峨眉山大火成岩省大理核心区早期喷发环境解译与火山机构重建

Mantle plume and large igneous province theory has long attracted major interests in global geoscience field. The ~260 Ma Emeishan Large Igneous Province (ELIP) located in Southwest China has been specially focused for it once was the best example of mantle plume induced crustal uplift, but recently has been challenged by increasing observations of mafic volcaniclastic deposits. A hot debating is fueled without consensus, largely due to poorly understanding on physical volcanology of the ELIP. Therefore we set the goal of our research as to reconstruct information of the volcanic edifice in the central ELIP and hence its evolution history. Main aspects include (1) identification of newly found volcaniclastic facies and following mechanism interpretations, (2) revealing the paleoenvironment during the early eruption period, (3) constraining a precise onset and termination time of the entire eruption process and (4), locating the spatial distribution of the central vent site and building an dynamic evolution model of the volcanic edifice. Volcanology and volcanostratigraphy are considered as main research methods associated with biostratigraphy, isotopic geochronology and geochemistry to give systematic assistances. The goal of this research is to give a precise understanding on volcanism, temporal duration and spatial distribution of the ELIP central volcanic system, which is aimed to make new contribution for the innovation and progress of LIP and mantle plume theories.

地幔柱-大火成岩省理论是当今国际地学界的热点问题，而峨眉山大火成岩省（ELIP）更是当下聚焦的前沿。发育于~260 Ma 的ELIP曾一度被认为是记录地幔柱诱发地壳隆升行为的国际最佳实例之一，然而近期却受到野外火山学观察证据的质疑，至今没有达成共识。本研究的研究目标便是：在扎实的野外火山学研究基础上，厘定大理核心区的早期火山喷发环境和火山机构样式及其演化史。主要内容包括：（1）新型火山碎屑岩相的发现与机制解译、（2）早期火山喷发时期的地质环境解译、（3）中央带精确起止延限的厘定以及（4）确认中央火口的空间方位并建立完整火山机构的展布演化模型。本研究以火山地质学为主要方法，同时综合生物地层学、同位素年代学、主微量-同位素地球化学等方法，以期建构峨眉山大火成岩省大理地区中央火山机构的时间-空间展布模模型，恢复其演化过程，创新发展峨眉大火成岩省和地幔柱相关理论。

大火成岩省是远古的巨型熔岩体，发源于地球历史上那些大规模的火山喷发。峨眉山大火成岩省（以下简称ELIP, Emeishan Large Igneous Province）发育于扬子板块西缘，紧邻古特提斯洋东侧分支，此区域不仅保存了精确古环境信息的二叠纪浅水碳酸盐台地序列，且以大理-丽江地区为主，保存着良好的ELIP中央带早期喷发层序，总厚度约5400米，含有应答ELIP起源机制的潜在信息。本研究在该地区进行了火山-沉积相研究，以系统识别大理地区下部层系里典型MVD为基础，对ELIP 早期喷发环境进行判别，搭建了喷发前至喷发早期的火山-沉积相演化格架，揭示了两套关键现象：（1）在ELIP下伏的茅口组顶部，浅水碳酸盐台地层序被深水碳酸盐重力流沉积骤然所覆。（2）ELIP早期喷发层序上覆于碳酸盐重力流，呈现出四套由深水向浅水建隆的旋回，旋回内层序以枕状熔岩筑底、向上过渡为淬碎角砾岩、浅水水汽爆破层序，最终旋回式加深，记录了深水至浅水喷发的渐隆过程及期间持续的拉张构造活动。以上述火山学研究位基础，结合空间逐步规律分析，得到以下主要认识：在茅口晚期，地壳开始活动，形成一条沿N-S走向展布的狭窄裂谷，谷内水深骤然加剧。ELIP中央带早期火山活动便起始于该裂谷深水环境中，裂谷作用随后和火山活动共同持续，导致喷发环境水深呈现旋回特征。在火山层序2700米层位，喷发相由水下全面转为陆上，标志着水下裂谷充填完成、喷发早期结束。随后，本地和扬子板块广大区域一同进入以陆相玄武质熔岩溢流为特征的主喷发期。项目还首次在大理核心区识别并确认了受地幔柱热源诱发的壳源熔融花岗岩，并在天山博格达地区新发现peperite系列，为限定古生代晚期构造背景提供了可靠证据，也为推广peperite研究做出新贡献。

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