Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea

摘要: The SiO2-undersaturated lavas from Lihir island, Papua New Guinea, like most arc are highly enriched in Sr, Ba, K, Rb, and Cs depleted Hf, Ta, Nb, Ti relative to ocean floor basalts oceanic island basalts. These alkali-rich have trace element signatures Nd, Pb isotopic systematics. However, they not a product of present-day subduction, as this volcanism has tapped mantle which was by prior subduction episodes. narrow range (206pb/204pb, 18.74–18.76) Nd (143Nd/144Nd, 0.51297–0.51304) compositions suggest cogenetic origin for these lavas. During the fractionation primitive lavas, elements normally considered incompatible (i.e., light rare earth (LREE), Th, P) high bulk solid/melt partition coefficients (0.15–1.5). Relatively higher during forma-tion evolved produced crossing (REE) patterns, abundances than This results (1) changes amount apatite fractionating assemblage (2) abnormal partitioning into microphenocrysts nucleate at crystal-liquid interface rapidly growing clinopyroxene phenocrysts. lower alkali, Cs, LREE other Tabar-Feni They derived less source rather degree melting similar that islands. similarity Sm/Nd ratios undersaturated those tholeiitic calc-alkaline moderate chondrite-normalized La/Yb (La/Ybcn = 3–7) indicates there been limited enrichment heavy REE generation arc-modified mantle. alkaline nature reflects their generation, tensional tectonic environment, “fossil” region undergone extreme alkali two earlier