3.45 Ga granitic gneisses from the Yangtze Craton, South China: Implications for Early Archean crustal growth

Abstract

Zircon U–Pb–Lu–Hf–O isotopic compositions of two granitic gneisses from the Kongling Terrain in the Yangtze Craton, South China were determined by SIMS, LA-ICP-MS and LA-MC-ICP-MS. Whole rocks of the two samples were analyzed for major and trace element compositions. The SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4, 3.3, 2.9, 2.7, and 2.0Ga for both gneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical 176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-ICP-MS. Taken together, zircons from the two samples with 98–102% age concordance give weighted average SIMS ages of 3434.3±9.6Ma (2σ, MSWD=13, n=8) for Group A, 3446.0±8.8Ma (2σ, MSWD=10.7, n=15) for Group B, and 3479±26Ma (2σ, MSWD=0.49, n=2) for Group C. Groups A and B together yield an upper intercept age of 3457±14Ma (2σ, MSWD=0.85, n=23). The LA-ICP-MS data yield weighted average ages of 3442±19Ma (2σ, MSWD=0.17, n=7) for Group A and 3435±11Ma (2σ, MSWD=0.44, n=16) for Group B. They yield an upper intercept age of 3443±13Ma (2σ, MSWD=0.63, n=23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrain by 150Ma.The 3.4Ga zircons show near chondritic ɛHf (t) (−0.7±1.0, 2σ, MSWD=1.14, n=8), which is below the coeval value of the depleted mantle. This suggests that the granitic magma contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1–6.4‰) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4Ga zircons vary from 3.9 to 3.6Ga with a weighted average of 3703±27Ma (2σ, MSWD=1.05, n=7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8Ga.Among the younger age groups, the 3.3Ga zircons exhibit 176Hf/177Hf (t) and δ18O values similar to the 3.4Ga zircons, suggesting that they were altered from the 3.4Ga zircons. The 2.9 and 2.7Ga zircons in both samples are rare and magmatic. Their 176Hf/177Hf (t) ratios are distinct from the 3.4Ga zircons, indicating different sources. These two age groups are consistent with the 2.9Ga trondhjemitic–tonalitic–granodioritic and the 2.7Ga A-type granitic magmatism in the Kongling Terrain. The 2.0Ga metamorphic zircons, regardless of being concordant or discordant, have 176Hf/177Hf (t) ratios overlapping those of the 2.7Ga zircons, suggesting a common source. In contrast, δ18O of the 2.0Ga zircons is strongly variable and positively correlated with age concordance. The low δ18O (down to 3.1‰) requires interaction with hydrothermal fluid. These results suggest that at least some of the 2.0Ga zircons were likely to have been altered from the 2.7Ga zircons by hydrothermal fluid.