Late Cretaceous volcaniclastic rocks at DSDP Holes 74-525A and 74-528@en

Volcaniclastic rocks of Late Cretaceous age occur in four out of five sites (525, 527, 528, 529) drilled on the crest and the northwest flank of the Walvis Ridge during Leg 74. They are mostly interlayered with and overlie basement in the lowermost 10-100 m of the sedimentary section. Rocks from Holes 525A and 528 were studied megascopically and microscopically, by XRD, and XRF chemical analyses of whole-rock major and trace elements were undertaken. The dominant rock of Hole 528 volcaniclastics is a fine-grained (silt to fine sand), mostly matrix-bearing (partly matrix-rich) vitric \"tuff,\" occurring as 5-110 cm thick, partly graded layers, some of which are distinctly bedded. Volcaniclastics of Hole 525A are generally richer in sanidine crystals. Most rocks contain some nonvolcanic clasts, chiefly foraminifers and lesser amounts of shallow-water fossil debris. Scoria shards, clasts of tachylite, and fine-grained basalts as well as chemical analyses suggest a basaltic to intermediate composition for most rocks of Hole 528, whereas volcaniclastics of Hole 525A are more silicic. The occurrence of tachylite and epiclastic, coarse-grained, basaltic clasts throughout the volcaniclastic sequence at Site 528 indicates shallow-water eruptions and perhaps even ocean island volcanism. The minor occurrence in Hole 528 of trachytic? pumice shards with phenocrysts of K-feldspar and the abundance of such shards in rocks from Hole 525A indicate Plinian eruptions characteristic of more mature stages of ocean island evolution. The sedimentary structures of volcaniclastic layers and their occurrence within deep sea calcareous oozes indicate a mass flow origin. […]晚白垩世（Late Cretaceous）火山碎屑岩（volcaniclastic rocks）见于第74航次（Leg 74）在沃尔维斯海岭（Walvis Ridge）峰顶及西北翼钻取的5个站位中的4个（525、527、528、529）。该类岩石多与沉积剖面最下部10~100米范围内的基底互层并覆于基底之上。对525A孔与528孔的岩石开展了宏观与微观岩石学研究，并通过X射线衍射（X-ray diffraction, XRD）分析及X射线荧光光谱（X-ray fluorescence, XRF）全岩主量与微量元素化学分析完成了组分测试。528孔火山碎屑岩的主导岩性为细粒级（粉砂至细砂级）、多含基质（部分富基质）的玻质‘凝灰岩’，以厚度5~110厘米、部分具粒序层理的层状产出，其中部分层段具清晰的层理构造。525A孔的火山碎屑岩普遍含更丰富的透长石（sanidine）晶屑。多数岩石含有少量非火山碎屑，主要为有孔虫，辅以少量浅水化石碎屑。火山渣屑、玄武玻璃（tachylite）岩屑、细粒玄武岩以及化学分析结果均表明，528孔多数岩石的成分为玄武质至中性；而525A孔的火山碎屑岩则更富硅质。528站位整个火山碎屑岩序列中广泛产出玄武玻璃岩屑及外生粗粒玄武岩屑，指示了浅水喷发环境，甚至可能为洋岛火山活动。528孔中少量产出含钾长石斑晶的疑似粗面岩质浮岩屑，而525A孔岩石中此类岩屑极为发育，这指示了洋岛演化成熟阶段典型的普林尼式喷发（Plinian eruptions）。火山碎屑岩层的沉积构造及其产于深海钙质软泥之中的产出特征，表明其为块体流成因。……