Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

Abstract

A reconstruction of the pressure–temperature–time (P–T–t) path of high-pressure eclogite-facies rocks in subduction zones may reveal important information about the tectono-metamorphic processes that occur at great depths along the plate interface. The majority of studies have focused on prograde to peak metamorphism of these rocks, whereas after-peak metamorphism has received less attention. Herein, we present a detailed petrological, pseudosection modeling and radiometric dating study of a retrograded eclogite sample from the Sumdo ultrahigh pressure belt of the Lhasa terrane, Tibet. Mineral chemical variations, textural discontinuities and thermodynamic modeling suggest that the eclogite underwent an exhumation–heating period. Petrographic observations and phase equilibria modeling suggest that the garnet cores formed at the pressure peak (∼2.5GPa and ∼520°C) within the lawsonite eclogite-facies and garnet rims (∼1.5GPa and <650°C) grew during post-peak amphibole eclogite-facies metamorphism. The metamorphic evolution of the Sumdo eclogite is characterized by a clockwise P–T path with a heating stage during early exhumation, a finding that conflicts with previously reported heating-compression P–T paths for the Sumdo eclogite. A garnet–whole rock Lu–Hf age of 266.6±0.7Ma, which is consistent with the loosely constrained zircon U–Pb age of 261±15Ma within uncertainty, was obtained for the sample. The peak metamorphic temperature of the sample is lower than the Lu–Hf closure temperature of garnet, which combined with the general core-to-rim decrease in the Mn and Lu concentrations and the occurrence of a second maximum Lu peak in the inner rim, is consistent with the Lu–Hf system skewing to the age of the garnet inner rim. Thus the Lu–Hf age likely reflects late eclogite-facies metamorphism. The new U–Pb and Lu–Hf ages, together with previously published radiometric dating results, suggest that the overall growth of garnet spans an interval of ∼7millionyears, which is a minimum estimate of the duration of the eclogite-facies metamorphism of the Sumdo eclogite.