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highly fractionated granites:Recognition and research

Science China Earth Sciences 2017年第7期60卷 1201-1219页

作者： WU FuYuan LIU XiaoChi JI WeiQiang WANG JiaMin YANG Lei State Key Laboratory of Lithospheric Evolution Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China Chinese Academy of Sciences Center for Excellence in Tibetan Plateau Earth Sciences Beijing 100101 China College of Earth Sciences University of Chinese Academy of Sciences Beijing 100049 China

granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture,structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation,mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, flow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated(including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that h

关键词： Fractional crystallization Accumulation highly fractionated granite Granitic magma Continental crustal evolution

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Crystal fractionation of granitic magma during its non-transport processes： A physics-based perspective

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Science China Earth Sciences 2018年第2期61卷 190-204页

作者： CHEN Chen DING Xing LI Rui ZHANG WeiQi OUYANG DongJian YANG Lei SUN WeiDong CAS Key Laboratory of Mineralogy and Metallogeny Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China University of Chinese Academy of Sciences Beijing 100049 China State Key Laboratory of Isotope Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China State Key Laboratory of Lithospherie Evolution Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China Center of Deep Sea Research Institute of Oceanography Chinese Academy of Sciences Qingdao 266071 China Laboratory for Marine Mineral Resources Qingdao National Laboratory for Marine Science and Technology Qingdao 266237 China CAS Center for Excellence in Tibetan Plateau Earth Sciences Chinese Academy of Sciences Beijing 100101 China

Granitic continental crust distinguishes the Earth from other planets in the Solar System. Consequently, for understanding terrestrial continent development, it is of great significance to investigate the formation and evolution of *** fractionation is one of principal magma evolution mechanisms. Nevertheless, it is controversial whether crystal fractionation can effectively proceed in felsic magma systems because of the high viscosity and non-Newtonian behavior associated with granitic magmas. In this paper, we focus on the physical processes and evaluate the role of crystal fractionation in the evolution of granitic magmas during non-transport processes, i.e., in magma chambers and after emplacement. Based on physical calculations and analyses, we suggest that general mineral particles can settle only at tiny speed(~10^(-9)–10^(-7) m s^(-1))in a granitic magma body due to high viscosity of the magma; however, the cumulating can be interrupted with convection in magma chambers, and the components of magma chambers will tend to be homogeneous. Magma convection ceases once the magma chamber develops into a mush(crystallinity, F>~40–50%). The interstitial melts can be extracted by hindered settling and compaction, accumulating gradually and forming a highly silicic melt layer. The high silica melts can further evolve into high-silica granite or high-silica rhyolite. At various crystallinities, multiple rejuvenation of the mush and the following magma intrusion may generate a granite complex with various components. While one special type of granites, represented by the South China lithium-and fluoride-rich granite, has lower viscosity and solidus relative to general granitic magmas, and may form vertical zonation in mineral-assemblage and composition through crystal fractionation. Similar fabrics in general intrusions that show various components on small lengthscales are not the result of gravitational settling. Rather, the flowage differentiation may pla

关键词： granite Crystal fractionation Magma convection Layering structure Mush model highly fractionated granite granite complex Li-F granite

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