Multi-system geochronology of North Dabie eclogite: Ineffective garnet ‘shielding’ on rutile inclusions under multi-thermal conditions

Abstract

It has been shown that certain host minerals can robustly shield mineral inclusions and prevent loss of petrochronological information during subsequent thermal events. Rutile inclusions in garnet may crystallize under a wide range of metamorphic conditions and thus rutile is expected to be a useful geochronometer for constraining different stages of a metamorphic P-T path. Hence, it is important to ascertain whether rutile inclusions shielded by garnet retain the original crystallization age information despite later retrograde cooling or subsequent new thermal events. In this study, we test the “shielding effect” of garnet on rutile inclusions from the Baizhangya eclogite in the North Dabie Zone (China), which underwent a multi-stage thermal evolution. Secondary ion mass spectrometry (SIMS) UPb dating of inherited zircon core domains constrains a Neoproterozoic formation age (~770 Ma) of the eclogite protolith. SIMS UPb dating of metamorphic zircon and complementary thermal-ionization mass spectrometry (TIMS) SmNd isochron dating of garnet revealed Triassic eclogite-facies metamorphic ages (245–210 Ma). An Early Cretaceous SIMS UPb age of ca. 130 Ma, yielded by matrix rutile grains and rutile inclusions, suggests significant isotopic alteration since crystallization. This indicates that shielding was ineffective, since the UPb ages derived from the rutile inclusions, although hermetically armored by garnet, were influenced by a subsequent Early Cretaceous tectono-thermal event. Further transmission electron microscope (TEM) analysis revealed the presence of hematite microlites along the garnet-rutile boundaries, which may have acted as a potential reservoir to store the expelled radiogenic Pb from rutile. This study suggests that newly formed minerals located along mineral boundaries seem to be crucial for understanding the shielding effect.