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 ~...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.展开更多
Dehydration melting of subducted continental crust is significant during exhumation,and its study from both experimental and petrological observations is of great importance to our understanding of continental geodyna...Dehydration melting of subducted continental crust is significant during exhumation,and its study from both experimental and petrological observations is of great importance to our understanding of continental geodynamics.Dehydration melting experiments were carried out on ultrahigh-pressure(UHP)eclogite from Bixiling in the Dabie orogen using a piston cylinder at 1.5–3.0GPa and 800–950°C to investigate partial melting of eclogite induced by phengite breakdown.The phengite-bearing eclogite started to melt at T 800–850°C and P=1.5–2.0 GPa and produced about 3%granitic melt.The products of dehydration melting vary with temperature and pressure.Such results provide valuable constraints on the micro-texture related to partial melting of UHP rocks in the Dabie-Sulu orogenic belt.Three types of polyphase inclusions were identified in garnet from the Shuanghe UHP eclogite.K-feldspar and quartz inclusions are interpreted to represent the products of segregation and crystallization of minor amounts of melt that formed during dehydration melting of phengite by the inferred reaction Phengite+Omphacite±Quartz→Amphibole±Garnet+Melt(K-feldspar+Quartz±Plagioclase).Polyphase inclusions of phengite and K-feldspar+Quartz inclusions were also found in zoisite/clinozoisite and garnet from the Shuanghe garnet-bearing paragneiss.These polyphase inclusions provide evidence for a continuous process from sub-solidus dehydration to partial melting within the UHP gneissic rocks.The compositional variation of garnets demonstrates that breakdown of epidote-group minerals may have played a crucial role during dehydration melting reaction of phengite.The Ti-in-zircon thermometry and Si content of phengite in zircon suggest that partial melting would occur at 783–839°C and 2.0–2.5 GPa.Therefore,both experimental results and petrological observations indicate that dehydration melting and fluid activity within the Dabie UHP rocks at micro-scale are controlled by the breakdown of phengite.展开更多
基金Supported by National Natural Science Foundation of China(Grant Nos.40802046and90714005)National Basic Research Program of China(Grant No.2009CB825003)State Key Laboratory of GPMR Foundation(Grant No.200610)
文摘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.
基金supported by the National Key Basic Research Program from the MOST(2009CB825003)the National Natural Science Foundation of China(40802046 and 41272078)the Fundamental Research Funds for the Central Universities,China University of Geosciences(Wuhan)(CUG120121)
文摘Dehydration melting of subducted continental crust is significant during exhumation,and its study from both experimental and petrological observations is of great importance to our understanding of continental geodynamics.Dehydration melting experiments were carried out on ultrahigh-pressure(UHP)eclogite from Bixiling in the Dabie orogen using a piston cylinder at 1.5–3.0GPa and 800–950°C to investigate partial melting of eclogite induced by phengite breakdown.The phengite-bearing eclogite started to melt at T 800–850°C and P=1.5–2.0 GPa and produced about 3%granitic melt.The products of dehydration melting vary with temperature and pressure.Such results provide valuable constraints on the micro-texture related to partial melting of UHP rocks in the Dabie-Sulu orogenic belt.Three types of polyphase inclusions were identified in garnet from the Shuanghe UHP eclogite.K-feldspar and quartz inclusions are interpreted to represent the products of segregation and crystallization of minor amounts of melt that formed during dehydration melting of phengite by the inferred reaction Phengite+Omphacite±Quartz→Amphibole±Garnet+Melt(K-feldspar+Quartz±Plagioclase).Polyphase inclusions of phengite and K-feldspar+Quartz inclusions were also found in zoisite/clinozoisite and garnet from the Shuanghe garnet-bearing paragneiss.These polyphase inclusions provide evidence for a continuous process from sub-solidus dehydration to partial melting within the UHP gneissic rocks.The compositional variation of garnets demonstrates that breakdown of epidote-group minerals may have played a crucial role during dehydration melting reaction of phengite.The Ti-in-zircon thermometry and Si content of phengite in zircon suggest that partial melting would occur at 783–839°C and 2.0–2.5 GPa.Therefore,both experimental results and petrological observations indicate that dehydration melting and fluid activity within the Dabie UHP rocks at micro-scale are controlled by the breakdown of phengite.
