Ancient isotopic characteristics of Neogene potassic magmatism in Western New Guinea (Irian Jaya, Indonesia)

In the Central Ranges of western New Guinea a suite of 3–7 Ma collision-related magmatic rocks were emplaced in, or erupted through, an orogenic belt associated with the partial subduction of the northern continental margin of Australia beneath a south-facing oceanic island arc. Intermediate calc-alkaline to low-K shoshonitic rocks are most common throughout this belt, although other, less common, associations include lamprophyric, high-K shoshonitic and syenitic rocks. This suite of collision-related magmatic rocks is characterized by unradiogenic Nd (εNd=−3.4 to −22.0) and relatively radiogenic Sr (87Sr/86Sr=0.70578 to 0.71355) isotopic compositions. Pb isotopic compositions of these rocks are quite variable, with 206Pb/204Pb varying between 17.268 and 19.025, 207Pb/204Pb between 15.515 and 15.789, and 208Pb/204Pb between 37.952 and 40.071. The isotopic characteristics of these rocks are unique among Neogene magmatic rocks from New Guinea and its immediate environs; only Neogene lamproites and kimberlites from Western Australia share some of the isotopic characteristics observed in this suite of igneous rocks. The development of this range of isotopic characteristics is modeled to reflect interaction between at least three components. Parental magmas to this suite are believed to have isotopic compositions reflecting mixing between melts derived from a depleted mantle source with 2–3% melt derived from an ancient, enriched mantle reservoir. This ancient, enriched reservoir is interpreted to reside within the continental lithospheric mantle and is similar to the source of lamproites and kimberlites in Western Australia. Subsequent evolution of these magmatic systems involved interaction with a reservoir characterized by unradiogenic Pb, Sr and Nd isotopic compositions. This reservoir is interpreted to be comprised of either Proterozoic or Archean lower crustal material. Assimilation–fractional crystallization models indicate that the amount of assimilation necessary to achieve the observed isotopic compositions to be between approximately 10 and 17% (if an Archean assimilant is assumed) or 35 and 75% (if a Proterozoic assimilant is assumed). The incorporation of Precambrian materials in at least one of these magmatic centers has been confirmed through the presence of inherited cores in zircons having 207Pb/206Pb ages of 1295–1773 Ma.