U-PB ZIRCON DATING OF MAGMATISM AND METAMORPHISM IN THE CHARLESTON METAMORPHIC GROUP, NEW ZEALAND

The Charleston Metamorphic Group (CMG) is in the footwall of the Paparoa Metamorphic Core Complex (PMCC) situated within the Buller Terrane of the South Island, New Zealand. Gneisses and foliated granitoids from the lower plate of the PMCC were exhumed in the Early Cretaceous due to motion on oppositely dipping low angle detachment faults during the breakup of Gondwana. One banded orthogneiss sample collected from Parsons Hill and three foliated granitoids from Constant Bay and Joyce Bay were U-Pb zircon dated using LA-ICPMS to determine crystallization and metamorphic ages of the CMG. The orthogneiss from Parsons Hill yields prominent zircon populations at 360, 493, and 1050 Ma. A foliated porphyritic tonalite that crosscuts a two-mica foliated granite from Joyce Bay yields a pronounced peak at 111 Ma and contains inherited cores with Carboniferous through Proterozoic dates. The two-mica foliated granite displays a prominent peak at 113 Ma with another pronounced peak at c. 329 Ma, similar in age to the Carboniferous Foulwind Suite granitoids. A foliated porphyritic tonalite from Constant Bay yields a prominent peak at 113 Ma, with smaller populations at 323 and 489 Ma.

We interpret the c. 360 Ma peak from the Parsons Hill orthogneiss to be the crystallization age of a biotite granite possibly related to the Karamea Suite, however U/Th for zircons from this population are typically >>10 suggesting metamorphism and lead loss may have affected the dates. The three foliated granitoids yield similar Cretaceous peaks (111-113 Ma) with older populations that correspond well with ages of the Carboniferous Foulwind Suite granitoids and Cambro-Ordovician Greenland Group crustal sources. Most of the Cretaceous ages come from well-developed rims (up to ~65 µm thick) that are dark under CL (high U) and/or display oscillatory zoning and overgrow one or more generations of zircon cores with variable CL signatures. These results suggest that the foliated granitoids were either 1) intruded in the Carboniferous (Foulwind Suite) and grew exceptionally large zircon rims during metamorphism in the Cretaceous, or 2) they are Cretaceous intrusions, with inherited cores sourced from partial melting of Carboniferous granites and Cambro-Ordovician Greenland Group metasedimentary rocks.