The Bogda Range (hereafter referred to as the Bogda) is located in the Eastern Tianshan. Interpreting its tectono-thermal history is critical to understanding the intra-continental evolution of the Tianshan. In this study, we report new apatite fission track data from the late Paleozoic–Mesozoic sedimentary rocks in the northern Bogda and the Late Paleozoic granites in the southern Bogda to investigate the exhumation history of the Bogda. Apatite fission track ages dominantly range from the Jurassic to earliest Cenozoic (~143–61 Ma), except for one siliciclastic sample from the Early Permian strata with an older age of ~251 Ma. Thermal history modeling, together with detrital apatite fission track age peaks, reveal that the Bogda underwent three episodes of cooling during the Late Triassic, the Late Cretaceous, and the Late Miocene. The Late Triassic rapid cooling may represent the initial building of the Bogda, which is probably related to the final closure of the Paleo-Asian Ocean. During the Late Cretaceous, the Bogda may have experienced a moderate exhumation, which was possibly triggered by the extensive tectonic extension in the central–eastern Asian regime during the Cretaceous. The Late Miocene rapid cooling may be a response to the rapid uplift of the whole Tianshan, due to the far-field effect of the continuous India–Eurasia collision since the beginning of the Cenozoic.

The Austroalpine mega-unit contains the type locality of eclogites (Haüy, 1822) but their protolith age is largely unknown except that of the Permian Bärofen eclogite, for which three Sm-Nd ages between 275 ± 18 and 275 ± 18 Ma have been reported (Thöni & Jagoutz, 1993, Geochim. Cosmochim. Acta 56, 347–368; Miller & Thöni, 1997, Chem. Geol. 137, 283–310). Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe and Sieggraben Complexes, which are considered to represent a previously coherent subducted and then exhumed fragment of a continental rift, which led to the formation of the late Middle Triassic Meliata oceanic basin. A combined zircon U–Pb and Hf isotopic study, whole-rock geochemistry of two complexes revealed a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) in the Sieggraben Complex, and 283 ± 5 Ma, 255 ± 3 Ma (Early and Late Permian), 251 ± 3 Ma, and 241 ± 3 Ma (Early to Middle Triassic) in the Saualpe-Koralpe Complex. Magmatic zircons from the Sieggraben eclogites have 176Hf/177Hf ratios of 0.283067–0.283174, εHf(t) values of +15.7 to +19.4, and that from Saualpe-Koralpe eclogite have 176Hf/177Hf ratios of εHf(t) 0.282935–0.283090, εHf(t) values of +10 to +17.4 showing their juvenile mantle source rather than significant crustal assimilation. In both complexes, N-MORB geochemical characteristics are established. Associated ultramafic rocks of Sieggraben eclogites as part of oceanic or Permian subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of Sieggraben eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during the Cretaceous. A trondhjemite dike cutting through the eclogite gives a crystallization age of 82.19 ± 0.4 Ma and is formed by partial melting of likely eclogite during decompression. The host metasedimentary rocks of Sieggraben and Saualpe-Koralpe Complexes are interpreted as old continental crust close to the margin of the Meliata basin and were affected by Permian migmatitic metamorphism. Metamorphic zircons of one eclogite from the Saualpe-Koralpe Complex give an age of 87–93 Ma (peak at 91 ± 1.2 Ma). The results of this study combined with previous results are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The Austroalpine Sieggraben and Saualpe-Koralpe Complexes represent a location on the distal continental margin during Permian to Middle Triassic rifting. The mafic rocks are associated with numerous Permian and potential Triassic acidic pegmatites, whereas structurally separated thick Triassic sedimentary cover successions lack any magmatism, likely excluding the present-day eclogite-bearing units as Triassic basement of the sedimentary cover successions.The now eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times.Acknowledgment： The study is financially supported by NSFC (91755212).

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al₂O₃/(MgO+FeO^T) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

The pre-Mesozoic basements in the Eastern Alps overprinted by the Variscan and alpine metamorphism (Neubauer and Frisch, 1993), which still remained the pre-Variscan tectonic evolution evidences. Many of these basements left away from their lithospheric roots due to large-scale tectonic activities (von Raumer et al., 2001), whereas their origin and tectonic history can be recorded by detailed geochemistry and geochronology. Here we present a study on the Schladming Complex, one part of Silvretta-Seckau nappe system in Austroalpine Unit, that located in the northern part of Alps to discuss their ages, origin, and tectonic relationship with the Proto-Tethys Ocean.The Schladming Complex basement mainly comprises biotite-plagioclase gneiss, hornblende-gneiss, mica-schists, together with some amphibolites, orthogneisses, paragneisses, metagabbro and migmatites, which covered by sequence of metasedimentary (Slapansky and Frank, 1987). It underwent the medium- to high-grade metamorphism during the Variscan event and is overprinted by the greenschist facies metamorphism during the Alpine orogeny (Slapansky and Frank, 1987).Granodioritic gneisses (539~538 Ma) and fine-grained amphibolite (531±2 Ma) in the basement represent a bimodal magmatism. Geochemical data show that the granodioritic gneisses belong to A2-type granite and originated from the lower crust, while the fine-grained amphibolites have an E-MORB affinity and the magma origined from the lithospheric mantle and contaminated by the arc-related materials. The data implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian.A medium-grained amphibolite gives an age of 495±5 Ma, exhibits ocean island basalt-like geochemical features and zircons positive εHf(t) values (+5.3~+10.9) indicating that the medium-grained amphibolite derived from a depleted mantle source. The monzonite granitic gneiss and plagioclase gneiss yields ages of 464±4 Ma for and 487±3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive εHf(t) values of +5.9~+7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite.In summary, our data presents record of the Cambrian to Ordovician magmatism in the Schladming Complex, which provided new evidence of tectonic evolution history between Proto-Tethys and Gondwana. According to the data, we proposed that a series of rift developed in the northern margin of Gondwana during 540-530 Ma, the rifts continually expanded into a back-arc ocean in ~490 Ma and was closed around 460 Ma with S-type granitic magma intruded.ReferencesNeubauer, F., Frisch, W. 1993. The Austroalpine metamorphic basement east of the Tauern window.  In: Raumer, J. von & Neubauer, F. (eds.): Pre-Mesozoic Geology in the Alps. Berlin (Springer), pp. 515–536.von Raumer, J., Stampfli, G., Borel, G., Bussy, F., 2001. Organization of pre-Variscan basement areas at the north-Gondwanan margin. International Journal of Earth Sciences 91, 35-52.Slapansky, P., Frank, W. 1987. Structural evolution and geochronology of the northern margin of the Austroalpine in the northwestern Schladming crystalline (NE Tadstädter Tauern). In: Flügel, H. W. & Faupl, P. (eds.), Geodynamics of the Eastern Alps, pp. 244-262.