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

Abstract

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.

Ultra‐fast early Miocene exhumation of Cavalli Seamount, Northland Plateau, Southwest Pacific Ocean

“…On a separate biological cruise by the National Institute of Water and Atmospheric Research (NIWA), rocks were recovered as "by-catch" at several stations on the seamount's flat top, and bottom camera traverses were made of the seabed. Our new samples, analyses, and images give more confidence to, and supplement, the interpretations made by Mortimer et al (2003).…”

“…F, Strontium isochron diagram (no useful age information can be derived from the isochron lines, they are plotted for reference only). Reference data from Ruddock (1990), McCulloch et al (1994), Palmer et al (1995), Nicholson et al (2000), Nicholson & Black (2004), adams et al (2005, and Mortimer et al (1998Mortimer et al ( ,2003Mortimer et al ( ,2006.…”

“…Paragneisses Mortimer et al (2003) discussed the usefulness and limitations of petrography and of whole rock geochemical and isotopic analyses in helping correlate the metasedimentary rocks with specific geological units. Basically, the banded biotite gneisses in this study have broadly similar compositions to the 1999 biotite schists, but overall are slightly more silicarich.…”

“…In a cruise in 1999, samples of schist were recovered from two dredges on the east slopes of Cavalli Seamount. Post-cruise analysis and interpretation of those rocks led Mortimer et al (2003) to propose a Miocene metamorphic core complex model. Key results leading to this model were establishment of (1) Late Cretaceous-Paleogene sedimentary protolith and (2) rapid early Miocene exhumation.…”

“…This paper is an update of Mortimer et al (2003) and is based on new, previously unpublished results from two cruises made in 2002 (Clark et al 2002;Herzer et al 2004) (Table 1). One, a dedicated geological cruise by GNS Science, made one rock dredge up the south flank of the western part of Cavalli.…”