Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

Abstract

AbstractComposite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.