Sediment contribution in post-collisional high Ba-Sr magmatism: Evidence from the Xijing pluton in the Alxa block,NW China |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources and School of Earth Science, China University of Geosciences, Wuhan 430074, PR China;2. Department of Earth Sciences, University of Bristol, Bristol BS81RJ, UK;3. Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, PR China;1. Geology Department, Faculty of Science, Beni-Suef University, Beni-Suef 62521, Egypt;2. Geology Department, Faculty of Science, Helwan University, 11790 Cairo, Egypt;3. Geology Department, Faculty of Science, Cairo University, Giza 12613, Egypt;4. Economic Geology Research Unit (EGRU), Department of Earth and Oceans, James Cook University, Townsville, 4011, QLD, Australia;5. Department of Economic Geology and Petrology, Institute of Mineralogy, Technische University Bergakademie, Freiberg, Brennhausgasse 14, 09596 Freiberg/Sachsen, Germany;1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;1. Programa de Pós-graduação em Geologia, Universidade Federal do Paraná, Av. Coronel Francisco Heráclito dos Santos 210, 81531-970 Curitiba, Brazil;2. Géosciences Montpellier, CNRS and Université de Montpellier, Place Eugène Bataillon, 34090 Montpellier, France;3. Instituto de Geociências, Universidade de São Paulo, Rua do Lago 562, 05508-080 São Paulo, Brazil;1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | High Ba-Sr granitoids occur in a number of tectonic settings, and variable petrogenetic models have been proposed. Those from post-collisional settings are rarely studied and are the focus of this study. Zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Hf isotopic data are presented for a suite of high Ba-Sr granitoids from the southern margin of the Alxa block, NW China. U-Pb zircon dating shows that the Xijing high Ba-Sr granitoids were emplaced in two periods at ~440 Ma and ~410 Ma, and they are characterized by elevated Ba and Sr contents. The older ~440 Ma high Ba-Sr granitoids range from monzodiorite to quartz monzonite, and the younger ~410 Ma high Ba-Sr granites tend to be more evolved. Both suites have similar enriched Sr-Nd-Hf isotopic ratios, low Ba/Th and Sr/Th ratios, and high Th contents and Th/Ce ratios indicative of a sediment contribution. This is potentially linked to previous subduction of the North Qilian ocean slab. Major and trace element calculations suggest that the older ~440 Ma granitoids experienced up to ~60% fractionation at 4–6 kbar with a crystallizing assemblage dominated by amphibole and plagioclase. The younger ~410 Ma granites could in turn have been formed by a further 80% fractional crystallization at lower pressures (1–2 kbar). Trace element and Sr-Nd isotopic modeling suggest incorporation of ~5% slab-derived sediment into mantle magma source of these high Ba-Sr granitoids. We note that post-collisional granitoids with high Ba and Sr characteristics may also reflect a sediment contribution in their source region, and this may be a key aspect of why such granites plot in the field of post-collisional granites in the Rb vs Y + Nb diagram. Some sanukitoids with high Ba and Sr contents in the late Archean also appear to reflect sediment subduction and they hence may represent early stages of crustal recycling. |
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