Rifting-related volcanism in an oceanic post-collisional setting: the Tabar–Lihir–Tanga–Feni (TLTF) island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain occurs in a zone of lithospheric extension superimposed on a post-collisonal tectonic setting along the Pacific and Indo-Australian plates northeast of Papua New Guinea. We present geochemical and Sr, Nd, and Pb isotope data for volcanic rocks from these islands and three recently discovered seamounts located at Lihir island. Major element data document an alkalic affinity of the sample suite and trachybasalts as the predominant rock type. Negative Nb-anomalies in extended trace element patterns, enrichment of the light rare earth elements, and Ce/Pb ratios of about 4 are typical of the values in calc alkaline island arc volcanics and support an origin from subduction-modified mantle. ⁸⁷Sr/⁸⁶Sr ratios of 0.7037 to 0.7044 and _Nd values of +5.6 to +6.8 indicate that the upper mantle evolved with a time-integrated depletion in LREE, however, not as severe as that recorded in basalts from the East Pacific Rise. Variable ⁸⁷Sr/⁸⁶Sr ratios at less variable ¹⁴³Nd/¹⁴⁴Nd ratios suggest that ⁸⁷Sr/⁸⁶Sr ratios of the melts were modified by secondary processes, such as assimilation of seawater Sr from crustal rocks. The Pb isotope ratios are uniform, moderately radiogenic (²⁰⁶Pb/²⁰⁴Pb ca. 18.7 to 18.8), and similar to those reported for the active Mariana arc. Elevated ²⁰⁷Pb/²⁰⁴Pb ratios relative to Pacific MORB suggest melting of small amounts of subducted sediments (ca. 1–2 wt.%). An important control of subducted sediment on the chemistry of the melts can also be inferred from the ratios of highly incompatible trace elements (e.g., Th, U, Pb, La, and Nb). Additional mantle enrichment by subduction derived fluids is reflected in high values of highly incompatible trace element ratios between fluid mobile (e.g., Ba) and fluid immobile elements (e.g., Th, Nb). The results of this study document that the chemical composition of igneous rocks from post-collisional tectonic settings are strongly influenced by previous plate tectonics. This conclusion implies that the information conveyed by tectonic discrimination diagrams for these rocks must be interpreted with care.     

巴布亚新几内亚利希尔岛拉多拉姆(Ladolam)金矿矿床地质特征及找矿标志

巴布亚新几内亚利希尔岛拉多拉姆金矿处于太平洋板块与印澳板块的板块边界部位,为西南太平洋地区最大的碱性侵入岩有关的低硫型浅成低温热液型金矿。矿床赋存在Luise火山内,成矿与更新世碱性二长闪长岩侵入体有关,该岩体形成于俯冲带环境下被流体交代的上地幔的部分熔融。矿床的矿化可分为三个阶段,在岩体侵入后形成斑岩铜矿化,并在之后逐渐过渡为浅成低温热液型金矿化。该类矿床在成矿过程和矿体的分布明显受到区域性构造、蚀变带和成矿有关角砾岩等成矿结构面的控制,在找矿标志上具有特定的物化探和地质特征,具有较好的找矿远景。