The Colville Ridge: New Zealand’s ancient offshore arc and its role in present day Kermadec arc volcanism

Abstract:

The intraoceanic Kermadec arc subduction zone system northeast of New Zealand comprises a number of major bathymetric features, including a ~1,300 km long Miocene remnant arc –the Colville Ridge– that bounds the Havre Trough back-arc to the west. Until recently, seafloor exploration in this region was largely focused on the Kermadec arc front volcanoes and little was known about the Colville Ridge. In 2013 and 2015, detailed bathymetric mapping and rock sampling was conducted between 36˚S and 33˚S on two expeditions with the R/V Tangaroa. The new bathymetric maps reveal that a number of split volcanoes and few intact volcanic cones are dotted along Colville Ridge, consistent with their formation during different episodes of volcanism.

Despite their formation at different times, all volcanic rocks recovered from the Colville Ridge to date are basalts to basaltic andesites that have typically higher Th/Yb and La/Sm and lower Ba/Yb values at a given wt.% MgO than Kermadec arc front lavas. This suggests that the mantle wedge underlying the Colville Ridge was less depleted in fluid immobile high field strength elements than that beneath the recent Kermadec arc front. With unradiogenic Pb and Nd isotopic compositions the Colville lavas are distinctly different to the generally more radiogenic Pb and Nd isotopic composition in present day Kermadec arc lavas, the reason of which remains enigmatic. In addition, because the Colville Ridge represents the ancient proto-Kermadec arc, the question arises whether old mantle or crustal domains are still present affecting the geochemical composition of recently erupted Kermadec arc lavas. The recent discovery of fundamental geochemical differences between the Kermadec arc front volcanoes Rumble II East and West, forming a ~20 km trench perpendicular transect, revealed that both the subducting Pacific Plate (Hikurangi Plateau) and pre-existing mantle heterogeneities may play a role in the production of todays arc lavas. Crustal fragments with lower seismic velocities have been identified in the Havre Trough, supporting the presence of old crustal blocks in this region. The Colville Ridge therefore provides another piece of the puzzle to understand the geochemical evolution of the Kermadec arc system and its role in present day Kermadec arc volcanism.