Petrogenesis of Late Carboniferous gabbroic intrusions in the Xilinhot region of Inner Mongolia: Products of partial melting of a hydrous mantle source in an intracontinental extensional setting

Abstract

The petrogenesis and geodynamic setting of Late Carboniferous magmatism in Inner Mongolia, China, hold a key to understanding the final closure of the Paleo-Asian Ocean and formation of the Xing' an-Inner Mongolia Orogenic Belt (XMOB). This study carried out secondary ion mass spectrometer (SIMS) zircon U-Pb geochronological, and bulk-rock geochemical and Sr-Nd-Hf isotopic analyses of gabbroic diorites sampled in the Xilinhot region with the aim of investigating its petrogenesis and unravelling the geodynamic setting of its emplacement. Cathodoluminescence (CL) images show that the analyzed zircon grains are euhedral to subhedral in shape, with paint zoning or oscillatory zoning (Th/U = 0. 3 ∼ 2. 5). SIMS U-Pb zircon dating shows that Xilinhot gabbroic diorites were intruded at 316.9 ± 2. 2Ma, suggesting a Late Carboniferous magmatic event in the XMOB. The effects of crystal accumulation and crustal contamination on the whole-rock composition of the gabbroic rocks are insignificant, whereas fractional crystallization of olivine and clinopyroxene played an important role in magma differentiation. The source of the rocks display depleted signatures as evidenced by presence of low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7034 ∼ 0.7041) and MORB-mantle source-like positive ϵNd(t) (+5.58 ∼ +6.88) and ϵHf(t) values (+ 12. 07 ∼ + 13. 44), and they also have enriched features as characterized by enrichment of fluid-mobile elements (Rb, U, Sr, Pb), but depletion of fluid-immobile elements (Nb, Ta). Such co-existence of depleted Sr-Nd-Hf isotopes and enriched trace element signatures suggests the enrichment of mantle source by water-rich fluids may occur during or shortly prior to the melting event, i. e. a recent metasomatic event. The gabbroic rocks are also characterized by relatively high SiO2(51. 7% ∼53.2%), Cr (138.4 × 10 ⁻⁶ ∼757. 2 × 10⁻⁶), Ni (50.4× 10⁻⁶ ∼141.1 × 10⁻⁶) and Zn/Fe ratios (10.8∼11.5), but relatively low A12O3 (13. 1% ∼16. 8%) contents, indicating that they derived from a mixed source composed of an orthopyroxene-rich pyroxenite vein-plus-peridotite source. A range of geological evidence indicates an intracontinental extensional origin for the Late Carboniferous gabbroic intrusions in Xilinhot area, rather than a subduction setting. Together with the high water contents (up to 4. 41%) of contemporaneous basalts, the geochemical and isotopic characteristics of the studied gabbroic rocks indicate that the Late Carboniferous magmatism in Xilinhot and adjacent areas was generated by partial melting of mantle sources hydrated by water-rich fluids released from subducted slabs stagnated in mantle transition zone. Therefore, we proposed a deep-Earth water cycling process to account for mantle hydration and subsequent Late Carboniferous magmatism in XMOB, supporting a geodynamic link between deep-Earth water cycling, and post-orogenic magmatism and lithospheric extension.