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Journal of the Geological Society; 2003; v. 160; issue.6; p. 971-983;
DOI: 10.1144/0016-764902-157
© 2003 Geological Society of London

Original Article

Cavalli Seamount, Northland Plateau, SW Pacific Ocean: a Miocene metamorphic core complex?

N. Mortimer1, R.H. Herzer2, N.W. Walker1, A.T. Calvert3,4, D. Seward5 & G.C.H. Chaproniere6

1 1Institute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, New Zealand (email: n.mortimer@gns.cri.nz)
2 2Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand
3 3Department of Geological Sciences, University of California Santa Barbara, Santa Barbara, CA 93106, USA
4 4Present address: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA
5 5Department of Earth Sciences, ETH Zentrum, 8092 Zurich, Switzerland
6 6Department of Geology, Australian National University, Canberra, A.C.T. 0600, Australia

Cavalli Seamount (34°06'S, 174°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 °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.

Key Words: New Zealand • Pacific Ocean • Miocene • tectonics • petrology