Thesis

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, as well as Ti-in-zircon and Zr-in-titanite 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. Rims that grew on zircon in samples from the Arthur River Complex indicate that Early Cretaceous metamorphism initiated at 134.9 ± 3.1 Ma (T = 720°C) and lasted until 126.5 ± 2.7 Ma (T = 700 °C). Subsequent granulite‐facies metamorphic zircon growth occurred from 121.3 ± 1.6 to 116.0 ± 2.3 Ma in Bligh and George Sounds at 830-700°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 Western Fiordland Orthogneiss from c. 125‐115 Ma during the final stages of Median Batholith magmatism. 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 120.6 ± 4.4 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 112.0 ± 2.4 Ma and a temperature of 760°C which overlaps with values reported in central Fiordland from Caswell to Breaksea Sounds. Younger dates of 104.8 ± 1.6 and 94.0 ± 2.3 Ma also occur in Bligh and George Sounds, and yield temperatures of 930 and 840°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.

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