Abstract

Cenozoic geology of Woodlark Island was influenced by events prior to the onset of continental rifting at ca 8 Ma and the slightly later commencement of seafloor spreading in the eastern Woodlark Basin between ca 6 and 3.6 Ma. The embryonic Woodlark Island consisted of latest Paleocene(?)–Eocene shallow marine, deep-water shelf and turbidite deposits (Loluai Formation) and small submarine and emergent mafic volcanic centres (Utavai Basalt Member). Platform carbonates extended across the Woodlark Basin in the late Oligocene–middle Miocene (Nasai Limestone). Onset of middle Miocene eruptions in the north-northeast-trending Okiduse Volcanic Zone (OVZ) was localised in the similarly trending Kulumadau Horst and Graben Zone (KHGZ). Volcanic centres flanked major structures in the 12.5 km wide horst and graben zone, suggesting a link between volcanism and extensional tectonics. It was not an island arc settling, and there are neither andesites nor an obvious subduction zone. Offshore the KHGZ is connected with the similarly trending Nubara Fault, the plate boundary between the Solomon Sea Plate and Woodlark Plate. Contrasting volcanic deposits were erupted from volcanoes of the OVZ, viz. Mt Kabat Eruptive Centre, Uvarakoi Caldera, Watou Mountain Eruptive Centre and Suloga Harbour Tuff Cone. The epithermal Busai and Kulumadau gold deposits are located in the KHGZ. They are hosted by porous and permeable crystal lithic lapilli tuff beds in the Monasiy Tuff (Uvarakoi Caldera). Emplacement of thick dolerite sills (Auyed Dolerite) occurred post-middle Miocene. Sill emplacement coincided with the ca 6–3.6 Ma commencement of seafloor spreading in the eastern Woodlark Basin. A major phase of extensional tectonics along the KHGZ commenced post-middle Miocene and has persisted to the Holocene. Uplift was accompanied by rapid erosion and the widespread deposition of poorly to chaotically sorted gravels (Na’ku’iel Conglomerate).

1. KEY POINTS

2. Middle Miocene volcanism occurred from four volcanic centres in the north-northeast-trending Okiduse Volcanic Zone.

3. Volcanism was confined to the similarly north-northeast-trending Kulumadau Horst and Graben Zone. Volcanism was not located in an island arc setting, as there are neither andesites nor an obvious subduction zone.

4. A major phase of extensional tectonics along the horst and graben zone commenced post-middle Miocene and has persisted to the Holocene.

5. Both the epithermal Busai and Kulumadau gold deposits exhibit lithological controls (porosity and permeability) on ore deposition.

摘要

伍德拉克岛的新生代地质是由前大陆裂谷的发病事件在影响CA 8 Ma和海底的稍晚开始之间的东部伍德拉克盆地传播CA6和3.6 Ma。原始的伍德拉克岛由最新的古新世（？）–始新世的浅海，深水层架和浊积岩沉积物（Loluai组）以及小型海底和新兴的镁铁质火山中心（Utavai玄武岩成员）组成。在渐新世-中新世中期（Nasai石灰岩）中，台地碳酸盐岩延伸至整个Woodlark盆地。北部向北的Okiduse火山带（OVZ）中中新世爆发发生在类似趋势的Kulumadau Horst和Graben带（KHGZ）中。火山中心位于12.5 km的霍斯特和格陵兰地带的主要构造两侧，表明火山作用与伸展构造之间存在联系。这不是岛上的弧形沉降，也没有安山岩，也没有明显的俯冲带。海上KHGZ与类似趋势的Nubara断层相连，所罗门海板块和伍德拉克板块之间的板块边界。OVZ的火山喷出了不同的火山沉积物，即 Kabat山喷发中心，Uvarakoi火山口，Watou山喷发中心和Suloga海港凝灰岩。釜山和库鲁马道的超热金矿床位于KHGZ。它们由Monasiy Tuff（Uvarakoi Caldera）中的多孔和可渗透的晶体石质拉菲凝灰岩床主持。中新世后发生了厚的钙铝石基石（Auyed Dolerite）的进入。基台位正好与伍德拉克盆地东部海底扩张的开始时间大约6–3.6 Ma相吻合。KHGZ沿伸展构造的一个主要阶段开始于中新世中期以后，一直持续到全新世。隆起伴随着快速的侵蚀以及从不良到混乱分类的砾石的广泛沉积（Na'ku'iel砾岩）。

1. 关键点

2. 中新世中层火山活动发生在北向北的Okiduse火山区的四个火山中心。

3. 火山活动仅限于北风向北的库鲁马道霍斯特和格拉本地区。由于没有安山岩，也没有明显的俯冲带，所以火山活动不位于岛弧环境中。

4. 沿霍斯特和格伦布带的伸展构造的一个主要阶段开始于中新世中期以后，一直持续到全新世。

5. 釜山超热金矿和库鲁马道金矿在沉积时均表现出岩性控制（孔隙度和渗透率）。