Geochemical characteristics of the metapelites from the Xingxingxia group in the Eastern Segment of the Central Tianshan: Implications for the provenance and paleoweathering

摘要

Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan, Northwestern China, were analyzed for major and trace elements, including rare earth elements. Compared with post-Archean shales, the metapelites are enriched in Sr, Hf and Zr. The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale (PAAS). The characteristics of Al_2O_3/TiO_2, Cr/Zr, Cr/Th, Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic materials in nature. These rocks display highly fractionated REE patterns, and average (La/Yb)_n = 18.6. Several samples have slightly negative Eu anomalies (Eu/Eu~* = 0.7-0.84) in comparison with PAAS (Eu/Eu~* = 0.65), indicating contribution from young immature materials. The low K_2O/Al_2O_3 values of these metapelites suggest that their source materials could contain minimal alkali feldspar. In the A-CN-K triangular diagram, the sample data array intersects the feldspar join, indicating that the source had a plagioclase: K-feldspar ratio of approximately 5:1, representing weathered products from components that could approximately correspond to granodio-rites and tonalites. The low Chemical Index of Alteration (CIA) and high Index of Compositional Variability (ICV) values suggest low degrees of weathering of the source and low compositional maturity of the sediments, respectively. The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.