Advanced Search

Indexed by SCI、CA、РЖ、PA、CSA、ZR、etc .

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

Citation:

Petrogenesis, Tectonic Evolution and Geothermal Implications of Mesozoic Granites in the Huangshadong Geothermal Field, South China

  • Mesozoic multi-stage tectono-magmatic events produced widely distributed granitoids in the South China Block. Huangshadong (HSD) is located in south-eastern South China Block, where closely spaced hot springs accompany outcrops of Mesozoic granites. New data on whole-rock geochemistry, zircon U-Pb geochronology, and zircon Lu-Hf isotopes are presented, to study the petrogenesis and tectonic evolution of the granites, and to explore the relationship between granites and geothermal anomalies. Zircon U-Pb isotopes display three periods of granites in the HSD area:Indosinian (ca. 253 Ma, G4) muscovite-bearing monzonitic granite, early Yanshanian (ca. 175-155 Ma, G5 and G3) monzonitic granite and granodiorite, and late Yanshanian (ca. 140 Ma, G1 and G2) biotite monzonitic granite. In petrogenetic type, granites of the three periods are Ⅰ-type granite. Among them, G1, G2, G3, and G4 are characterized by high fractionation, with high values of SiO2, alkalis, Ga/Al, and Rb/Sr, and depletion in Sr, Ba, Zr, Nb, Ti, REEs, with low (La/Yb)N, Nb/Ta, and Zr/Hf ratios and negative Eu anomalies. In terms of tectonic setting, 253 Ma G4 may be the product of partial melting of the ancient lower crust under post-orogenic extensional tectonics, as the closure of the Paleo-Tethys Ocean resulted in an intracontinental orogeny. At 175 Ma, the subduction of the Pacific Plate became the dominant tectonic system, and low-angle subduction of the Paleo-Pacific Plate facilitated partial melting of the subducted oceanic crust and basement to generate the hornblende-bearing Ⅰ-type granodiorite. As the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to back-arc extension, inducing intense partial melting of the lower crust at ca. 158 Ma and resulting in the most frequent granitic magmatic activity in the South China hinterland. When slab foundering occurred at ca. 140 Ma, underplating of mantle-derived magmas caused melting of the continental crust, generating extensive highly fractionated granites in HSD. Combining the granitic evolution of HSD and adjacent areas and radioactive heat production rates, it is suggested that highly fractionated granites are connected to the enrichments in U and Th with magma evolution. The high radioactive heat derived from the Yanshanian granites is an important part of the crustal heat, which contributes significantly to the terrestrial heat flow. Drilling ZK8 reveals deep, ca. 140 Ma granite, which implies the heat source of the geothermal anomalies is mainly the concealed Yanshanian granites, combining the granite distribution on the surface.
  • 加载中
  • Arth, J. G., 1976. Behaviour of Trace Elements during Magmatic Processes— A Summary of Theoretical Models and Their Applications. Journal of Research of the U.S. Geological Survey, 4: 41-47
    Ballouard, C., Poujol, M., Boulvais, P., et al., 2016. Nb-Ta Fractionation in Peraluminous Granites: A Marker of the Magmatic-Hydrothermal Transition. Geology, 44(3): 231-234. https://doi.org/10.1130/g37475.1
    Bao, Z. W., Zhao, Z. H., 2003. Geochemistry and Tectonic Setting of the Fugang Aluminous A-Type Granite, Guangdong Province, China—A Preliminary Study. Geology-Geochemistry, 31(1): 52-61 (in Chinese with English Abstract)
    Bau, M., 1996. Controls on the Fractionation of Isovalent Trace Elements in Magmatic and Aqueous Systems: Evidence from Y/Ho, Zr/Hf, and Lanthanide Tetrad Effect. Contributions to Mineralogy and Petrology, 123(3): 323-333. https://doi.org/10.1007/s004100050159
    Bea, F., 1996. Residence of REE, Y, Th and U in Granites and Crustal Protoliths: Implications for the Chemistry of Crustal Melts. Journal of Petrology, 37(3): 521-552. https://doi.org/10.1093/petrology/37.3.521
    Bonnetti, C., Liu, X. D., Mercadier, J., et al., 2018. The Genesis of Granite-Related Hydrothermal Uranium Deposits in the Xiazhuang and Zhuguang Ore Fields, North Guangdong Province, SE China: Insights from Mineralogical, Trace Elements and U-Pb Isotopes Signatures of the U Mineralisation. Ore Geology Reviews, 92: 588-612. https://doi.org/10.1016/j.oregeorev.2017.12.010
    Chappell, B. W., 1999. Aluminium Saturation in I- and S-Type Granites and the Characterization of Fractionated Haplogranites. Lithos, 46(3): 535-551. https://doi.org/10.1016/s0024-4937(98)00086-3
    Chappell, B. W., Bryant, C. J., Wyborn, D., et al., 1998. High- and Low-Temperature Ⅰ-Type Granites. Resource Geology, 48(4): 225-235. https://doi.org/10.1111/j.1751-3928.1998.tb00020.x
    Chappell, B. W., White, A. J. R., 2001. Two Contrasting Granite Types: 25 Years Later. Australian Journal of Earth Sciences, 48(4): 489-499. https://doi.org/10.1046/j.1440-0952.2001.00882.x
    Chen, J. Y., Yang, J. H., 2015. Petrogenesis of the Fogang highly Fractionated Ⅰ-Type Granitoids: Constraints from Nb, Ta, Zr and Hf. Acta Petrologica Sinica, 31: 846-854 (in Chinese with English Abstract)
    Chen, C. H., Lee, C. Y., Shinjo, R., 2008. Was there Jurassic Paleo-Pacific Subduction in South China? Constraints from 40Ar/39Ar Dating, Elemental and Sr-Nd-Pb Isotopic Geochemistry of the Mesozoic Basalts. Lithos, 106(1/2): 83-92. https://doi.org/10.1016/j.lithos.2008.06.009
    Chen, J. F., Jahn, B. M., 1998. Crustal Evolution of Southeastern China: Nd and Sr Isotopic Evidence. Tectonophysics, 284(1/2): 101-133. https://doi.org/10.1016/s0040-1951(97)00186-8
    Chen, L., Zhao, Z. F., Zheng, Y. F., 2014. Origin of Andesitic Rocks: Geochemical Constraints from Mesozoic Volcanics in the Luzong Basin, South China. Lithos, 190/191: 220-239. https://doi.org/10.1016/j.lithos.2013.12.011
    Deering, C. D., Bachmann, O., 2010. Trace Element Indicators of Crystal Accumulation in Silicic Igneous Rocks. Earth and Planetary Science Letters, 297(1/2): 324-331. https://doi.org/10.1016/j.epsl.2010.06.034
    Deng, J. F., Mo, X. X., Zhao, H. L., et al., 1999. The Yanshanian Lithosphere- Asthenosphere Catastrophe and Metallogenic Environment in East China. Mineral Deposits, 18(4): 309-311 (in Chinese with English Abstract)
    Dill, H. G., 2015. Pegmatites and Aplites: Their Genetic and Applied Ore Geology. Ore Geology Reviews, 69: 417-561. https://doi.org/ 10.1016/j.oregeorev.2015.02.022 doi: 10.1016/j.oregeorev.2015.02.022
    Ding, X., Hu, Y. H., Zhang, H., et al., 2013. Major Nb/Ta Fractionation Recorded in Garnet Amphibolite Facies Metagabbro. The Journal of Geology, 121(3): 255-274. https://doi.org/10.1086/669978
    Dostal, J., Kontak, D. J., Gerel, O., et al., 2015. Cretaceous Ongonites (Topaz- Bearing Albite-Rich Microleucogranites) from Ongon Khairkhan, Central Mongolia: Products of Extreme Magmatic Fractionation and Pervasive Metasomatic Fluid: Rock Interaction. Lithos, 236/237: 173-189. https://doi.org/10.1016/j.lithos.2015.08.003
    Frindt, S., Trumbull, R. B., Romer, R. L., 2004. Petrogenesis of the Gross Spitzkoppe Topaz Granite, Central Western Namibia: A Geochemical and Nd-Sr-Pb Isotope Study. Chemical Geology, 206(1/2): 43-71. https://doi.org/10.1016/j.chemgeo.2004.01.015
    Gao, P., Zheng, Y. F., Zhao, Z. F., 2016. Distinction between S-Type and Peraluminous Ⅰ-Type Granites: Zircon versus Whole-Rock Geochemistry. Lithos, 258/259: 77-91. https://doi.org/10.1016/j.lithos.2016.04.019
    Gelman, S. E., Deering, C. D., Bachmann, O., et al., 2014. Identifying the Crystal Graveyards Remaining after Large Silicic Eruptions. Earth and Planetary Science Letters, 403: 299-306. https://doi.org/10.1016/j.epsl.2014.07.005
    Gilder, S. A., Gill, J., Coe, R. S., et al., 1996. Isotopic and Paleomagnetic Constraints on the Mesozoic Tectonic Evolution of South China. Journal of Geophysical Research: Solid Earth, 101(B7): 16137-16154. https://doi.org/10.1029/96jb00662
    Green, T. H., 1995. Significance of Nb/Ta as an Indicator of Geochemical Processes in the Crust-Mantle System. Chemical Geology, 120(3/4): 347-359. https://doi.org/10.1016/0009-2541(94)00145-x
    Gu, H. L., Yang, X. Y., Deng, J. H., et al., 2017. Geochemical and Zircon U-Pb Geochronological Study of the Yangshan A-Type Granite: Insights into the Geological Evolution in South Anhui, Eastern Jiangnan Orogen. Lithos, 284/285: 156-170. https://doi.org/10.1016/j.lithos.2017.04.007
    Guangdong Geological Bureau, 1982. Regional Hydrogeological Survey Report of the Peopleʼs Republic of China (1 : 200 000 Huiyang F-50-(7)). Geological Publishing House, Beijing (in Chinese)
    Hacker, B. R., Ratschbacher, L., Webb, L., et al., 1998. U/Pb Zircon Ages Constrain the Architecture of the Ultrahigh-Pressure Qinling-Dabie Orogen, China. Earth and Planetary Science Letters, 161(1/2/3/4): 215-230. https://doi.org/10.1016/s0012-821x(98)00152-6
    Halliday, A. N., Davidson, J. P., Hildreth, W., et al., 1991. Modelling the Petrogenesis of High Rb/Sr Silicic Magmas. Chemical Geology, 92(1/2/3): 107-114. https://doi.org/10.1016/0009-2541(91)90051-r
    Hasterok, D., Chapman, D. S., 2011. Heat Production and Geotherms for the Continental Lithosphere. Earth and Planetary Science Letters, 307(1/2): 59-70. https://doi.org/10.1016/j.epsl.2011.04.034
    Hawkesworth, C. J., Kemp, A. I. S., 2006. Using Hafnium and Oxygen Isotopes in Zircons to Unravel the Record of Crustal Evolution. Chemical Geology, 226(3/4): 144-162. https://doi.org/10.1016/j.chemgeo.2005.09.018
    Hou, J. C., Cao, M. C., Liu, P. K., 2018. Development and Utilization of Geothermal Energy in China: Current Practices and Future Strategies. Renewable Energy, 125: 401-412. https://doi.org/10.1016/j.renene.2018.02.115
    Hu, S. B., Wang, J. Y., 1994. Crustal Heat Generation Rate and Mantle Heat Flow in Southeastern China. Science in China (Series B), 24(2): 185-193 (in Chinese)
    Huang, J., Ren, J., Jiang, C., et al., 1980. The Geotectonic Evolution of China. Science Press, Beijing. 1-124 (in Chinese)
    Huang, J., Xiao, Y., Gao, Y., et al., 2012. Nb-Ta Fractionation Induced by Fluid-Rock Interaction in Subduction-Zones: Constraints from UHP Eclogite- and Vein-Hosted Rutile from the Dabie Orogen, Central- Eastern China. Journal of Metamorphic Geology, 30(8): 821-842. https://doi.org/10.1111/j.1525-1314.2012.01000.x
    Huang, L. C., Jiang, S. Y., 2014. Highly Fractionated S-Type Granites from the Giant Dahutang Tungsten Deposit in Jiangnan Orogen, Southeast China: Geochronology, Petrogenesis and Their Relationship with W-Mineralization. Lithos, 202/203: 207-226. https://doi.org/10.1016/j.lithos.2014.05.030
    Huang, S. P., 2012. Geothermal Energy in China. Nature Climate Change, 2(8): 557-560. https://doi.org/10.1038/nclimate1598
    Huang, S. P., 2014. Opportunities and Challenges of Geothermal Energy Development in China. Energy of China, 36(9): 4-8, 16 (in Chinese with English Abstract)
    Jahn, B. M., 1974. Mesozoic Thermal Events in Southeast China. Nature, 248(5448): 480-483. https://doi.org/10.1038/248480a0
    Jahn, B. M., Wu, F. Y., Capdevila, R., et al., 2001. Highly Evolved Juvenile Granites with Tetrad REE Patterns: The Woduhe and Baerzhe Granites from the Great Xingʼan Mountains in NE China. Lithos, 59(4): 171-198. https://doi.org/10.1016/s0024-4937(01)00066-4
    Ji, W. B., Lin, W., Faure, M., et al., 2017. Origin of the Late Jurassic to Early Cretaceous Peraluminous Granitoids in the Northeastern Hunan Province (Middle Yangtze Region), South China: Geodynamic Implications for the Paleo-Pacific Subduction. Journal of Asian Earth Sciences, 141: 174-193. https://doi.org/10.1016/j.jseaes.2016.07.005
    Jiang, S. H., Bagas, L., Hu, P., et al., 2016. Zircon U-Pb Ages and Sr-Nd-Hf Isotopes of the Highly Fractionated Granite with Tetrad REE Patterns in the Shamai Tungsten Deposit in Eastern Inner Mongolia, China: Implications for the Timing of Mineralization and Ore Genesis. Lithos, 261: 322-339. https://doi.org/10.1016/j.lithos.2015.12.021
    Jiang, X. Y., Luo, J. C., Guo, J., et al., 2018. Geochemistry of I- and A-Type Granites of the Qingyang-Jiuhuashan Complex, Eastern China: Insights into Early Cretaceous Multistage Magmatism. Lithos, 316/317: 278-294. https://doi.org/10.1016/j.lithos.2018.07.025
    Kelkar, S., WoldeGabriel, G., Rehfeldt, K., 2016. Lessons Learned from the Pioneering Hot Dry Rock Project at Fenton Hill, USA. Geothermics, 63: 5-14. https://doi.org/10.1016/j.geothermics.2015.08.008
    Kinny, P. D., 2003. Lu-Hf and Sm-Nd Isotope Systems in Zircon. Reviews in Mineralogy and Geochemistry, 53(1): 327-341. https://doi.org/10.2113/0530327
    Kostitsyn, Y. A., 2001. Sources of Rare Metals in Peraluminous Granites: A Review of Geochemical and Isotopic Data. Geochemistry International, 39: 43-59
    Li, J., Huang, X. L., 2013. Mechanism of Ta-Nb Enrichment and Magmatic Evolution in the Yashan Granites, Jiangxi Province, South China. Acta Petrologica Sinica, 29(12): 4311-4322 (in Chinese with English Abstract)
    Li, J. H., Zhang, Y. Q., Zhao, G. C., et al., 2017. New Insights into Phanerozoic Tectonics of South China: Early Paleozoic Sinistral and Triassic Dextral Transpression in the East Wuyishan and Chencai Domains, NE Cathaysia. Tectonics, 36(5): 819-853. https://doi.org/10.1002/2016tc004461
    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., 1997. Timing of the Cathaysia Block Formation: Constraints from SHRIMP U-Pb Zircon Geochronology. Episodes, 20(3): 188-192 doi: 10.18814/epiiugs/1997/v20i3/008
    Li, X. H., Li, W. X., Wang, X. C., et al., 2009. Role of Mantle-Derived Magma in Genesis of Early Yanshanian Granites in the Nanling Range, South China: In situ Zircon Hf-O Isotopic Constraints. Science in China Series D: Earth Sciences, 52(9): 1262-1278. https://doi.org/10.1007/s11430-009-0117-9
    Li, X. H., Li, Z. X., Li, W. X., et al., 2007. U-Pb Zircon, Geochemical and Sr-Nd-Hf Isotopic Constraints on Age and Origin of Jurassic I- and A-Type Granites from Central Guangdong, SE China: A Major Igneous Event in Response to Foundering of a Subducted Flat-Slab?. Lithos, 96(1/2): 186-204. https://doi.org/10.1016/j.lithos.2006.09.018
    Li, Z. L., Zhou, J., Mao, J. R., et al., 2013. Zircon U-Pb Geochronology and Geochemistry of Two Episodes of Granitoids from the Northwestern Zhejiang Province, SE China: Implication for Magmatic Evolution and Tectonic Transition. Lithos, 179: 334-352. https://doi.org/10.1016/j.lithos.2013.07.014
    Li, Z. X., Li, X. H., 2007. Formation of the 1 300-km-Wide Intracontinental Orogen and Postorogenic Magmatic Province in Mesozoic South China: A Flat-Slab Subduction Model. Geology, 35(2): 179-182. https://doi.org/10.1130/g23193a.1
    Lin, W. J., Gan, H. N., Wang, G. L., et al., 2016. Occurrence Prospect of HDR and Target Site Selection Study in Southeastern Coast of China. Acta Geologica Sinica, 90(8): 2043-2058 (in Chinese with English Abstract)
    Lin, W. J., Liu, Z. M., Wang, W. L., et al., 2013. The Assessment of Geothermal Resources Potential of China. Geology in China, 40: 312-321 (in Chinese with English Abstract)
    Lin, L. F., Sun, Z. D., Wang, D., et al., 2017. Radioactive Geochemistry of Mesozoic Granitic from Nanling Region and Southeast Coastal Region and Their Constraints on Lithospheric Thermal Structure. Acta Petrologica et Mineralogica, 36(4): 488-500 (in Chinese with English Abstract)
    Linnen, R. L., Keppler, H., 2002. Melt Composition Control of Zr/Hf Fractionation in Magmatic Processes. Geochimica et Cosmochimica Acta, 66(18): 3293-3301. https://doi.org/10.1016/s0016-7037(02)00924-9
    Liu, S. A., Li, S. G., He, Y. S., et al., 2010. Geochemical Contrasts between Early Cretaceous Ore-Bearing and Ore-Barren High-Mg Adakites in Central-Eastern China: Implications for Petrogenesis and Cu-Au Mineralization. Geochimica et Cosmochimica Acta, 74(24): 7160-7178. https://doi.org/10.1016/j.gca.2010.09.003
    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
    Liu, Y. S., Hu, Z. C., Gao, S., et al., 2008. In situ Analysis of Major and Trace Elements of Anhydrous Minerals by LA-ICP-MS without Applying an Internal Standard. Chemical Geology, 257(1/2): 34-43. https://doi.org/10.1016/j.chemgeo.2008.08.004
    London, D., Evensen, J. M., 2002. Beryllium in Silicic Magmas and the Origin of Beryl-Bearing Pegmatites. Reviews in Mineralogy and Geochemistry, 50(1): 445-486. https://doi.org/10.2138/rmg.2002.50.11
    Lu, S. M., 2018. A Global Review of Enhanced Geothermal System (EGS). Renewable and Sustainable Energy Reviews, 81: 2902-2921. https://doi.org/10.1016/j.rser.2017.06.097
    Lund, J. W., 2008. Development and Utilization of Geothermal Resources. Episodes, 31(1): 140-147 doi: 10.18814/epiiugs/2008/v31i1/019
    Maniar, P. D., Piccoli, P. M., 1989. Tectonic Discrimination of Granitoids. Geological Society of America Bulletin, 101(5): 635-643. https://doi.org/10.1130/0016-7606(1989)101 < 0635:tdog > 2.3.co; 2 doi: 10.1130/0016-7606(1989)101<0635:tdog>2.3.co;2
    Mao, J. R., Li, Z. L., Ye, H. M., 2014. Mesozoic Tectono-Magmatic Activities in South China: Retrospect and Prospect. Science China Earth Sciences, 57(12): 2853-2877. https://doi.org/10.1007/s11430-014-5006-1
    Mao, X. P., Wang, X. W., Li, K. W., et al., 2018. Sources of Heat and Control Factors in Geothermal Field. Earth Science, 43(11): 4256-4266 (in Chinese with English Abstract)
    McLaren, S., Sandiford, M., Powell, R., et al., 2006. Palaeozoic Intraplate Crustal Anatexis in the Mount Painter Province, South Australia: Timing, Thermal Budgets and the Role of Crustal Heat Production. Journal of Petrology, 47(12): 2281-2302. https://doi.org/10.1093/petrology/egl044
    Merino, E., Villaseca, C., Orejana, D., et al., 2013. Gahnite, Chrysoberyl and Beryl Co-occurrence as Accessory Minerals in a Highly Evolved Peraluminous Pluton: The Belvís de Monroy Leucogranite (Cáceres, Spain). Lithos, 179: 137-156. https://doi.org/10.1016/j.lithos.2013.08.004
    Pearce, J. A., 1996. Sources and Settings of Granitic Rocks. Episodes, 19: 120-125 doi: 10.18814/epiiugs/1996/v19i4/005
    Pearce, J. A., Harris, N. B. W., Tindle, A. G., 1984. Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks. Journal of Petrology, 25(4): 956-983. https://doi.org/10.1093/petrology/25.4.956
    Peccerillo, A., Taylor, S. R., 1976. Rare Earth Elements in East Carpathian Volcanic Rocks. Earth and Planetary Science Letters, 32(2): 121-126. https://doi.org/10.1016/0012-821x(76)90050-9
    Pérez-Soba, C., Villaseca, C., 2010. Petrogenesis of highly Fractionated Ⅰ-Type Peraluminous Granites: La Pedriza Pluton (Spanish Central System). Geologica Acta, 8: 131-149
    Qi, C. S., Deng, X. G., Li, W. X., et al., 2007. Origin of the Darongshan- Shiwandashan S-Type Granitoid Belt from Southeastern Guangxi: Geochemical and Sr-Nd-Hf Isotopic Constraints. Acta Petrologica Sinica, 2: 403-412 (in Chinese with English Abstract)
    Regenauer-Lieb, K., Yuen, D. A., Qi, S. H., et al., 2015. Foreword: Toward a Quantitative Understanding of the Frontier in Geothermal Energy. Journal of Earth Science, 26(1): 1-4. https://doi.org/10.1007/s12583-015-0601-4
    Ren, J. S., 1990. On the Geotectonics of Southern China. Acta Geologica Sinica, 65(4): 275-288 (in Chinese with English Abstract)
    Rybach, L., 1976. Radioactive Heat Production in Rocks and Its Relation to other Petrophysical Parameters. Pure and Applied Geophysics, 114(2): 309-317. https://doi.org/10.1007/bf00878955
    Rybach, L., 1988. Determination of Heat Production Rate. In: Haenel, R., Rybach, L., Stegena, L., eds., Handbook of Terrestrial Heat Flow Density. Kluwer Academic Publishers, Holland. 125-142
    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)
    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
    Sun, T., 2006. A New Map Showing the Distribution of Granites in South China and Its Explanatory Notes. Geological Bulletin of China, 25(3): 332-335 (in Chinese with English Abstract)
    Sylvester, P. J., 1998. Post-Collisional Strongly Peraluminous Granites. Lithos, 45(1/2/3/4): 29-44. https://doi.org/10.1016/s0024-4937(98)00024-3
    Tao, J. H., Li, W. X., Li, X. H., et al., 2013. Petrogenesis of Early Yanshanian Highly Evolved Granites in the Longyuanba Area, Southern Jiangxi Province: Evidence from Zircon U-Pb Dating, Hf-O Isotope and Whole-Rock Geochemistry. Science China Earth Sciences, 56(6): 922-939. https://doi.org/10.1007/s11430-013-4593-6
    Teixeira, R. J. S., Neiva, A. M. R., Silva, P. B., et al., 2011. Combined U-Pb Geochronology and Lu-Hf Isotope Systematics by LAM-ICPMS of Zircons from Granites and Metasedimentary Rocks of Carrazeda de Ansiães and Sabugal Areas, Portugal, to Constrain Granite Sources. Lithos, 125(1/2): 321-334. https://doi.org/10.1016/j.lithos.2011.02.015
    Wang, D. Z., 2004. The Study of Granite Rocks in South China: Looking back and forward. Geological Journal of China Universities, 10(3): 305-314 (in Chinese with English Abstract)
    Wang, D. Z., Shen, W. Z., 2003. The Genesis of Granites and Crustal Evolution in Southeast of China. Earth Science Frontiers, 10(3): 209-220 (in Chinese with English Abstract)
    Wang, D. Z., Zhou, X. M., 2002. Crustal Evolution and Petrogenesis of Late Mesozoic Granitic Volcanic-Intrusive Complexes in Southeastern China. Science Press, Beijing (in Chinese)
    Wang, G. L., Zhang, W., Liang, J. Y., et al., 2017. Evaluation of Geothermal Resources Potential in China. Acta Geoscientica Sinica, 38(4): 448-459 (in Chinese with English Abstract)
    Wang, L. X., Ma, C. Q., Zhang, C., et al., 2018. Halogen Geochemistry of I- and A-Type Granites from Jiuhuashan Region (South China): Insights into the Elevated Fluorine in A-Type Granite. Chemical Geology, 478: 164-182. https://doi.org/10.1016/j.chemgeo.2017.09.033
    Wang, L. X., Ma, C. Q., Zhang, C., et al., 2014. Genesis of Leucogranite by Prolonged Fractional Crystallization: A Case Study of the Mufushan Complex, South China. Lithos, 206/207: 147-163. https://doi.org/10.1016/j.lithos.2014.07.026
    Wang, Y. J., Fan, W. M., Zhang, G. W., et al., 2013. Phanerozoic Tectonics of the South China Block: Key Observations and Controversies. Gondwana Research, 23(4): 1273-1305. https://doi.org/10.1016/j.gr.2012.02.019
    Wang, Y. J., Wu, C. M., Zhang, A. M., et al., 2012. Kwangsian and Indosinian Reworking of the Eastern South China Block: Constraints on Zircon U-Pb Geochronology and Metamorphism of Amphibolites and Granulites. Lithos, 150: 227-242. https://doi.org/10.1016/j.lithos.2012.04.022
    Whalen, J. B., Currie, K. L., Chappell, B. W., 1987. A-Type Granites: Geochemical Characteristics, Discrimination and Petrogenesis. Contributions to Mineralogy and Petrology, 95(4): 407-419. https://doi.org/10.1007/bf00402202
    Whalen, J. B., Jenner, G. A., Longstaffe, F. J., et al., 1996. Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-Type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians. Journal of Petrology, 37(6): 1463-1489. https://doi.org/10.1093/petrology/37.6.1463
    Wu, F. Y., Ji, W. Q., Sun, D. H., et al., 2012. Zircon U-Pb Geochronology and Hf Isotopic Compositions of the Mesozoic Granites in Southern Anhui Province, China. Lithos, 150: 6-25. https://doi.org/10.1016/j.lithos.2012.03.020
    Wu, F. Y., Li, X. H., Zheng, Y. F., et al., 2007. Lu-Hf Isotopic Systematics and Their Applications in Petrology. Acta Petrologica Sinica, 23(2): 185-220 (in Chinese with English Abstract)
    Wu, F. Y., Lin, J. Q., Wilde, S. A., et al., 2005. Nature and Significance of the Early Cretaceous Giant Igneous Event in Eastern China. Earth and Planetary Science Letters, 233(1/2): 103-119. https://doi.org/10.1016/j.epsl.2005.02.019
    Wu, F. Y., Liu, X. C., Ji, W. Q., et al., 2017. Highly Fractionated Granites: Recognition and Research. Science China Earth Sciences, 60(7): 1201-1219. https://doi.org/10.1007/s11430-016-5139-1
    Xi, Y. F., Zhao, Y. B., David, A. Y., et al., 2018. Geothermal Structure Revealed by Curie Isothermal Surface under Guangdong Province, China. Journal of Earth Science. https://doi.org/10.1007/s12583-017-0967-6
    Xie, Y. S., Tan, K. X., Tang, Z. P., et al., 2014. Tectono-Magmatic Activization and Fractal Dynamics of Ore-Forming of Hydrothermal Uranium Deposits in South China. Acta Geologica Sinica—English Edition, 88(Suppl. 2): 1695-1696. https://doi.org/10.1111/1755-6724.12385_47
    Xu, T. F., Hu, Z. X., Li, S. T., et al., 2018. Enhanced Geothermal System: International Research Progress and Research Status of China. Acta Geologica Sinica, 92: 1936-1947 (in Chinese with English Abstract)
    Yan, C. L., Shu, L. S., Michel, F., et al., 2017. Early Paleozoic Intracontinental Orogeny in the Yunkai Domain, South China Block: New Insights from Field Observations, Zircon U-Pb Geochronological and Geochemical Investigations. Lithos, 268/271: 320-333. https://doi.org/10.1016/j.lithos.2016.11.013
    Yan, J., Liu, J. M., Li, Q. Z., et al., 2015. In situ Zircon Hf-O Isotopic Analyses of Late Mesozoic Magmatic Rocks in the Lower Yangtze River Belt, Central Eastern China: Implications for Petrogenesis and Geodynamic Evolution. Lithos, 227: 57-76. https://doi.org/10.1016/j.lithos.2015.03.013
    Yang, J. H., Liu, L., Liu, J., 2017. Current Progresses and Prospect for Genesis of Extensive Mesozoic Granitoid and Granitoid-Related Multi-Metal Mineralization in Southern China. Acta Mineralogica Sinica, 37: 791-800 (in Chinese with English Abstract)
    Yang, X. Y., Sun, W. D., 2018. Jurassic and Cretaceous (Yanshannian) Tectonics, Magmatism and Metallogenesis in South China: Preface. International Geology Review, 60(11/12/13/14): 1321-1325. https://doi.org/10.1080/00206814.2018.1479891
    Yuan, Y. S., Ma, Y. S., Hu, S. B., et al., 2006. Present-Day Geothermal Characteristics in South China. Chinese Journal of Geophysics, 49(4): 1005-1014. https://doi.org/10.1002/cjg2.922
    Yurimoto, H., Duke, E. F., Papike, J. J., et al., 1990. Are Discontinuous Chondrite-Normalized REE Patterns in Pegmatitic Granite Systems the Results of Monazite Fractionation?. Geochimica et Cosmochimica Acta, 54(7): 2141-2145. https://doi.org/10.1016/0016-7037(90)90277-r
    Zen, E. A., 1986. Aluminum Enrichment in Silicate Melts by Fractional Crystallization: Some Mineralogic and Petrographic Constraints. Journal of Petrology, 27(5): 1095-1117. https://doi.org/10.1093/petrology/27.5.1095
    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
    Zhang, L., Chen, Z. Y., Li, S. R., et al., 2017. Isotope Geochronology, Geochemistry, and Mineral Chemistry of the U-Bearing and Barren Granites from the Zhuguangshan Complex, South China: Implications for Petrogenesis and Uranium Mineralization. Ore Geology Reviews, 91: 1040-1065. https://doi.org/10.1016/j.oregeorev.2017.07.017
    Zhang, L., Chen, Z. Y., Li, X. F., et al., 2018. Zircon U-Pb Geochronology and Geochemistry of Granites in the Zhuguangshan Complex, South China: Implications for Uranium Mineralization. Lithos, 308/309: 19-33. https://doi.org/10.1016/j.lithos.2018.02.029
    Zhang, X. B., Hu, Q. H., 2018. Development of Geothermal Resources in China: A Review. Journal of Earth Science, 29(2): 452-467. https://doi.org/10.1007/s12583-018-0838-9
    Zhao, Z. H., Akimasa, M., Shabani, M. B., 1992. Tetrad Effects of Rare-Earth Elements in Rare-Metal Granites. Acta Geochimica, 3: 221-233 (in Chinese with English Abstract)
    Zhao, J. L., Qiu, J. S., Liu, L., et al., 2016. The Late Cretaceous I- and A-Type Granite Association of Southeast China: Implications for the Origin and Evolution of Post-Collisional Extensional Magmatism. Lithos, 240-243: 16-33. https://doi.org/10.1016/j.lithos.2015.10.018
    Zhao, P., Wang, J. Y., Wang, J. A., et al., 1995. Characteristics of Heat Production in SE China. Acta Petrologica Sinica, 11(3): 292-303 (in Chinese with English Abstract)
    Zhao, Z. F., Zheng, Y. F., 2009. Remelting of Subducted Continental Lithosphere: Petrogenesis of Mesozoic Magmatic Rocks in the Dabie-Sulu Orogenic Belt. Science in China Series D: Earth Sciences, 52(9): 1295-1318. https://doi.org/10.1007/s11430-009-0134-8
    Zheng, Y. F., Xiao, W. J., Zhao, G. C., 2013. Introduction to Tectonics of China. Gondwana Research, 23(4): 1189-1206. https://doi.org/10.1016/j.gr.2012.10.001
    Zheng, Y. F., Zhang, L. F., McClelland, W. C., et al., 2012. Processes in Continental Collision Zones: Preface. Lithos, 136-139: 1-9. https://doi.org/10.1016/j.lithos.2011.11.020
    Zhou, X. M., 2003. My Thinking about Granite Geneses of South China. Geological Journal of China Universities, 9: 556-565 (in Chinese with English Abstract)
    Zhou, X. M., Li, W. X., 2000. Origin of Late Mesozoic Igneous Rocks in Southeastern China: Implications for Lithosphere Subduction and Underplating of Mafic Magmas. Tectonophysics, 326(3/4): 269-287. https://doi.org/10.1016/s0040-1951(00)00120-7
    Zhou, X. M., Sun, T., Shen, W. Z., et al. 2006. Petrogenesis of Mesozoic Granitoids and Volcanic Rocks in South China: A Response to Tectonic Evolution. Episodes, 29: 26-33 doi: 10.18814/epiiugs/2006/v29i1/004
    Zhou, Z. M., Ma, C. Q., Xie, C. F., et al., 2016. Genesis of Highly Fractionated Ⅰ-Type Granites from Fengshun Complex: Implications to Tectonic Evolutions of South China. Journal of Earth Science, 27(3): 444-460. https://doi.org/10.1007/s12583-016-0677-3
    Zhu, J. L., Hu, K. Y., Lu, X. L., et al., 2015. A Review of Geothermal Energy Resources, Development, and Applications in China: Current Status and Prospects. Energy, 93: 466-483. https://doi.org/10.1016/j.energy.2015.08.098
  • jes-31-1-141-TablesS1-S4.xlsx
  • 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(15)

Article Metrics

Article views(66) PDF downloads(6) Cited by()

Related
Proportional views

Petrogenesis, Tectonic Evolution and Geothermal Implications of Mesozoic Granites in the Huangshadong Geothermal Field, South China

    Corresponding author: Shihua Qi, shihuaqi@cug.edu.cn
  • 1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China
  • 2. School of Environmental Studies, China University of Geosciences, Wuhan 430074, China
  • 3. No. 935 Geological Brigade, Geology Bureau for Nonferrous Metals of Guangdong Province, Huizhou 516000, China

Abstract: Mesozoic multi-stage tectono-magmatic events produced widely distributed granitoids in the South China Block. Huangshadong (HSD) is located in south-eastern South China Block, where closely spaced hot springs accompany outcrops of Mesozoic granites. New data on whole-rock geochemistry, zircon U-Pb geochronology, and zircon Lu-Hf isotopes are presented, to study the petrogenesis and tectonic evolution of the granites, and to explore the relationship between granites and geothermal anomalies. Zircon U-Pb isotopes display three periods of granites in the HSD area:Indosinian (ca. 253 Ma, G4) muscovite-bearing monzonitic granite, early Yanshanian (ca. 175-155 Ma, G5 and G3) monzonitic granite and granodiorite, and late Yanshanian (ca. 140 Ma, G1 and G2) biotite monzonitic granite. In petrogenetic type, granites of the three periods are Ⅰ-type granite. Among them, G1, G2, G3, and G4 are characterized by high fractionation, with high values of SiO2, alkalis, Ga/Al, and Rb/Sr, and depletion in Sr, Ba, Zr, Nb, Ti, REEs, with low (La/Yb)N, Nb/Ta, and Zr/Hf ratios and negative Eu anomalies. In terms of tectonic setting, 253 Ma G4 may be the product of partial melting of the ancient lower crust under post-orogenic extensional tectonics, as the closure of the Paleo-Tethys Ocean resulted in an intracontinental orogeny. At 175 Ma, the subduction of the Pacific Plate became the dominant tectonic system, and low-angle subduction of the Paleo-Pacific Plate facilitated partial melting of the subducted oceanic crust and basement to generate the hornblende-bearing Ⅰ-type granodiorite. As the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to back-arc extension, inducing intense partial melting of the lower crust at ca. 158 Ma and resulting in the most frequent granitic magmatic activity in the South China hinterland. When slab foundering occurred at ca. 140 Ma, underplating of mantle-derived magmas caused melting of the continental crust, generating extensive highly fractionated granites in HSD. Combining the granitic evolution of HSD and adjacent areas and radioactive heat production rates, it is suggested that highly fractionated granites are connected to the enrichments in U and Th with magma evolution. The high radioactive heat derived from the Yanshanian granites is an important part of the crustal heat, which contributes significantly to the terrestrial heat flow. Drilling ZK8 reveals deep, ca. 140 Ma granite, which implies the heat source of the geothermal anomalies is mainly the concealed Yanshanian granites, combining the granite distribution on the surface.

0.   INTRODUCTION
  • Countries worldwide have paid close attention to geothermal resources due to their pollution-free, sustainable and resource-intensive properties recently (Xu et al., 2018; Zhang and Hu, 2018; Kelkar et al., 2016; Regenauer-Lieb et al., 2015). Chinese government has proposed to improve the level of alternative clean energy in "The 13th Five-Year Plan for Energy Development (2016-2020)", aiming to use more than 15% of renewable energy and simultaneously reduce 18% of CO2 emissions by 2020 (Hou et al., 2018). Geothermal energy reserves in the upper 10 km of the Earthʼs crust are approximately 1.3×1027 J (Lund, 2008). Assuming the global energy consumption rate is 6.5×1020 J/year (Lu, 2018), the geothermal energy could supply the global energy consumption for 2 Ma. There are 2.78×1020 J of shallow geothermal energy in 287 Chinese prefecture-level cities, and the annual resources available are 2.89×1012 kWh. Meanwhile, 2.5×1025 J of hot dry rock resources are stored at depths of 3-5 km crust of the Chinese mainland, which is 2.6×105 times the total annual energy consumption in China (Lin et al., 2013). At present, the annual utilization of geothermal resources in China is equivalent to 21 million tons of standard coal, and the exploitation rate of hydrothermal geothermal resources is only 0.2%, and shallow geothermal energy is 2.3%. There is great potential for the utilization of geothermal resources (Wang et al., 2017). China was number one in direct geothermal utilization, reaching 25.2% of the capacity of all countries by 2015, while the installed capacity of geothermal power generation was only 27.78 MWe, accounting for 0.2% of the total and ranking 18th in the world (Zhu et al., 2015; Huang, 2014, 2012).

    Consistent conclusions indicate that radioactive heat generation is an important component of geothermal energy. Radioactive heat generation in the lithosphere from the decay of U, Th, and K, accounts for an estimated 30%-40% of the heat loss at the Earth's surface (Hasterok and Chapman, 2011). Studies have shown that the contents of radioactive elements in acidic magmatic rocks are generally higher than those of other type rocks (Zhao et al., 1995; Rybach, 1988). High terrestrial heat flow and frequent distribution of hot springs consistently show geothermal anomalies in South China (Lin et al., 2016), where Mesozoic granites are widely distributed (Mao et al., 2018; Xi et al., 2018).

    In the Phanerozoic Eon, South China had experienced three significant tectono-magmatic events in the Middle Paleozoic, Triassic and Jurassic-Cretaceous, which are also regarded as the "Kwangsian", "Indosinian" and "Yanshanian" movements in the Chinese literatures, respectively (Wang et al., 2012; Ren, 1990; Huang et al., 1980). Kwangsian granitoids were mainly emplaced in the Wuyi, Baiyun, Yunkai and Wugong domains between 410 and 467 Ma with an age peak of 436 Ma (Wang et al., 2013). Indosinian peraluminous granitoids were predominantly exposed in the Nanling and Yunkai domains and ranged from 202 to 248 Ma, with the age peaks of 239 and 220 Ma. Yanshanian granitoids, with the most widespread occurrence, were mainly emplaced east of the Anhua-Luocheng fault. Statistically, there are three age clusters of 152-180, 120-130 and 87-102 Ma with age peaks of 158, 125 and 93 Ma, respectively (Wang et al., 2013). Plentiful petrological and geochemical studies have been conducted on the chronology, petrogenesis, mineralization, and tectonic setting of the South China granites (e.g., Bonnettia et al., 2018; Zhang L et al., 2018, 2017; Li et al., 2017; Wu et al., 2017; Mao et al., 2014; Xie et al., 2014; Zhang G W et al., 2013; Shu, 2012; Wang, 2004; Zhou, 2003; Deng et al., 1999). However, viewpoints on the Mesozoic tectono-magmatic mechanism of South China granites have not reached a consensus (Jiang et al., 2018; Li et al., 2017; Yan et al., 2017; Mao et al., 2014; Wang et al., 2013; Zhang et al., 2013; Zheng et al., 2013, 2012; Shu, 2012; Zhou, 2003). Moreover, limited studies have been carried out on the relationship between the granites and geothermal anomalies. Both Indosinian and Yanshanian granitoids crop out in the study area (Figs. 1, 2), and studying the petrogenesis of the granitoids is the key to tracing the geodynamic mechanism and deciphering the geothermal anomalies from the perspective of petrogeochemistry.

    Figure 1.  (a) Sketch map showing the tectonic affinity of the South China Block (simply modified from GS(2016)1569, http://bzdt.ch.mnr.gov.cn/); (b) geological sketch map showing the distribution of Yanshanian granites in Guangdong (modified from Sun, 2006) and the location of sampling area. ① Cathaysia Block; ② Yangtze Block.

    Figure 2.  Geological map of the sampling area with the location of ZK8 (modified from Guangdong Geological Bureau, 1982). Pz. Paleozoic; Z. Sinian; Є. Cambrian; D. Devonian; C. Carboniferous; T. Triassic; J. Jurassic; K. Cretaceous; Q. Quaternary; HSD. Huangshadong geothermal field.

1.   GEOLOGICAL BACKGROUND
  • The South China Block is located at the south-eastern edge of the Eurasian continent and formed by the amalgamation of the Yangtze Block in the northwest with the Cathaysia Block in the southeast during 1.0-0.9 Ga (Zhang et al., 2013; Shu, 2012). South China's remarkable geological signature is multi-cycle tectonic-magmatic events, resulting in massive granitoids distributed on the surface (Yang and Sun, 2018; Li et al., 2017; Xie et al., 2014; Wang et al., 2013; Zhang et al., 2013; Wang and Shen, 2003; Zhou, 2003). The collision of the South China Block and the North China Block along the Qinling-Dabie orogenic belt formed nearly EW-striking folded orogenic belts and foreland basins (Zheng et al., 2013; Hacker et al., 1998). The disappearance of the Paleo-Tethys Ocean in the Late Paleozoic and the subduction of the Pacific Plate in the Mesozoic had significant impacts on South China (Zheng et al., 2013). In the Indosinian period, the continental collision among the South China Block, the North China Block and the Indochina Block created the tectonic setting of the South China granitoids (Wang et al., 2013; Ren, 1990). In the Middle to Late Jurassic, the crust was shortened and thickened due to tectonic compression, forming large folds, overthrust tectonics and metamorphism (Chen et al., 2008; Jahn, 1974). Numerous rifted basins and extensional fornix structures formed in the later Cretaceous extensional setting (Gilder et al., 1996), which induced intense intrusion and volcanism (Jiang et al., 2018; Yan et al., 2017; Li, 2000).

    Field investigations and regional geological data show that the sedimentary strata in the study area mainly comprise Sinian- Cambrian metasandstone and phyllitic shale; Devonian, Carboniferous, Triassic, and Jurassic sandstone, siltstone, and shale; and Cenozoic conglomerate, sandstone, and clay layers. Granites intruded into the Cambrian-Sinian metamorphic siltstone or shale in the north region, while they intruded into the Early Jurassic strata in the south region. Due to the influence of regional tectonic events, the attitudes of strata are complex and changeable, and small-scale faults are widely distributed in the area (Fig. 2). NE-striking major faults restrict the distribution of granitoids and strata. The pre-Jurassic strata experienced thermal metamorphism and dynamic metamorphism under multiple stages of magmatism and tectonic stress. A statistical analysis of the relationship between the distribution of geothermal fields and magmatic intrusions is conducted on 315 hot springs in Guangdong Province. There are 132 (42%) hot springs located in the contact zones with exposed intrusions, not taking into account the situation that intrusions are covered by sedimentary rocks, which has a high possibility. In terms of tectonic sites, 238 of them are closely related to major deep faults. The geothermal field is also related to the age of intrusive rocks. There are 110 hot springs distributed in the contact zones of Jurassic intrusions, 16 geothermal fields related to the Ordovician-Triassic intrusions, and only 14 geothermal fields related to Late Cretaceous-Quaternary intrusions.

    The terrain of the HSD area is a graben basin, belonging to the fault-fold zone of the coastal south-eastern China, of which east and west sides are convex. The granitoids are mainly distributed in highlands, and low-lying terrain exposes sedimentary strata. The HSD geothermal field is approximately 30 km southeast of Heyuan fault, 12 km southeast of Zijin-Boluo fault, and 40 km northwest of Lianhuashan fault. Among them, Zijin- Boluo fault controls the distribution of Yanshanian granite bodies and conversely cuts them. Dense broken fissures have been created by repeated regional tectonic stresses. The water temperatures of natural hot springs in HSD are 56.0-63.7 ℃, with a flow rate of 0.38 L/s. The increased water temperature in geothermal wells can reach 40 ℃/km (Lin et al., 2016). Hot springs are always discovered in the broken fissures and the contact zones of Mesozoic granitoids and sedimentary strata. In October 2013, Guangdong Geological Technology Consulting Company drilled a 591.5 m geothermal well (ZK8), with an artesian water temperature of 98.2 ℃, and the highest temperature at the bottom of the hole was 118.2 ℃.

2.   SAMPLES AND PETROGRAPHY
  • Systematic sampling was carried out in HSD and the adjacent areas, including two muscovite-bearing monzogranite samples (Id-30 and Id-33, G4), two granodiorite samples (Is-01 and Is-02, G5), and fourteen biotite monzogranite samples from intrusive mass G1, G2, and G3 (Fig. 2), and a total of twenty monzogranite samples were collected in the study area. G1 was determined as a complex massif of Late Jurassic to Early Cretaceous. Detailed field investigation shows the fine- to medium-grained granites distributed in the west of the complex, while medium- to coarse-grained granites in the east side. Exposure area of G2 is the largest in all magmatic bodies (200 km2), it is medium- to coarse- grained biotite monzogranite, comprising small-scale fine-grained monzogranite. G3 has medium- to coarse-grained heterogeneous texture, and G4 has the meso-grain equigranular texture, intruding into Sinian and Cambrian meta- sandstone or phyllite. G5 is the fine- to medium-grained granodiorite with heterogeneous texture. Id-33 collected from borehole ZK8 is characterized by medium- fine heterogranular texture, exposed by drilling into Precambrian meta-sandstone under ca. 500 m.

    Lithologic classification for the analyzed granites is based on the visually estimated mode in hand specimens and thin sections (Fig. 3). The granodiorites comprise K-feldspar (5%-10%), quartz (20%-25%), plagioclase (40%-50%), amphibole (3%-5%) and biotite (5%-10%), as well as accessory zircon, sphene, and Fe-Ti oxides (Fig. 3, Is-01, Is-02). The biotite monzogranites comprise K-feldspar (25%-40%), quartz (30%-35%), plagioclase (20%-30%), and biotite (~10%), with accessory minerals: zircon, sphene, allanite, apatite, and Fe-Ti oxides (Fig. 3, Id-21, Id-22). Muscovite-bearing monzogranites comprise K-feldspar (20%-25%), quartz (30%-35%), plagioclase (30%-35%), muscovite (5%-10%), and garnet (3%-5%). All samples were collected from weakly weathered outcrops, in some instances, hydrothermal alteration is indicated by sericitization of feldspar and chloritization of biotite. The intrusions didn't undergo obvious metamorphism, whereas undulatory extinction of a small portion of quartz indicates minor deformation occurred under low-grade metamorphic condition. The granodiorites and monzogranites contain early crystallizing hornblende, sphene, muscovite, and garnet, but lack peraluminous cordierite. On petrography, plagioclase and alkali feldspar have euhedral or subhedral platy texture, and sericitization or clayization occurred on mineral surface. Plagioclases show exquisite polysynthetic twinning, and hypautomorphic K-feldspar grains mainly are perthite. Quartz has xenomorphic granular texture, with smooth and clean surface, having no cleavage and alteration, often develops irregular cracks with undulatory extinction. Biotite has a brown schistose texture, the euhedral grains reside in feldspars. It has obvious pleochroism and absorptivity, showing an unidirectional eminent cleavage. Some biotite grains are choritization with unobtrusive interference color. Muscovite has xenomorphic or subhedral schistose texture with an unidirectional eminent cleavage, showing distinct twinkling, bright interference colors, and parallel-axial extinction in the microplariscope. Muscovite grains bend to form kink band under stress. Garnet has rotund granule surface developing irregular cracks, non-directionally distributed in the plagioclase and quartz with less content, showing complete extinction under crossed nicol. Amphibole is characterized by pale-green euhedral or subhedral columnar texture with a complete cleavage.

    Figure 3.  Microscope photographs of the granites. (a) Id-30 is collected from G4, medium-grained garnet-bearing monzogranite, having homogranular texture; (b) Id-33 is a core sample of borehole ZK8 in HSD, fine- to medium-grained heterogranular muscovite monzogranite; (c) Id-21 is sampled from G1, fine- to medium- grained heterogranular two-mica monzogranite; (d) Id-22 is sampled from G1, fine- to medium-grained heterogranular biotite monzogranite; (e) Is-01, fine-grained granodiorite porphyrite; (f) Is-02 is sampled from G5, fine- to medium-grained granodiorite. Qz. Quartz; Pl. plagioclase; Af. alkali feldspar; Bt. biotite; Mus. muscovite; Amp. amphibole.

3.   ANALYTICAL METHODS
  • Zircon grains are extracted from the crushed samples using heavy liquid and magnetic separation techniques. Cathodoluminescence (CL) images of the zircon grains are photographed in order to study their structure and morphology and select proper area for U-Pb dating and Lu-Hf isotope analysis. It is conducted at Wuhan Sample Solution Analytical Technology Company, Wuhan, China, using a detector (GATAN MINICL) attached to a scanning electron microscope (JSM-IT100). U-Pb isotope analyses were measured at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China, using the Geo-Las-Pro laser-ablation system (2005) and Agilent 7500a ICP-MS (Agilent Technologies, Japan). Zircon 91500 is used as external standard to correct instrumental mass discrimination and elemental fractionation, and zircon GJ-1 is used for quality control. Lead abundance of zircon is externally calibrated using 29Si as internal standard. Raw data reduction is performed off-line by using the Isoplot software. Detailed analytical procedures were described by Liu Y S et al. (2010). The weighted mean of the 206Pb/238U ages are quoted at the 95% confidence level.

  • A Neptune Plus MC-ICP-MS combines a Geo-Las-Pro laser-ablation system is used for in-situ zircon Lu-Hf isotopic analysis at Wuhan Sample Solution Analytical Technology Company. Laser-ablation system uses a spot size of 44 μm with repetition rate of 8 Hz. Standard zircon 91500 yields a weighted average 176Hf/177Hf ratio of 0.282 307 9±0.000 018 (2σ, n=99), while GJ-1 yields weighted average of 0.282 018 1±0.000 003 5 (2σ, n=28). 176Lu decay constant adopts 1.865×10-11 yr-1, and εHf(t) values are calculated using the present-day chondritic ratios of 176Hf/177Hf=0.282 772 and 176Lu/177Hf=0.033 2. Single-stage model ages (TDM1) are calculated by referring to a depleted mantle with a present-day 176Hf/177Hf ratio of 0.283 25 and 176Lu/177Hf ratio of 0.038 4. Two-stage model ages (TDM2) are calculated with an assumed 176Lu/177Hf ratio of 0.015 for the average continental crust (Wu et al., 2007).

  • Granite samples are crushed into 200 mesh-size for major and trace element analysis. The experiments are conducted at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China. Major elements are determined by X-ray fluorescence spectrometry using an SHIMADZU XRF-1800 spectrometer. To demonstrate the reliability for the analytical results, two standard substances (GBW07104, GBW07110) are assayed, the relative standard deviation is less than 2%. Test is repeated every ten samples for quality control. Trace elements are determined using an inductively coupled plasma mass spectra (ICP-MS, PE 350D) after acid digestion for samples in teflon bombs. Five reference materials (AGV-2, BCR-2, BHVO-2, GSR-5, GSR-6) were analyzed, the relative standard deviation is less than 5%. The detailed analytical procedures can be referred to Liu et al. (2008).

4.   RESULTS
  • Representative granite samples were collected for U-Pb dating. The zircon grains are subhedral to euhedral, transparent and colorless under optical microscopy, and mostly are approximately 50-300 μm in length, with elongation ratios of 1.5 : 1 to 4 : 1. Typical oscillatory growth zoning suggestive of a magmatic origin, and some inherited cores have been observed in CL images. The measured 206Pb/238U ratios are in good agreement within analytical errors. The U-Pb isotope data for analysis of crystallization ages refer to Table S2, and the results are displayed in Figs. 4 and 5. Sample Id-08 collected from G2, is fine- to medium-grained biotite monzogranite, in which 16 zircons yield a weighted mean 206Pb/238U age of 139.5±1.9 Ma (Fig. 4); sample Id-12 was sampled from G2, fine- to medium-grained biotite monzogranite, in which 12 zircons yield a weighted mean 206Pb/238U age of 138.4±1.7 Ma (Fig. 4); sample Id-10 from G2, is medium- to coarse-grained biotite monzogranite, in which 17 zircons yield a weighted mean concordant 206Pb/238U age of 141.3±3.1 Ma (Fig. 4); sample Id-11 from G2, is medium- to coarse-grained biotite monzogranite, and its 16 zircons yield a weighted mean concordant 206Pb/238U age of 139.8±1.6 Ma (Fig. 4); Id-22 from G1, is fine- to medium-grained biotite monzogranite, in which 14 zircons yield a weighted mean concordant 206Pb/238U age of 140.9±1.7 Ma (Fig. 4); sample Id-24 from G1, is fine- to medium-grained biotite monzogranite, in which 15 zircons yield a weighted mean concordant 206Pb/238U age of 143.0±1.5 Ma (Fig. 4). Fifteen zircons from sample Id-01 yield a weighted mean 206Pb/238U age of 174.2±1.7 Ma (Fig. 5); 20 zircons of sample Is-02 collected from G5 yield a weighted mean 206Pb/238U age of 176.7±1.8 Ma (Fig. 5); sample Id-04 from G3 is a medium- to coarse-grained biotite monzogranite, in which 13 zircons yield a weighted mean 206Pb/238U age of 155.8±1.8 Ma (Fig. 5); sample Id-30 from G4 is a medium-grained granodiorite, and analyses of 15 zircons yield a weighted mean 206Pb/238U age of 253.0±5.0 Ma (Fig. 5). Meanwhile, a borehole sample Id-33 was analyzed, and it's a fine- to medium-grained granodiorite. Analyses of 9 zircons yield a weighted mean 206Pb/238U age of 143.6±2.8 Ma (Fig. 5).

    Figure 4.  Concordia diagrams of zircon U-Pb ages with weighted mean 206Pb/238U ages. Id-08, Id-12, Id-10 and Id-11 are sampled from intrusion G2, and Id-22 and Id-24 are from G1.

    Figure 5.  Concordia diagrams of zircon U-Pb ages with weighted mean 206Pb238U ages. Sample Id-01 is collected in Gongzhuang, 40 km away from HSD in northwest, beyond the range of the Fig. 2; Is-02 is collected from G5; Id-04 is collected from G3; Id-17 is sampled in Yirong where is 30 km north away from G3; Id-30 is collected from intrusion G4; Id-33 is exposed in the borehole ZH8.

  • Twenty granitoid samples were analysed for major (wt.%) and trace element contents (ppm), and the results are listed in Table S1. Before plotting, the major element results were normalized to 100% accounting for the loss on ignition (LOI). The monzogranites have high SiO2 contents (71.17 wt.%-76.71 wt.%), while the granodiorites have 58.90 wt.%-63.55 wt.%. The monzogranites are alkali-rich (K2O+Na2O, 7.59 wt.%-8.55 wt.%), while the granodiorites have the contents of 4.07 wt.%-6.15 wt.%, and all samples plot in the high-K calc-alkaline field on the K2O vs. SiO2 diagram (Fig. 6a). Based on the molar ratios of A/CNK, the monzogranites are weakly peraluminous, with A/CNK ratios from 0.98 to 1.13 for G1, G2, and G3, while G4 is strongly peraluminous, with A/CNK ratios from 1.10 to 1.15 (Fig. 6b). Major elements show that as SiO2 increases, the content of TiO2, Al2O3, CaO, Fe2O3, MgO, and K2O decreases. The chondrite-normalized rare earth element (REE) distribution pattern for the monzogranites is presented in Fig. 7a, characterized by right-inclined shapes for G1 and G2 with slight REE fractionation [(La/Yb)N, 3.2-9.2, 2.6-12.3, respectively] and relatively flat REE patterns and REE tetrad effects for G3 and G4, with (La/Yb)N values of 1.3 and 0.8, respectively. The samples have wide ranges of total REE contents (ΣREE): 200 ppm-401 ppm for G1, 120 ppm-233 ppm for G2, 121 ppm-146 ppm for G3, and 51 ppm-84 ppm for G4, with various degrees of negative δEu anomalies (0.03-0.86). Primitive mantle-normalized trace element spider diagrams for G1, G2, G3, and G4 (Fig. 7b) show relatively enriched Th, U, Rb, and K, and depleted Sr, Ba, P, and Ti.

    Figure 6.  Classification of rock series. (a) K2O vs. SiO2 diagram with field boundaries between tholeiite, calc-alkaline, high-K calc-alkaline and shoshonitic series (Peccerillo and Taylor, 1976); (b) A/NK vs. A/CNK diagram showing the weakly peraluminous nature of the granites. A. Al2O3; N. Na2O; K. K2O; C. CaO (all in molar proportion, Maniar and Piccoli, 1989).

    Figure 7.  (a) Chondrite-normalized REE diagram; (b) primitive-mantle-normalized incompatible element spidergram for G1, G2, G3, and G4. The normalization values after Sun and McDonough (1989).

  • A large proportion of U-Pb dating zircons were also analysed for Lu-Hf isotopes. The results are listed in Table S3. The 176Lu/177Hf ratios of most zircons are less than 0.002, indicating low radioactive growth of 176Hf. Initial 176Hf/177Hf ratios and εHf(t) values were calculated for their mean U-Pb ages. Two distinct sets of Hf isotope compositions are found for G1: 176Hf/177Hf ratios of sample Id-24 vary between 0.282 447 and 0.282 508, corresponding to ɛHf(t) values varying between -6.4 and -8.4 and TDM2 between 1 600 and 1 733 Ma (mean value, 1 206 Ma), while 17 zircons from samples Id-20 and Id-22 have 176Hf/177Hf ratios between 0.282 447 and 0.282 843, corresponding to ɛHf(t) values varying between 2.7 and -3.0 and TDM2 between 1 021 and 1 385 Ma (mean value, 1 665 Ma). The εHf(t) values and TDM2 model ages of G2 show unimodal distributions: 43 zircons from samples Id-08, Id-10, Id-11, and Id-12 of G2 yield 176Hf/177Hf ratios between 0.282 357 and 0.282 611, ɛHf(t) values between -2.7 and -11.8 and TDM2 between 1 367 and 1 941 Ma (mean value, 1 626 Ma). Sixteen zircons from sample Id-30 of G4 have varied Hf isotope compositions, with 176Hf/177Hf ratios between 0.282 125 and 0.282 656, ɛHf(t) values between -4.4 and -17.7 and TDM2 between 1 559 and 2 396 Ma. A bimodal pattern is presented by the εHf(t) values and TDM2 model ages of G4, with average TDM2 ages of 1 698 and 2 252 Ma.

5.   DISCUSSION
  • Granites are commonly divided into different genetic types according to their petrochemistry, geochemistry, discriminating minerals or tectonic setting. They are classified as Ⅰ-type and S-type, according to the magma extracted from igneous protoliths or sedimentary protoliths (Chappell and White, 2001). A-type granite refers to a source rock derived from dehydrated continental crust, and M-type granite is derived directly from the melting of subducted oceanic crust or mantle (Whalen et al., 1987). The I-S-M-A classification system has become the most widely used, but it is difficult to identify the genetic types of highly fractionated granites because they tend to have similar geochemistry characteristics (Wang et al., 2018; Wu et al., 2017; Zhou et al., 2016; Li et al., 2007). Existing studies show that genetic types of the South China granites are diverse, including I-, S-, and A-type granites, and different conclusions have even been drawn for the same pluton (e.g., Fogang granitic complex, Chen and Yang, 2015; Li et al., 2007; Bao and Zhao, 2003). Many classifications have been proposed to distinguish the genetic types (e.g., Wang et al., 2018; Wu et al., 2017; Dill, 2015; Whalen et al., 1996, 1987). Geochemically, ΣREE and the ratios of light REEs and heavy REEs (LREE/HREE) decrease in highly fractionated granites, with obvious negative Eu anomalies (Zhou et al., 2016; Gelman et al., 2014; Huang and Jiang, 2014), as shown in Fig. 6a for G3 and G4. Some trace element ratios have been measured for the fractionation degree, such as the values of Rb/Sr, Rb/Ba, and K/Rb (Halliday et al., 1991); the mean values are 6.6, 2.8, 154 for G1; and 21.7, 14.6, 107 for G2; and 7.1, 2.8, 106 for G3; and 71.8, 36.1, 58 for G4, respectively. Another important feature of highly fractionated granites is the mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, and REEs (Wang et al., 2018; Zhang et al., 2017; Huang and Jiang, 2014; Merino et al., 2013; London and Evensen, 2002). Based on these characteristics and ratios, the Mesozoic granites are highly fractionated granites.

    A reference value of A/CNK=1.0 has been taken to distinguish between metaluminous and peraluminous granites (Zen, 1986). When extensive fractional crystallization occurs in felsic melts, S-type melts are more peraluminous than Ⅰ-type granites. It seems easy to judge G4 as S-type granite. However, Ⅰ-type granite can also be peraluminous due to fractional crystallization, and the P content of S-type granites increases during fractional crystallization, while P decreases to very low in Ⅰ-type granites (Chappell et al., 1998). Because apatite is soluble in peraluminous melts, P becomes progressively more abundant in S-type melts. This feature leads to easy contrasts in the abundances of P vs. Y, REE, and Th between highly fractionated granites and S-type granites (Gao et al., 2016; Chappell, 1999); hence, the granitoids in the study area are not S-type granites, having low contents of P2O5. As shown in the Zr vs. 10 000Ga/Al diagram and (K2O+Na2O)/CaO vs. Zr+Nb+Ce+Yb diagram, G1, G2, G3 and G4 fall in the overlap region of A-type and highly fractionated I/S type granites (Figs. 8a, 8b). Evidently, the discrimination of A-type granites and highly fractionated granites cannot be based on a few geochemical diagrams (Wu et al., 2017). The REE distribution of Ⅰ-type granites is generally flat or uniform when normalized against chondrite, with a very strong negative Eu anomaly (Chappell et al., 1998). Furthermore, the high Rb/Sr and Rb/Ba ratios show that the four intrusions are not A-type granites, but rather highly fractionated Ⅰ-type granites (Whalen et al., 1987). REE characteristics indicate that G4 underwent F-Cl- rich fluid alteration in the late stage of magmatism (Zhao et al., 1992). Overall, the Yanshanian granites are characterized by high SiO2, alkalis, and FeOt/MgO, and low CaO. In terms of the trace elements, high field strength elements such as Zr, Nb and Ta are significantly enriched, while Ti, Ba, Sr, P and Eu are depleted (Fig. 7b), with high Ga/Al and Y/Nb ratios. Wu et al. (2017) proposed that different evolutionary trends can distinguish A-type granites from other highly fractionated granites. In Figs. 8c and 8d, G1, G2 and G3 manifest the evolutionary trend of Ⅰ-type granites. A comprehensive analysis shows that the Yanshanian granites in the study area are highly fractionated Ⅰ-type granites.

    Figure 8.  Discriminant diagrams of petrogenetic types. (a) Zr vs. 10 000 Ga/Al diagram for discrimination of I-, S-, A-, or M-type granites, the dashed line is referred to Wu et al. (2017); (b) (K2O+Na2O)/CaO vs. (Zr+Nb+Ce+Y) diagram (Whalen et al., 1987); (c) P2O5 vs. SiO2 diagram (Chappell and White, 2001); (d) Y vs. Rb diagram. FG. Fractionated granites; OGT. unfractionated granites. Symbols of G1, G2 and G3 are the same as shown in Fig. 6.

  • Calc-alkaline Ⅰ-type granite is usually generated from partial melting of either mafic or intermediate igneous sources (Chappell and White, 2001). Kostitsyn (2001) proposed that anatectic melting of crustal material could explain high concentrations of many lithophile elements in Ⅰ-type granites. Frindt et al. (2004) deemed that radioactive isotope ratios provide the main criterion to discuss the magma sources of fractionated granites. Zircon Lu-Hf isotope data can provide a more precise identification of continental crustal evolution than Sm-Nd isotope data (Teixeira et al., 2011; Hawkesworth and Kemp, 2006). Magmas derived from depleted mantle or juvenile crust are characterized by high 176Hf/177Hf and εHf(t) values, while those derived from ancient crust are characterized by low 176Hf/177Hf and εHf(t) values (Tao et al., 2013; Li et al., 2009; Kinny, 2003). The magmatic origin of ca. 140 Ma G1 has two end-members: The higher 176Hf/177Hf ratios and εHf(t) values of samples Id-20 and Id-22 indicate that G1 has an end-member of juvenile crustal material or depleted mantle, while the lower 176Hf/177Hf ratios and εHf(t) values of Id-24 make it clear that ca. 1.7 Ga ancient crust is another end-member. Unimodal Lu-Hf isotopes suggest that ca. 1.7 Ga ancient crust is the magmatic source that formed ca. 140 Ma G2; it is also the dominant magmatic origin of the granites in the study area, as shown by G1 and G4 (Fig. 9). The Late Paleoproterozoic was an important epoch of crustal growth in South China, as tholeiite and alkaline amphibolites (1.77 Ga) have been identified from the Cathaysia basement (Li, 1997). Another TDM2 end-member of ca. 253 Ma G3 is 2.25 Ga ancient material. The zircon εHf(t) values of G4 are within the scope of overall Indosinian granites (-2.0 to -20.2, Qi et al., 2007). We conclude from the above analysis that there are diverse compositions in the formation of granitic complexes. The source rocks of the Yanshanian granites are relatively juvenile and may have been affected by the addition of mantle-derived magma. It is interpreted that the upwelling of high-temperature mantle-derived magma facilitated the formation of granite provinces in South China, causing large-scale partial melting of the lower crust. Recent research on Lu-Hf isotopes of granite zircons also indicates that mantle- derived magma was involved in the formation of South China granites (e.g., Yang et al., 2017; Tao et al., 2013; Li et al., 2007).

    Figure 9.  Frequency numbers of εHf(t) values and Hf model ages (TDM2) of G1, G2, and G4.

  • Element variations in granites are controlled by magmatic evolution. It is clearly demonstrated that the K/Rb ratios of the HSD Yanshanian biotite monzogranites are low, 90-123 for G1, 122-191 for G2, and 96-112 for G3; and the K/Ba ratios are strikingly high, 296-2 579 for G1, and 57-1 194 for G2, and 172-502 for G3; and the Zr/Hf ratios are reduced to 22.4 for G1, and 25.8 for G2, and 18.9 for G3 (Table S1). Overall, the G1, G2, G3 and G4 granites have low Nb/Ta ratios and high Rb/Sr ratios, which reflects the strong crystal fractionation in combination with the low REE contents and LREE/HREE ratios. The REE tetrad effect in G4 was caused by F-Cl-rich fluid alteration in the late stage of magmatism (Zhao et al., 1992); likewise, fluid alteration induced the enrichment in Ta relative to Nb (Green, 1995). All these characteristics illustrate that the HSD granites have experienced extensive fractionation. The conspicuous negative Eu anomalies cannot be solely attributed to feldspar separation, although it is commonly known to have large positive Eu anomalies in its REE distribution coefficients. Magma-fluid interaction, the primary cause of the tetrad effect, could lead to all constituent minerals depleted Eu, caused severely negative Eu anomalies in the rocks (Jiang et al., 2016; Jahn et al., 2001). The granite, ca. 140 Ma, is the most widely distributed in the study area, and sample Id-33 exposed by borehole ZK8 is also from the same period; hence, the fractional crystallization of granites from this period is described for emphasis. The diagram of Rb vs. Sr reveals the crystallization of K-feldspar or plagioclase (Fig. 10a). In the Ba vs. Sr plot (Fig. 10b), Sr and Ba decrease sharply from 239 ppm to 11 ppm for Sr and from 774 ppm to 16 ppm for Ba, which is interpreted as a result of the fractionation of plagioclase and biotite more than K-feldspar. The proportion of K-feldspar increases during magmatic evolution, as shown in the petrography. Fractionation of plagioclase supports the above analysis in the Eu vs. Sr plot (Arth, 1976). The low MgO contents in these granites suggest fractionation of mafic minerals, and P depletions are mainly related to the fractionation of apatite. LREEs decrease more than HREEs in G1 and G2, implying a higher fractionation degree of monazite than zircon (Yurimoto et al., 1990). Depletions in HREEs in the granites may have been affected by the fractionation of apatite or zircon, and the total contents of REEs in the granites are low due to fractionation of accessory minerals such as monazite, apatite or zircon (Bea, 1996). However, the relative enrichment in HREEs in G4 granites is consistent with the occurrence of garnet. The plot of (La/Yb)N vs. La (Fig. 10c) displays the fractionation of titanite, zircon, and apatite in G2 granites, while G1 granites show the fractionation trends of allanite and monazite. In summary, plagioclase, K-feldspar, hornblende, apatite, monazite/allanite, and Fe-Ti oxides were fractionated from the magma during the formation of the highly fractionated monzogranites.

    Figure 10.  Fractional crystallization diagrams. (a) Rb vs. Sr plot; (b) Ba vs. Sr plot; (c) (La/Yb)N vs. La plot. Pl. Plagioclase; Kf. K-feldspar; Bt. biotite; Al. allanite; Mo. monazite; Ap. apatite; Sp. sphene; Zr. zircon. Symbols of G1 and G2 as shown in Fig. 6.

    Geochemically, SiO2, MgO/(MgO+FeO), Rb/Sr, and Rb/Ba usually measure the degree of magma fractionation (Halliday et al., 1991). The K/Rb, Zr/Hf, Nb/Ta, and Y/Ho (i.e., twin elements) are consistent during common evolution (Green, 1995), but these ratios decrease when the magma changed in nature due to intensive differentiation (Dostal et al., 2015; Deering and Bachmann, 2010; Linnen and Keppler, 2002; Bau, 1996). Nb-Ta fractionation occurs during subduction of a slab, and low Nb/Ta fluids are released during the blueschist to amphibolite transformation in deeper portions of the subduction zone; the enrichment in Ta relative to Nb is revealed as the granitic magma differentiates (Ding et al., 2013; Li and Huang, 2013; Huang et al., 2012). Therefore, it has been proposed that the ratios of Zr/Hf and Nb/Ta of the whole rock could be regarded as signs of the fractionated degree of granitic magma (Ballouard et al., 2016; Pérez-Soba and Villaseca, 2010). Combining results of this paper with the data for the large-scale Yanshanian Fengshun and Fogang complexes nearby the study area, characterized by highly fractionated Ⅰ-type granite, the relationship between the evolution of the granite composition and radioactive elements Th and U is explored in Fig. 11 (the raw data are collected in the ESMs). It shows that both Th and U contents have a tendency to increase as granitic magma differentiates. Among them, the covariant relationship between Nb/Ta and radioactive elements is relatively vague, which may be related to the complex Na/Ta differentiation.

    Figure 11.  The radioactive elements of three Yanshanian granite complexes in Guangdong as magmatic differentiation. The covariant relations are indicated by dashed lines. Fogang Complex data were quoted from Li et al. (2007); Fengshun Complex data were quoted from Zhou et al. (2016).

  • Geochemical discrimination diagrams of tectonic setting for granites are sometimes ambiguous (Maniar and Piccoli, 1989). In the Rb vs. (Y+Nb) diagram (Fig. 12), G2 granites plot in the post-COLG field, whereas G1, G3 and G4 plot in the WPG field. This inconsistency suggests that the diagrams of tectonic setting are not straightforward but are combined with specific geological settings. Peraluminous granite is traditionally considered to form during continent-continent collisional events (Pearce et al., 1984) or in post-collisional settings after the climax of crustal thickening (Sylvester, 1998). In the past 40 years, the dynamic setting of Mesozoic multi-stage tectonic-magmatic activity in South China has been controversial. Some of the most controversial arguments include the key driving mechanisms involved in the Indosinian orogenesis. In the South China interior, the Indosinian granites are mainly exposed to the west of the Zhenghe-Dapu fault (ZDF) and along two sides of the Jiangshan-Shaoxing fault (JSF) (Fig. 1a); these rocks were traditionally classified as S-type granites (Wang et al., 2013; Wang and Zhou, 2002; Chen and Jahn, 1998). Li and Li (2007) proposed that flat-slab subduction of the Pacific Plate formed the 1 300 km folded orogenic belt of South China and that subsequent melting of the subducting plate dominated the Late Mesozoic magmatism. However, Zhao and Zheng (2009) proposed that Mesozoic granitic rocks in South China were derived from anatexis of the Precambrian crustal basement due to lithospheric extension in response to subduction of the Pacific Plate. Alternatively, the closure of Paleo-Tethys Ocean resulted in the Triassic intracontinental orogeny, and a comprehensive compilation of published magmatic and metamorphic ages showed no obvious spatial propagation of Triassic deformation and metamorphism across South China (Wang et al., 2013). In addition, the Triassic NE-trending dextral shear zones and east-trending folds in Cathaysia manifest a NNE-directed shortening is inconsistent with the flat subduction model, and thus supports the continental collision model (Li et al., 2017). Based on geodynamical setting, we attribute the Middle Triassic dextral transpression to collisions of the South China Block with the North China Block and the Indochina Block. The 253 Ma sample G4 may be the product of partial melting of the ancient lower crust under post-orogenic extensional tectonics and affected by late fluid metasomatism.

    Figure 12.  Rb vs. (Y+Nb) discrimination diagram (Pearce, 1996) for samples from G1, G2, and G3. VAG. Volcanic-arc granite; syn-COLG. syn-collisional granite; WPG. within-plate granite; ORG. ocean-ridge granite; post-COLG. post-collisional granite. Symbols of G1, G2, and G3 as shown in Fig. 6.

    There is also a persistent controversy over the tectonic setting of the Early Yanshanian period, focusing on the dynamic mechanics by which Jurassic granites and volcanic rocks consistently display NE and E-W trends, while the majority of geologists accept that the Late Yanshanian magmatism was dominated by the subduction of the Paleo-Pacific Plate (Jiang et al., 2018; Wang et al., 2018; Zhou et al., 2016). Chen et al. (2008) deemed that the Jurassic mantle beneath the Cathaysia Block was rather homogeneous and undisturbed and was not affected by the Paleo-Pacific subduction system; therefore, the postorogenic extension of the Indosinian orogeny might have resulted in the Jurassic granitic magmatism in the Cathaysia Block. Zheng et al. (2013) stressed that Yanshanian magmatism was affected by the subduction and roll-back of the Pacific Plate and that the ancient continental margin material underwent reconstruction (deformation, metamorphism and anatexis) in a new intracontinental tectonic setting. Recently, the hypothesis related to the subduction of the Paleo-Pacific Plate has been widely accepted to decipher the Yanshanian granites (Jiang et al., 2018; Wang et al., 2018; Ji et al., 2017; Zhao et al., 2016; Chen et al., 2014; Mao et al., 2014; Zhou and Li, 2000). Zhou et al. (2006) proposed a model with low-angle initial subduction of the Paleo-Pacific Plate and gradual steepening of the regressive plate to explain the Late Mesozoic magmatic evolution of South China, which well explained the migration of magmatic activity from inland to coast since the Middle Jurassic. For 190-180 Ma, the magmatic activity was distributed merely in the E-W-trending Nanling fold belt. The Early Jurassic basalts beneath the Cathayasian inland and Wuyi Mountain belt are characterized by similar enrichment in Nd isotopes, indicating the underlying continental lithospheric mantle was ancient and homogeneous, and was not significantly affected by the subduction of the Pacific Plate (Mao et al., 2014; Chen et al., 2008).

    Numerous ca. 175 Ma adakitic rocks related to large-scale Cu-Fe-Au deposits are distributed along the Gan-Hang belt, indicating that there was a syn-tectonic compressional crustal thickening event (Mao et al., 2014). Geochemically, the rocks belonging to the high-potassium calc-alkaline series, have affinity with island arc-type rocks, and exhibit a high degree of chemical differentiation with relatively enriched large ion lithosphile elements and LREEs, and depleted high field strength elements (Zheng et al., 2013). Combining these data with the fact that ca. 175 Ma was a period of frequent magmatic activity in South China inland, we consider ca. 175 Ma represents the initial effect of the subducting Pacific Plate entering the dominant tectonic system (Mao et al., 2014). Low-angle subduction of Paleo- Pacific Plate facilitated partial melting of the subducted oceanic crust and basement to generate the hornblende-bearing Ⅰ-type granodiorite (G5), as shown in Fig. 13a (e.g., Jiang et al., 2018; Wang et al., 2014; Wu et al., 2012; Liu S A et al., 2010). As subduction continued, the dip angle of the subducting plate increased as the rock was metamorphosed into high-density eclogite; the continental arc tectonic setting was transformed to back-arc extension, inducing intense partial melting of the lower crust from 165-155 Ma that peaked at 158 Ma (Wang et al., 2013; Li et al., 2007) and resulting in the most frequent granitic magmatic activity in the South China hinterland, as shown Fig. 13b (Mao et al., 2014; Wang et al., 2013). Jurassic rocks in eastern South China are mainly composed of high-K calc-alkaline Ⅰ-type granites and subordinate syenites and A- and S-type granites (e.g., Jiang et al., 2018; Yang and Sun, 2018; Yang et al., 2017; Zhou et al., 2016; Wang et al., 2013; Li et al., 2007). As slab foundering and crustal thinning occurred at ca. 140 Ma, as shown in Fig. 13c, underplating of mantle-derived magmas drove the crust-mantle interaction (Jiang et al., 2018; Gu et al., 2017; Yan et al., 2015; Li et al., 2013). Then, the melting of the continental crust produced the extensive highly fractionated granites of Huizhou and adjacent regions, initiating the late stage of the Late Yanshanian magmatic event that involved eastward rejuvenation along the coast of South China (Wu et al., 2005).

    Figure 13.  Tectonic-magmatic evolution model for Huizhou Yanshanian granites. (a) In ca. 175 Ma, low-angle subduction of Paleo-Pacific Plate facilitated partial melting of subducted oceanic crust and basement generated the hornblende-bearing Ⅰ-type granodiorite (G5); (b) as subduction continued, the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to back-arc extension, inducing intense partial melting of the lower crust in 165-155 Ma, and resulting in the most frequent granitic magmatic activity in South China hinterland; (c) as slab foundering and crustal thinning occurred in ca. 140 Ma as shown in Fig. 13c, the melting of the continental crust produced extensive highly fractionated granites of Huizhou.

  • Terrestrial heat flow in a passive continent is composed of crustal heat flow and mantle heat flow (Hu and Wang, 1994). Compared with the relatively stable mantle heat flow, crustal heat flow is more likely to result in surface thermal anomalies. The calculation formula for the radioactive heat generation rate (A) proposed by Rybach (1976) is used to measure Yanshanian granites of the HSD area and the Fengshun and Fogang complexes; the results are shown in Table S4. As shown in Fig. 11, the content of radioactive elements increases obviously as magma evolves in the highly fractionated Ⅰ-type Yanshanian granites. Therefore, itʼs easy to see that there is a significant positive correlation between the radioactive heat generation rate and magmatic fractionation in Fig. 14. In order to make a qualitative assessment for the radioactive heat generation rates of the Yanshanian highly fractionated granites in the Guangdong hinterland, their covariant curves are drawn as dashed lines. The radioactive heat generation rates of the Yanshanian granites are characterized by an average of 6.7 μW/m3, which is much higher than the mean values of worldwide granites (2.5 μW/m3, McLaren et al., 2006) and south-eastern China granites (4.2 μW/m3, Zhao et al., 1995). According to existing research results, the terrestrial heat flow in the study area is an average value for South China (mean value of 73 mW/m2 for Guangdong Province, Xi et al., 2018; Yuan et al., 2006), although the crustal heat flow is typically assumed to be approximately 20-40 mW/m2 (McLaren et al., 2006). Assuming that the average thickness of Yanshanian granites is 2-3 km, the geothermal energy generated by the Yanshanian granites accounts for 18%-27% of the terrestrial heat flow. Such an approximate estimate qualitatively shows that radioactive heat generation in the Yanshanian granites contributes significant heat flow to geothermal anomalies in the study area. It has been realized that the granites have high radioactive heat generation rates corresponding to the large-scale Yanshanian granitic plutons, such as the Fogang Complex (Lin et al., 2017; Wang et al., 2013). We conclude that the larger the granitic body is, the longer the cooling crystallization process is, and the more conducive to enrichment in U and Th caused by long-time extraction.

    Figure 14.  The radioactive heat generation rate of three Yanshanian granite complexes as magmatic evolution. Symbols are as shown in Fig. 11.

  • By studying the geochronology, geochemistry and tectono- magmatic evolution of the granites in the HSD area, it is speculated that the heat source of the HSD geothermal field mainly comes from the deep concealed Yanshanian granites, and the age of sample Id-33 exposed by ZK8 reinforces this prediction. At the same time, the granites have high thermal conductivity and act as the medium for the conduction of thermal energy from the deep earth, making contributions to the high geothermal gradient (Mao et al., 2018). In contrast, the pre-Yanshanian granites exposed at the Earth surface have limited distribution in the study area (Fig. 2), and there is no sedimentary cover to store geothermal energy (Mao et al., 2018).

    Based on the above analysis and collected data, a simple geological geothermal model is proposed. As shown in Fig. 15, dense tectonic fissures and broken contact zones in the HSD area provide favorable channels for large amounts of atmospheric precipitation to enter subsurface runoff. Geophysical data obtained from our geothermal exploration project show that there is a thick crystalline rock below the tuff as an impermeable layer, which is consistent with the fact that the granites and diorites have been exposed under ca. 300 (Id-33) and 500 m (Is-05), respectively. Figure 15 displays the measured geothermal well temperature of ZK8. It is divided into three sections from shallow to deep: (1) 20-180 m with a water temperature gradient of 27.5 ℃/hm; (2) 180-320 m with a water temperature gradient of 17.1 ℃/hm; and (3) 320-591.5 m with a water temperature gradient of 6.0 ℃/hm. ZK8 reveals that there are tuff layers at 94-260 and 400-470 m in HSD and that the tuff layers accept groundwater as aquifers. Overall, the water temperature increases significantly with depth (D), indicating that deep circulating geothermal water is dominant in HSD groundwater. The highest water temperature gradient of the first section may be related to the rapid migration of geothermal water in Cambrian metamorphic rocks with low porosity; while the tuff's porosity is relatively higher, its geothermal water circulation becomes slow and cooling in the second section. The water temperature gradient of the third section approximates the geothermal gradient and might indicate that it is close to the position reached by the groundwater circulation.

    Figure 15.  Geothermal genesis model based on geological survey and drilling data in Huangshadong geothermal field. Cyan dashed lines with arrow represent the water flow direction. Tectonic fissures provide favorable channel for the atmospheric precipitation by subsurface runoff. The water temperature increases through deep circulating, and tuff layer accepts groundwater as an aquifer. The heat mainly comes from the radioactive heat generation and effect of thermal conductivity of concealed Yanshanian granites.

6.   CONCLUSIONS
  • (1) Zircon LA-ICP-MS U-Pb dating results indicate that there are three periods of granites distributed in the HSD geothermal field: Indosinian (ca. 253 Ma) muscovite-bearing monzonitic granite, Early Yanshanian (ca. 175-155 Ma) monzonitic granite and granodiorite, Late Yanshanian (ca. 143-140 Ma) biotite monzonitic granite, respectively.

    (2) The Yanshanian granites (G1, G2, and G3) are characterized by high SiO2, alkalis, and FeOt/MgO, and low CaO in major elements and enrichment in high field strength elements such as Zr, Nb and Ta, while depletion in Ti, Ba, Sr, P and Eu, with high Ga/Al, Y/Nb, Rb/Sr, and Rb/Ba ratios and low ΣREE, Nb/Ta, and LREE/HREE ratios. Based on these characteristics and ratios, the granites are highly fractionated Ⅰ-type granites. The REE tetrad effect indicates that G4 granites underwent F-Cl-rich fluid alteration in the late stage of magmatism.

    (3) The 253 Ma granite G4 may be the product of partial melting of the ancient lower crust under post-orogenic extensional tectonics, affected by late fluid metasomatism. At ca. 175 Ma, low-angle subduction of the Paleo-Pacific Plate facilitated partial melting of subducted oceanic crust and basement to generate the hornblende-bearing Ⅰ-type granodiorite. As the dip angle of the subducting plate increased, the continental arc tectonic setting was transformed to back-arc extension, inducing intense partial melting of the lower crust at 158 Ma and resulting in the most frequent granitic magmatic activity in inland Guangdong. As slab foundering occurred, underplating of mantle-derived magmas caused the melting of the continental crust and produced extensive highly fractionated granites at ca. 140 Ma in the HSD area.

    (4) Geochemistry and Hf isotopes data suggest that the Late Paleoproterozoic lower crust was a magmatic source that formed granites in the study area and an important crustal accretion event in South China occurred at ca. 1.7 Ga. The bimodal distribution of Lu-Hf isotopes for G1 indicates that a mantle-derived component may have added to the formation of the Yanshanian granites.

    (5) Plagioclase, K-feldspar, hornblende, apatite, monazite/ allanite, and Fe-Ti oxides were fractionated from the magma during the formation of the highly fractionated monzogranites. SiO2, Rb/Sr, Rb/Ba, Zr/Hf and Nb/Ta of the whole rocks could be regarded as signs of the fractionated degree of granitic magmas. It shows that both Th and U contents have a tendency to increase as the Yanshanian granites differentiate.

    (6) The radioactive heat generation rates of the highly fractionated Yanshanian monzogranites are characterized by an average of 6.7 μW/m3, which is much higher than the mean values of worldwide granites (2.5 μW/m3) and south-eastern China granites (4.2 μW/m3). There is a significant positive correlation between the radioactive heat generation rate and magmatic fractionation. By studying the geochronology, geochemistry and tectono-magmatic evolution of the granites in the HSD area, it is speculated that the heat source of the HSD geothermal anomalies mainly comes from the deep concealed Yanshanian granites.

ACKNOWLEDGMENTS
  • This study was financially supported by the China Geological Survey (No. 1212011220014). We deeply appreciate constructive comments and suggestions from Prof. Changqian Ma and other anonymous reviewers, which help us to improve the manuscript significantly. David A. Yuen, Balachandar Subramaniyan, Selvarajah Marimuthu and Jianhua Wang gave excellent guidance in the manuscript writing, Lanlan Jin assisted the element analyses. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1242-9.

    Electronic Supplementary Materials: Supplementary materials (Tables S1-S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1242-9.

Reference (121)

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return