LATEST JURASSIC TO EARLY CRETACEOUS COOLING OF THE BARTLE FRERE PLUTON, NORTH QUEENSLAND AUSTRALIA, REVEALED BY (U-TH)/HE ANALYSIS OF APATITE AND ZIRCON

We employed the apatite (U-Th)/He (AHe) and zircon (ZHe) low temperature thermochronometers to explore the exhumation history of the North Queensland segment of Australia’s Great Escarpment. The goal of this work was to better decipher the evolution of this rifted margin and how it compares with that of better-studied segments of this same escarpment. Previous workers have attributed escarpment development to Cretaceous-Paleogene rifting during opening of the Tasman and Coral seas. Our 1500m vertical transect up the escarpment sampled the I-type Bartle Frere pluton of the ca. 280 Ma Bellenden Ker Batholith is exposed from the base to the top of Mount Bartle Frere.

A previous apatite fission track (AFT) study determined that the escarpment base cooled through ~110 °C at 142.3 ±9.9Ma. Our preliminary ZHe analysis of the same outcrop reveals that it passed through ~180 °C at ca. 155 Ma. These data indicate that an episode of relatively rapid exhumation occurred during the latest Jurassic to earliest Cretaceous, which brought the pluton up from approximately 6 km to 3.5 km depth (assuming a 30 °C/km geothermal gradient).

Samples throughout the entire transect yielded AHe dates ranging from 72Ma to 182Ma, with no apparent elevation-date relationship. These data suggest that the pluton continued to cool during the Cretaceous to below ~65 °C, indicating unroofing to less than ~2 km depth. The data scatter makes it difficult to resolve the details of this later stage of cooling, however the fact that we obtain Cretaceous AHe ages across the entire 1500 m vertical transect suggests that the Great Escarpment in North Queensland has existed at approximately its current location and height since at least the Late Cretaceous. Based solely on AFT results from low elevation samples, previous workers concluded, by contrast, that this segment of the Great Escarpment developed after he Cretaceous. Our Cretaceous age for this segment is similar to the age determined by other AHe workers for the segment in southern New South Wales.