Thermochronologic data for diabase recovered during Leg 180 indicates that the diabase crystallized at 66.4 Ma. Although highly heterogeneous in terms of K alteration and (40Ar/36Ar)i, 40Ar/39Ar step heat experiments on plagioclase from diabase and gabbro from the Moresby Seamount yielded an age of 58.9 ± 5.8 Ma for the least altered sample (Core 180-1109D-45R); the data are interpreted to represent cooling following crystallization. A 40Ar/39Ar isochron age for highly altered plagioclase from diabase Core 180-1118A-70R provides an estimate of the age of subsequent hydrothermal alteration at 31.0 ± 0.9 Ma. The diabase has not been thermally affected by Miocene-Pliocene rift-related events, supporting our inference that these rocks have remained at shallow (i.e., cool) depths in the crust since they were partially reset as a result of middle Oligocene hydrothermal alteration. Although the relationship of Leg 180 diabases with those from eastern PNG is not firmly established, a reexamination of similar mafic rocks from the Papuan Ultramafic Belt and the Milne Basic Complex is required in light of the results presented here. This, along with further geochemical comparisons, will establish more firmly the nature and timing of the tectonic evolution of the Papuan Peninsula.
40Ar/39Ar and 238U/206Pb dating of rhyolite clasts (Cores 180-1110B-3X and 180-1111A-16R) derived from the vicinity of the Moresby Seamount provide evidence for volcanic activity at 15.7 ± 0.3 Ma followed by hydrothermal alteration at 10-11 Ma. These ages are interpreted to reflect arc magmatism possibly related to subduction at the Trobriand Trough. A microgranite clast (Core 180-1108B-6R) provides evidence for synrift felsic magmatism, as the clast was derived from a rapidly cooled, shallowly emplaced 3.0-Ma protolith in the vicinity of the Moresby Seamount.