Petrogenesis of Mesozoic granitoids in the northeastern North China Craton: Constraints from apatite trace elements and in-situ Nd isotopic data

Apatite can act as a sensitive recorder for reflecting the composition of the host magma at the time of its crystallization. In this paper, we report apatite geochemical and in-situ Nd isotopic data of Mesozoic granitoids in the northeastern North China Craton, to provide new insights into their petrogenesis. Based on apatite geochemistry and previous zircon U-Pb ages, we divided them into six groups: Late Triassic granitoids, Early Jurassic granitoids, Late Jurassic granitoids, and three groups of Early Cretaceous granitoids. Morphologically and compositionally, apatites from these granitoids are mostly of magmatic origin, but apatites from Early Jurassic granitoids and part of group 2 of Early Cretaceous granitoids are influenced by late metamorphism. Based on the competitive relationships of element distribution in the crystallization process of minerals (e.g., plagioclase, apatite, allanite, and titanite), we found that Sr and rare earth elements (REEs) in apatite can be utilized as geochemical indexes to reveal the mineral crystallization sequence. Apatites from Late Triassic granitoids and group 3 of Early Cretaceous granitoids crystallized earlier than plagioclase and were affected by the crystallization of allanite and titanite. Late Jurassic granitoids and group 1 of Early Cretaceous granitoids apatites co-crystallized with plagioclase. Group 2 of Early Cretaceous granitoids apatites preceded the crystal-lization of plagioclase. Furthermore, apatites from Late Triassic granitoids, Early Jurassic granitoids, and group 2 and group 3 of Early Cretaceous granitoids show high Sr, Sr/Y, and Eu/Eu* values, which are similar to those of the adakitic rocks worldwide, referring to an adakitic property; this is consistent with previous discrimination based on the whole-rock Sr-Y contents. Therefore, apatite Sr, Y, Eu and heavy REEs can be as typical geochemical proxies to identify the adakitic property. The variable in-situ Nd isotopic compositions, together with the occurrence of mafic microgranular enclaves in the studied granitoids reflected the different mantle contributions in the formation of these granitoids.