite Diorite
    SiO2 52.2 52.42 52.3 53.01 54.78 53.14
    TiO2 1.04 0.96 0.93 0.99 0.94 0.97
    Al2O3 15.69 15.77 15.79 15.86 14.9 15.64
    Fe2O3T 8.84 8.46 8.48 8.89 8.3 8.5
    MnO 0.14 0.14 0.15 0.15 0.15 0.15
    MgO 7.16 6.92 7.03 7.58 6.56 7.14
    CaO 6.49 5.9 6.41 5.06 5.42 5.71
    Na2O 3.97 3.86 4.03 4.08 3.71 4.14
    K2O 1.59 1.85 1.76 1.97 1.97 1.78
    P2O5 0.29 0.27 0.26 0.27 0.25 0.27
    LOI 2.3 2.71 2.67 2.61 2.67 2.8
    Total 99.71 99.26 99.81 100.47 99.65 100.24
    Mg# 65 66 66 67 65 66
    Na2O+K2O 5.56 5.71 5.79 6.05 5.68 5.92
    MgO+K2O 8.75 8.77 8.79 9.55 8.53 8.92
    A/NK 1.90 1.89 1.85 1.79 1.81 1.79
    A/CNK 0.78 0.83 0.78 0.88 0.82 0.82
    La 22.60 22.20 22.30 22.50 24.70 22.90
    Ce 49.80 48.00 48.00 48.10 52.30 49.70
    Pr 5.98 5.70 5.67 5.73 6.04 5.93
    Nd 24.70 22.90 23.10 23.30 24.30 24.00
    Sm 5.44 4.96 4.99 5.08 5.08 5.08
    Eu 1.46 1.28 1.33 1.29 1.36 1.34
    Gd 4.96 4.54 4.52 4.73 4.60 4.73
    Tb 0.81 0.73 0.73 0.76 0.74 0.75
    Dy 4.50 4.12 4.12 4.26 4.17 4.26
    Ho 0.93 0.85 0.84 0.87 0.83 0.88
    Er 2.67 2.42 2.39 2.51 2.44 2.50
    Tm 0.40 0.36 0.36 0.38 0.37 0.37
    Yb 2.31 2.19 2.16 2.19 2.22 2.26
    Lu 0.34 0.32 0.32 0.34 0.33 0.33
    ∑REE 126.9 120.57 120.83 122.04 129.48 125.03
    Eu/Eu* 0.84 0.81 0.84 0.79 0.84 0.82
    (La/Yb)N 7.02 7.27 7.41 7.37 7.98 7.02
    (La/Sm)N 2.68 2.89 2.89 2.86 3.14 2.91
    (Gd/Yb)N 1.78 1.71 1.73 1.79 1.71 1.73
    Ce/Ce* 1.03 1.02 1.02 1.01 1.02 1.02
    Ti 6 233.23 5 753.75 5 573.95 5 933.55 5 633.88 5 813.68
    Li 56.40 63.70 58.00 44.00 62.70 62.80
    Be 1.35 1.38 1.38 1.24 1.57 1.42
    Sc 18.00 18.90 18.30 26.80 16.80 16.70
    V 187 178 179 195 163 173
    Cr 404 414 418 449 358 402
    Co 37.30 36.10 35.90 38.90 34.90 36.70
    Ni 113 110 110 128 110 120
    Cu 41.70 23.6 20.6 22.6 39.9 17.5
    Zn 82.20 81.80 82.20 85.30 81.60 84.90
    Ga 19.1 18.9 18.7 18.9 19.0 19.1
    Rb 48.20 62.50 57.80 76.70 70.00 57.70
    Sr 250 317 293 399 314 286
    Y 24.40 22.10 21.90 23.80 22.30 22.50
    Zr 120 124 122 121 136 129
    Nb 10.50 10.20 9.92 9.92 11.20 11.00
    Sn 1.38 1.36 1.39 1.33 1.15 1.28
    Cs 1.31 1.60 1.68 2.23 1.60 1.43
    Ba 428 610 545 835 598 530
    Hf 3.51 3.53 3.50 3.47 3.78 3.67
    Ta 0.70 0.68 0.67 0.66 0.73 0.71
    Tl 0.30 0.35 0.30 0.41 0.36 0.31
    Pb 5.28 6.94 7.64 5.16 15.70 5.34
    Th 4.69 4.90 4.79 5.37 5.49 4.88
    U 1.12 1.14 1.11 1.05 1.20 1.13
    Rb/Sr 0.19 0.20 0.20 0.19 0.22 0.20
    Ga/Al 2.30 2.26 2.24 2.70 2.41 2.31
    Y/Yb 10.56 10.09 10.14 10.87 10.05 9.96
    Note: Major element values are in wt.%. Trace element values are in ppm. LOI=loss on ignition; ALK=K2O+Na2O; A/CNK=molar[Al2O3/(CaO+Na2O+K2O); A/NK=molar[Al2O3/(Na2O+K2O)].

    Table 3.  Whole-rock major and trace elements analyses of the Pengshan diorite

    Figure 7.  Na2O+K2O vs. SiO2 (a), K2O vs. SiO2 (b) and A/NK vs. A/CNK (c) diagrams of the Pengshan diorites. A/NK=Al/(Na+K) (molar ratio). A/CNK=Al/(Ca+Na+K) (molar ratio). Abbreviations of different numbers: 1. Olivine gabbro; 2. alkaline gabbro; 3. subalkaline gabbro; 4. diorite; 5. granodiorite; 6: granite; 7. quartzolite; 8. monzogabbro; 9. monzodiorite; 10. monzonite; 11. quartz monzonite; 12. syenite; 13. feldspathoid gabbro; 14. feldspathoid monzodiorite; 15. feldspathoid monzosyenite; 16. feldspathoid syenite; 17. feldspathoid plutonite; 18. tawite.

    The Pengshan diorites have a relatively narrow range of trace elements with 18.7 ppm-19.1 ppm Ga, 48.2 ppm-76.7 ppm Rb, 120 ppm-136 ppm Zr, 9.92 ppm-11.20 ppm Nb, 21.9 ppm-24.4 ppm Y (Table 3). The total REEs contents of the Pengshan diorites are low (Fig. 8), and show narrow variations (ΣREE=120.6 ppm−129.5 ppm, average value =124.1 ppm). The Pengshan diorites have enriched light REEs and low heavy REEs patterns, and weak negative Eu anomalies (Eu/Eu*=0.79- 0.84) (Fig. 8a) as shown in the chondrite-normalized diagram. The Pengshan diorites have distinctly negative anomalies of Ti, Nb, Ta and Sr and positive Pb anomalies as described in the primitive mantle-normalized figure (Fig. 8b).

    Figure 8.  Chondrite-normalized REE distribution patterns (a) and primitive mantle-normalized spidergrams (b) of the Pengshan diorites. The data of lower continental crust values from Rudnick and Gao (2003). The data of N-MORB, E-MORB and OIB and normalization values from Sun and McDonough (1989).

5.   DISCUSSION
  • The collected zircons from the Pengshan diorite are characterized by variably high Th/U ratios (> 4, Table 1) and oscillatory zoning textures (Fig. 5). The zircons have narrow ranges of Pb contents (11.1-151.5 ppm), fractionated REE patterns, and negative and positive anomalies for Eu and Ce, respectively (Fig. 6c), indicating that the zircons are magmatic (Wu and Zheng, 2004). The upper intercept age is 771±10 Ma (MSWD= 0.27) (Fig. 6a). The weighted mean 206Pb/238U age is 768±8 Ma (MSWD=0.29) (Fig. 6b), which is similar to the upper intercept age of 771±10 Ma. Therefore, we propose the weighted mean 206Pb/238U age of 768 ± 8 Ma as the formation age of the diorite in the Pengshan area of the Jiangnan orogenic belt.

  • It is very significant to evaluate the sample alteration and element mobility in order to efficiently assess the petrogenesis and tectonic setting of the analyzed samples. The geochemical analyses were conducted on six samples of the Pengshan diorites. All the samples are fresh with the least weathered surfaces. The zircons CL images show lack of metamorphic rims (Fig. 5), indicating low effect from latter metamorphic events. Furthermore, all the analyzed samples have low values of loss on ignition (LOI) (< 3 wt.%). (Table 3). All the above features suggest that the samples experienced very weak alteration (Polat and Hofmann, 2003). Figure 5 shows that the trace elements have relatively uniform distribution pattern, indicating that those trace elements are immobile. Therefore, the geochemical analysis in this study can be applied to discuss the formation environment and petrogenesis of the Pengshan diorite.

  • It is generally considered that the genesis of dioritic magma includes partial melting of mantle wedge peridotite metasomatized by aqueous fluid, AFC (assimilation and fractional crystallization) process of mantle-derived magma and partial melting of crustal material (Wang et al., 2019b; Thompson., 1996; Arnaud et al., 1992). Diorites in this study have relatively high MgO content (6.56 wt.%-7.58 wt.%, 7.07 on average) and Mg values (65-67, 65.8 on average). In addition, a small amount of pyroxene can be seen under the microscope. These evidences suggest that mantle material was involved in the formation of the diorites. The diorites are metaluminous, high K calc-alkaline series rocks with high contents of K2O (1.59 wt.%-1.97 wt.%) and total alkali (Na2O+K2O=5.56 wt.%-6.05 wt.%). The diorites have Nd/Th ratio of 4.34-5.27 (4.74 on average), which is higher than that of crust-derived rocks (Nd/Th≈3, Bea et al., 2001) and lower than mantle-derived rocks (Nd/Th > 15, Bea et al., 2001). The Rb/Sr ratio (0.19-0.22, 0.20 on average) in Pengshan diorite is slightly lower than crust (0.40, Taylor and McLennan, 1995), but significantly higher than upper mantle (0.03), which indicates that the crust-derived materials make major contribution to the formation of Pengshan diorite, and there should be a small amount of mantle- sourced materials involved (Wang et al., 2011; King et al., 2001; Patiño Dounce, 1997; Sun and McDonough, 1989).

    If the crustal components of Pengshan dioritic magma are derived from AFC (assimilation and fractional crystallization) process during the magma rise, their Nb/La and La/Sm ratios will increase, which will approach the ratios of crust (Nb/La > 0.5, La/Sm > 5.0; Lassiter and Depaolo, 1997). However, the Nb/La and La/Sm ratios in this study are 0.44-0.48 and 4.15-4.86, respectively, and do not show a positive correlation. Thus, it is unlikely that the crust-derived components came from the contamination of the surrounding rocks.

    Diorites are characterized by right-declined REE pattern with weak Eu anomalies (0.79-0.84) and Ce anomalies (1.01-1.03). The enriched LILE (eg., Ba, K), and LREE (La/SmN=2.68-3.14), depleted HFSE (eg., Ta, Ti) in the Pengshan diorite, similar to the typical island arc rocks, indicate that they were possibly produced at an arc-related subduction environment (Wang et al., 2019a). In the Zr versus Ti diagram (Pearce and Cann, 1973), they all plot in the field of volcanic arc setting (Fig. 9a). In the Y+Nb versus Rb diagram (Pearce, 1996), these samples fall into the VAG (volcanic arc granite) field and show the features of post-collisional granite (Fig. 9b). Collectively, we suggest that Pengshan diorite was possibly formed in an extensional environment. The mantle-sourced rocks have been modified by crust-derived fluid/melt, generating an enriched mantle source. During the Neoproterozoic, the metasomatized mantle melted due to extension of the Jiangnan orogenic belt and produced these rocks with both crustal- and mantle-like features (Wang et al., 2011; King et al., 2001; Patiño Dounce, 1997; Sun and McDonough, 1989).

    Figure 9.  Tectonic discrimination of the Pengshan diorites ((a) after Pearce and Cann, 1973; (b) after Pearce, 1996). Syn-COLG. syn-collisional granite; Post-COLG. post-collisional granite; WPG. intra plate granite; VAG. volcanic arc granite; ORG. ocean ridge granite.

  • There are many felsic and mafic rocks that have similar age to Pengshan diorite (Table 4) in the Jiangnan orogenic belt. In general, most of the acidic magmatic rocks have characteristics of A-type granite (Deng et al., 2016). They have generally high content of SiO2, low content of Al2O3, MgO, CaO, P2O5 and A/CNK values (1.1-1.6). The trace elements are enriched in REE, Th, Zr and Gd, but depleted in Nb, Ta, Ti, Sr and P. The distribution of chondrite-normalized REE contents of these granites show a slight enrichment in LREEs and a flat HREEs pattern, with negative Eu anomalies and weak differentiation between LREE and HREE. These studies suggest that the formation of A-type granitic rocks is related to the extensional tectonic setting (Wu et al., 2007). The basic rocks in the study area are characterized by MORB and intraplate basalts (Deng et al., 2016), which reveals the post-orogenic extensional setting (Deng et al., 2016; Wang et al., 2008; Li et al., 2003).

    Rock Units GPS location Age Method References
    Longitude (E) Latitude (N) (Ma)
    Lingshan granite porphyries 118°28′48″ 29°28′48″ 791.8±2.6 LA-ICP-MS Deng et al., 2016
    Shiershan granite 118°22′8″ 29°23′58″ 776±6 LA-ICP-MS Wu et al., 2005
    118°28′59″ 29°52′53″ 778±11 LA-ICP-MS Wu et al., 2005
    118°33′19″ 29°55′31″ 777±11 LA-ICP-MS Wu et al., 2005
    118°19′38″ 29°32′10″ 779±11 Shrimp Li et al., 2003
    Daolinshan granite 120°05′33″ 29°49′56″ 794±9 Shrimp Li et al., 2008
    120°13′26″ 29°50′9″ 792.0±5.5 LA-ICP-MS Yao et al., 2014
    Lianhuashan granite 120°22′8″ 29°23′58″ 771±17 Shrimp Zheng et al., 2008
    120°33′19″ 29°55′31″ 777±7 LA-ICP-MS Zheng et al., 2008
    Longsheng gabbro Unknow 761±8 TIMS Ge et al., 2001
    Tongdao ultramafic rocks Unknow 772±11 Shrimp Wang et al., 2008
    Guzhang dolerite 109°49′6.5″ 28°29′41.8″ 768±28 LA-ICP-MS Zhou et al., 2007

    Table 4.  Neoproterozoic igneous rocks from the Jiangnan orogen

    All the Pengshan doirite samples fall into the post- collisional granite field as shown in the the diagram of Rb/(Y+Nb) (Fig. 9b), which were produced from an extensional environment (Pearce, 1996). The cumulated minerals of hornblende (Fig. 3b) in the Pengshan diorite suggest they were formed in a water-rich environment, which is consistent with post-collisional setting. The Jiangnan orogenic belt has experienced four main stages of tectonic evolution including ocean-ocean subduction (970-880 Ma), arc-continent collision (880-860 Ma), back-arc basin deposition (860-825 Ma) and intra-plate extensional rifting (after 810 Ma) (Wang X L et al., 2017). The formation age of the Pengshan diorite is 768±8 Ma, which falls into the stage of intra-plate rifting. Combined with the geochemical characteristics of the Pengshan diorites, we suggest that the Pengshan diorites represent the magmatic activities after the final collision between the Yangtze and Cathaysia blocks to form the Jiangnan orogenic belt. Therefore, the Pengshan diorites are suggested to be produced by the extensional rifting event.

6.   CONCLUSIONS
  • (1) Whole-rock major and trace elements data show that the Pengshan diorites fall into high-K calc-alkaline series and have metaluminous features.

    (2) The diorites in the Pengshan area have zircon weighted mean 206Pb/238U age of 768±8 Ma, and may have been mainly derived from crustal materials with a small amount of mantle- originated materials involved.

    (3) The diorites were possibly produced by an extension event following the collage to form the Jiangnan orogenic belt.

ACKNOWLEDGMENTS
  • This study was supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences, Wuhan (CUGW) (Nos. CUGCJ1707 and CUGL180406), NNSFC fund (No. 41602234), China Geological Survey's projects (Nos. DD20160036 and DD20190261) and Open Fund from the State Key Laboratory of Geological Processes and Mineral Resources, CUGW (No. GRMR20 1901). We thank the four anonymous reviewers for their comments that greatly assisted in improving the manuscript. Editors Shuhua Wang and Ge Yao are thanked for the hard editing work. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-0875-z.

Reference (55)

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return