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Crustal delamination induced by eclogitization of orogenic roots, in addition to plate subduction, is an important process that transfers crustal materials back into the mantle. Many previous studies have demonstrated a causal link between crustal delamination and eclogitization, but actual magmatic records of the process are rarely documented. Here we present detailed studies on two distinct late Paleozoic adakite suites in the Awulale Mountains (Chinese Western Tianshan), that is, the suites with high (HN) and low (LN) Nb/Ta ratios, to constrain the regional crustal delamination. Both igneous suites have adakitic affinity and formed by partial melting of the juvenile thickened lower crust with different residual mineral assemblages in their source regions. The late Carboniferous HN adakite has superchondritic Nb/Ta (21.4–23.3) and high Dy/Yb (2.66–2.88) ratios and may have formed via rutile‐bearing, eclogite‐controlled melting of the lower crust, implying crustal thickening and eclogitization. In contrast, the Permian LN adakites have varying and lower ranges of Nb/Ta (12.9–20.0) and Dy/Yb ratios (1.35–2.53), which were probably derived from mixing of garnet‐bearing amphibolite with minor rutile‐bearing, eclogite‐controlled melts, implying crustal thinning and the end of eclogitization. The succession of these two adakite suites suggests that during the late Carboniferous to early Permian, eclogitization of the orogenic root, accompanying successive crustal delaminations, occurred in this region. Accordingly, our results imply that adakites with different geochemical features are potential tracers for the mineralogy of orogenic roots and, importantly, provide convincing evidence for eclogitization‐induced crustal delamination in orogenic belts. Key Points: Two distinct adakite groups (with super/subchondritic Nb/Ta ratios) are present in the Awulale Mountains (Chinese Western Tianshan)Adakite geochemical features are controlled by the residual mineral assemblages in the magma sourcesAdakites in orogenic belts could be used to trace eclogitization‐induced crustal delamination events [ABSTRACT FROM AUTHOR]

The Awulale Mountains, the conjunction point of the Tarim Plate, Junggar Plate and Yili-Central Tianshan Block, are the crucial place to decipher the tectonic evolution of the western Tianshan. We report results of petrologic, whole-rock geochemical, Sr–Nd isotopic data and in-situ zircon U–Pb and Hf isotopes for the Wuling (poikilitic) hornblende gabbro from the Awulale Mountains, western Tianshan. Two stage crystallization processes are applied to interpret its mineral composition. Zircon U–Pb ages indicate that the Wuling hornblende gabbro was emplaced during the late Carboniferous (313 Ma ± 3 Ma). The Wuling hornblende gabbro is characterized by tholeiitic affinity with low 87 Sr/ 86 Sr ratios, positive ε Nd ( t ) and zircon ε Hf ( t ) values, resembling its coeval mafic rocks in the Awulale Mountains. They are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), indicating a subduction-related setting for their generation and their mantle source were mainly fluxed by fluids released by the down-going slab. The flat REE and depleted Sr–Nd–Hf isotopic signatures imply a shallow and heterogeneous mantle source with the involvement of a deep MORB-like component. We propose that an arc–nascent back-arc system had been developed between the Nalati Mountains and the Awulale Mountains during the late Carboniferous, where the magma derived from decompression melting of MORB-like mantle caused by the slab roll-back and flux-induced melting of mantle wedge were both contributed to their generation. The arc-type features of them were inherited from the strong subduction input at the initiation of back-arc rifting. However, the back-arc rifting ceased at the end of late Carboniferous because of the compressional stress from the collision between the Tarim Plate and the Yili-Central Tianshan Block. [ABSTRACT FROM AUTHOR]

A-type magmatism is an effective lithoprobe for constraining the tectonic evolution of orogenic belts. In this study, we have identified two episodes of Late Paleozoic A-type magmatism in Qunjisayi, western Tianshan. Zircon U–Pb geochronology has yielded Late Carboniferous (rhyolites, 306 ± 2 Ma)- and Early Permian (granites, 296 ± 3 Ma) ages. Both the Qunjisayi rhyolites and granites have depleted Sr–Nd–Hf isotopic compositions, moderate A/CNK ratios (0.91–1) and negative Nb and Ta anomalies. They are also characterized by high K 2 O + Na 2 O (8.72–10.6 wt.%), FeO T /(FeO T + MgO) (0.86–0.99) and Y/Nb (1.42–2.34) values, similar to those of typical A 2 -type granitoids. The Qunjisayi rhyolites may have been formed by partial melting of a juvenile calc-alkaline granitic crustal source induced by subduction roll-back in shallow depth. The succeeding Qunjisayi granites were likely to be derived from a lower crustal source that contains depleted mantle-derived components. Asthenospheric mantle upwelling, triggered by post-subduction slab break off, may have been important in the Qunjisayi granites formation. We conclude that the Late Paleozoic Qunjisayi A-type magmatism was genetically linked to the geodynamic transformation from late subduction to collision. [ABSTRACT FROM AUTHOR]