2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 195-7
Presentation Time: 9:40 AM

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


GEBAUER, Samantha K., Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, CA 91330, SCHWARTZ, Joshua J., Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, KLEPEIS, Keith A., Geology, University of Vermont, Trinity Campus, Burlington, VT 05405, STOWELL, Harold H., Department of Geological Sciences, University of Alabama, BOX 870338, Tuscaloosa, AL 35487, COBLE, Matthew A., Department of Geological Sciences, Stanford University, Stanford, CA 94305, KYLANDER-CLARK, Andrew, Department of Earth Science, University of California, Santa Barbara, CA 93106 and HACKER, Bradley R., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, samantha.gebauer.513@my.csun.edu

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.