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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]

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]

We report here for the first time the occurrence of sapphirine-bearing granulites within the Qinling Group of the Qinling-Tongbai orogen and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions. We document the UHT indicator of sapphirine and spinel in a mafic granulite consisting of orthopyroxene, biotite, plagioclase, amphibole and rutile/ilmenite. The ferromagnesian minerals in the sapphirine-bearing granulite have high X Mg [Mg/(Mg + Fe)] (orthopyroxene X Mg = 0.84–0.95; biotite X Mg = 0.81; amphibole X Mg = 0.87–0.96). The phase equilibria modeling demonstrates that the early spinel-bearing assemblage is stable at 923–950 °C and 6.7–8.9 kbar, and the peak assemblage of Opx + Pl + Spr/Spl + Amp + Bt + Ilm (+ melt) defines a field at 922–947 °C and 8.4–10.2 kbar. Rutiles have variable Zr concentrations but mostly cluster at ca. 1,500 and 3400 ppm. Zr-in-rutile geothermometry yielded high temperatures of up to 890–940 °C. Zircon U–Pb dating of the granulite constrains the timing of the immediate post-peak and retrograde metamorphic stages as 429 ± 7 Ma and 412 ± 4 Ma, respectively. The UHT metamorphism, together with extensive occurrence of coeval magmatic suites suggests that the Tongbai orogen experienced a Paleozoic Andean-type orogeny probably derived from mid-oceanic ridge subduction of the Qinling Ocean. [ABSTRACT FROM AUTHOR]

High‑uranium (~4200 to 8500 ppm U) columbite-group minerals (CGMs) from No. 1 pegmatite at Dakalasu, the Chinese Altay orogen were used to assess the effect of metamictization on the U Pb isotopic systematics. Backscattered electron (BSE) imaging shows that the high U concentrations are due to the U in the crystal structure itself. In the rims, the part of CGM next to cracks or pores filled by titan-uranoan fluorcalciomicrolite is altered. The microlite was likely formed via a dissolution-reprecipitation process that resulted in fractionation between U and Pb and potential discordance of the U Pb isotopic systematics in the altered CGM. Raman spectroscopy indicates that the columbite samples are metamictized to various extents. However, the CGM samples dated by laser ablation – inductively coupled plasma – mass spectrometer (LA-ICP-MS) show excellent U Pb age concordance, with thirty eight out of forty analyses displaying concordant U Pb ages (229.0 ± 1.0 Ma to 228.1 ± 0.6 Ma) overlapping within analytical uncertainty (2σ). The alpha-decay doses are estimated to be 1.53 to 2.31 × 1015 α-decay events/mg, indicating that the CGMs are at a transition stage from being crystalline to completely metamict states. Although spatial separation between U and radiogenic Pb isotopes caused by α-recoil is inevitable, the displacement is on sub-micron scale, much smaller than the scale of laser ablation pits (43 μm) for LA-ICP-MS dating. Therefore, the U Pb isotopic systematics of partially metamict, U-rich CGM may still remain concordant for CGMs that have not suffered hydrothermal alteration or metasomatism and do not contain U- or Pb-rich inclusions within the laser ablation pits. Additionally, the chemical compositions of the CGM have no control over the U Pb isotopic systematics. • The effect of metamictization on columbite U Pb isotopic systematics is investigated. • The degree of metamictization is quantified using α-decay dose and displacements per atom. • Hydrothermal alteration can result in discordance of columbite U Pb isotopic systematics. [ABSTRACT FROM AUTHOR]