Ancient subcontinental ultra-depleted mantle-derived crust and its role
in Phanerozoic magmatism of Central Asian Orogenic Belt
Abstract
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
εNd(t)–εHf(t) and is regarded as the
largest site of Phanerozoic crustal growth on Earth. Ancient (ca. 1.7
Ga) subcontinental UDM with extremely high
εNd(t)–εHf(t) values was identified
beneath the CAOB for the first time based on finding of 1.7 Ga diorite
intrusions with extremely high εNd(t),
εHf(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 εNd(t)–εHf(t) values, and
subsequent episodic remelting of this crust generated the Phanerozoic
high εNd(t)–εHf(t) granitoids in the
CAOB. Phanerozoic crustal growth in accretionary orogens as exemplified
by the CAOB may have been overestimated.