Multi-stage Jurassic magmatism in the Liaodong Peninsula: Constraints on crustal evolution beneath the eastern North China Craton

Abstract

Whole-rock geochemical and in situ zircon UPb and HfO isotope compositions have been measured for the Jurassic granitoids in the Liaodong Peninsula, in order to investigate their genesis and reveal the Jurassic crustal evolution of the eastern North China Craton. Our new data, combined with field and petrological observations, indicate multi-stage Jurassic magmatism, now represented by Early to Middle Jurassic biotite granites and monzogranites, and Late Jurassic two-mica monzogranites. The Early to Middle Jurassic granitoids have relatively low SiO2 contents (64.4–69.6 wt%) and K2O/Na2O ratios (0.4–1.0), adakitic characteristics with enriched LREE and depleted HREE contents, high Sr/Y ratios (52–189), positive Eu and Ba anomalies, and negative Nb and Ta anomalies. Together with their highly negative zircon ɛHf(t) (−26.6 to −16.1) and mantle-like δ18O (4.6‰ to 6.3‰) values, partial melting of a basaltic lower crust, which formed at ca. 2.5 Ga, can be proposed for the magma source of the Early to Middle Jurassic granitoids. These source rocks have not experienced significant water–rock alteration after formation. In contrary, the Late Jurassic two-mica granitoids exhibit relatively differentiated geochemical characteristics, with high SiO2 contents (>72 wt%) and K2O/Na2O ratios and variable Eu anomalies. Their parental magma was also enriched in LREEs and depleted in HREEs, with high Sr/Y (49.8–134) and (La/Yb)N (22.6–38.0) ratios. The magmatic zircons have extremely negative ɛHf(t) (−37.5 to −19.6) and relatively high δ18O (6.9‰ to 9.6‰) values, indicating the involvement of an ancient crustal component with high δ18O values, such as metasedimentary rocks. We propose, therefore, that the Late Jurassic two-mica monzogranites originated from partial melting of reworked ancient crustal materials at the depth of lower crust, and with the magma subsequently evolving as a result of plagioclase-dominated fractional crystallization. During Early to Middle Jurassic, westward subduction of the Paleo-Pacific plate, resulted in the partial melting of the mafic lower crust and the burial of ancient crustal materials in to deep crust, while crustal stretching in Late Jurassic might account for the decompression melting of the ancient crustal source. Thus, multi-stage Jurassic magmatism in the Liaodong Peninsula recorded a complex crustal evolution history, which closely related to the westward subduction of the Paleo-Pacific plate.