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-rel ated magmatic rocks were emplaced in, or erupted through, an orogenic belt associated with the partial subduction of the northern continental margin o f Australia beneath a south-facing oceanic island are. Intermediate calc-al kaline to low-K shoshonitic rocks are most common throughout this belt, alt hough other, less common, associations include lamprophyric, high-K shoshon itic and syenitic rocks. This suite of collision-related magmatic rocks is characterized by unradiogenic Nd (epsilon(Nd) = - 3.4 to - 22.0) and relati vely radiogenic Sr (Sr-87/Sr-86 = 0.70578 to 0.71355) isotopic compositions . Pb isotopic compositions of these rocks are quite variable, with Pb-206/P b-204 varying between 17.268 and 19.025, Pb-207/Pb-204 between 15.515 and 1 5.789, and Pb-208/Pb-204 between 37.952 and 40.071. The isotopic characteri stics of these rocks are unique among Neogene magmatic rocks from New Guine a and its immediate environs; only Neogene lamproites and kimberlites from Western Australia share some of the isotopic characteristics observed in th is suite of igneous rocks. The development of this range of isotopic charac teristics is modeled to reflect interaction between at least three componen ts. Parental magmas to this suite are believed to have isotopic composition s reflecting mixing between melts derived from a depleted mantle source wit h 2-3% melt derived from an ancient, enriched mantle reservoir. This ancien t, enriched reservoir is interpreted to reside within the continental litho spheric mantle and is similar to the source of lamproites and kimberlites i n Western Australia. Subsequent evolution of these magmatic systems involve d interaction with a reservoir characterized by unradiogenic Pb, Sr and Nd isotopic compositions. This reservoir is interpreted to be comprised of eit her Proterozoic or Archean lower crustal material. Assimilation-fractional crystallization models indicate that the amount of assimilation necessary t o achieve the observed isotopic compositions to be between approximately 10 and 17% (if an Archean assimilant is assumed) or 35 and 75% (if a Proteroz oic assimilant is assumed). The incorporation of Precambrian materials in a t least one of these magmatic centers has been confirmed through the presen ce of inherited cores in zircons having Pb-207/Pb-206 ages of 1295-1773 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.