Geochemistry and minerals at DSDP Leg 81 Holes

Three phases of volcanism have been recognized in the lower Paleogene sequence of the southwest Rockall Plateau which are related to the onset of seafloor spreading in the NE Atlantic. The earliest, Phase 1, is marked by a sequence of tholeiitic basalts and hyaloclastites which form the dipping reflector sequence in Edoras Basin. Phase 2 is characterized by tuffs and lapilli tuffs of air-fall origin, ranging in composition from basic to intermediate. They were generated by highly explosive igneous activity due to magma-water interaction, and terminate at the level of a major transgression. Subsequently, volcanism reverted to tholeiitic basalt type, producing the thin tuffs and minor basalt flows of Phase 3. Alteration of the volcanic glass and diagenesis of the tuffs and lapilli tuffs has been considerable in many cases, with a large number of diagenetic mineral phases observed, including smectite, celadonite, analcime, phillipsite, clinoptilolite, mordenite, and calcite. Although calcite is the latest observed diagenetic cement, it nevertheless occurred relatively early, in one case totally preserving basaltic glass from alteration.在西南罗卡尔海台（southwest Rockall Plateau）的下古近系地层（lower Paleogene sequence）中，已识别出三期火山作用，其与北大西洋东北部（NE Atlantic）海底扩张（seafloor spreading）的启动密切相关。最早的第一期火山作用以一套拉斑玄武岩（tholeiitic basalts）和玻质碎屑岩（hyaloclastites）为特征，该套岩石构成了埃多拉斯海盆（Edoras Basin）内的倾斜反射层序（dipping reflector sequence）。第二期火山作用以降落成因（air-fall origin）的凝灰岩（tuffs）和火山砾凝灰岩（lapilli tuffs）为特征，其成分范围从基性至中性；该期火山作用由岩浆-水相互作用（magma-water interaction）引发的强烈爆炸性火成活动形成，终止于大规模海侵（major transgression）层位。随后，火山作用回归拉斑玄武岩类型，形成了第三期的薄层凝灰岩与少量玄武岩流。 在多数案例中，火山玻璃（volcanic glass）的蚀变以及凝灰岩与火山砾凝灰岩的成岩作用（diagenesis）均较为显著，已观测到大量成岩矿物相（diagenetic mineral phases），包括蒙脱石（smectite）、绿鳞石（celadonite）、方沸石（analcime）、钙十字沸石（phillipsite）、斜发沸石（clinoptilolite）、丝光沸石（mordenite）与方解石（calcite）。尽管方解石是目前观测到的最晚形成的成岩胶结物（diagenetic cement），但其形成时间相对较早，在其中一个案例中，它完全保留了玄武质玻璃未发生蚀变。