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Abstract: The Chinese Tianshan occupies the southernmost part of the Central Asian Orogenic Belt (CAOB). High-grade metamorphic rocks are widely distributed in its central uplift zones, composing the crustal basement of the Central Tianshan Block of the Chinese Tianshan. However, the origin of the crustal basement and whether the high-grade metamorphism was associated with the Paleozoic orogeny are still unresolved. Here, we present precise LA-ICP-(MC)-MS zircon U–Pb dating and Hf isotopic data for three meta-sedimentary rocks and one orthogneiss from the Xingxingxia Complex, which represents the Precambrian basement of the Central Tianshan Block. Zircon U–Pb dating results show that the protolith age of the orthogneiss is ca. 880Ma and that the meta-sedimentary rocks consist dominantly of 0.8–1.0Ga and 1.3–2.0Ga materials with minor early Paleoproterozoic components. In combination with the zircon Hf isotopic data, it is suggested that the crustal basement of the Central Tianshan Block was formed in the early Mesoproterozoic, which is evidently different from the Archean basement formation of the Tarim Craton. We suggest that the Central Tianshan Block may have formed through a magmatic arc accretionary orogen along the continental margin of Baltica during the Mesoproterozoic. Moreover, both the meta-sedimentary rocks and the orthogneiss of the Xingxingxia Complex simultaneously suffered a metamorphic overprint at ca. 380Ma. Zircon REE and Hf isotopic data indicate that the metamorphic zircon rims were formed by new zircon overgrowths in partial melts. Geothermobarometry and average P–T calculations using THERMOCALC on the Grt–Bt gneiss yielded ca. 720–730°C and 4–6kbar, suggesting amphibolite- to granulite-facies conditions. The Late Devonian metamorphism of the Xingxingxia Complex is possibly related to orogenic low-pressure/high-temperature metamorphism in the middle or upper crust during the closure of the eastern segment of South Tianshan Ocean. Our new data, together with the occurrences of Ordovician–Silurian eclogites and HP mafic granulites along the northeastern margin of the Tarim Craton, indicate that progressive continental collisions occurred in the southern CAOB during the Paleozoic. [Copyright &y& Elsevier]

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U–Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic–pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granulites, and orthopyroxene–clinopyroxene-bearing granulites. Average P–T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600–650 °C at 3.2–4.2 kbar and peak metamorphism of 750–840 °C at 2.9–6.3 kbar, indicating a rather high geothermal gradient of ca. 60 °C/km. Zircon U–Pb LA–ICP–MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380 Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution. [ABSTRACT FROM AUTHOR]

Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U–Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO 2 = 60.48–78.92 wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U–Pb crystallization ages from ca. 1.45 to 1.40 Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon ε Hf ( t ) values between − 1.0 and + 8.6 and the zircon Hf model ages of 1.95 to 1.55 Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40 Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U–Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo- to Mesoproterozoic times. However, the southern Beishan microcontinents and the CTA have similar zircon U–Pb and Hf isotopic signatures, indicating a similar Precambrian tectonothermal evolution for both. This finding provides new and important constraints on the Precambrian crustal evolution of the microcontinents in the southern CAOB. [ABSTRACT FROM AUTHOR]