Evaluating the Timescales of Tectonic Mélange-forming Processes Using Sm-Nd Garnet Geochronology

The Catalina Schist (Santa Catalina Island, CA) amphibolite-facies mélange zone is interpreted to represent a paleo-subduction interface. The zone consists of cm-m scale tectonic blocks encapsulated by a heterogeneous metasomatized matrix. Blocks throughout the mélange zone record amphibolite-facies metamorphic conditions with small, yet resolvable (up to 80°C; 1), differences in peak metamorphic temperature. The heterogeneous distribution of these temperatures throughout the zone suggests the material was chaotically mixed at the subduction interface, although the differences in peak metamorphic temperature are small relative to other exhumed tectonic mélange zones such as the Franciscan Complex (CA). If the tectonic mixing model is correct, blocks would be expected to record different peak metamorphic ages. We test this hypothesis using Sm-Nd garnet geochronology to determine the time scale over which mixing occurred. Five garnet amphibolite tectonic blocks with a wide peak metamorphic temperature range of 646-726°C (estimated using Zr-in-rutile thermometry) were selected. Sm concentrations in garnet were determined by LA-ICP-MS and range from 0.3-4.6 ppm, showing a general increase in concentration from core to rim. Nd concentrations range from 0.1-0.9 ppm, and do not show any systematic changes from core to rim. Preliminary geochronologic data from one sample yield moderate ¹⁴⁷Sm/¹⁴⁴Nd ratios in garnet and garnet leaches between 0.45-0.99, with an apparent age of 108.8 ± 1.7 Ma. This age is younger than previous estimates of 114.5 ± 0.6 Ma using Lu-Hf garnet [2] and 111.7-114.1 Ma using U-Pb titanite [3], suggesting that age variations may occur throughout the mélange zone. Potential effects of ¹⁴³Nd/¹⁴⁴Nd diffusional resetting will be evaluated, although rapid cooling to c. 550-600 °C based on ⁴⁰Ar/³⁹Ar on hornblende [4] and U-Pb rutile of 105-110 Ma suggests the effect is negligible. Sm vs. Lu intragrain zonation will also be considered. Additional geochronologic data being collected for the samples will provide precise constraints on the timing of peak metamorphism for each block, elucidating the time scale over which the mélange zone formed.

[1] Penniston-Dorland et al. (2018) EPSL[2] Anczkiewicz et al. (2004) EPSL[3] Mattinson (1986) GSA Memoirs[4] Grove & Bebout (1995) Tectonics