Genetic links between granitic and related dioritic rocks in Liaodong Peninsula, China: Sr–Nd–Hf–O isotopic evidence

Abstract

Syenogranite and related dioritic rocks (dioritic microgranular enclaves and quartz diorite) from the Early Cretaceous Shihuzhen–Jiudaogou pluton, NE China, were studied to investigate the formation and evolution of the granitic magma driven by the mantle-derived magma. All selected rocks were formed contemporaneously, with zircon UPb ages of 125–121 Ma. Field and petrographic observations show that magma mixing happened between mafic and felsic magmas represented by dioritic enclaves and syenogranite. The dioritic enclaves with quenching textures have low SiO2 and high MgO contents, variable zircon εHf(t) compositions (−8.7 to +2.4), coupled with low initial 87Sr/86Sr ratios (0.70647–0.70772) and variable εNd(t) values (−8.8 to −3.9) for apatite, suggesting they were derived from an enriched mantle source with contamination of crustal materials. The syenogranites have relatively high 87Rb/86Sr ratios (2.92–22.0), variable zircon εHf(t) compositions (from −11.1 to −3.6), coupled with low initial 87Sr/86Sr ratios (0.70727 to 0.70791) and variable εNd(t) values (from −9.9 to −7.3) for apatite, requiring a juvenile crust origin with participation of mantle-derived magma. They were characterized by high SiO2 (> 74.0 wt.%) and low MgO, P2O5 contents, with conspicuous negative Ba, Sr, and Eu anomalies, implying a late fractionation phase. The quartz diorites have relatively high initial 87Sr/86Sr ratios (0.70781–0.70928) for apatite, with negative zircon εHf(t) values (−11.6 to −7.8) and apatite εNd(t) values (−10.5 to −8.9), which are identical to those of the felsic parental magma of syenogranite. They show complementary geochemical compositions with the syenogranite, including low SiO2 (< 60.0 wt.%), high MgO (Mg# up to 44.2) and Sr (> 744 ppm) contents, with obvious positive Ba and Sr anomalies and weakly positive Eu anomalies. Coupled with petrological observations (e.g., cumulate texture), we argue the crystal-melting segregation model account for the genetic link between the quartz diorite and syenogranite, representing residual crystal accumulations and segregated interstitial melt from a crystal-rich magma chamber.The partial melting of juvenile lower crust and the subsequent crystallization differentiation of intermediate-acid magma developed a crystal-rich magma chamber. The mantle-derived magma then drove the rejuvenation of pre-exist magma chamber, promoted the crystal-melt segregation process and mixed with the felsic magma, and then formed quartz diorite and syenogranite in the Shihuzhen-Jiudaogou pluton. Mantle-derived magma played an important role in the formation of the granitic rocks, providing heat for partial melting, materials, and the driving force for their compositional evolution.