Major and trace element zoning of euhedral garnet in high-grade (> 900 °C) mafic granulite from the Song Ma Suture zone, northern Vietnam

摘要

In this study, mafic granulite and related leucosomes from the Song Ma Suture zone in northern Vietnam were investigated. Euhedral garnet appears in both granulites and leucosomes, whereas the former is partly to mostly replaced by orthopyroxene + plagioclase symplectite. Most garnets occurring both in granulite and leucosome contain an Mg-rich and Ca-poor core as well as an Mg-poor and Ca-rich mantle. Garnets in leucosomes have an Mg-rich and Ca-poor thin rim. Amphibole + quartz association is observed only in the core and mantle and not in the rim and matrix. Garnet core is characterized by flat middle to heavy rare earth element (REE) patterns and depletion of light REEs. The rim also contains high concentrations of middle REEs, whereas light and heavy REEs are further depleted in comparison with the core. The REE patterns of the mantle are completely different from core and rim, which show considerable depletion in light to middle REEs and extreme enrichment in heavy REEs. The petrographical features and REE patterns of the associated mineral phases suggest that the complex chemical zonation of garnet might be formed by dehydration melting of amphibole and multiple recrystallization of garnet.