A new model for the granite–pegmatite genetic relationships in the Kaluan–Azubai–Qiongkuer pegmatite-related ore fields, the Chinese Altay
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Pegmatite
Igneous differentiation
Fractional crystallization (geology)
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Muscovite
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Incompatible element
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Aso caldera (SW Japan) was formed incrementally over a period of ∼300 kyr, by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits (Aso 1–4). These units are collectively referred to as the Aso Pyroclastic Flow Deposit (APFD) in which tholeiitic and calc-alkaline magmas have been erupted in close temporal and spatial proximity. Detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between Aso 1 and 4 as the caldera-forming stage evolved, changing from a predominantly tholeiitic to a calc-alkaline signature. The mineralogical and geochemical characteristics of Aso 1–4 suggest that magmatic evolution within each unit is controlled by fractional crystallization, associated with magma mixing ± crustal assimilation. By altering the fractional crystallization mineral assemblage, as well as the relative amounts of fractional crystallization, magma mixing and crustal assimilation, the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced. This suggests that the coexistence of tholeiitic and calc-alkaline magma series can be attributed to shallow-level intracrustal processes.
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Abstract Magma differentiation in arc settings has usually been attributed to an interplay of processes (fractional crystallization, assimilation, and magma mixing). Homogeneous fractional crystallization has been widely used to model the magmatic evolution of volcanic systems in arc settings due to its simplicity, even though boundary layer fractionation (BLF) has been proposed as a preponderant process of differentiation in hydrous magmatic systems. Both models produce distinct compositional paths and the application of the wrong model yields erroneous estimates of parameters like pressure–temperature-H2O conditions and primary melt compositions. Melt inclusion (MI) populations corrected for post-entrapment processes have the potential to help discriminate between these two types of fractional crystallization, as their compositions are not affected by crystal accumulation and should capture the magmatic evolution as crystallization occurs. In this study, olivine-hosted MIs are used to assess the differentiation trends of basic arc magmas in northern Japan. Differentiation trends from five arc volcanic systems in northern Japan show that BLF is ubiquitous. Homogeneous fractionation models are unable to explain the liquid lines of descent of minor elements, like TiO2 and P2O5. To reproduce these differentiation trends, the presence of accessory phases like titanomagnetite or apatite are required, which in many cases are not equilibrated by the melt or need to be fractionated in amounts that are incompatible with homogeneous fractionation. The prevalence of BLF in all studied arc magmas of northern Japan indicates that solidification fronts are key environments in the crustal evolution of some hydrous subduction zone magmas.
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Fractional crystallization (geology)
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PetroGram is an Excel© based magmatic petrology program that generates numerical and graphical models. PetroGram can model the magmatic processes such as melting, crystallization, assimilation and magma mixing based on the trace element and isotopic data. The program can produce both inverse and forward geochemical models for melting processes (e.g. forward model for batch, fractional and dynamic melting, and inverse model for batch and dynamic melting). However, the program uses a forward modeling approach for magma differentiation processes such as crystallization (EC: Equilibruim Crystallization, FC: Fractional Crystallization, IFC: Imperfect Fractional Crystallization and In-situ Crystallization), assimilation (AFC: Assimilation Fractional Crystallization, Decoupled FC-A: Decoupled Fractional Crystallization and Assimillation, A-IFC: Assimilation and Imperfect Fractional Crystallization) and magma mixing. One of the most important advantages of the program is that the melt composition obtained from any partial melting model can be used as a starting composition of the crystallization, assimilation and magma mixing. In addition, PetroGram is able to carry out the classification, tectonic setting, multi-element (spider) and isotope correlation diagrams, and basic calculations including Mg#, Eu/Eu∗, εSr and εNd widely used in magmatic petrology.
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AssrRAcr Linkages between the fertile granite and associated pegmatites, linkages among pegmatites, and processes instrumental in defining their textural, chemical, and mineralogical uniqueness are obscured by their coarse grain-size and the transitional nature between magmatic and hydrothermal regimes. Our studies of the Harney Peak granite-pegmatite system in the Black Hills, South Dakota, indicate that these systems are a cuhnination of distinctive processes of partial melting and fractional crystallization. Both the Harney Peak Granite and the associated pegmatite field are mineralogically and chemically zoned. Superimposed on the general zoning in the pegmatite field are swanns of pegmatites that appear to define distinct compositional-textural arrays. These may be related to distinct fractional crystallization trajectories. The Harney Peak Granite and numerous pegmatites define a single trajectory of fractional crystallization, whereas Li-, Rb-, Cs-enriched zoned pegmatites and F-, Sn-, Be-enriched pegmatites represent trajectories of fractional crystallization involving chemically distinct magma-types. A fractionation trajectory (from biotite ganites to tourmaline granites) does not represent an evolutionary path of a single magma, but most likely represents paths of fractional crystallization of similar batches of magmas. Within each trajectory of fractional crystallization are coherent sequences of textural and mineralogical characteristics. The extreme rare-element enrichments observed in pegmatites can in part be modeled by moderate to high degrees of fractional crystallization (up to 70-90q0) of a suite of volatile-rich magmas. In addition to fractional crystallization, partial melting appears to be important in controlling the potential content and composition of the volatile component, and the incompatible element character of tle distinct magma-types. Varying dqrees of detrydrarion melting of compositionally diverse metasediments appear to be the most likely model for the production of these compositional diverse parental granitic magmas.
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Fractional crystallization (geology)
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Igneous differentiation
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