An experimental study of dehydration melting of phengite-bearing eclogite at 1.5–3.0 GPa

Abstract

Dehydration melting experiments were performed on ultrahigh-pressure eclogite from Bixiling in the Dabie orogen at 1.5–3.0 GPa and 800–950°C using piston cylinder apparatus. The results show that (1) eclogite with ∼5% phengite started to melt at T⩽800–850°C and P = 1.5–2.0 GPa and produced about 3% granitic melt; (2) the products of dehydration melting of phengite-bearing eclogite vary with temperature and pressure. Fluid released from dehydration of phengite and zoisite leads to partial melting of eclogite and formation of plagioclase reaction rim around kyanite at pressures of 1.5–2.0 GPa and temperatures of 800–850°C; (3) phengite reacted with omphacite and quartz and produced oligoclase, kyanite and melt at elevated temperatures. Oligoclase is the primary reaction product produced by partial melting of phengite in the eclogite; and (4) the dehydration melting of phengite-bearing eclogite at pressures of 1.5–3.0 GPa and temperatures ⩾900°C results in formation of garnets with higher molar fraction of pyrope (37.67 wt.%–45.94 wt.%). Potassium feldspar and jadeite occur at P = 2.4–3.0 GPa and T⩾900°C, indicating higher pressure and fluid-absent conditions. Our results constrain the solidus for dehydration melting of phengite-bearing eclogite at pressures of 1.5–3.0 GPa. Combining experimental results with field observations of partial melting in natural eclogites, we concluded that phengite-bearing eclogites from the Dabie-Sulu orogen were able to partially molten at P = 1.5–2.0 GPa and T = 800–850°C during exhumation. The ultrahigh-high pressure eclogites would have experienced partial melting in association with metamorphic phase transformation under different fluid conditions.