Geochemistry of the alkaline volcanicsubvolcanic rocks of the Fernando de Noronha Archipelago, southern Atlantic Ocean

The Fernando de Noronha Archipelago presents, on its main island, a centrally-located stratigraphic unit, the Remédios Formation (age around 8 - 12 Ma) constituted by basal pyroclastic rocks intruded by dikes, plugs and domes of varied igneous rocks, capped by flows and pyroclastics of mafic to ultramafic rocks of the Quixaba Formation (age around 1 - 3 Ma), which is limited from the underlying unit by an extensive irregular erosion surface. A predominant sodic Remédios series (basanites, tephrites, tephriphonolites, essexite, phonolites) can be separated from a moderately potassic Remédios sequence (alkali basalts, trachyandesites, trachytes), both alkaline series showing mostly continuous geochemical trends in variation diagrams for major as well as trace elements, indicating evolution by crystal fractionation (mainly, separation of mafic minerals, including apatites and titanites). There are textural and mineralogical evidences pointing to hybrid origin of some intermediate rocks (e.g., resorbed pyroxene phenocrysts in basaltic trachyandesites, and in some lamprophyres). The primitive Quixaba rocks are mostly melanephelinites and basanites, primitive undersaturated sodic types. Geology (erosion surface), stratigraphy (two distinct units separated by a large time interval), petrography (varied Remédios Formation, more uniform Quixaba unit) and geochemistry indicate that the islands represent the activity of a protracted volcanic episode, fueled by intermittent melting of an enriched mantle, not related to asthenospheric plume activity.

费尔南多-迪诺罗尼亚群岛（Fernando de Noronha Archipelago）主岛上发育一处位于中部的地层单元——雷梅迪奥斯组（Remédios Formation，形成年代约8~12 Ma），其由基底火山碎屑岩构成，并被多种火成岩组成的岩墙、岩颈及穹隆侵入，顶部被基克萨巴组（Quixaba Formation，形成年代约1~3 Ma）的镁铁质至超镁铁质岩流与火山碎屑岩覆盖；该地层与下伏单元以大范围不规则侵蚀面为界。雷梅迪奥斯组可划分为以钠质为主的岩系（包括碧玄岩、安粗岩、碱玄质响岩、正霞正长岩、响岩）与中等钾质序列（包括碱性玄武岩、粗安岩、粗面岩）；两类碱性岩系在主量元素与微量元素的变异图解中均呈现出连续的地球化学变化趋势，表明其演化主要通过结晶分异作用实现，即镁铁质矿物（包括磷灰石与榍石）的分离结晶。部分中性岩存在结构与矿物学证据，表明其具有混合成因，例如玄武质粗安岩与部分煌斑岩中可见熔蚀的辉石斑晶。基克萨巴组的原始岩石以黑云霞石岩与碧玄岩为主，属于不饱和钠质原始岩石类型。该群岛的地质特征（侵蚀面）、地层结构（两大独立地层单元间隔漫长地质时限）、岩石学特征（雷梅迪奥斯组岩性多样，基克萨巴组岩性更为均一）以及地球化学特征均表明，该群岛是一次持续漫长火山活动期的产物，其动力来源于富集地幔的间歇性熔融，与软流圈柱活动无关。