The transition of the magma plumbing system of Tianchi shield-forming basalts, Changbaishan Volcanic Field, NE China: Constraints from dynamic Fe-Mg diffusion modelling in olivine
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Abstract The depths of crustal reservoirs within volcanic systems may experience transitions over time. Here, we report the crystal and bulk rock compositions of the shield-forming basaltic lavas of the Tianchi composite volcano in the intraplate Changbaishan Volcanic Field, NE China to constrain the crustal magmatic evolution with time. We investigated samples covering the entire basaltic stratigraphic sequence, consisting of the Toudao (TD), Baishan (BS), and Laofangzixiaoshan (LFZ) units from bottom to top, respectively. The core compositions of olivine macrocrysts vary among the three units, i.e., the TD and BS olivine phenocrysts can both be divided into two populations: a high-Fo population (~Fo76-80) and a low-Fo population (~Fo72-74). The LFZ unit only exhibits a high-Fo population (~Fo77-80). Phase equilibria modelling using rhyolite-MELTS suggests that the high-Fo populations were stored at depths of ~20 km for the TD and BS units and ~15 km for the LFZ unit. The low-Fo populations crystallized at shallow depths, i.e., ≤15 km for the TD unit and ≤13 km for the BS unit. We employ a dynamic Fe-Mg interdiffusion modelling with constantly adapting boundary conditions in zoned olivine macrocrysts to constrain the magmatic environments and timescales during the pre-eruption and post-eruption, enabling clarify the magmatic histories recorded by two olivine populations. The dynamic Fe-Mg interdiffusion modelling considers the variable boundary condition caused by crystal growth and composition variation of melts during magma cooling. Calculated results suggest that the high-Fo populations from the TD and BS units recorded prolonged timescales ranging from six months to more than two years with lower cooling rates and slower crystal growth 37 rates. These characteristics reflect a relatively hot and slow-cooling magmatic environment; and the modelled timescales correspond to the sum time including shallow storage, magma ascent, and further cooling within the lava flows. Conversely, the high-Fo population from the LFZ unit and the low-Fo populations from the TD and BS units record shorter timescales (<140 days) with higher cooling rates and faster crystal growth rates. These results indicate relatively cold and highly undercooling magmatic environments; hence the timescales record magma ascent in the conduits and further cooling during lava emplacements. Our study demonstrates that the Tianchi basaltic plumbing system experienced a structural transition over time. In detail, the TD and BS magmas experienced multi-stage stalling and ascent, first accumulating in deep reservoirs and then transferring to shallow reservoirs for storage before the eruption. The LFZ magmas accumulated in a mid-crustal reservoir, followed by a direct ascent to the surface without additional residence.Dike
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标明日期的高精确锆石 U-Pb 显示那主要恼人的身体(Tongan， Niumiao， Huashan， Lisong ) ，并且在 Huashan-Guposhan 建筑群的一块 mafic microgranular 被外国领地包围的土地在 160163 妈被形成。从 Huashan 花岗石的锆石的 ? Hf (t) 价值变化从 ? 2.8 变化到从 Lisong 花岗石的 +0.3 和那些从 ? 2.3 到 +0.3，它与从 Lisong 花岗石的 mafic 被外国领地包围的土地的那些价值(到 +7.4 的 +2.6 ) 显然是不同的。这些 Hf 同位素的数据显示 mafic 被外国领地包围的土地和主机花岗石从岩浆的不同来源被使结晶，为 mafic-felsic 岩浆混合提供证据处理。最高 ? 从 mafic 被外国领地包围的土地的锆石的 Hf (t) 价值直到 +7.4，显示 mafic 岩浆从相对弄空的披风来源被发源。在这个区域的地区性的地质学和同时期的 mafic 和碱的岩石上的研究显示 mafic 岩浆没从玄武岩的少年外壳重做，但是从披风的部分融化被发源。然而， mafic 岩浆是否从岩流圈或 lithospheric 披风的部分融化被导出，尚待被决定。Huashan 花岗石和 Lisong 花岗石从导出披风、导出外壳的岩浆的混合岩浆被形成，并且 mafic 被外国领地包围的土地在混合过程期间被看作导出披风的岩浆的遗体。从 Niumiao 闪长岩的锆石的 ? Hf (t) 价值变化从 ? 1.1 变化到从 Tongan 石英二长岩的 +2.1，和那些从 ? 1.7 到 +1.7。这些价值是比从 mafic 被外国领地包围的土地的那些低的，建议闪长岩和二长岩从有 mafic 被外国领地包围的土地的不同导出来源的岩浆被形成。为 Niumiao 闪长岩和 Tongan 二长岩的 ? Hf (t) 价值仅仅比为 Hushan 花岗石和 Lisong 花岗石的那些稍微高。丰富的 mafic 被外国领地包围的土地也发生在 Niumiao 闪长岩并且在 Tongan 二长岩。因此，我们建议 Niumiao 闪长岩和 Tongan 二长岩也可能从象花岗石的一样的混合岩浆被形成，但是通过部分结晶化和外壳的污�
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2 . Based on image interpretation, Aubele (2) stated that shield fields may have deep magma sources with low magmatic flux; comparitively, shield plains may have relatively shallow, and more widespread magma sources. Shield fields may be either strati- graphically older, younger, or concurrent with the plains units that surround them. In contrast, shield plains consistently appear stratigraphically above densely lineated plains and below wrinkle-ridged plains (3).
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Current theories for the generation of voluminous granitoid intrusions state that mafic magmas can provide a localized, external source of heat for crustal melting. Regardless of their age or tectonic setting, many alkaline, metaluminous, and peraluminous granitoid bodies occur with contemporaneous satellite mafic intrusions and show spatial relationships with mafic magmas in the form of synplutonic mafic bodies and mafic igneous enclaves. Physical juxtaposition of the intruded mafic magmas and resulting anatectic granitoid melts also allows for the transfer of chemical components to produce hybridized lithologies by mechanical interaction and/or chemical diffusion. In this multidisciplinary study, identification of all these established criteria for mafic-granitoid genetic relationships, in combination with one-dimensional thermal models of mafic intraplating, allows the assessment of a role for intruded mafic magma as a source of chemical components and, ultimately, heat in a suite of granitoid rocks.
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