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

Miocene marine volcaniclastic deposits occur in Mahia Peninsula (North Isla nd, New Zealand) and were sedimented in a forearc setting related to the Hi kurangi trench subduction system. These deposits are interbedded with hemip elagic marls, and correspond to simple or amalgamated centimetric- to metri c-thick turbiditic sequences. Volcaniclastic material is mainly composed of vitric particles, with crystals (quartz, plagioclase with minor biotite, a mphibole, pyroxene and oxides), which are well represented in the coarse-gr ained fraction. The glass shards are mainly rhyolitic in composition. Three types of volcaniclastic turbidites were distinguished with geochemical dat a, because distinction is impossible on sedimentary characteristics. (1) Pr imary monomagmatic turbidites contain both magmatic (bubble wall pumice and shards) and phreatomagmatic (blocky shards with few vesicles and hydroclas tically 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 eve nts and are probably related to the entrance of hot subaerial pyroclastic f lows into the sea, which also led to their transformation into subaqueous g ravity flows. (2) Secondary monomagmatic turbidites never contain phreatoma gmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites. There is a strong linear correlation in the compositions tha t suggest that the glass particles are derived from the same magma. Crystal s also show a compositional homogeneity. These deposits reflect the success ion 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 multimagnatic turbidites do not display compositio nal homogeneity of their vitric and mineral components. This implies that t he volcaniclastic material has been stored during a relative long period on the shelf before remobilization, and that this sedimentation records the v olcanic activity of multiple magmatic sources. Consequently, it has been po ssible to distinguish primary volcaniclastic turbidites that are directly r elated to the volcanic activity, from secondary turbidites that result from reworking of previously deposited material on the shelf. Monomagmatic turb idites can be used as stratigraphic and magmatic markers whereas multimagma tic cannot. Multimagmatic turbidites, however, record the evolution of the volcanic arc during longer periods. This leads to the conclusion that the M ahia Peninsula volcaniclastic turbidites distally record the evolution of t he source volcanic arc. (C) 2001 Elsevier Science B.V. All rights reserved.