Geochemical characteristics and genesis of plagiogneiss from the 0 similar to 2000 to main hole of the Chinese Continental Scientific Drilling Project.

The Plagiogneiss, which was used to regard as paragneiss in the Sulu-Dabie orogen, is also one of the important rock types in the main hole of the Chinese Continental Scientific Drilling Project. It not only comprises the principal part of the plagiogneiss lithological segment (738 similar to 1113 m), but also exists as "interlayer" in other lithological segments. Almost all of the plagiogneisses are similar in geochemical characteristics with those of the acidic members of Late Proterozoic bimodal volcanics remained in the north margin of the Yangtze craton except minority of them which may formed by retrograde metamorphism of the eclogites or by metamorphism of pelitic sedimentary rocks, that means the protolith of the most paragneisses may be acidic ignimbrite. obviously, some elements are more mobile than others during UHP metamorphism. The contents of SiO2, TiO2, Al2O3, FeO*, CaO, Cr, Ni, V, Co, Fe, Mn, Cu, Zn and Sc in the plagiogneisses show well correlation with MgO, these elements and Mg belong to immobile elements. The heavy rare earth and Y also belong to immobile elements. No or weak correlations with MgO have been seen for K2O, Pb, Rb, Ba, U, Th (i. e., LILE), so these elements belong to mobile elements during UHP metamorphism. The mobilities of Na2O, Sr, Nb, Ta, Zr and Hf are between immobile and mobile. Although with some degree mobility for LREE, the REE patterns had not been inflected radically during UHP metamorphism, therefore they can be used for protolith reconstruction. The plagiogneisses obviously differ from granitic gneisses in the drilling hole in geochemistry, the contents of MgO, TiO2, CaO and transition elements in the plagiogneisses are much higher than in the orthogneisses. Particularly the lower K2O/Na2O and Rb/Sr ratios in the plagiogneisses are greatly different A from the granitic gneisses. There are also great differences in REE patterns and trace element distributions between the two types of gneisses in the hole, the plagiogneisses are characterized by medium negative Eu anomalies (delta Eu > 0.5) and positive Ba anomalies as a whole, but the granitic gneisses with strong negative Eu anomalies (delta Eu = 0.21-0.26) and negative Ba anomalies. So most of the granitic gneisses and the plagiogmeisses (> 90%) can be discriminated on the basis of their whole rock geoehemical characteristics.