Geochronological and geochemical study of the high-grade orthogneissic rocks from the Northern Fuyun Complex: Implications for crustal architecture and Early-Middle Paleozoic evolution of the Chinese Altai

High-grade metamorphic complexes are critical for the understanding of regional crustal configuration and accretionary history of the Altai orogenic belt. The orthogneissic rocks from the Northern Fuyun Complex, southern Chinese Altai, are investigated in this study. Zircon UPb isotopic analysis yields ages of ca. 411–398 Ma, arguing against previous views that these rocks represent old Precambrian basement. All the samples are calc-alkaline, metaluminous to weakly peraluminous (ASI = 0.80–1.06), and can be classified as I-type granitoids. They have low to medium Al2O3 and relatively high ferromagnesian compositions, and most of them exhibit weak heavy rare-earth element (HREE) fractionation and variably negative Eu anomalies. The above geochemical compositions are consistent with dehydration melting of meta-basaltic or amphibolitic crustal rocks followed by later fractional crystallization for their derivation. In addition, their positive zircon εHf(t) (average = 2.58–8.38) and mantle- to CHUR (chondritic uniform reservoir)-like whole-rock εNd(t) (−1.6 to +2.9) values indicate a relatively juvenile signature of their source rocks, which suggests that the deep crustal section of the Chinese Altai may be dominated by juvenile mantle-derived materials. The high MgO contents (2.06–3.66 wt%) of some samples and the occurrence of the mafic microgranular enclave may reflect the involvement of mantle materials in their petrogenesis, implying possible anatexis of the lower crust driven by basaltic underplating. By synthesizing magmatic, metamorphic, and structural geological evidence, we interpret that the Chinese Altai experienced a lithospheric-scale extensional event in the Devonian, which resulted in massive heat influx, emplacement of juvenile magmas, syn-orogenic crustal anatexis, and formation of mature and layered continental crust. The available data further suggest that alternating contraction and extension have probably operated on the Chinese Altai in the Early-Middle Paleozoic, which is possibly critical for the evolution of accretionary systems worldwide.