Ultra-depleted mantle (UDM), characterized by Nd-Hf isotopically more depleted than mid-ocean ridge basalt, is ubiquitous beneath the oceanic crust, but no UDM has been reported underneath the continental crust. The Central Asian Orogenic Belt (CAOB) preserves large volumes of Phanerozoic granitoids with elevated ε_Nd(t)–ε_Hf(t) and is regarded as the largest site of Phanerozoic crustal growth on Earth. Ancient (ca. 1.7 Ga) subcontinental UDM with extremely high ε_Nd(t)–ε_Hf(t) values was identified beneath the CAOB for the first time based on finding of 1.7 Ga diorite intrusions with extremely high ε_Nd(t), ε_Hf(t) and δ¹⁸O values, and significant Nd-Hf isotope decoupling. Partial melting of 1.7 Ga subcontinental UDM produced widely distributed 1.7 Ga crust with ultrahigh ε_Nd(t)–ε_Hf(t) values, and subsequent episodic remelting of this crust generated the Phanerozoic high ε_Nd(t)–ε_Hf(t) granitoids in the CAOB. Phanerozoic crustal growth in accretionary orogens as exemplified by the CAOB may have been overestimated.