2.6–2.7 Ga crustal growth in Yangtze craton, South China

A combined study of zircon U–Pb and Lu–Hf isotopes and whole rock major and trace elements and Sr–Nd isotopes has been conducted for 10 tonalitic–trondhjemitic–granodioritic (TTG) and granitic gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ∼2.6–2.7 Ga, in addition to the previously reported ∼2.9 Ga and ∼3.2–3.3 Ga magmatism. The ∼2.6–2.7 Ga rocks show relatively high REE (530–1074 ppm), apparently negative Eu anomaly (Eu/Eu* = 0.22–0.35), low #Mg (19.51–22.63) and low LaN/YbN (10.3–24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O + Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10,000 × Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ∼2.6–2.7 Ga zircon grains have ɛHf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ∼2.6–2.7 Ga granitoids compared to the ∼3.2–3.3 Ga and ∼2.9 Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65–2.76 Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛNd(t) values (−3.74 to 1.59) and the zircon ɛHf(t) values (−11.18 to 3.55) for the ∼2.9 Ga TTG and the Hf isotopes of ∼3.2–3.3 Ga igneous zircons (−7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions.

► We identify, for the first time, ∼2.6–2.7 Ga magmatic event in the Yangtze craton.
► Archean A-type granites (∼2.6–2.7 Ga) are firstly reported in the Yangtze craton.
► Hf isotope reveals a significant crustal growth at ∼2.6–2.7 Ga.
► The ∼2.9 Ga magmatic event formed the major part of the Kongling terrain.
► The ∼3.2–3.3 Ga inherited igneous zircons also exist in the eastern part of the Kongling terrain.