Primary or secondary distal volcaniclastic turbidites: how to make the distinction? An example from the Miocene of New Zealand (Mahia Peninsula, North Island)

Abstract

Miocene marine volcaniclastic deposits occur in Mahia Peninsula (North Island, New Zealand) and were sedimented in a forearc setting related to the Hikurangi trench subduction system. These deposits are interbedded with hemipelagic marls, and correspond to simple or amalgamated centimetric- to metric-thick turbiditic sequences. Volcaniclastic material is mainly composed of vitric particles, with crystals (quartz, plagioclase with minor biotite, amphibole, pyroxene and oxides), which are well represented in the coarse-grained fraction. The glass shards are mainly rhyolitic in composition. Three types of volcaniclastic turbidites were distinguished with geochemical data, because distinction is impossible on sedimentary characteristics. (1) Primary monomagmatic turbidites contain both magmatic (bubble wall pumice and shards) and phreatomagmatic (blocky shards with few vesicles and hydroclastically fragmented pyroclasts) vitric particles. The chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin. These turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea, which also led to their transformation into subaqueous gravity flows. (2) Secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites. There is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma. Crystals also show a compositional homogeneity. These deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf. (3) Secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components. This implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization, and that this sedimentation records the volcanic activity of multiple magmatic sources. Consequently, it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity, from secondary turbidites that result from reworking of previously deposited material on the shelf. Monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot. Multimagmatic turbidites, however, record the evolution of the volcanic arc during longer periods. This leads to the conclusion that the Mahia Peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc.