U-Pb zircon dating and whole-rock plus mineral geochemistry of granitoids from coastal exposures of the Paparoa Metamorphic Core Complex lower plate, Westland, New Zealand

Sagar, Matthew W.

The Paparoa Metamorphic Core Complex (PMCC) of northern Westland, South Island, New Zealand, developed during mid-Cretaceous continental extension in the New Zealand sector of Gondwana, which preceded fragmentation of the southern supercontinent. Coastal exposures of the PMCC lowerplate between Charleston and Belfast Creek include small granitic plutons and numerous dikes whose relations to established metamorphic, deformation and magmatic events associated with the separation ofNew Zealand from Gondwana are largely unknown. To remedy this, a small (~200m2) pluton of biotite-muscovite-garnet monzogranite (Little Beach granite) intruding the Devonian Pecksniff Metasedimentary Gneiss and a syn-tectonic two-mica syenogranite dike (Doctor Bay dike) cutting the foliated mid-Cretaceous Charleston orthogneiss near Charleston, along with the muscovite±biotite locally foliated leucocratic Redjacket Granite that intrudes Pecksniff Metasedimentary Gneiss at Belfast Creek, were sampled for x-ray florescence spectrometry (XRF) whole-rock major element geochemistry, and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon dating plus whole-rock trace element geochemistry analyses. The Little Beach granite and Doctor Bay dike were both intruded in the mid-Cretaceous, at 112 ± 2 Ma and 108 ± 2 Ma, respectively. Emplacement of the Redjacket Granite occurred at 353 ± 6 Ma, synchronous with metamorphism of the host Pecksniff Metasedimentary Gneiss. All three samples exhibited significant zircon inheritance. The ages obtained via LA-ICP-MS U-Pb zircon dating are consistent with those determined previously by other U-Pb zircon and monazite methods, and Rb-Sr dating. The syntectonic relationship of the Doctor Bay dike to its host rocks, constrains the timing of metamorphism and deformation affecting the Charleston orthogneiss to prior to ~ 108 Ma. The ages of the Little Beach granite and Doctor Bay dike are indistinguishable from each other and of the syn-tectonic Buckland Granite, indicating intrusion coeval with development of the PMCC. Furthermore, the geochemistry (e.g., Na20/K20 ~ 1 and moderate-SY), zircon inheritance patterns and mineralogy of the Doctor Bay dike and Little Beach granite suggest correlation with Rahu Suite granitoids, in particular the Buckland Granite. The Redjacket Granite is probably a member of the Karamea Suite based on its earliest Carboniferous age, zircon inheritance patterns and geochemistry (e.g., high KzO and low Sr).

155 leaves, : col. ill., maps ; 30 cm

2008Sagar