Petrological, geochemical and chronological constraints for the origin and geological significance of Neoarchean TTG gneiss in the Songshan area, North China Craton
河南嵩山地区新太古代TTG质片麻岩的成因及其地质意义：来自岩石学、地球化学及同位素年代学的制约

Neoarchean TTG gneisses widely occurred in the Songshan area, Henan Province. They mainly consist of granitoid and minor diorite, with a crystallization age of 2600~2500Ma by zircon SHRIMP U-Pb technique. The TTG gneisses are rich in SiO2(>67%) and Na2O (3% to 7%), but have low K2O/Na2O ratios and low FeOT and MgO contents. Their A/NCK ratios range from 0.97 to 1.21, consistent with tonalities to trondhjemite trend. The Songshan TTG gneisses are geochemically similar to high-Al TTG assemblage. The Songshan TTG gneisses have low REE contents, with differentiated LREE/HREE patterns and no Eu anomalies on the chondrite-normalized REE patterns. The Songshan TTG have high Sr (433×10-6 on the average) and Sr/Y ratios (165 on the average) and low Rb/Sr ratios, and display negative Nb, Ta and Ti anomalies on the primitive-mantle normalized trace element patterns. Their Nb/Ta ratios are averaged to be about 14 and La/Nb ratios are 7. These indicate that the Songshan TTG may have formed in an island arc or a continent marginal arc. The whole rock Nd isotopic compositions and zircon Hf isotopic compositions indicate that the Songshan TTG may originated from a juvenile crust derived from a depleted mantle at 2.6Ga with minor assimilation of ancient crust, while the variable Mg# values of the Songshan TTG indicate different degrees of contamination of mantle wedge. Therefore, the Songshan TTG gneiss may have formed by partial melting of a subducted basaltic slab with garnet and amphibolite left in the residual magmas. The emplacement of the Songshan TTG indicates an ancient oceanic crust may have existed at 2.53 Ga before the collisions of different micro-continents in North China Craton.