Revisiting Early–Middle Jurassic igneous activity in the Nanling Mountains,South China: Geochemistry and implications for regional geodynamics |
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| Institution: | 1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;2. School of Earth and Ocean Sciences, University of Victoria, PO Box 1700, STN CSC, Victoria, BC V8W 2Y2, Canada;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;2. Department of Geological Sciences, Indiana University, IN 47405, USA;3. Geological Team 723, Bureau of Geology and Mineral Resources of Guangdong Province, Meizhou 514089, China;1. School of Earth Sciences, Center for Global Tectonics, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;2. Three Gorges Research Center for Geo-hazards, China University of Geosciences, Wuhan 430074, China;3. Department of Geological Engineering, Middle East Technical University, Ankara, Turkey;4. Department of Earth and Environmental Sciences, University of Windsor, Ontario, Canada;1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;2. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;3. No. 2 Geological Team of Guangdong Bureau of Geology, Shantou 515300, Guangdong, China |
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| Abstract: | Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust. |
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