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Intensive Devonian felsic magmatism is recorded within the southwestern Mongolian collage system of the Central Asian Orogenic Belt (CAOB). The voluminous magmas have isotopic compositions of juvenile materials from the mantle, thus manifesting significant mantle–crust interaction and continental growth at this time. Here, we present systematic Sr–Nd–Hf–Pb isotopic data for the Devonian mafic intrusions in the Chinese Altai, a key region within the southwestern Mongolian collage system to decipher the evolution of the mantle during this important tectonothermal event. The Keketuohai gabbro (409 ± 5 Ma) and type I mafic dykes (376 ± 5 Ma) within the Habahe complex have high ( 87 Sr/ 86 Sr) i , ( 206 Pb/ 204 Pb) t , ( 207 Pb/ 204 Pb) t and ( 208 Pb/ 204 Pb) t ratios, and decoupled Nd–Hf isotopic compositions; e.g., low ε Nd (t) values (− 2.5 to + 5.4) combined with high ε Hf (t) (+ 2.6 to + 15.1) values. These rocks have low Ba/La and high La/Yb and Th/Yb ratios, and are enriched in Pb, the light rare earth elements (LREE) and Th. They formed from magmas generated from the depleted lithospheric mantle metasomatised by hydrous melts from subducted sediments. In comparison, the gabbroic samples from the Habahe complex (369 ± 3 Ma) are enriched in the LREE, Th and Ba and have high La/Yb, Th/Yb and Ba/La ratios. They do not show significant Pb anomalies, and have depleted isotopic compositions that include low initial 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios and high ε Nd (t) (+ 7.4 to + 7.8) and ε Hf (t) (+ 13.4 to + 15.3) values. These rocks are thought to have formed from magmas derived from the lithospheric mantle metasomatised by hydrous melts from subducted oceanic crust. The type II mafic dykes within the Habahe complex are depleted in the LREE and Th, have high Ba/La ratios, and are enriched in Pb, Ba, Sr, and U. They have positive ε Nd (t) (+ 7.6 to + 8.1) and ε Hf (t) (+ 14.1 to + 15.4) values, high initial 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios, and are thought to have formed from a magmas generated by the partial melting of the depleted lithospheric mantle metasomatised by fluids derived from subducted sediments. The melting and dehydration of subducted oceanic crust and sediments indicate the Devonian subduction of a hot slab beneath the Chinese Altai, which could be related to a ridge subduction model previously proposed. The involvement of hydrous melts and fluids promoted the melting of the lithospheric mantle which was responsible for intensive felsic magmatism with abrupt increase of juvenile component and high temperature metamorphism in the region. [ABSTRACT FROM AUTHOR]

The NW–SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D 1 is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D 2 is represented by a shallowly dipping fabric and related ∼ NW–SE stretching lineations oriented sub-parallel to the strike of the orogen. D 2 foliations are folded by ∼ NW–SE folds (F 3 ) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D 3 ). Two samples of mica schists yielded youngest detrital zircon peaks at ∼322 Ma, placing a maximum constraint on the timing of D 1 –D 3 deformation. A ∼ NE–SW granitic dyke swarm (∼252 Ma) crosscuts D 3 fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D 3 transpressional deformation took place prior to ∼252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D 1 –D 3 at ∼322–252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D 1 ), collapse (D 2 ), and transpressional deformation (D 3 ) during the convergence between the Chinese Altai and the East Junggar. On a larger scale, late Paleozoic sinistral shearing (D 3 ), together with dextral shearing farther south, accommodated the eastward migration of internal segments of the western CAOB, possibly associated with the amalgamation of multiple arc systems and continental blocks during the late Paleozoic. [ABSTRACT FROM AUTHOR]