Origin of the Early Permian zircons in Keping basalts and magma evolution of the Tarim Large Igneous Province (northwestern China)

Abstract:

The Tarim continental flood basalts (CFBs) provide important clues about the genesis and magmatic evolution of the Early Permian Tarim Large Igneous Province (Tarim LIP) in northwestern China. Here we present results of LA–MC–ICPMS Lu–Hf isotope analysis on Early Permian (ca. 290Ma) zircons extracted from the Tarim CFBs in the Keping area, northwest of the Tarim Basin. Zircons from two sub-groups of Keping basalts (Groups 1a and 1b) have similar Lu–Hf isotopic compositions and exhibit a relatively large range of 176Hf/177Hf ratios between 0.282422 and 0.282568. Their negative εHf(t) values (−6.8–−1.4) are generally lower than the whole-rock εHf(t) values of their host basalts (−2.8–2.1), and are distinct from other known intrusive rocks (−0.3–7.1) in the Tarim LIP and their hosted zircons (4.9–8.8). Systematic studies of Hf isotopic data from Tarim and its adjacent regions reveal that these zircons are probably xenocrysts, sourced from coeval igneous rocks in the South Tianshan Orogen (e.g., the Lower Permian Xiaotikanlike Formation volcanic and pyroclastic rock suite). This, together with the presence of Precambrian zircons in Keping basalts, clearly indicates crustal contamination during their eruptions and provides hints about the potential contaminant sources. Geochemical modeling further suggests that the earlier erupted Group 1b basalts experienced more contamination, predominantly by some high Th–U–Pb rock components, most likely from the South Tianshan Orogen. The later erupted Group 1a basalts in the Keping area have been less contaminated with mainly the Tarim Precambrian rocks. Another group of the Tarim CFBs in the Northern Tarim Uplift (Group 2) appears to have undergone negligible crustal contamination but possesses evidence for variable source compositions. The modeling also indicates that the uncontaminated parental magmas of various Tarim LIP rocks (from the picrites and basalts to ultramafic–mafic and syenitic intrusive rocks) exhibit a wide range of εNd(t) values (ca. −5–5), reflecting source isotopic heterogeneity, which may be a consequence of plume–lithosphere interaction during the generation of the Tarim LIP.

Notes:

Special Issue Permian large igneous provinces: Characteristics, mineralization and paleo-environment effects

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