Syn- and post-tectonic cooling and exhumation in the Larsemann Hills, East Antarctica

Abstract

In this paper, we present 40Ar/39Ar hornblende, biotite, and K-feldspar and apatite fission track thermochronological data to constrain the cooling and exhumation of high-grade rocks of the Larsemann Hills, Antarctica, synchronous with and postdated regional intense orogenesis of the early Paleozoic. Combined with Pb-Pb, U-Pb, Sm-Nd and 40Ar/39Ar age data published, a 40Ar/39Ar plateau age of hornblende recorded cooling below c. 520°C at 514 ± 2 Ma at a rate of c. 40°C/Ma. This rapid cooling can be attributed to the tectonically driven exhumation along the ductile shear zoons. By 514 Ma, c. 10-15 km of exhumation had achieved. Slowing of the cooling rate from c. 9°C/Ma to c. 3°C/Ma occurred during 500-490 Ma, when the geotherm dropped through the closure temperature for 40Ar/39Ar system in biotite at c. 300°C. Documented by an 40Ar/39Ar plateau age of K-feldspar, further drop of the geotherm below c. 160°C took place around 455 Ma, by which erosionally controlled exhumation had become dominant, as suggested by the very slow cooling rate at c. 3-0.3°C/Ma. The final cooling through apatite partial annealing zone happened at c. 210 Ma and could correspond to an early period of aborted rifting in the Lambert Garben region.