Grain size composition and geochemistry of volcaniclastic sediments in the Lau Basin

Early Pliocene to Pleistocene volcaniclastic sediments recovered during Ocean Drilling Program Leg 135 from Sites 834 to 839 in the Lau Basin show a wide range of chemical and mineralogical compositions extending the spectrum previously known from the Lau Basin, Lau Ridge and Tofua Arc. The following major types of volcaniclastics have been distinguished: (1) primary fallout ashes originating from eruptions on land, (2) epiclastic deposits that resulted from subaerial and submarine eruptions, (3) subaqueous fallout and pyroclastic flow deposits resulting from explosive submarine eruptions, and (4) hyaloclastites resulting from mechanical fragmentation and spalling of chilled margins of submarine pillow tubes and sheet-lava flows. Vitric shards are mostly basaltic andesitic to rhyolitic and broadly follow two major trends in terms of K2O enrichment: a low-K series (LKS) with about 1 wt% K2O at 70 wt% SiO2, and a very low-K series (VLKS) with only about 0.5 wt% K2O at 70 wt% SiO2. Sites 834 and 835 on "old" backarc basin crust, >4.2 and 3.4 m.y. old, comprise LKS rhyolites >3.3 m.y. old. Calc-alkaline basaltic turbidites originating from the Lau Ridge flowed in at 3.3 Ma. In the period from 3.3 to 2.4 Ma basaltic andesitic to rhyolitic, fine-grained LKS and VLKS volcaniclastics were deposited by turbidity currents and subaerial fallout. Three thin, discrete fallout layers (2.4-3.2 m.y. old) with high-K calc-alkaline compositions probably erupted in New Zealand. Volcaniclastics from Site 836, all <0.6 m.y. old, make up 24% of the sediments and comprise local basaltic andesitic to andesitic hyaloclastites with low Ba/Zr ratios of 0.9 to 1.4 and polymict andesitic sediments with Ba/Zr ratios of up to 5.5, containing clasts altered to lower greenschist facies. In Sites 837-839, drilled on young crust (1.8-2.1 m.y. old), volcaniclastics make up 45%-64% of the total sediment. Glass compositions are often bimodal with a mafic and a rhyolitic population. Large-volume rhyolitic, silt- to lapilli-sized volcaniclastics are interpreted as pyroclastic flows from explosive eruptions on a seamount 25-50 km away from the sites. Ba/Zr ratios are 2 to 4, partially overlapping with some Lau Basin basement lavas that show an "arc" signature, and they can reach values >5 in thin volcaniclastic layers <0.6 m.y. old.本数据集涵盖了大洋钻探计划（Ocean Drilling Program, ODP）第135航次在劳盆地（Lau Basin）834至839站位所获取的上新世早期至更新世火山碎屑沉积物。这些沉积物具有广泛的化学与矿物组成特征，拓展了此前已知的劳盆地、劳脊（Lau Ridge）与汤加弧（Tofua Arc）的火山碎屑谱系。 研究已识别出以下四类主要火山碎屑类型：（1）源自陆上喷发的原生降落火山灰；（2）由陆上及海底喷发形成的表生碎屑沉积；（3）由爆炸性海底喷发形成的水下降落与火山碎屑流沉积；（4）由海底枕状熔岩席及块状熔岩流的淬冷边缘经机械碎裂、剥落形成的玻屑岩（hyaloclastites）。 玻璃屑主要为玄武安山质至流纹质，在氧化钾（K₂O）富集规律上大致遵循两条主要趋势：其一为低钾系列（low-K series, LKS），在二氧化硅（SiO₂）含量为70 wt%时，氧化钾含量约为1 wt%；其二为超低钾系列（very low-K series, VLKS），在二氧化硅含量同为70 wt%时，氧化钾含量仅约0.5 wt%。 位于年龄大于4.2百万年与3.4百万年的“古老”弧后盆地地壳上的834与835站位，其沉积物以年龄大于3.3百万年的低钾系列流纹岩为主。源自劳脊的钙碱性玄武质浊积岩于3.3 Ma时沉积于此。在3.3 Ma至2.4 Ma期间，玄武安山质至流纹质的细粒低钾系列与超低钾系列火山碎屑通过浊流与陆上降落作用沉积下来。三层离散的薄降落层（年龄为2.4~3.2 Ma）具有高钾钙碱性组成，其喷发源大概率位于新西兰。 836站位的火山碎屑沉积物年龄均小于0.6 Ma，占该站位总沉积物的24%，主要包括两类：其一为本地形成的玄武安山质至安山质玻屑岩，其钡/锆（Ba/Zr）比值介于0.9至1.4之间；其二为多碎屑安山质沉积物，其钡/锆比值最高可达5.5，且含有被改造至低绿片岩相的岩屑。 钻取于年龄1.8~2.1 Ma的年轻地壳上的837至839站位，火山碎屑沉积物占总沉积物的45%~64%。玻璃组成通常呈双峰分布，分别对应铁镁质与流纹质两类组分。大体积的粉砂级至火山砾级流纹质火山碎屑被认为源自距离站位25~50 km处一座海山的爆炸性喷发所形成的火山碎屑流。该区域的钡/锆比值介于2至4之间，部分与部分具有“弧型”地球化学特征的劳盆地基底熔岩重叠；在年龄小于0.6 Ma的薄火山碎屑层中，钡/锆比值可超过5。