Modification of subcontinental lithospheric mantle above continental subduction zone: Constraints from geochemistry of Mesozoic gabbroic rocks in southeastern North China

Abstract

The subcontinental lithospheric mantle (SCLM) in the southern margin of the North China Block was modified by the Triassic subduction of the South China Block, but less is known about subsequent events. This study presents a geochemical study of its anatectic products, Mesozoic gabbroic rocks from the southeastern edge of the North China Block. Zircon U–Pb dating gave ages of 119±4 to 131±1Ma for magma crystallization. The gabbroic rocks have high contents of MgO, Cr and Ni as well as high Mg# values, but low contents of SiO2. They exhibit arc-like trace element patterns, enriched Sr–Nd–Hf isotope compositions, and high δ18O values. These geochemical features indicate their derivation from partial melting of a fertile mantle source that is enriched not only in large ion lithophile elements and light rare earth elements but also in radiogenic isotopes. In particular, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are similar to those of ultrahigh-pressure metaigneous rocks in the Dabie–Sulu orogenic belt. This suggests a genetic relationship in precursor origin between them. Therefore, the mantle source for the gabbroic rocks would be generated by crustal metasomatism through underplate reaction of the overlying SCLM-wedge peridotite with felsic melts derived from subducting continental crustal rocks of the South China Block during the Triassic continental collision. The fertile and enriched signatures in the mantle source were imparted by the crustally derived melts in the continental subduction channel, giving rise to the metasomatised SCLM in the Triassic. Because of its fertile and enriched properties, the metasomatised SCLM was susceptible to partial melting in an extensional tectonic setting, resulting in gabbroic magmatism in the Early Cretaceous. Therefore, these gabbroic rocks provide petrological and geochemical records of the crust–mantle interaction above a continental subduction zone. The spatial effect of continental deep subduction on the overlying SCLM wedge may extend to 200–300km distant from the collisional orogen. The melt metasomatism would have modified the chemical and physical properties of the SCLM in the southern North China Block, facilitating the SCLM thinning in the Mesozoic.