Subduction, peak and multi-stage exhumation metamorphism: Traces from one coesite-bearing eclogite, Tso Morari, western Himalaya

• Coesites in garnet rims are as large as 60 μm.
• Inclusions in garnet were mapped and directly correlated to the compositional zoning.
• REE concentrations were obtained for garnet, its inclusions and major matrix minerals.
• REE pattern reflect sequential change in matrix minerals and their proportions.
• We link prograde, peak pressure (44–48 kbar/560–760 °C) and retrograde conditions.

Ultrahigh-pressure (UHP), coesite-bearing eclogites in the Himalaya have been documented from the Kaghan Valley in Pakistan and the Tso Morari area in northwest India. These complexes are part of the northern edge of the Indian plate that has been subducted to, and metamorphosed at, mantle depths of more than 100 km before being exhumed. Both UHP complexes are located today directly adjacent to the Indus–Tsangpo suture zone and are not separated by non-metamorphosed sequences of Tethyan sediments from the Asian margin. Herein, we present new data for one fresh coesite-bearing eclogite from the Tso Morari massif. Therein, garnets are zoned reflecting their growth during prograde and peak metamorphism and showing a thin retrograde overgrowth. Inclusions can be directly correlated to the compositional zoning and are seen as either relicts of the protolith mineral paragenesis and as “snap shots” of the mineral paragenesis during subduction and under peak conditions. Rare earth element concentrations (REE) were obtained for garnet, mineral inclusions in garnet and matrix minerals. The REE pattern in garnet reflects a sequential change in matrix minerals and their proportions due to net transfer reactions during subduction and peak metamorphism. Using conventional geothermobarometry, a peak pressure of ca. 44–48 kbar at 560–760 °C followed by an S-shaped exhumation curve has been deduced. Gibbs free energy minimization modelling was used to supplement our analytical findings.