Crystallization of hydrous mantle magmas at the base of extending continental crust: evidence from the Mafic Complex of the Ivrea-Verbano Zone (Italian Alps)

The composition of the lowermost continental crust is still enigmatic, mostly due to its typical inaccessibility. The archetypal Ivrea-Verbano Zone from Italian Alps exposes a nearly complete sequence of lower continental crust. The Ivrea-Verbano Zone encases the Permian Mafic Complex, an 8 km thick gabbro-dioritic batholith, crystallized during the post-collisional transtensional phase of the Variscan orogeny. The lower section of the Mafic Complex (hereafter referred to as Lower Mafic Complex) is remarkably mainly composed of amphibole gabbros. Evidence for crystallization of hydrous mantle magmas within or at the base of the extending continental crust was rarely reported. The Lower Mafic Complex from the Ivrea-Verbano Zone enables to shed light on the magmatic processes associated with deep-seated emplacement of hydrous mantle magmas at the base of the extending continental crust. The present contribution wishes to ascertain the nature of the H2O-rich magmas forming the Lower Mafic Complex, and the processes driving their chemical differentiation. The whole dataset also allowed us to provide evidence for involvement of primary mantle magmas with different compositions in the building of the whole Mafic Complex. In a broader perspective, the present study aims at acquiring new insights into the accretion of the lower continental crust by magmatic underplating in extensional continental settings. This submission contains a pdf text file with the analytical methodologies used; a pdf file containing supplementary figures; supplementary tables; and references. However, a brief description of the analytical methodologies used is provided within the manuscript.大陆下地壳的组成至今仍属未解之谜，这主要源于其通常难以被直接获取。产自意大利阿尔卑斯山的典型伊夫雷亚-韦尔巴诺带（Ivrea-Verbano Zone）出露了近乎完整的大陆下地壳序列。该伊夫雷亚-韦尔巴诺带（Ivrea-Verbano Zone）包裹着二叠纪镁铁质杂岩（Permian Mafic Complex）——这是一处厚度达8千米的辉长闪长质岩基（gabbro-dioritic batholith），形成于华力西造山运动（Variscan orogeny）的后碰撞张扭阶段。该镁铁质杂岩的下部（下文简称下镁铁质杂岩（Lower Mafic Complex））主体由角闪辉长岩组成，特征显著。此前鲜有报道关于伸展大陆地壳内部或其底部含水幔源岩浆（hydrous mantle magmas）结晶的相关证据。来自伊夫雷亚-韦尔巴诺带（Ivrea-Verbano Zone）的下镁铁质杂岩（Lower Mafic Complex），可为揭示与伸展大陆地壳底部含水幔源岩浆深位侵位相关的岩浆作用过程提供研究契机。本研究旨在明确形成下镁铁质杂岩（Lower Mafic Complex）的富水岩浆的本质，以及驱动其发生化学分异的作用过程。本数据集还可为不同成分的原始幔源岩浆参与构建完整镁铁质杂岩的过程提供佐证。从更宏观的视角来看，本研究旨在为理解伸展型大陆环境下通过岩浆底侵（magmatic underplating）作用实现的大陆下地壳增生过程提供新的认识。 本提交材料包含一份记载所用分析方法的PDF文档、一份包含补充图件的PDF文件、补充表格以及参考文献。不过，本文手稿中已对所用分析方法进行了简要说明。