Constraints on the age of dome-and-keel structures in the Pilbara Craton through integrated garnet geochronology and microstructural analyses

Abstract

The dome-and-keel granite-greenstone structures in the East Pilbara Terrane are commonly interpreted to result from a crustal overturn process that underpinned the growth and stabilization of continental crust on early Earth. We investigate the timing and timescales of this overturn process using integration of garnet and zircon geochronology with microstructural analyses. We use this integrated approach to provide insight into the evolution of dome-and-keel structures in the East Pilbara Terrane. Garnet Lu-Hf and Sm-Nd geochronology shows that two discrete periods of fabric development related to dome formation occurred between ∼ 3.42 to 3.39 Ga and 3.34 to 3.30 Ga. The older population of garnet is mechanically disaggregated, and we interpret that their growth is related to an early metamorphic event in the East Pilbara that occurred prior to the main phase of dome-and-keel formation. The younger population of garnet, between 3.34 and 3.30 Ga, include garnet with post-kinematic growth structures. By coupling these microstructures and garnet Lu-Hf ages, we show that the main phase of dome formation is dated by this younger event of garnet growth at about 3.34 and 3.30 Ga. The Sm-Nd ages of these garnets are systemically younger than the Lu-Hf ages, consistent with protracted cooling of the system after dome formation. Together, these data show that dome-and-keel structures in the East Pilbara formed quickly, perhaps in a few tens of million years.