Zetland Diorite, Karamea Batholith, west Nelson: field relationships, geochemistry and geochronology demonstrate links to the Carboniferous Tobin Suite

Abstract

Field observations, petrography, geochemistry and U–Pb geochronology of the I-type Zetland Diorite from New Zealand's Western Province reveal a Carboniferous age of emplacement and chemical affinity with the Tobin dioritic suite. New U–Pb zircon dating by isotope-dilution thermal-ionisation mass spectrometry (ID-TIMS) provides a robust age of 347.19±0.24 Ma for a sample of Zetland Diorite. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating of a Zetland quartz leucodiorite sample is complicated by significant inheritance from Karamea and Paringa granitoid suites but yields an estimated crystallisation age of 350.6±8.7 Ma, consistent with the field evidence for emplacement coeval with the dated mafic dioritic phase. Our results show that Zetland Diorite is at least c. 10 Ma younger and geochemically distinct from the 369–360 Ma I-type dioritic intrusives of the Paringa Suite, including the Riwaka Complex, as well as from the 370–368 Ma Karamea Suite S-type granites with which the Zetland Diorite had previously been deemed coeval. It therefore cannot serve as a mafic end-member source component of the Karamea Suite and its emplacement does not constrain the minimum age of amalgamation of the Buller and Takaka terranes.