Formation age and geodynamic setting of the Neoproterozoic Shalong iron formation in the Central Tianshan, NW China: Constraints from zircon U–Pb dating, geochemistry, and Hf–Nd isotopes of the host rocks

The recently discovered Shalong iron deposit in the Central Tianshan with a reserve of 14 Mt of iron at 25–30 wt.% Fe shows the typical iron formation geological characteristics: The host rocks are mainly metamorphosed volcanic rocks including quartz hornblende schist, mica quartz schist, and plagioclase amphibolite; the iron orebodies are stratiform and bedded, indicative of strata-controlled deposits; metallic minerals are dominated by magnetite with haematite and ilmenite in minor quantities, whereas gangue minerals are quartz, amphibole, chloritoid, and plagioclase; and ore textures are banded and laminated. Here, we present LA-ICP-MS zircon U–Pb ages, whole-rock geochemistry, Sm–Nd isotope, and zircon Lu–Hf isotope analyses on the host rocks in order to constrain the timing and geodynamic setting of the deposit. The geochemical characteristics, Sm–Nd isotope, and zircon Lu–Hf isotope compositions of the host rocks indicate that their protoliths were likely basic and felsic volcanic rocks. The bimodal kind rock types, the juvenile Nd–Hf isotopic characters, and the within-plate trace element geochemical features of the host rocks together indicate that these rocks formed in a rift-like extensional setting. Zircon U–Pb dating of the host rock interlayer suggests that the Shalong iron formation was formed in the Neoproterozoic (ca. 760 Ma). Combined with geological characteristics, close association with meta-volcanic rocks, absence of glacial deposits, and within-plate geochemical and juvenile isotopic signatures of the host rocks, the Shalong iron deposit is interpreted to be an Algoma-type Neoproterozoic iron formation. The formation of this deposit can be linked to rift-like volcanic activities related to the breakup of the Rodinia supercontinent. Our results in conjunction with previous studies suggest that the Central Tianshan should be related to the breakup of Rodinia, as indicated by the Neoproterozoic rift-related igneous events of the Central Tianshan.

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Whole-rock geochemistry and zircon Hf isotope of Late Carboniferous–Triassic sediments in the Bogda region, NW China: Clues for provenance and tectonic setting

Detailed petrological, whole-rock geochemical, and zircon Hf isotopic researches were conducted on the Late Carboniferous–Triassic sedimentary rocks in the Bogda region to evaluate the effects of weathering, sorting, and alteration, as well as to understand their provenance and tectonic setting. These rocks show variable major element compositions, but most are generally similar to post-Archean Australian shales. The relatively narrow range of TiO2–Zr variation and good correlation between Th/Sc and Zr/Sc signify no obvious sorting and recycling of the sedimentary rocks. The relatively high index of compositional variability (0.8–3.1) and low chemical index of alteration values (44.4–76.4) of the sedimentary rocks indicate that they are immature and probably undergo weak to moderate chemical weathering. In the chondrite-normalized diagrams, almost all the samples are distinguished by moderately enriched light rare earth element and flat heavy rare earth element patterns with negative Eu anomalies (Eu/Eu* = 0.5–0.8). In conjunction with their Th/Sc, Zr/Sc, La/Th, Zr/Al2O3, TiO2/Zr, Co/Th, and La/Sc ratios, we infer that the major sources of these sedimentary rocks were the intermediate–acidic igneous rocks from the North Tianshan and Yili–Central Tianshan. Combining the orientation of the Bogda region in the Palaeozoic that was roughly perpendicular to the Tianshan and/or East Junggar orogens with the sedimentologic, petrologic, and tectonic researches, we suggest that the Bogda region was a rift basin that occurs at high angles to the orogenic belt. On the other hand, the zircons from these sediments have minor Precambrian grains (<0.2%), positive εHf(t) values (mostly +7.1–+15.0) and young 2-stage Hf model ages (major peak <1,000 Ma), suggesting that a juvenile continental crust exists as the basement of the Junggar Basin.

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