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 similar to 2.6-2.7 Ga, in addition to the previously reported similar to 2.9 Ga and similar to 3.2-3.3 Ga magmatism. The similar to 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 La-N/Yb-N (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 x 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 similar to 2.6-2.7 Ga zircon grains have epsilon(Hf)(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 similar to 2.6-2.7 Ga granitoids compared to the similar to 3.2-3.3 Ga and similar to 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 epsilon(Nd)(t) values (-3.74 to 1.59) and the zircon epsilon(Hf)(t) values (-11.18 to 3.55) for the similar to 2.9 Ga TTG and the Hf isotopes of similar to 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. (c) 2012 Elsevier B.V. All rights reserved.