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The widespread Early Cretaceous plutons intruding along the southern Great Xing'an Range (SGXR) provide evidence for tectonic evolution of the region. Petrological, geochemical, zircon U-Pb geochronology, and zircon Hf isotopic studies are conducted on intrusions from Bianjiadayuan and Hongling areas. These suites classify as A2-type granites and monzodiorites, respectively. The 138-133 Ma A2-type granites originated from partial melting of continental crustal materials at high temperatures and shallow depths with significant addition of juvenile mafic lower crust sourced from a metasomatized mantle. The 136-134 Ma monzodiorites originated from the partial melting of an enriched mantle that was modified by melts of a previously subducted slab coupled with crustal contamination. The Early Cretaceous magmatism in the SGXR occurred in two periods: ~145-136 Ma (peak at ~139 Ma; [[epsilon].sub.Hf] (t) = 5 to 10) and ~136-130 Ma (peak at ~131 Ma; [[epsilon].sub.Hf] (t) = -10 to 15). The Early Cretaceous granite--monzodiorite suite in the SGXR suggests a bimodal magmatism in an extensional setting. The ~145-130 Ma magmatism may have been triggered by asthenospheric upwelling induced by the Mongol--Okhotsk oceanic slab breakoff and large-scale lithospheric delamination resulting from post-orogenic extension. The variation of subduction direction of the Paleo-Pacific Ocean likely triggered a change in stress regime at ca. 136 Ma and likely promoted the lithospheric delamination beneath the SGXR resulting in intense magmatism originating from various sources. As such, the Paleo-Pacific Oceanic subduction likely played an important role in the Early Cretaceous magmatism in the SGXR. Key words: southern Great Xing'an Range, the Early Cretaceous, A2-type granite, bimodal magmatism, asthenospheric upwelling, lithospheric delamination. Les nombreux plutons d'âge crétacé précoce mis en place le long de la chaîne du Grand Khingan méridionale (CGKM) témoignent de l'évolution tectonique de la région. Des études pétrologiques, géochimiques, de géochronologie U-Pb sur zircons et d'isotopes de Hf sur zircons sont effectuées sur des intrusions des secteurs de Bianjiadayuan et Hongling. Ces suites sont, respectivement, des granites de type A2 et des monzodiorites. Les granites de type A2 de 138-133 Ma sont issus de la fusion partielle de matériaux de la croûte continentale à des températures élevées et de faibles profondeurs, avec un ajout important de matériau mafique juvénile de la croûte inférieure prenant sa source dans du manteau métaso-matisé. Les monzodiorites de 136-134 Ma sont issues de la fusion partielle de manteau enrichi modifié par des magmas issus d'une plaque préalablement subductée, jumelés à une conttion crustale. Le magmatisme crétacé précoce dans la CGKM s'est effectué en deux phases, soit à ~145-136 Ma (apogée à ~139 Ma; [[epsilon].sub.Hf] (t) = 5 à 10) et ~136-130 Ma (apogée ~131 Ma; [[epsilon].sub.Hf] (t) = -10 à 15). La suite de granite-monzodiorite d'âge crétacé précoce dans la CGKM indiquerait un magmatisme bimodal dans un milieu d'extension. Le magmatisme à ~145-130 Ma pourrait avoir été déclenché par la remontée asthénosphérique induite par le bris de la plaque océanique de Mongolie-Okhotsk et une délamination lithosphérique à grande échelle résultant de l'extension postorogénique. Les variations de la direction de subduction de l'océan Paléo-Pacifique ont vraisemblablement engendré un changement du régime de contraintes vers 136 Ma et favorisé la délamination lithosphérique sous la CGKM, ce qui s'est traduit par un magmatisme intense de sources variées. Ainsi, la subduction de l'océan Paléo-Pacifique a probablement joué un important rôle dans le magmatisme dans la CGKM au Crétacé précoce. [Traduit par la Rédaction] Mots-clés : chaîne du Grand Khingan méridionale, Crétacé précoce, granite de type A2, magmatisme bimodal, remontée asthénosphérique, délamination lithosphérique., Introduction Northeastern China is located between the North China Craton and the Siberian Craton and is a part of the eastern segment of the Central Asian Orogenic Belt (CAOB) (Fig. [...]

The North Qaidam Orogenic Belt (NQOB) is characterized by widespread Palaeozoic magmatism and high to ultrahigh-pressure metamorphism that provides important information regarding the continental crust growth and tectonic evolution in the North Qaidam. Our work focuses on the geology, geochronology, whole-rock geochemistry, and Sr–Nd isotopy of granitoids in the Tanjianshan area in the northern NQOB. The results indicate that the 486–469 Ma adakitic and TTG-like granitoids originated from the partial melting of Middle–Late Mesoproterozoic lower crustal materials with the addition of juvenile mafic lower crust at high pressures under an active continental margin setting. The 396–392 Ma I-type granitoids originated from the partial melting of juvenile crustal materials with the addition of ancient crustal component in a post-collisional setting. Geochronological studies show that there are significant regional differences in magmatism between the northern and rest parts of the NQOB. In the northern part, extensive and prolonged magmatism was widespread during oceanic subduction (~526–460 Ma) and post-collisional extension (~420–350 Ma) but magmatism in the stage of orogenic collision was absent. The Early Palaeozoic adakitic magmatism suggest a significant continental crustal reworking in a subduction zone during 486–469 Ma along the NQOB. The initial oceanic subduction may have occurred prior to ca. 526 Ma. An intense and multiple-stage extension developed at ~420–350 Ma in the northern part of the NQOB following the orogenic collapse. Lithospheric delamination and subsequent mafic magma underplating may be the main geodynamic mechanism resulting in the uprooting of the continental crust and ore mineralization in response. [ABSTRACT FROM AUTHOR]

The volcanic rocks of the Elashan Formation provide important evidence for the Late Triassic post-collisional magmatism in the East Kunlun Orogenic Belt. This study focused on the geochronology, geochemistry, and petrogenesis of volcanic rocks of the Elashan Formation in the Dulan–Xiangride Basin. The 227.9 ± 0.4 Ma dacite exhibits calc-alkaline I-type characteristics, with low Sr/Y and (La/Yb)N ratios and prominent negative Eu anomalies. We suggest that the dacite was derived from the partial melting of lower crust at normal crustal thickness. The crystalline tuff (225.0 ± 0.4 Ma) has negative εHf(t) values, implying that it was likely formed by the partial melting of the ancient crust. The rhyolites (221.3 ± 1.1 and 221.9 ± 0.6 Ma) are peraluminous and calc-alkaline to high-K calc-alkaline. The high SiO2 and Na2O + K2O contents, and high K2O/Na2O and Ga/Al ratios of these rhyolites have an affinity with A-type granite. The rhyolites were formed by the partial melting of Late Paleoproterozoic–Mesoproterozoic lower crust induced by mantle-derived mafic magma underplating. The andesite and basaltic andesites formed at 221.1–217.9 Ma. The zircon Hf isotopic characteristics show that the andesite was likely derived from the partial melting of the Late Paleoproterozoic mafic lower crust. The combination of our new data and previously obtained geological data suggests that the East Kunlun Orogenic Belt underwent three magmatic activities corresponding to three tectono-magmatic events (slab breakoff, the delamination of unstable lower lithospheric mantle, and the intense asthenosphere upwelling with further delamination and continuous crustal thinning) during the Late Triassic. [ABSTRACT FROM AUTHOR]