Abstract: In the southwestern Tianshan, the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures. The Mada'er area is located in the Kuokesaleling belt, Southwestern Tianshan. The cherts from the former Wupata'erkan Group contain abundant radiolarian fossils, including 10 species which are identified as late Devonian to early Carboniferous in age. Eleven chert samples have SiO 2 contents ranging from 88.80 wt% to 93.28 wt%, and 2.02 wt% to 3.72 wt% for Al 2 O 3 . The SiO 2 /Al 2 O 3 ratios of all samples vary from 23.84 to 46.11, much lower than those of the pure cherts (80–1400). These values suggest that the cherts contain high ratios of terrigenous materials. The Al 2 O 3 /(Al 2 O 3 +Fe 2 O 3 ) ratios vary between 0.64 and 0.77, whereas V and Cu concentrations range from 10.92 ppm to 26.7 ppm and from 2.15 ppm to 34.1 ppm respectively. The Ti/V ratios vary from 25.53 to 44.93. The total REE concentrations of the cherts are between 30.78 ppm and 59.26 ppm, averaging 45.46 ppm. The (La/Ce) N ratios range from 0.81 to 1.12, and 0.88–1.33 for (La/Yb) N , averaging 1.09, which suggests a continental margin environment. Consequently, it is inferred that the cherts formed in a residual sea environment during the late Devonian to early Carboniferous time, which suggests that the collision between the Karakum‐Tarim and Kazakhstan‐Junggar plates did not occur at the time. In addition, the regional geological information indicates that the study area experienced a post‐collision stage during the early Permian, and thus it is likely that the collision between the two plates took place in the late Carboniferous.
Abstract Subduction‐induced metasomatism may increase heterogeneity in the subcontinental lithospheric mantle (SCLM) over time. Syenite and its relevant varieties are typically derived from the most metasomatized portions of the SCLM and thus provide a key to decipher its heterogeneity. The only circum‐cratonic syenite province in the world, the Late Triassic belt around North China craton, is unique and offers an opportunity to investigate heterogeneity of the SCLM right before its destruction in the Mesozoic. New and previous data of whole‐rock Sr‐Nd and zircon in situ Hf‐O isotopes of syenite intrusions encircling the craton indicate two distinct types of metasomatism characterized by an alteration of juvenile or ancient materials. The spatiotemporally distinct sources can be discriminated by melts from oceanic slabs subducting beneath much of North China followed by deep subduction of South China craton only in the south. This unique setting of a circum‐cratonic syenite belt proves in a single case study that subduction‐induced metasomatism may cause craton‐scale SCLM heterogeneity.
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