The island of New Guinea lies in an area of complex tectonic interaction between the relatively stable Australian continental landmass and the geologically active circum-Pacific Belt. Thus the Palaeo-zoic - Mesozoichistory of the southern half of the island is closely related t o the Palaeozoic - Mesozoic development of northeastern Australia, whereas in the northern half of the island there are Cainozoic geosynclinal and volcanic belts which cross-cut the Palaeozoic - Mesozoic basement and are parallel to the circum-Pacific margin in this region. In mainland Papua New Guinea, intrusive igneous activity before Tertiary time was restricted to the southern part of the Central Highlands and, from the data available, appears to have been rather sporadic. The Upper Permian Kubor Granodiorite, dated at about 2 4 0 m.y., represents the largest and oldest-known igneous massif in New Guinea. A number of small Mesozoic granitic bodies were emplaced in the Early Jurassic (190m.y), Early to Mid-Jurassic (172m.y) , and mid-Cretaceous (90m.y). Increased tectonism in the mid-Tertiary is evidenced by several late Oligocene and early Miocene results (26 to 20 m.y. range) in four metamorphic terrains and in granitic intrusives in the south Sepik area. Volcanic and plutonic igneous activity reached a climax in the mid - Miocene , between 12 and 15 m.y. ago. K - Ar dating on some of these volcanic rocks in close stratigraphic relationship with palaeontologically controlled sedimentary rocks provided the basis for constructing a physical time scale for the East Indies letter stages. The 12-15 m.y. (mid - Miocene) pulse of plutonic activity continued to a lesser degree in the late Miocene and Pliocene. These Miocene - Pliocene bodies lie in an arcuate northwest-trending belt (New Guinea Mobile Belt) over 700km long. Porphyry copper and gold mineralization events are temporally and spatially associated with the middle Miocene and Pliocene high-level intrusives in this belt. All the volcanic and plutonic rocks have low initial Sr 87 / Sr 86 ratios, suggesting that they were derived from mantle sources with little or no contamination by crustal material. A close relation between the major tectonic processes of faulting, volcanism, and plutonism in the New Guinea Mobile Belt, over a relatively short time span of a few million years in the mid - Miocene, corresponds to the waning stages of the Papuan Geosyncline sedimentary history and the beginning of the main mountain-building processes in the New Guinea highlands. It is hypothesized that this climactic upsurge in the tectonic history of New Guinea was triggered in the early to mid - Miocene by interaction and collision between the Pacific Plate and the northward-moving Australian Plate, which has New Guinea at its leading edge.
