Metamorphic evolution and tectonic implications of high-pressure metapelites from the Song Ma suture zone, between the South China and Indochina cratons

Abstract

High-pressure metapelites from the Song Ma suture zone provide crucial insights into the tectonometamorphic evolution of convergent plate boundaries between the South China and Indochina cratons. Two distinct metamorphic stages (M₁ and M₂) are identified. M₁ is characterized by relict inclusions of garnet, quartz, white mica, chlorite, monazite within albite porphyroblasts, with internal fabrics (S₁) ranging from straight to sigmoidal and discontinuous to the major foliations (S₂) indicating pre-S₂ formation. M₂ assemblages include porphyroblasts of garnet, albite, quartz and white mica, chlorite, monazite in the matrix, almandine is higher than spessartine from core to rim of garnet. The P–T conditions of M₁, constrained by isochemical phase diagrams and geothermobarometric data, are estimated at 15–17 kbar and 440–475 °C, whereas those of M₂ are 11–12.5 kbar and 440–490 °C, suggesting blueschist facies metamorphism followed by the epidote–amphibolite facies. Monazite grains associated with M₁ and M₂ exhibit compositional and textural differences, indicating distinct metamorphic events. U–Th–Pb isotopic data yield ages of 244 ± 10 Ma (M₁) and 234 ± 10 Ma (M₂) by SHRIMP, and 241 ± 5 Ma (M₁) and 221 ± 4 Ma (M₂) by LA–MC–ICP–MS. These results collectively record the burial and subduction of metasedimentary protoliths during the initial stages of crustal convergence, followed by subsequent exhumation during the collisional amalgamation of the Indochina and South China cratons and closure of the Song Da Ocean.