Apatite as a Proxy for Reconstructing Magma Mixing Histories in Granitoids

Abstract

Apatite is a common accessory mineral in granites and mafic microgranular enclaves (MMEs), and its textural and compositional characteristics can record dynamic magmatic evolution processes. Apatites from the Early Cretaceous Sanguliu granite and its MMEs in the Liaodong Peninsula exhibit pronounced compositional heterogeneities. In this study, we systematically analyzed textures, in situ trace elements, and O isotopes of apatites in the Sanguliu granite and MMEs to reveal magma mixing processes. Cathodoluminescence (CL) images reveal clear core‐rim textures and compositional zoning in apatites, defining four types: Ap1(G) and Ap2(G) in granite as well as Ap1(M) and Ap2(M) in MMEs. From core to rim, these apatites show abrupt changes in REE, Si, and Sr contents, suggesting their formation involved multiple episodes of magma mixing. In situ oxygen isotope analyses yield relatively high δ ¹⁸ O values of 7.19–8.45‰, 6.97–7.81‰, 5.32–8.75‰, and 7.77–8.86‰ for Ap1(G), Ap2(G), Ap1(M), and Ap2(M), respectively. Integrating the high Mg# values (53–56), consistent Sr‐Nd isotopic compositions (I _Sr  = 0.7145–0.7148; ε _Nd ( t ) = −18.6 to −18.3), and zircon Hf isotopes ( ε _Hf ( t ) = −21.5 to −19.6) of both the granite and MMEs, our geochemical data indicate crust‐mantle mixing and cognate magma mixing. Modeling calculations suggest that the crust‐mantle mixing involved approximately 71% lower crustal melt and 29% mantle‐derived melt, followed by 50% fractional crystallization of lower crustal melts and subsequent mixing with 28% hybrid magma. This study highlights that apatite serves as a sensitive petrogenetic proxy and provides critical insights into the petrogenesis of MME‐bearing granitoids.