The age, isotopic signature and significance of the youngest Mesozoic granitoids in the Jiaodong Terrane, Shandong Province, North China Craton

Abstract

Most Cretaceous granitoids in the Jiaodong Terrane of Shandong Province, North China Craton, were intruded between ~ 130 and 122 Ma. However, four granitoid batholiths are significantly younger with an average zircon U–Pb SHRIMP emplacement age of 116 ± 2 Ma, these are: the Sanfoshan Batholith — 118 ± 1 Ma, the Aishan Batholith — 116 ± 1 Ma, the Yashan Pluton — 113 ± 2 Ma and the Laoshan Batholith — 115 ± 2 Ma. These granitoids post-date proposed delamination that occurred prior to ~ 125 Ma in the eastern NCC and their geochemistry thus provides information about the nature of the continental crust at this time. The rocks are mostly monzogranites and syenogranites of I-type affinity, with local alkali-feldspar granite of A-type affinity. In addition, a monzogranite sample from the Aishan Batholith records an age of 125 ± 3 Ma, indicating a direct association with the slightly older granitoids in the area. Indeed, they all contain inherited zircons with ages of ~ 125–120 Ma, indicating interaction with these earlier granitoids. Most samples are metaluminous, enriched in light rare earth elements and depleted in high-field strength elements. Paleo- to Neoarchean T DM2 model ages (ranging from 3900 Ma to 2491 Ma) and negative ε Hf(t) values imply the involvement of Archean basement and older recycled continental crustal material in the magma source region. They were likely derived from partial melting of the lower or middle crust due to mafic magma underplating followed by various degrees of interaction between mantle-derived mafic magma and felsic crustal magma, accompanied by fractional crystallization. We propose that lithospheric thinning and extension in the Early Cretaceous, due to roll-back of the Pacific Plate beneath the eastern North China Craton, led to asthenospheric upwelling and basaltic magmatism. This underplating in turn led to crustal melting and evolution of the granitic intrusions described here.