Elemental and Nd isotopic compositions of zoned titanite in mafic microgranular enclaves of the Early Cretaceous Sanguliu granitic pluton in the North China Craton: Insights into magma mixing process

Widespread Mesozoic granitic plutons in the North China Craton (NCC) are products of remelting of the crust associated with large-scale Mesozoic destruction of the NCC. Mafic microgranular enclaves (MMEs) are common in these plutons and are attributed to magma mixing. However, the involved magmatic processes of the mixing are poorly known. In this paper, we reported complex zoning patterns, in situ U–Pb ages, and elemental and Nd isotopic compositions of titanite grains in the MMEs and host monzogranite of the Sanguliu pluton in the NCC. Titanite grains in the MMEs and monzogranite have similar U–Pb ages of ca.130 Ma, and display different internal textures and compositions. They can thus be divided into four types, i.e., types 1 and 2 from the host monzogranite, and types 3 and 4 from the MMEs. Type 1 is euhedral in shape, whereas type 2 has a euhedral inner domain and a thin deuteric rim with a sharp contact between them. Types 3 and 4 are angular grains. Type 3 shows a typical core-mantle-rim texture with a resorbed core, a mantle and an irregular rim, whereas type 4 contains a homogeneous inner domain and a discontinuous rim. Most titanite grains of the four types display oscillatory, fir-tree and sector zoning and have crystallization temperatures ranging from 680 to 750 °C based on the Zr-in-titanite thermometry, indicating a magmatic origin. The core of type 3 titanite has the highest εNd(t) of −11.3 to −12.5 among others, close to that of the mafic dykes (εNd(t) = −7.8 to −11.6) that intrude the Sanguliu pluton. The core can be interpreted as crystallized from the mixed magmas with more mafic components that were derived from metasomatized subcontinental lithospheric mantle (SCLM) beneath the NCC. The mantle of type 3 and the inner domain of type 4 titanite have εNd(t) (−13.0 to −15.3) nearly identical to that of the MMEs (−14.3 to −15.4), but distinctly higher than that of types 1 and 2 titanite (−15.9 to −18.3). The rims of types 3 and 4 titanite have εNd(t) (−16.1 to −18.3), identical to that of type 1 and the inner domain of type 2 titanite, however, they have REE concentrations and Th/U and Nb/Ta similar to the rim of type 2 titanite, clearly indicative of crystallization from evolved, hydrated, granitic magmas. A three-stage growth model is thus proposed to explain the core-mantle-rim texture of the titanite in the MMEs by a magma mixing process. The core likely crystallized from the mixed magma with more mafic components. It was then partially or totally resorbed by mixed, dioritic magma due to chemical disequilibrium, which was followed by the re-precipitation from the dioritic magma, forming the mantle. The rim is an overgrowth from the evolved, hydrated, granitic magma. This study demonstrates that the complex zoning patterns and compositions of titanite in the MMEs can be used to investigate magmatic processes including magma mixing.