Granulite-facies overprinting of ultrahigh-pressure metamorphic rocks, northeastern Su-Lu region, eastern China

Abstract

Secondary enstatite is present in ultrahigh-pressure (UHP) metamorphic rocks at two localities of Rongcheng County, in the northeastern Su-Lu region, eastern China. An enstatite-bearing eclogitic rock consists mainly of large grains of garnet and clinopyroxene, and enstatite is present as fine-grained coronas around quartz in the matrix. The enstatite coronas tend to develop near garnet grains, and plagioclase forms between the two minerals. In the second enstatite-bearing eclogite, the assemblage of enstatite + diopside is present as coronas around quartz. Textural relationships indicate that enstatite formed under plagioclase-stable and Si-saturated conditions after the peak-P metamorphism. Several reaction curves constrain the enstatite-forming conditions to be 700-800°C and 0.7-1.2 GPa. Application of thermometers to enstatite coronas also indicates high-T (~700-800°C) conditions. In these rocks, kyanite has been partially replaced by spinel + anorthite symplectites. Similar development of spinel is common in kyanite eclogite from Rongcheng County, suggesting that all share a similar decompressional history. Granulite-facies overprinting of UHP eclogite is probably a common phenomenon in the northeastern Su-Lu region. Equilibrium temperature at the peak-P stage is approximately the same as that recorded at the granulite stage, implying nearly isothermal decompression. Although such an adiabatic path generally requires rapid exhumation of the UHP rocks, the scale of the exhuming body is also important. Calculation of the length scale for thermal conduction indicates that UHP rocks must be greater than ~ 10 km in scale to avoid loss or gain of heat during the exhumation. Individual UHP eclogitic blocks are smaller than required for adiabatic exhumation in Rongcheng County, and hence they probably ascended together with the surrounding orthogneiss.