PLIOCENE CU-AU-BEARING IGNEOUS INTRUSIONS OF THE GUNUNG-BIJIH (ERTSBERG) DISTRICT, IRIAN-JAYA, INDONESIA - K-AR GEOCHRONOLOGY

Nine distinct potassic intermediate igneous bodies have been identifie d during detailed surface and underground mapping of 100 km(2) in the Gunung Bijih (Erstberg) Cu-Au district in the highlands of Irian Jaya, Indonesia (west New Guinea). Fourteen biotite K-Ar ages range from 2. 6 to 3.8Ma, while another from a small isolated pluton is distinctly o lder at 4.4 Ma. These ages represent times of intrusion and crystalliz ation, which followed regional kilometer-scale folding that resulted f rom collision at a subduction zone of the northern edge of the Austral ian continental margin with the Melanesian oceanic island are complex. The Grasberg and Ertsberg intrusions are cut by numerous faults with only minor (<hundreds of meters) offset. Throughout the island of New Guinea, dated Neogene igneous rocks form two clusters, separated in ag e and location. A distinct belt of Miocene (20-10 Ma) plutons (Maramun i Arc) stretches the entire length of the northern highlands of Papua New Guinea and recurs in western Irian Jaya across a little-studied ga p of 430 km. This Miocene magmatism probably represents a subduction-r elated are above a SW-dipping Benioff zone. Magmatism that is 7 Ma and younger, including that in the Gunung Bijih (Ertsberg) district, lies farther to the south, in that portion of New Guinea underlain by Aust ralian continental crust. This Latest Miocene and younger activity doe s not appear to be directly related to subduction, but instead to an e pisode of asthenospheric upwelling somehow resulting from are-continen t collision.