Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt

•Zircon xenocrysts from the northern Alxa and its nearby regions were statistical studied.•The basement of the north part of the region (ZS) is similar to CAOB and the south part (YNH) to the Alxa Block.•The southern boundary of CAOB to the Alxa block is located between the ZS and YNH zones.•Isotopic mapping of zircon xenocrysts is an effective way to trace deep crust and to separate tectonic units.