Thermal and Temporal Constraints on the Development of Dome and Keel Structures in the Eastern Pilbara Craton Using U-Pb Thermochronology

Abstract:

Continued study of Archean rocks is important for calibrating the thermal history of the mantle and crust, and the resulting geodynamic settings of lithosphere formation. The Pilbara craton in northwestern Australia contains an important record of Mesoarchean tectonics as it preserves granite-gneiss domes bound by greenschist facies supracrustals that typify many Archean terranes. These structures and their comparison with modern orogenic belts are key to the debate regarding the secular evolution of tectonics on Earth.

In order to better understand the thermo-tectonic evolution of the eastern Pilbara craton, we obtained ID-TIMS apatite and titanite U-Pb ages (with nominal T_c of 350-550 °C) from three of the most prominent granite-gneiss domes. Our data are compared to existing models for dome and keel formation, which hypothesize thermal incubation of the middle crust leading to gravitational instability and partial convective overturn ca. 3.3 Ga.

The three ca. 30-50 km diameter domes currently under investigation record diverse apatite cooling ages. They show:

 (1) Radial cooling patterns, where the center of the dome cooled ~15 Ma after the outer rim, supporting diapiric emplacement or post-intrusion cooling (Corunna Downs dome). (2) Large ranges in cooling ages between 3.3 and 2.95 Ga, that indicate portions of these domes were above ~400 ˚C after 3.3 Ga (Mt. Edgar dome). Preliminary thermal modeling shows that partial resetting of apatite due to post tectonic intrusions is possible but unlikely, potentially indicating that young cooling ages reflect tectonic exhumation. (3) Older cooling ages of ~3.44 Ga that indicate early emplacement, deformation and exhumation into the upper crust, which are not well explained by current models (Muccan dome). These thermochronological data provides new temporal constraints on the thermal and deformation history of the eastern Pilbara craton, which are key for constraining increasingly sophisticated models for Mesoarchean geodynamics.