The origin and geodynamic significance of the Mesozoic dykes in eastern continental China

Abstract

We sampled 22 Mesozoic dykes in eastern continental China and carried out a detailed study on these samples, including K–Ar and zircon U–Pb geochronology, and elemental and Sr-Nd-Pb-Hf isotope geochemistry. Their K–Ar and zircon ages of 130-110 Ma are broadly consistent with the timing of the lithosphere thinning and the emplacement ages of widespread granitoids in the vast region, explicitly pointing to a common cause in space and time. The dykes represent evolved alkaline basaltic melts intruding the Mesozoic granitoids. Their rare earth element (REE) and multi-element patterns differ from the present-day ocean island basalts (OIB), but show strong arc-like signatures (e.g., enrichment in Rb and Pb and depletion in Nb, Ta and Ti). They show high (87Sr/86Sr)i (0.7048 to 0.7103), low εNd(t) (−12.3 to −5.7), low εHf(t) (−16.5 to −8.0) and low (206Pb/204Pb)i (18.79–18.85). These Mesozoic dykes are best understood as resulting from melting of geochemically enriched sub-continental lithospheric mantle (SCLM), whose geochemical enrichment is consistent with prior metasomatism with the agent being hydrous melt coming from subduction of the Paleo-Pacific plate at ~120 Ma or earlier. Similar to the present-day situation, the paleo-Pacific slab may have also existed stagnant in the mantle transition zone in the Mesozoic. The slab dehydrated and released water in the form of hydrous melt that percolated through and metasomatized the mantle lithosphere, and weakened the base of the lithosphere while producing basaltic melts that evolved to intermediate-felsic compositions of these dykes. The basaltic magmas that underplated and melted the lower crust to generate the widespread Mesozoic granitoids in eastern continental China.