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The Altai Mountains are a key area for understanding the development of the Altai Tectonic Collage and accretionary orogen. However, the orogenic processes, particularly their early stage, have not been well understood. In this work, we undertake zircon U-Pb dating of six Paleozoic synorogenic plutons in order to better define the early magmatic and tectonic evolution of the Chinese Altai Mountains. The results revealed three Paleozoic granitic plutonic events at ca. 460, 408, and 375 Ma. These ages, along with the structural patterns of the plutons, suggest two periods of regional deformation, 460–410 Ma and 410–370 Ma. The granitoids mainly follow the tholeiitic and calc-alkaline trends and are mostly I type. Sr-Nd isotopic analyses indicate that the sources of the granitoids contain both old continental and younger (juvenile) mantle-derived components. Chemical, isotopic, and structural features suggest that the plutons were formed mainly in continental arc settings and that the subduction and accretion processes began at ca. 460 Ma and culminated at ca. 408 Ma. Thus, the Altai orogen was mainly built up during early-middle Paleozoic time, rather than during late Paleozoic time. Furthermore, the southern Altai terrane comprises not only Silurian to Devonian island arcs but also old continental fragments. With these new constraints, we present a new model to account for the tectonic evolution of the Altai orogen. This model proposes that early-middle Paleozoic Altai orogenic processes could have experienced formation of an active continental margin, the splitting of this margin to form a back-arc oceanic basin, and the final closing of the back-arc basin. Consequently, the opening and closure of back-arc basins along active margins is probably a common process in the central Asian accretionary orogen. [ABSTRACT FROM AUTHOR]

The Altai orogen is an important constituent of the Central Asian Orogenic Belt (CAOB). The main orogenic processes occurred mainly in the early to middle Paleozoic and involved a series of northward subduction and terrane accretion. However, termination of the accretionary and post-accretionary processes remains poorly defined. The Chinese Altai is located in the southern part of this orogeny, which is widely intruded by Permian granitic plutons. These plutons are approximately circular in shape, free of deformation and generally cutting pre-Permian structures, suggesting a post-tectonic formation. We report the results of geochronological and geochemical data from five specific granitic plutons (Buerjin, Xibodu, Daqiaonan, Aweitan, and Adenbluk), which all yielded magmatic zircon U-Pb ages of about 270 Ma. These plutons are composed of high-K calc-alkaline rocks, including K-feldspar megaphyric granite, biotite granite and monzogranite that have metaluminous to weakly LREE-enriched, coupled with negative Eu anomalies. Significant negative anomalies of Ba, Sr, P, and Ti are also observed in the primitive-mantle normalized diagram. They have positive whole-rock εNd(t) (+1.3 to +7.2) and zircon εHf(t) values (+5.6 to +12.9), yielding Sm-Nd model ages of ⩽0.9 Ga. Therefore, these granitoids are proposed to have been generated by differentiation of mantle-derived magmas with variable crustal contamination. In view of the field occurrence, structural analysis, regional tectonics and geochemical characteristics, these Permian plutons are concluded to be post-accretionary or post-collisional. Asthenospheric upwelling after the collision and amalgamation of the Altai and Junggar blocks could have caused the mantle-derived magmas that evolved to form the granitoids. We note diat Permian intrusions are not only widespread but also voluminous in the CAOB. They mostly are post-collisional products, and some of them might have been related to the large igneous province activity in the Tarim Block. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB) is the world's largest Phanero-zoic accretionary orogen and is the most important site for juvenile crustal growth in die Phanerozoic. In this work, we employed U-Pb zircon geochronology to identify the early and middle Mesozoic intraplate granitic intrusive events in the Chinese Altai segment of die southern CAOB in order to better understand the crustal architecture of the CAOB. We also used whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data to constrain the generation for these granitic rocks and to evaluate the implications for vertical crustal growth in this region. The Early Mesozoic granitic intrusions were emplaced between 220 and 200 Ma in the central Altai "microcontinental terrane" (also widely referred to as Units 2 and 3). The granites have shoshonitic and high-K calc-alkaline affinities and show the characteristics of differentiated I-type granite. The whole-rock initial 87Sr/86Sr ratios (0.7058-0.7128) and εNd(210) values (-0.6 to -4.3), as well as the zircon eHf(t) values (-4.0 to +5.0) and two-stage Hf model ages (0.94-1.52 Ga), suggest that die granitic magmas were produced from a mixed source widi both mande-derived and recycled crustal components. The middle Mesozoic granites were emplaced at ∼150 Ma in die soudiern Altai "accretionary terrane" (Units 4 and 5). They show A-type characteristics widi the REE tetrad effect and have positive εNd(151) whole-rock values of +1.0 to +5.2 and two-stage Nd model ages (TDM2) of 0.6 to 1.0 Ga. Zircon Hf data show positive zircon εHf( 151) values of+ 1 to +8 and two-stage Hf model ages of 0.6 to 1.2 Ga. The Nd-Hf isotopic data suggest that the granitic magmas were derived from short-lived juvenile mande-derived materials. Thus, the isotopic signatures of all the Mesozoic granites from the central (old terrane) and southern (young accretional terrane) Altai suggest diat the basement of both terranes has retained its original nature. The data furdier imply that die Altai orogen has kept its original architecture of Paleozoic horizontal accretion during Mesozoic time, as commonly observed in accretionary orogens where horizontal tectonics are dominant. All the early Mesozoic intrusions in die Altai were emplaced in an intraplate anorogenic setting; hence are distinguished from the contemporaneous syn- or post-orogenic magmatism in the eastern CAOB. We conclude that the early Mesozoic granites in the CAOB were emplaced in a variety of tectonic settings. [ABSTRACT FROM AUTHOR]