Early-Middle Triassic Intrusions in Western Inner Mongolia, China: Implications for the Final Orogenic Evolution in Southwestern Xing-Meng Orogenic Belt

Abstract

The end-Permian to Early-Middle Triassic magmatic rocks in Inner Mongolia can provide valuable insights into the relationships between the collisional processes and the magmatic responses during the final orogenic evolution of Xing-Meng orogenic belt (XMOB). This paper presents zircon U-Pb ages and Hf isotopes, whole rock geochemical and Sr-Nd-Pb isotopic data for the Early-Middle Triassic diabases and monzogranites from the Langshan area, southwestern XMOB. Our results suggest that the studied diabases and monzogranites were respectively formed during Early Triassic and Middle Triassic. The Early Triassic diabases are characterized by “arc-like” geochemical signatures, including enrichment in Rb, U and K, and depletion in Nb, Ta, P and Ti. They have negative to weak positive ε_Nd (t) values (−3.1 to +1.5) and relatively high initial ratios of ²⁰⁸Pb/²⁰⁴Pb (35.968–37.346), ²⁰⁷Pb/²⁰⁴Pb (15.448–15.508) and ²⁰⁶Pb/²⁰⁴Pb (16.280–17.492), indicating a subduction-metasomatized enriched lithospheric mantle source. Their low Ba/Rb (2.72–6.56), Ce/Y (0.97–1.39) and (Tb/Yb)_N ratios (1.31–1.45) suggest that the parental magma was likely originated from low degree partial melting of the phlogopite-bearing lherzolite in a spinel-stability field. The Middle Triassic monzogranites show high Sr/Y ratios, low MgO, Cr and Ni contents, high Zr/Sm ratios (40–64), negative zircon ε _Hf(t) values (−25.8 to −8.8), as well as relatively flat heavy rare earth element patterns. They were likely derived from low degree partial melting of a moderately thickened ancient lower crust. The diabases and the slightly postdated high Sr/Y granites in this study represent the magmatic responses to the final orogenic evolution in the southwestern XMOB. Together with regional works, we propose that the slab break-off of the Paleo-Asian oceanic lithosphere following the terminal collision between the North China Craton and the South Mongolia terranes triggered asthenospheric upwelling, and the ongoing convergence further initiated moderately crustal thickening and uplift in the XMOB.

Keywords

diabase / granite / high Sr/Y / Early-Middle Triassic / Xing-Meng orogenic belt / slab break-off

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