Crust–mantle interaction induced by deep subduction of the continental crust: geochemical and Sr–Nd isotopic evidence from post-collisional mafic–ultramafic intrusions of the northern Dabie complex, central China

Interaction of deeply subducted continental blocks with the upper mantle peridotite is a likely process, but it has never been demonstrated. New geochemical and isotope tracer analyses of post-collisional mafic–ultramafic rocks from the Dabie terrane in central China show that they could have been generated by melting of such metasomatized mantle as a result of crust–mantle interaction. Isotopic dating using different techniques (Rb–Sr, Sm–Nd and Ar–Ar) has established that these mafic–ultramafic rocks were emplaced post-tectonically in early Cretaceous (≈120–130 Ma), nearly contemporaneous with the massive intrusions of granitic plutons. They did not form as part of the early Paleozoic arc complex, nor did they undergo UHP metamorphism at about 220 Ma. The strong enrichment of light rare earth elements (REE) and the highly negative εNd(T) values (about −15 to −20) for all mafic and ultramafic rocks indicate their derivation from an enriched mantle source. Significant negative Nb anomalies observed in the spidergrams and other `crustalʹ signatures of these rocks suggest an important role of continental material in their petrogenesis. We interpret that the singular geochemical and isotopic characteristics witness a post-collisional interaction between the subducted ancient crust (Yangtze craton) and the mantle peridotite (asthenosphere). Partial melting of such metasomatized mantle produced the basic magmas, in response to the same thermal pulse that was responsible for the massive Cretaceous granitic intrusions and resetting of some isotopic clocks in UHP metamorphic rocks. Taking into consideration all geochemical and isotopic constraints, a tectonic model is presented with emphasis on the post-collisional crust–mantle interaction and possible heat sources required for massive Cretaceous granitic intrusions. We also advocate that the digestion of deeply subducted continental blocks in the upper mantle may represent an efficient way of crustal recycling when dealing with the problem of continental growth and destruction.