Thursday, June 16, 2016

Evidence of Archean Sediment Deposition During the Archean

U–Pb–Hf isotope systematics of detrital zircons in high-grade paragneisses of the Ancient Gneiss Complex, Swaziland: Evidence for two periods of juvenile crust formation, Paleo- and Mesoarchaean sediment deposition, and 3.23 Ga terrane accretion


Taylor et al


We present the first results of combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains, in high-grade paragneisses (Luboya-Kubuta granulites) from the ca. 3.66–3.22 Ga Ancient Gneiss Complex (AGC) in Swaziland. Uranium–Pb ages demonstrate that sedimentation along the SE margin of the proto-Kaapvaal Craton took place during two time intervals: during the Paleoarchaean ca. 3.53 and >3.43 Ga; and Mesoarchaean ca. 3.22 and >3.16 Ga. The Hf-isotope data show that the hinterland to the Paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca. 3.53 Ga (ƐHf3.53Ga values between −0.7 and +4.1), and by reworking of more ancient crust (TDM = 3.8–3.6 Ga). In contrast, detrital zircons in Mesoarchaean metasediments record juvenile crust formation at 3.32 Ga (ƐHf3.32Ga values between −1.5 and +6.0; TDM = 3.45–3.32 Ga), followed by internal reworking of this crust until 3.22 Ga. A small number of detrital zircons indicate limited input from older, Paleoarchaean AGC crust. Compared to sediments of the Barberton Greenstone Belt, i.e. Fig Tree and Moodies Group, the Mesoarchaean metasediments of Swaziland are more juvenile in character, and show greater complexity in their detrital zircon age spectra, pointing towards a different provenance. Our new U–Pb–Hf isotope data, together with field and petrological information from the study area, suggest that the Paleoarchaean metasediments were largely derived from the AGC; whereas the Mesoarchaean metasediments were derived from a ca. 3.45–3.32 Ga juvenile island-arc terrane (located SE of the AGC), which became accreted onto the Kaapvaal Craton at ca. 3.23 Ga, i.e. the onset of burial and high-grade metamorphism in the area.

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