ABSTRACT

The Xingzi Group, exposed in Lushan, southeast China, is the oldest basement in the Late Proterozoic Jiangnan orogen and composed mainly of metamorphosed psammites and pelitic schists with the grade of amphibolite facies. Eight quartz and felsic veins from two locations in this group have been chosen to investigate trace element behaviour during metamorphic dehydration. One location is at Xixiansi, where the wallrocks are completely metamorphosed psammites, and the other at Huangyansi, where the wallrocks are mainly pelitic schists. According to their field and petrologic characteristics, all the veins studied from the two locations are synmetamorphic in origin and were deposited by metamorphic fluids derived from their wallrocks. This is further indicated by the similar oxygen isotopic compositions of mineral separates between the veins and their immediate wallrocks. The estimated mean δ¹⁸O values of the fluids equilibrated with minerals are 11.2‰ at Xixiansi and 10.2‰ at Huangyansi, supporting no pervasive fluid infiltration on a relatively large scale across whole the Xingzi Group during metamorphism. The much different Nb/Ta, Zr/Hf and Pb/Nd ratios in the veins from those in their wallrocks as well as the REE tetrad effects in the veins at Huangyansi indicate fractionation of the REEs and several geochemical pairs (Nb-Ta, Zr-Hf, and Pb-Nd) during metamorphic dehydration. In addition, the large ion lithophile elements (LILEs) show a different behaviour during this process. The ratios of La/Ba, Sr/Nd and Th/Hf are similar in all veins and wallrocks. Rb/Ba, Ba/Nb, La/Nb, and La/Yb ratios are also similar in the quartz veins and wallrocks, but vary significantly in the felsic veins, suggesting their fractionation in the felsic veins. The various REE abundances in the veins confirm that the REEs in sedimentary rocks can obviously be mobilized during regional metamorphism of amphibolite facies. Based on differences in trace element characteristics of the veins between Xixiansi and Huangyansi, we conclude that the mineralogical compositions of the protoliths most likely play an important role in controlling the chemical compositions of the fluid, and in turn, controlling REE and other trace element behaviour during metamorphic dehydration.