Mesoproterozoic thermal evolution of the northern Gawler Craton from 40Ar/39Ar geochronology

Previous thermochronology studies of the northern Gawler Craton have interpreted cooling from mid-crustal temperatures in two phases; either cooling subsequent to a c. 1610-1580 Ma event or over the interval c. 1460-1440 Ma, however the extent and the significance of these ages has been uncertain. We have obtained nine new 40Ar/39Ar thermochronological dates on igneous and metamorphic rocks from drill hole samples across the region. The new data show that different fault-bound blocks have different thermal histories, and that cooling to below biotite closure to argon diffusion over the interval c. 1460-1415 Ma was widespread across the region. Arrhenius data from step heating experiments in this study yield closure temperature estimates for biotite samples that range from 436 to 310 degrees C. Cooling rates ranging from 3 to 5 degrees C/Myr are widespread across the region, with one block having slightly faster cooling, up to 14 degrees C/Myr. These rates suggest a combination of cooling related to thermal relaxation and modest erosion rates, with faster cooled zones likely resulting from moderate degrees of active exhumation along shear zone systems. This cooling may be linked to early stages of break-up of the Nuna supercontinent as c. 1460-1415 Ma tectonic activity is also documented across many terranes of the Nuna supercontinent including the Terre Adelie Craton in Antarctica and terranes of Laurentia.

Automated summary

This study reveals that different fault-bound blocks in the northern Gawler Craton have different thermal histories, with widespread cooling below biotite closure to argon diffusion occurring over the interval c. 1460-1415 Ma. Cooling rates range from 3 to 5 degrees Celsius per million years across the region, indicating a combination of cooling related to thermal relaxation and modest erosion rates.