Chemical composition of Late Cenozoic volcanic and volcaniclastic deposits from the western Woodlark Basin area, SW Pacific

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene–Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga– Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. […]

源自巴布亚半岛周边及半岛内部火山源的火山灰沉降层（Tephra fallout layers）与火山碎屑沉积（volcaniclastic deposits），构成了伍德拉克海盆（Woodlark Basin）海相沉积序列的重要组成部分。大洋钻探计划（Ocean Drilling Program）第180航次在伍德拉克海盆西部开展的取样工作，为我们记录火山成因组分的分布、评估其年代学、地球化学与同位素组成提供了契机，进而获取有关火山源区与原始岩浆系统的相关信息。从本次取样的上新世-更新世（Pliocene–Pleistocene）沉积序列的57个火山成因层位中筛选出的玻璃碎屑，主体为流纹岩质（rhyolitic）成分，少量为玄武安山岩（basaltic andesites）、玄武粗安岩（basaltic trachy-andesites）、安山岩（andesites）、粗安岩（trachy-andesites）、英安岩（dacites）与响岩（phonolites）。仅依靠地球化学与年代学信息，仅能在各站位间对比出少量火山成因层位，这显然是因为该主动裂谷边缘（actively rifting margin）发育了高度分隔化的沉积域。在多数情况下，我们可通过化学与同位素组成，将第180航次所发现的火山组分与其喷发源区对应起来。就高钾钙碱性玻璃质而言，相当一部分火山碎屑沉积的可能源区为当特尔卡斯托群岛（D'Entrecasteaux Islands）区域的莫尔斯比火山与道森海峡火山（Moresby and Dawson Strait volcanoes）。道森海峡火山似乎是五层过碱性火山灰层的源区。有一层玄武安山质火山碎屑沉积，其地球化学特征与伍德拉克扩张脊端（Woodlark spreading tip）及切希尔海山（Cheshire Seamount）产出的玄武安山岩具有亲缘性。对于其余层位，虽无法明确识别其源区，但它们的同位素与地球化学特征显示其与西南太平洋俯冲火山作用（south-west Pacific subduction volcanism）——如新不列颠（New Britain）与汤加-克马德克岛弧（Tonga– Kermadec island arcs）——的火山作用具有相似性。特罗布里恩德弧（Trobriand Arc）上的火山岛（如伍德拉克岛、安姆利特群岛（Woodlark Island Amphlett Islands）及/或埃古姆环礁（Egum Atoll）），是1.5 Ma至3 Ma之间形成的多套火山碎屑沉积层的可能源区。卢桑凯群岛（Lusancay Islands）可被排除为第180航次所发现的火山成因层位的源区。总体而言，自3.8 Ma以来，巴布亚东部发育了大量钙碱性流纹质火山作用。然而，从135 ka开始，出现了过碱性火山灰层。这种源区特征的地球化学转变，可能反映了影响当特尔卡斯托群岛区域的大地构造环境（geotectonic regime）发生了转变——从地壳俯冲转为扩张作用。[...]