Dehydration melting experiments were performed on ultrahigh-pressure eclogite from Bixiling in the Dabie orogen at 1.5―3.0 GPa and 800―950℃ using piston cylinder apparatus. The results show that (1) eclogite with ~5% phengite started to melt at T≤800―850℃ 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℃; (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 ℃ 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 ℃ , 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 experi- mental 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℃ during exhumation. The ultrahigh-high pressure eclogites would have experienced partial melting in association with metamorphic phase transformation under different fluid conditions.
LIU Qiang1,2, JIN ZhenMin1,2 & ZHANG JunFeng1,2 1 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
The ultrahigh-pressure(UHP) eclogite and gneiss from the Dabie(大别)-Sulu(苏鲁) orogen experienced variable degrees of partial melting during exhumation.We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800-950 ℃.Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 ℃.About 10% granitic melt can be produced at 950 ℃.In contrast,the partial melting of phengite-bearing eclogite exists at slightly lower temperatures(800-850 ℃).The melt fraction is in general more in biotite gneiss than in phengite-bearing eclogite under similar pressure and temperature conditions.Both melts are rich in silica and alkali,but poor in FeO,MgO and CaO.These results suggest that low-degree partial melting of gneiss and eclogite is often associated with dehydration of hydrous mineral,such as micas.The dehydration temperature and melt composition can place important constraints on the partial melting phenomena(granitic leucosome and multi-phase mineral inclusions) recorded in UHP rocks.
Foliated garnet-bearing granite,usually associated with high pressure and ultrahigh-pressure(UHP) metamophic rocks,is a particular rock-type extensively exposed in the Mesozoic Dabie-Sulu orogenic belt of China.This study focuses on deformation features and SHRIMP zircon dating of foliated garnet granite in a high-pressure metamorphic unit from Huwan,western Dabie Mountains in order to resolve discrepancies in current versions of its petrogenesis and structural evolution.SHRIMP dating reveals a zircon age of 762 ± 15 Ma(MSWD=1.7) for Huwan granites,representing the Middle to Late Neoproterozoic age of intrusion and crystallization.Field and microstructural studies show that the Huwan granite body underwent multiple-stage deformation.The deformation was manifested by an early stage of rootless folding and imposition of relict foliation(S1);an Indosinian main stage marked by imposition of north-dipping penetrative gneissosity(S2) and development of ductile shear zones under NNE-SSW directed compression;and a final Indosinian stage of southward thrusting of the Huwan high-pressure unit.Shallow level extension prevailed after the Late Triassic,giving rise to south-dipping thrust faults and north-dipping normal faults.
YANG KunGuangLIU QiangXIE JianLeiDAN WeiSHE ZhenBingMA ChangQian
