Abstract
Monazite and magnetite are sensitive indicators of local fluid chemistry, pressure, and temperature during metasomatism. In this study, the role of fluids, during the metamorphism of a granite to metagranite, (Jiao-Liao-Ji orogenic belt, North China Craton), is explored via monazite, magnetite, and pyrite microtextures and mineral chemistry coupled with zircon and monazite Th–U–Pb dating. CL bright zircon cores (2163 ± 17 Ma) record the crystallization age of the granite. BSE dark monazite cores (1876 ± 36 Ma) are characterized by high U and Ca and low Nd contents. The surrounding BSE bright mantle (1836 ± 14 Ma) is characterized by abundant fine-grained huttonite inclusions, a high porosity, a high Th and Si content, and a low P, La, Ce, and Y content. The monazites are surrounded by a three-layered concentric corona consisting of first fluorapatite, followed by allanite, and then epidote. TiO2 in the primary magmatic magnetite (Mag1–1) has been mobilized to form a series of compositionally and texturally distinct magnetites (Mag1–2, Mag2, Mag3, Mag4, and Mag5) associated with ilmenite, rutile, and titanite reaction textures. Combined, these results suggest that external NaCl and sulphate-bearing fluids derived from a local sulphate-bearing evaporate infiltrated the granite and induced the formation of pyrite and enriched the pre-existing monazite in S at around 1904 Ma. In situ δ34S values for pyrite range from 13.03 ‰ to 13.41 ‰, which is typical of metamorphic pyrite. Sporadic synchysite-(Y) inclusions in the pyrite indicate a local CO2-rich component in the fluid. The BSE bright mantle around monazite formed from later fluids from the same local evaporite deposit during the decompression stage of the Jiao-Liao-Ji orogenic belt at around ~ 1840 Ma, which overlaps with zircon dark rims at 1849 ± 12 Ma. This same Na-bearing fluid induced the albitization of the feldspars, formation of apatite–allanite–epidote coronas around monazite, and formation of rutile–titanite–epidote alteration textures associated with magnetite and ilmenite exsolved from the magnetite. During subsequent much later greenschist facies metamorphism, muscovite, chlorite, and Mag5 were precipitated along mineral grain boundaries, mineral cleavage, micropores, and fractures and pyrite experienced partial alteration to goethite.
