Thermochronological insights into how the dome and keel structures of the Pilbara craton formed

Описание: Thermochronological insights into how the dome and keel structures of the Pilbara craton formed
Scott Maclennan
University of the Witwatersrand, Johannesburg, South Africa


Dome and keel structures, broadly circular granitoid-gneiss domes that are surrounded by supracrustal rocks, are a hallmark of Archean crustal terranes around the world but are best exemplified by those in the eastern Pilbara craton in western Australia. The map pattern and structural geology of dome and keel structures are important pieces of evidence for hypotheses that propose that the Archean crust experienced large degrees of internal mobility, and that vertical flow and volcanism were key heat loss mechanisms at this early point in Earth’s history.

I will present apatite and zircon U-Pb thermal ionisation mass spectrometry (TIMS) geo/thermochronological data on samples from three eastern Pilbara craton granitoid-orthogneiss domes that have magmatic ages between ca. 3.5 to 3.23 Ga. The apatite U-Pb system is sensitive to mid-crustal temperatures (400 - 500°C) and thus records transport or emplacement into the upper crust, and through the brittle-ductile transition zone.

Apatite U-Pb ages delineate a complicated and protracted exhumation history for the granitoid-gneiss domes. The main magmatic episode in the eastern Pilbara craton occurs at ca. 3.3 to 3.2 Ga and apatite U-Pb data reflect that many of these intrusions were emplaced directly into the upper crust at that time. Isolated areas of the granitoid-gneiss domes preserve ca. 3.4 to 3.5 Ga ages, requiring earlier ductile deformation and exhumation. Lastly, ca. 2.8 Ga post-tectonic intrusions appear to have thermally reset large areas of one dome, potentially through hydrothermal fluid movement rather than conduction alone.