SPATIO-TEMPORAL PATTERNS OF LOWER ARC COOLING AND METAMORPHISM, NORTHERN FIORDLAND, NEW ZEALAND

The exhumed root of the Triassic to Early Cretaceous continental arc in Fiordland, New Zealand preserves a record of deep crustal arc processes during and following high-flux magmatism from c. 124-115 Ma. We present new LASS-ICP-MS and SHRIMP-RG ²⁰⁶Pb/²³⁸U dates and temperatures for metamorphic zircon and titanite in order to (1) reconstruct the thermal history of northern Fiordland, and (2) illuminate spatio-temporal patterns in arc root metamorphism. Samples were collected over ~50 km along an arc-parallel transect from George Sound ( ~8 kbar) to Milford Sound (~16 kbar) in order to understand the thermal evolution of the arc as a function of paleocrustal depth.

Zircon rims that developed on the c. 181.1 ± 2.2 Ma Selwyn Orthogneiss indicate that Early Cretaceous metamorphism initiated at 143.7 ± 2.6 Ma and c. 685°C. Subsequent granulite-facies metamorphic zircon growth occurred from 124.8 ± 1.6 to 120.2 ± 2.3 Ma in Bligh and George Sounds at ~820°C. These dates and temperatures overlap with existing garnet Sm-Nd dates from the Pembroke granulite (Milford Sound) and likely reflect heating associated with deep crustal emplacement of the Worsley pluton of the Western Fiordland Orthogneiss from c. 125-120 Ma.

Titanite LASS chronology of metasedimentary rocks shows complex spatio-temporal patterns that reveal multiple pulses of titanite growth and/or recrystallization. East of Sutherland Sound, titanites yield a date of 123.6 ± 8.0 Ma and a Zr-in-titanite temperature of 765°C, which is similar to metamorphic zircon results in the region. At George Sound, titanites yield a date of 112.7 ± 4.3 Ma and a temperature of 760°C which overlaps with values reported in central Fiordland from Caswell to Breaksea Sounds. Younger dates of 106.4 ± 3.8 and 94.8 ± 2.8 Ma also occur in Bligh and George Sounds, and yield temperatures of 970 and 875°C, respectively. These dates and temperatures indicate that titanite growth and/or recrystallization occurred during multiple pulses of lower crustal heating which we speculate may have resulted from either lithospheric thinning related to extension and/or episodic foundering of a high-density arc root produced during the arc flare-up event from 124-115 Ma.