Abstract

Eclogites in the Donghai area of the Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China contain both disseminated rutile and rutile deposits. The rutile occurs as small inclusions in garnet, omphacite, phengite, and quartz, as larger euhedral to subhedral grains in equilibrium with garnet, omphacite, phengite, and kyanite, and as discrete veinlets, aggregates, and megacrysts in cracks and along grain boundaries. To investigate the processes of rutile mineralization and its relationship to the evolution of the Sulu UHP metamorphic belt, we sampled the exposed Maobei deposit at Donghai and drill core between 100 and 2000 m in the nearby Chinese Continental Scientific Drilling Project (CCSD) main hole to carry out a detailed infrared microthermometric fluid inclusion study. Three types of fluid inclusions were identified: aqueous (Type I), CO2-H2O (Type II), and CH4 (Type III). Type I and Type II inclusions are the most common. Primary and pseudosecondary Type I inclusions, which were trapped during recrystallization of the eclogite or early amphibolite facies retrograde metamorphism, have a narrow range of homogenization temperatures but a wide range of salinities. In contrast, secondary Type I inclusions, which formed entirely during retrograde amphibolite-facies metamorphism associated with exhumation of the UHP rocks, have higher and more varied homogenization temperatures but medium to low salinities. Type II inclusions formed contemporaneously with the Type I variety and are characterized by having small amounts of CO2 and N2 in addition to H2O. In addition, some of the rutile grains contain sparse inclusions composed of CH4 (Type III). Our results combined with those of previous studies, suggest a great diversity of fluid composition in the peak metamorphic to early retrograde stages, implying very limited fluid-rock interaction during syn- to post-peak high-pressure (HP) and UHP metamorphism. This study shows that infrared microthermometric investigation of fluid inclusions can provide information not only on the composition of titanium ore-forming fluids but also on the relationship between mineralization and the evolution of UHP metamorphic belt.

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