Varying contents of sources affect tectonic-setting discrimination of sediments: a case study from permian sandstones in the eastern Tianshan, Northwestern China

This article reports new geochemical data from Permian sandstones, most probably deposited during a period of postcollisional extension, from the Yamansu, Shaquanzi, and Xingxingxia areas in the Eastern Tianshan, northwestern China, to determine and characterize their provenance and source nature. The medium- to coarse-grained sandstones are composed mainly of angular to subangular quartz and volcanic fragments, with minor plagioclase and/or microcline, suggesting proximal deposition. Although the samples show relatively large variations in SiO₂(59.7–70.8 wt%), Al₂O₃(9.0–14.9 wt%), and Fe₂O_3T+ MgO (2.3–5.2 wt%) contents, they yield low chemical index of alteration values (41–55) but high index of compositional variability values (1.06–1.78), suggestive of their derivation mainly from compositionally immature sources with weak chemical weathering. Their relatively low Zr concentrations (99–225 ppm) and GdN/ YbN ratios (1.20–1.53) manifest insignificant zircon and monazite enrichments. Most of the samples are characterized by upper continental crust–like rare earth element (REE) patterns with dEu values of 0.63–0.99, reflecting dominant contributions from intermediate to felsic rocks. In particular, the Xingxingxia and Shaquanzi samples have relatively low SiO₂/Al₂O₃(4.51–5.82) and Th/U (2.67–3.77) ratios but high ΣREE (total REE) contents (71.8–141.1 ppm), indicative of a low degree of sedimentary sorting/recycling, whereas the Yamansu samples have higher SiO₂/Al₂O₃(6.30–6.57) and Th/U (3.93–5.16) ratios but low ΣREE contents (77.1–86.8 ppm), probably indicative of a higher degree of sedimentary recycling or more involvement of recycled materials. Tectonic-setting discrimination using geochemical data from the samples yielded inconsistent results, with the Xingxingxia and Shaquanzi samples dominantly plotted in or adjacent to the active continentalmargin or continental island arc field, while the Yamansu samplesmostly plotted in or adjacent to the passive margin field. Combining our findings with those of previous detrital zircon studies, we suggest that the inconsistent tectonic discriminations of the Permian sandstones most likely resulted from their receiving different proportions of source rocks that have distinct geochemical compositions. This study provides ample evidence that the geochemical compositions of sedimentary rocks within a continuous sequence or in the same depositional tectonic setting can vary significantly as a result of changes in contents of different source materials and thus that the use of tectonic-setting discrimination diagrams requires great caution.