T37 Deformation Localization throughout the Crust (Posters)

Sunday, 19 October 2014
9:00 AM - 5:00 PM
Exhibition Hall C

PUNCTUATED COOLING OF AN ARC ROOT DURING EXTENSIONAL OROGENIC COLLAPSE, CENTRAL FIORDLAND, NEW ZEALAND: INTEGRATING ZIRCON, TITANITE, AND RUTILE THERMOCHRONOLOGY

Paper number: T37-2
Program: Technical Sessions
Time: 9:00 AM – 5:00 PM

Authors

Abstract

Deformed granulite- to amphibolite-facies orthogneisses in the exhumed root of the Jurassic-Cretaceous arc in Fiordland, New Zealand contain metamorphic rutile that records cooling through amphibolite-facies conditions when plagioclase, the weakest phase of the lower crust, ceased to deform crystal plastically. We integrate new rutile LASS 207-corrected Pb/U dates and trace-element data with metamorphic zircon and titanite dates to (1) document conditions of high-temperature metamorphism in the arc root, (2) evaluate time scales of arc cooling from granulite-facies to below amphibolite-facies conditions, and (3) calculate lower-crustal cooling rates during extensional orogenic collapse. Thermochronology samples were collected adjacent to three amphibolite-facies shear zones that partially accommodated Cretaceous extension in Fiordland: the Nancy Sound (NS), Doubtful Sound (DS), and Resolution Island (RI) shear systems.

Results of Zr-in-rutile thermometry yield upper amphibolite- to granulite-facies temperatures for metamorphism in all samples. From north to south, temperatures and 2σ errors are 710 ± 30 °C (NS), 770 ± 30 °C (DS), and 720 ± 30 °C (RI). Weighted-mean rutile dates are 77.9 ± 4.2 Ma (NS), 84.0 ± 4.6 Ma (DS), and 87.8 ± 4.9 Ma (RI). Integrating our rutile results with our existing metamorphic zircon and titanite dates allows us to calculate cooling rates and determine timescales of arc root cooling during extension. Calculated cooling rates over the temperature range of 850 to 650 °C are 63 °C/Ma (NS), 77 °C/Ma (DS), and 330 °C/Ma (RI) suggesting rapid cooling from granulite-facies conditions. Calculated cooling rates from 650 °C to below amphibolite-facies temperatures are 5 °C/Ma (NS) and 6 °C/Ma (both DS and RI) indicating ten-times slower amphibolite-facies cooling. These data support previous garnet Nd-Sm ages, that granulite-facies metamorphism in the arc root was short lived (<10 Ma) and closely followed emplacement of mafic to intermediate magmas into the deep crust. In contrast, the arc root resided at amphibolite-facies conditions for ca. 25-35 Ma. This prolonged period at amphibolite-facies temperatures allowed the development of organized amphibolite-facies shear zones that accommodated extension leading to intraplate rifting and opening of the Tasman Sea at ca. 83 Ma.