Archean granite-greenstone terranes provide important information about how early continental crust was formed and preserved. Early Archean cratons–such as in the Pilbara Craton of western Australia–have a distinct dome-and-keel architecture in which large (~70 km) ovoid granitic batholiths are surrounded by curvilinear metavolcanic greenstone belts. Dome-and-keel structures are often interpreted to result from gravity-driven crustal overturn processes in the early Archean, which are intrinsically linked with the production of preservable sialic crust (Collins et al., 1998). The timescales of this process, however, have yet to be constrained using the combined approaches of garnet Lu-Hf and Sm-Nd geochronology and structural analyses. In this study, we focus in the Pilbara Craton on the granitic rocks in the Mt. Edgar Granitic Complex and on the adjacent deformed supracrustal rocks of the Warrawoona Syncline. To better understand the evolution of Paleoarchean dome-and-keel structures–and access the connection between these overturn events and making stable felsic crust–we use an approach integrating zircon and titanite U-Pb and garnet Sm-Nd Lu-Hf geochronology, Sm-Nd and Lu-Hf isotopic data, and microstructural analyses. Using garnet Lu-Hf and Sm-Nd geochronology, we show that two discrete periods of fabric development related to dome-and-keel formation occurred ~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 event prior to the main phase of dome-and-keel formation. The younger group of garnet, between 3.34 and 3.30 Ga, have microstructures consistent with pre-, syn-, and post-kinematic growth with respect to the development of the dome-and-keel structure. By coupling these microstructures and garnet Lu-Hf ages, we show that the main phase of dome formation is dated by growth of the younger garnet group between 3.33 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 until 3.17 Ga. We also focus on the Hf-Nd isotope compositions of the granitic rocks in the Mt. Edgar Granitic Complex which intruded during these intervals of garnet growth and doming. These rocks crystallized between 3.46 and 3.25 Ga and have well-correlated chondritic initial whole rock Hf-Nd isotopes (εHf(i)= +1.5 to -1.8, εNd(i)= +2.5 to -1.1) and zircon Hf isotopes (εHf(i)= +0.9 to -0.8). Our isotope data, in conjunction with several recently published datasets (Petersson et al., 2020), establishes that the Paleoarchean granitic material in the Pilbara was predominantly juvenile and extracted from a source with broadly chondritic Lu/Hf and Sm/Nd. Together, our dataset illustrates the Pilbara dome-and-keel structures are the cumulative result of multiple events in the Paleoarchean, two of which we date precisely using garnet geochronology. Further, we also show the syn-doming granitic intrusions have juvenile Hf-Nd isotope compositions indicating that dome-and-keel formation, crustal overturn, and juvenile sialic magmatism are all spatially and temporally related.

Reference(s)

Collins, W.J., Van Kranendonk, M.J., Teyssier, C., 1998, Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: Driving mechanisms and tectonic implications: J. Struct. Geol. 20, 1405–1424.

Petersson, A., Kemp, A. I. S., Gray, C. M., Whitehouse, M. J., 2020, Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia: Chem. Geol., 119757.