望天鹅火山酸性岩的成因与演化及其岩浆系统重构：来自岩石地球化学和热力学模拟的制约

Understanding the petrogenesis and evolution of continental silicic magmas is of particularly critical importance because they are a primary magma type involved in violent and destructive eruptions. Detailed knowledge of magma plumbing systems beneath continent volcanoes is key for understanding magma evolution and volcanic activities. Wangtian'e stratovolcano, just as Tianchi volcano, is also one of active eruption center of the Changbaishan volcanic field in Northeast China, but the eruption style and evolve trends of silicic magma are significantly different from Tianchi volcano. Since previous studies devoted mainly to Tianchi volcano, only a few studies have reported on Wangtian'e volcano. Meanwhile the petrogenesis and magma evolution of Wangtian'e silicic magmas is still controversial, and the magma plumbing systems are also not well constrained due to lack of high quality of geochemical data and reliable methods. Rhyolite-MELTS can quantitatively simulate the evolution trend of magma crystallization under a given conditions. Therefore, it can also be used to constrain the magma evolution processed and the pre-eruption conditions by fitting the geochemical data of a volcano. This project aims 1) to study the petrogenesis and magma evolution of Wangtian'e silicic magmas and 2) to reconstruct the silicic magma plumbing systems of Wangtian'e. The rhyolite-MELTS software will be used based on geochemical, petrological and isotopic studies, the evolution of Wangtian'e silicic magma will be quantitative simulated, and the depth of silicic magma chamber will be constrained. In addition, the results will compare with Tianchi volcano to explore their links with magma transport, storage, and other activities of the eruption, and to provide some support for the volcanic hazards assessment.

大陆酸性火山岩的成因和演化一直是火山学研究的热点。重构活火山地下岩浆系统对理解岩浆演化过程和火山活动至关重要。望天鹅火山是长白山火山区内规模仅次于天池火山的喷发中心，而且存在再喷发的危险性。前人对望天鹅火山酸性岩的岩石成因和演化的观点还存在争议，对酸性岩浆系统的认识还不够，与天池酸性岩演化趋势不同的原因也未查明。鉴于热力学模拟在岩浆演化过程和岩浆房深度约束上的优势，本项目拟结合火山地质学、岩石地球化学等传统研究方法，采用rhyolite-MELTS热力学模拟软件对望天鹅火山酸性岩的岩石成因和岩浆演化过程进行详细的模拟研究，约束望天鹅酸性岩浆的岩浆房深度，并与天池火山酸性岩进行对比研究，探寻火山区内酸性岩浆的演化、运移、存储、喷发等活动的规律，为火山灾害的预测提供理论依据。

大陆酸性火山岩的成因和演化一直是火山学研究的热点，因其和爆炸式的大喷发紧密相关。理解酸性火山岩的成因是这些复式活火山活动性评价的基础和关键。望天鹅复式火山是长白山火山区内规模仅次于天池火山的喷发中心，而且存在再喷发的危险性，但是对望天鹅火山的基础研究还很匮乏，尤其是对其酸性岩岩石成因和演化以及岩浆系统的认识都还不够深入，导致无法进行危险性评价。本项目通过利用火山地质学、岩石地球化学和用热力学模拟软件相结合的方法对望天鹅火山酸性岩的岩石成因和岩浆演化过程进行详细研究，并对其岩浆房系统做初步的约束。结果显示：作为望天鹅酸性岩主体的粗面岩，并不是简单的结晶分异形成。粗面岩的喷发层序中夹有玄武质岩浆喷发的混合体，其岩相和主量元素、微量元素和Sr-Pb同位素成分都落在了基性岩和碱流岩混合线上。这些证据都一致的支持望天鹅粗面岩是玄武质岩浆与重熔的斜长石堆晶体以及碱流质岩浆熔体混合的成因假说。热力学的模拟还揭示了更为精细的过程：望天鹅地下以拉斑玄武质成分的岩浆房为主，尽管从望天鹅火山岩演化趋势看拉斑玄武岩和碱性玄武岩两个演化趋势都有。同时，模拟显示拉斑玄武质岩浆在上地壳结晶分异形成大量富含斜长石和歪长石的堆晶体（不含透长石）以及形成残余的低钾的碱流质岩浆。而这些滞留在地壳中的残留岩浆房又被下地壳补给来的过渡玄武质岩浆吞噬，二者发生混合后喷出地表形成了望天鹅粗面岩。本项目还研究发现，不管是望天鹅的低钾酸性岩还是天池高钾酸性岩它们的形成和喷发的过程中基性岩的补给混合作用可能是必不可少的条件。这些研究成果和认识对长白山火山区活动性评估以及对火山学的发展都有十分重要的意义。