Search

U-Pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at Taupo volcano, as well as unexpected crustal structure beneath the Taupo Volcanic Zone. Ages from the xenolith in a 28 ka rhyolite dome match those from Early Cretaceous Pahau Terrane graywackes that crop out >75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process. Boundaries between graywacke terranes in the North Island are thus more complex than implied from map views. In contrast, ages from zircons in the 20 ka Omega dacite match the nearby surficial graywacke of the Jurassic Kaweka Terrane (55% of grains), Quaternary plutonic sources (39%), and Pahau Terrane graywacke (6%). Five of 72 grains analyzed contain graywacke cores and Pleistocene rims. Survival of euhedral xenocrystic zircons through the generation processes for Omega dacite magma reflects wholesale melting of crustal protoliths (including varieties not previously considered for petrogenetic modeling), open-system behavior, and crustal melts being generated and erupted within ~1-10 yr. doi: 10.1130/G31123.1

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic--Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95-100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends. Keywords: Cretaceous, southwest Pacific, Gondwana, dating, geochemistry, tectonics.

Cavalli Seamount (34[degrees]06'S, 174[degrees]10'E) is an irregular-shaped; flat-topped seamount on the Northland Plateau, SW Pacific Ocean. In contrast to Miocene lavas recovered from other seamounts in the area, the rocks dredged from Cavalli Seamount consist of biotite schist, with minor calc-silicate bands. Mineralogy, whole-rock chemistry, U-Pb dating of detrital zircons, and Sr and Nd isotopic composition collectively indicate a siliciclastic-carbonate protolith of latest Cretaceous to Paleogene stratigraphic age. Metamorphic index minerals are sillimanite, andesine, garnet, ilmenite and K-feldspar, and indicate peak P-T conditions of c. 650[degrees]C and c. 0.4 GPa. U-Pb, Ar/Ar and fission-track dating of minerals, and micropalaeontological dating of associated limestones, indicate rapid cooling in the interval 23-21 Ma. The schist contains chloritized and hematized slickenslides parallel to a penetrative lineation, s-c planes and conjugate microfractures, and has limestone geopetal infillings parallel to foliation. These features are consistent with the interpretation of Cavalli Seamount as part of the lower plate of an Early Miocene metamorphic core complex. Although other explanations are possible, the core complex interpretation fits with regional Early Miocene events including crustal thickening as a result of allochthon emplacement in northern New Zealand, crustal thinning associated with rapid extension in the South Fiji Basin, Pacific trench rollback, and abundant Early Miocene igneous activity. Keywords: New Zealand, Pacific Ocean, Miocene, tectonics, petrology.