Revisiting the boundary between the Central Asian Orogenic Belt and North China Craton in Alxa area, China: Insights from zircon U-Pb ages and Hf isotopes of Phanerozoic granitoids

Abstract

The Alxa Block, located in the southern Alxa area of Inner Mongolia, is generally accepted as the westernmost part of the North China Craton (NCC), but there is still a controversy over the boundary between the southern CAOB and the Alxa Block. Currently, either the Enger Us ophiolite belt or Quagan Qulu ophiolite belt in the mid-eastern Alxa area is a candidate for the NCC-CAOB boundary, and the extension of the two ophiolite belts in the western Alxa area is ambiguous. In this study, we present a compilation of newly obtained and published zircon U-Pb age and Hf isotopic data from the Ordovician-Triassic granitoids in the Alxa area and discuss the implications of two contrasting isotope provinces for the NCC-CAOB boundary. Specifically, the granitoids in the Zongnaishan-Shalazhashan belt between the two ophiolite belts and the North Beidashan belt in the western Alxa area share similar emplacement ages (Carboniferous to Triassic) and juvenile zircon Hf isotopic compositions (two-stage mode ages < 1.30 Ga), and both Carboniferous to Permian granitoids in the Yabulai-Honggueryulin belt to the south of Quagan Qulu ophiolite belt and Ordovician to Devonian granitoids in the South Beidashan belt of the western Alxa area are characterized by similar ancient zircon Hf isotopic features (two-stage mode ages > 1.30 Ga), but most Late Carboniferous to Permian granitoids in the South Beidashan belt are characterized by juvenile zircon Hf isotopes. The two contrasting isotope provinces (juvenile in the north and generally ancient in the south) for the Phanerozoic granitoids could not result from differential denudation, but drastic differences in compositions of deep crusts. The Yabulai-Honggueryulin and South Beidashan belts with ancient crust sources are parts of the Alxa Block, whereas the Zongnaishan-Shalazhashan and North Beidashan belts with juvenile crust sources could be parts of CAOB, and thus the boundary between the CAOB and the Alxa Block is inferred located in between.