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Original Article |
1 1Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
2 2Present address: Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH-Zentrum, NW81.2, Clausiusstrasse 25, 8092 Zürich, Switzerland (e-mail: gutjahr@erdw.ethz.ch)
3 3Institut für Mineralogie, Westfälische Wilhelms Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
4 4Present address, Universität Bonn, Mineralogisch-Petrologisches Institut, 53115 Bonn, Germany
5 5Australian National University, Canberra, ACT0200, Australia
The oldest rocks in New Zealand are the Mid- to Late Cambrian intra-oceanic island arc rocks of the Takaka terrane (Devil River arc). The provenance of Cambrian conglomerates stratigraphically above the exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental crust. The Dead Goat Conglomerate contains two distinct groups of igneous clasts: (1) intermediate to felsic volcanic clasts with moderately enriched light rare earth element (LREE) and high field strength element (HFSE) contents and positive
Nd500 (+2.1) that were derived from a medium-K calc-alkaline source, probably the main sequence of the Devil River arc; (2) dioritic to metagranitic plutonic clasts strongly enriched in LREE and HFSE and with Nd500 of +3.5 to +5.9 that were derived from a high-K arc source, probably the uppermost units of the Devil River arc. This is consistent with a new U–Pb sensitive high-resolution ion microprobe age of 496 ± 6 Ma. The Lockett Conglomerate also contains two distinct groups of igneous clasts: (1) ultramafic to intermediate igneous clasts identified as boninitic to transitional low-K calc-alkaline arc-related rocks based on depleted REE and HFSE abundances; (2) ‘I’-type metagranitoid clasts derived from a distinct Andean type continental margin, as indicated by Nd500 as low as –7.1. Both conglomerates contain sandstone clasts derived from a common old, multi-cycle continental source with Nd500 of –14.2 to –15.7, and no suitable source has been found in present-day New Zealand. The new provenance data from these conglomerates constrain the time of accretion of the Devil River arc to the palaeo-Pacific Gondwana margin and provide new information on the structural evolution of the accretionary event.
