The nature of orogenic lithospheric mantle: Geochemical constraints from postcollisional mafic–ultramafic rocks in the Dabie orogen

Abstract

Postcollisional mafic–ultramafic rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr–Nd–Pb isotopes in addition to zircon U–Pb ages and Lu–Hf isotopes. The results provide insights into the nature of orogenic lithospheric mantle in the continental collision zone. The zircon U–Pb dating gave consistent ages of 125±3 to 129±1Ma for magma crystallization. A few residual zircon cores exhibit U–Pb ages of 234±5Ma and 739±9Ma, respectively, in agreement with ages of tectonothermal events for ultrahigh-pressure metamorphism and protolith formation. The mafic–ultramafic rocks have high contents of MgO (up to 18.0wt.%), Cr (up to 1546ppm) and Ni (up to 349ppm), but low contents of SiO2 (41.0–51.9wt.%), and exhibit arc-like trace element distribution patterns and enriched Sr–Nd–Pb isotope compositions. These geochemical features indicate their derivation from partial melting of special mantle sources that are fertile in lithochemistry and enriched not only in LILE and LREE but also in Sr–Nd–Pb isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle and they would be generated by melt–peridotite reaction in the continental subduction channel. The enriched signatures of incompatible trace elements and radiogenic isotopes in the mantle sources would be caused by metasomatic reaction of the overlying subcontinental lithospheric mantle (SCLM) peridotite with different compositions of felsic melts derived from the subducted continental crust during the Triassic continental collision. In this regard, slab–mantle interaction is evident during the continental deep subduction, with the postcollisional mafic–ultramafic rocks as its derivatives. Significant differences in element and isotope compositions between different mafic–ultramafic intrusions suggest that the orogenic lithospheric mantle is geochemically heterogeneous, with possible existence of hornblende-rich and pyroxene-rich lithologies in the mantle sources. The heterogeneity is primarily attributed to differences in the compositions of felsic melts derived from the subducted crust with a tectonic affinity to the South China Block, whereas the same SCLM of the North China Block was involved in the crustal metasomatism in the subduction channel. Therefore, the postcollisional mafic–ultramafic rocks provide a snapshot of the compositional and lithological variations in the orogenic lithospheric mantle in the continental collision zone.