ISOTOPIC AGES FROM THE NELSON REGION OF SOUTH ISLAND NEW-ZEALAND - CRUSTAL STRUCTURE AND DEFINITION OF THE MEDIAN TECTONIC ZONE

Plutonic rocks in the Rotoroa Complex and Drumduan Terrane of South Island, New Zealand yield zircon U/Pb dates of 156 and 142 Ma, respectively, that are interpreted as crystallization ages. Hornblende and biotite 40Ar/39Ar dates of 140-130 Ma from the Rotoroa represent either emplacement ages, cooling ages or a metamorphic resetting event. These two units crop out between the Brook Street Terrane and the Separation Point Batholith and lack any clear affinity with tectonostratigraphic terranes of the New Zealand Western or Eastern provinces. The Rotoroa Complex and Drumduan Terrane are interpreted as part of a series of dismembered Mesozoic volcanic-plutonic arc complexes that are sandwiched between terranes of the Western and Eastern provinces, occupying a structural position here referred to as the Median Tectonic Zone (MTZ). Correlative units in Fiordland on the opposite side of the Alpine Fault include the Mackay Intrusives, Darran Complex, Largs Terrane, Lochburn Formation and the Halfway Peak Gabbro. Farther south on Stewart Island the Anglem Complex and Paterson Group are part of the same structural belt. The MTZ is an extension of the original concept of the Median Tectonic Line put forth by Landis and Coombs (1967). Dismemberment and juxtaposition of arc magmatic assemblages in the MTZ with Western and Eastern Province terranes is related to large-scale transcurrent faulting in the Early Cretaceous. Its essential features as a regional tectonostratigraphic terrane were established by similar to 117 Ma as indicated by stitching of the Rotoroa Complex to the Takaka Terrane (Western Province) by the Separation Point Batholith (117-114 Ma). The Echinus Granite yields a 310 Ma U/Pb zircon crystallization age that suggests the granite and associated gneisses are part of the Western Province which may constrain the position of the western margin of the MTZ near Nelson City.