Origin of transitional I-A-type syenite and its relationship to A-type intrusions in the Luzong Basin, the Lower Yangtze River Belt: Insights from geochemistry

There are potential genetic relationships between I- and A-type granitoids, which remain obscure. The origin of transitional I-A-type intrusions could provide new clues for this issue and are important in deciphering the origin of A-type intrusions. Here we report geochemical and isotopic compositions of transitional I-A-type syenites (Makou pluton) from the Luzong Basin in the Lower Yangtze River Belt (LYRB). The Makou pluton is composed of gabbros and syenites. Zircon UPb ages reveal that it was formed at ∼130 Ma, which was slightly earlier than typical A-type intrusions in the LYRB. Geochemical features indicate that the Makou syenites belong to transitional I-A-type igneous rocks. The Sr-Nd-O isotopic compositions and the correlation between major and trace elements of Makou gabbros and syenites suggest that they are cogenetic, whereas the syenites experienced further magma evolution. Geochemical characteristics, especially the high Nb content of gabbros, suggest that the mantle source of the Makou pluton was metasomatized by melts from subducted sediments and slabs, which is similar to Huangmeijian A-type intrusion in the Luzong Basin. Makou syenites have different evolution conditions, e.g., the temperature and oxygen fugacity, compared with the Huangmeijian A-type intrusion. The ferroan character of the Makou syenites was weakened by crystal fractionation at relatively high oxygen conditions to produce the transitional I-A-type intrusions, due to more addition of fluids derived from subducted slabs to the source. This is consistent with the partial melting of the metasomatized lithosphere mantle due to asthenosphere upwelling as a result of ridge subduction beneath the LYRB.