Ultra-high-pressure (UHP) marble and eclogite in the Su-Lu UHP terrane, eastern China

A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su-Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na-bearing garnet (up to 0.2  wt% Na₂O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T  =610–660 °C and P =2.5–3.5  GPa (for X _{CO=0.001). A reaction between dolomite and diopside to form talc under tremolite-unstable conditions indicates a temperature decrease under ultra-high-pressure conditions (} P >2.4  GPa, X _{CO<0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post-dating the ultra-high-pressure metamorphism. The temperature decrease under ultra-high-pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The} P – T  conditions of the amphibolitization stage are estimated at <0.9  GPa and <460 °C, and are similar to conditions recorded by the surrounding orthogneisses.