Post-orogenic bimodal volcanism along the Sulu orogenic belt in Eastern China

Abstract

The early Cretaceous volcanism occurring along the Sulu orogenic belt, east Shandong Province, demonstrates a bimodal characteristic. The volcanic rocks belong to high-K alkaline series, dominated by alkali basalt, basaltic trachyandesite, latite and trachyte with LILE (e.g. K, Sr and Ba) and LREE enrichment but HFSE depletion (especially for Nb and P) in the primitive mantle-normalized spidergrams and steeply right-declined REE patterns. The enriched initial Sr-Nd isotopic ratios (⁸⁷Sr/⁸⁶Sr = 0.70724 to 0.70750 and ε_Nd(t) = −17.0 to −15.9) of the basaltic samples suggest their origin from an enriched lithospheric mantle, which might had undergone a fluid metasomatism or source mixing by the continental crust during or shortly after the Triassic continental subduction. Significantly negative Nb anomalies observed in the spidergrams and other “crustal” signatures of these rocks suggest an important role of continental material in their petrogenesis. The felsic rocks demonstrate geochemical and Sr-Nd isotopic features (initial ⁸⁷Sr/⁸⁶Sr = 0.70814 to 0.70961 and ε_Nd (t) = −18.9 to −17.0) similar to those of the post-collisional granitic plutons, probably derived from the anatexis of lower/middle crust in response to basaltic magma underplating. The widespread melting of the metasomatized mantle was probably attributed to the thermal perturbation or lithospheric extension induced by the mega-large displacement along the Tan-Lu wrench fault system when northward strike-slipping movement of the Izanagi Plate occurred during the late Mesozoic.