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Situated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy a critical place to investigate the tectonic history of the middle segment of the southern Central Asian Orogenic Belt (CAOB). In this work, field-based petrological studies and zircon U-Pb dating results reveal several episodes of granitic magmatism from 400 to 230 Ma in the YZHAs. Whole-rock geochemical and zircon Lu-Hf isotopic data indicate that all the 400-230 Ma granitoids underwent intensive fractional crystallization and were generated by magma mixing involving different proportions of mantle- and crust-derived materials. The ∼400 Ma monzogranites show (high-K) calc-alkaline affinities, akin to S-type granitoids. They were most likely generated in a postcollisional setting, corresponding to the assembly of the YZHAs before the Early Devonian. The 298-290 Ma granitoids belong to transitional I/S-type to A-type, whereas the 280-277 Ma granitoids are typical I-type. These Permian granitoids show increasingly evolved zircon εHf(t) values and formed from crust-mantle magma mixing, suggesting an advancing subduction setting. The ∼230 Ma monzogranites exhibiting fairly positive zircon εHf(t) values (+6.26 to +10.49) and high contents of mafic compositions and transition elements probably formed in a postcollisional setting after the assembly of the YZHAs and the Alxa Terrane. We infer that the final assembly of the middle segment of the southern CAOB probably occurred in the Early-Middle Permian. [ABSTRACT FROM AUTHOR]

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 Fe2O3T + 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 δEu 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. [ABSTRACT FROM AUTHOR]