Thickness and geothermal gradient of Neoarchean continental crust: Inference from the southeastern North China Craton |
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| Affiliation: | 1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, PR China;2. School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, PR China;3. Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia;4. Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines, Department of Geology, School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, PR China;1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, PR China;2. Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines, Department of Geology, School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, PR China;1. Beijing Institute of Geology for Mineral Resources, Beijing 100012, PR China;2. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University, Beijing 100871, PR China;3. Department of Energy and Mineral Engineering, The Pennsylvania State University, College Park, PA 16802, USA;4. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, PR China;5. Department of Geology, School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, PR China;6. Research Institute of Petroleum Exploration & Development, PetroChina Liaohe Oilfield Company, Panjin 124000, PR China;1. Wuhan Center of Geological Survey, China Geological Survey, Wuhan 430205, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Department of Earth and Environmental Sciences, University of Windsor, Ontario, Canada;4. State Key Lab of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;5. Key Laboratory of Marine Hydrocarbon Resources Environmental Geology, Ministry of Natural Resources, Qingdao Institute of Marine Geology, Qingdao 266071, China;6. Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China;7. School of Earth Sciences, China University of Geosciences Wuhan, Wuhan 430074, China;1. Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong;2. College of Marine Geosciences, Ocean University of China, Qingdao 266003, China;3. State Key Laboratory of Continental Dynamics, Northwest University, Xi''an 710068, China |
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| Abstract: | The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granodiorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at ~2.70 Ga and ~2.55 Ga which were sourced from partial melting of juvenile crustal components. The ~2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)N = 39), whereas the ~2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)N = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P–T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44–51 km with geothermal gradients of 17 to 20 °C/km for the ~2.70 Ga TTG gneisses whereas the ~2.55 Ga TTG gneisses show lesser crustal thicknesses of 35–43 km with geothermal gradients of 19 to 26 °C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (~41 km, ~20 °C/km) of the modern average continental crust, indicating that a modern-style plate tectonic regime may have played an important role in the formation and evolution of the Neoarchean continental crust in the NCC. |
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