Slab–mantle interaction in continental subduction channel: Geochemical evidence from Mesozoic gabbroic intrusives in southeastern North China

Abstract

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic gabbroic intrusives in the southeastern edge of the North China Block (NCB). The zircon U–Pb dating yields consistent ages of 131±2 to 136±2Ma for magma crystallization. The gabbroic rocks are characterized by arc-like trace element distribution patterns, enrichment of LILE (large ion lithophile elements) and LREE (light rare earth elements) but depletion of HFSE (high field strength elements). They exhibit variable enrichment of whole-rock Sr–Nd and zircon Hf–O isotopic compositions relative to the depleted MORB (mid-ocean ridge basalts) mantle. These geochemical features indicate their derivation from enriched mantle sources with incorporation of continental crustal-derived materials. The arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotopic compositions for these Early Cretaceous mafic igneous rocks are comparable to those of Triassic ultrahigh-pressure metaigneous rocks in the Dabie–Sulu orogenic belt. Furthermore, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are characteristic time of crustal growth for the subducted continental crust of the South China Block (SCB). These results suggest contributions of the subducted SCB continental crust to the mantle sources for the gabbroic intrusives. It is thus inferred that the mantle sources were generated by melt-peridotite reaction in continental subduction channel, in which the overlying NCB subcontinental lithospheric mantle (SCLM) wedge peridotite was metasomatized by felsic melts derived from the subducted SCB continental crust during the Triassic continental collision. The gabbroic intrusives in the southeastern NCB edge exhibit geochemical gradients from southeast to northwest, with decreases of their 87Sr/86Sr, K2O/Na2O, Ce/Pb and La/Nb ratios but increase of their εNd(t) and εHf(t) values. This suggests the spatial effect of crustal metasomatism on the mantle sources with regard to distance from the Dabie–Sulu orogenic belt. Therefore, there are variable contributions from the subducted SCB continental crust to the mantle sources, with a possible range of about 200–300km from the continental collision zone for the slab–mantle interaction in the subduction channel.