Identical metamorphic record in distinct petrochemical systems: Case study of microscopically interlayered garnet amphibolite and metapelite from the Danba dome, SW China

Abstract

It is known that different metamorphic rocks may have different abilities in recording metamorphic processes. Here, we report microscopically interlayered garnet amphibolite and metapelite layers in a rock from the Danba dome in eastern Tibetan Plateau, SW China. Both rock types exhibit similar garnet zoning patterns, biotite and plagioclase compositions, as well as peak P-T conditions (7.3–8.0 kbar / 700–725 °C for metapelite and 7.1–8.4 kbar / 690–710 °C for garnet amphibolite, respectively), indicating a global thermodynamic equilibrium state and the same metamorphic record of the two distinct layers. However, medium-sized garnet exhibits homogeneous core compositions and biotite preserves lower Ti and Fe# [= Fe2+ / (Fe2+ + Mg)] at the lithological interface, possibly indicating more intense chemical homogenization at the metamorphic peak and retrograde modification at the interface caused by the possible presence of vast fluid. Sensitive high-resolution ion microprobe (SHRIMP) UPb dating of metamorphic zircon show a wide dispersion of ages (c. 200–155 Ma), overlapping previous age data reported for this region. These results indicate that differences in petrochemical compositions may not lead to significant distortion in the metamorphic records, including garnet zoning patterns and peak P-T conditions. This further suggests that metamorphic rocks originating from diverse protoliths, which exhibit varying peak P-T conditions within the same locality, could either be a result of the tectonic juxtaposition of distinct tectonic slices or the influence of varying fluid conditions.