Multiple metamorphic events recorded in the metamorphic terranes in central Inner Mongolia, Northern China: Implication for the tectonic evolution of the Xing’an-Inner Mongolia Orogenic Belt

Abstract

The tectonic evolution of the Xing’an-Inner Mongolia Orogenic Belt (XIMOB) in the southeastern segment of the Central Asian Orogenic Belt is not well understood. It is arguable whether the Paleo-Asian Ocean was closed in Devonian or Early Mesozoic and whether the tectonic regimes during Carboniferous and Permian were operated by continuous subduction-arc systems or extension after the orogenic process. Comprehensive studies of metamorphism for four metamorphic terranes including the Ondor Sum Group, Baoyintu Group, Xilingol Complex, and early Triassic metamorphic sequences in central Inner Mongolia provide good constraints on the arguable issues about the XIMOB. According to the zircon U-Pb dating analyses in this study, quartz schists of the Ondor Sum Group in the northern and southern orogenic belts (NOB and SOB) have a broad detrital zircon age range from late Archean to early Paleozoic with a similar youngest deposition age of 434–430 Ma, but have different zircon age populations with a ∼650 Ma peak for the NOB but not for the SOB. Biotite gneisses of the Xilingol Complex have age populations and the youngest deposition age of 436 Ma similar to the Ondor Sum Group in the NOB. Amphibolites in the complex have a metamorphic zircon age of 321 ± 3 Ma and hornblende 40Ar/39Ar age of 309 ± 2 Ma. The early Triassic metamorphic sequences, metabasites in Wulangou and mica-schists in Shuangjing both have broad zircon age ranges from the late Archean to late Paleozoic. Their protoliths are constrained to have formed later than 258 Ma and 261 Ma respectively and the metamorphism occurred soon afterwards in the Early Triassic constrained by an actinolite 40Ar-39Ar age of 241 ± 19 Ma in metabasite and a muscovite 40Ar-39Ar age of 240 ± 2 Ma in mica-schist. Combined with the results previously published, this study identified four phases of metamorphism in central Inner Mongolia since Paleozoic. The Silurian (430–426 Ma) high P/T metamorphism of the Ondor Sum Group was related to bidirectional subduction processes. The Devonian (∼400 Ma) medium P/T metamorphism of the Baoyintu Group suggests a crustal thickening orogeny. The Carboniferous (337–321 Ma) low P/T metamorphism of the Xilingol Complex is indicative of an extension process of a previous orogen. The Early Triassic (∼240 Ma) extensive medium-low P/T metamorphism along the Solonker suture zone was attributed to the closure of limited ocean basins. Thus, an alternative tectonic scenario for the metamorphic terranes in central Inner Mongolia involves: (i) the early Paleozoic trench–arc system (500–425 Ma); (ii) the Devonian collisional orogeny due to closure of the Paleo-Asian-Ocean (400–360 Ma); (iii) the Carboniferous and Permian extension after the collision (350–250 Ma), and (iv) the Early Triassic within-plate orogeny due to the closure of limited ocean basins (∼240 Ma).