Anatexis Of Orogenic Crust: Evidence From Migmatites In The North Dabie

The evolution of orogency is related of partial melting. Dabie-Sulu orogenic belt is a typical continent-continent collisional orogenic belt, anatexis took place during the evolution of different stages. This dissertation focuses on petrology and geochemistry of migmatites in the North Dabie Zone. With detail discussion of the formation mechanism of migmatite, the nature of partial melting, anatectic time, the evolution of melt and mineral reactions, the results provide powerful evidence of orogeny crust anatexis during exhumation and post-collisional stage and shed light on chemical geodynamics of orogeny.An integrated petrologic, mineralogical and geochemical study of amphibole-bearing migmatites from North Dabie Zone show that the migmatite formed by water-fluxed partial melting. The migmatites are characterized by large euhedral poikilitic amphiboles with abundant inclusions of plagioclase, biotite, and quartz in leucosome and melanosome. Hornblende-plagiocase thermobarometry indicates that the migmatites formed at P-T conditions of ～700-750℃ and 5 kbar, this P-T conditions is lower than the solidus of biotite breakdown, suggesting partial melting of a biotite+plagioclase+quartz-bearing protolith under water-fluxed conditions. The melt volume is significant higher than the melt volume which can be generated by hydrous mineral breakdown in gneiss, this also support the migmatite in generated under water-fluxed conditions. The inferred P-T conditions which are below the biotite dehydration melt reactions, the large melt fraction compared with the low amount of biotite in the protolith and the presence of peritectic hornblende in the leucosomes point to water-fluxed melting conditions in the NDZ migmatites. The leucosomes show large differences in mineralogical and geochemical compositons, which indicates that the melt have changed it original composition. The leucosomes range from tonalitic to granitic in composition having higher SiO2, Na2O, Sr, and Ba contents than the mesosome, but lower contents of CaO, FeO, MgO, TiO2, and MnO2. The granitic leucosomes are enriched in Ba, Rb, and K2O compared to the tonalitic leucosomes. The leucosomes have variable rare earth element patterns, Hornblende-free leucosomes generally have highly fractionated REE pattern with (La/Yb)N=66-119) and low HREE contents with weak Eu anomalies, which is attributed to partial melting of granitic orthogneisses with amphibole in the residue. Hornblende-bearing leucosomes are characterized by elevated HREE contents and low La/YbN ratios (11-69) which is generated by different degrees of amphibole entrainment into the melt. Leucosomes with positive Eu and Sr anomalies have lower total REE contents are the result of feldspar accumulation. Zircon U-Pb dating and isotopic geochemistry study of migmatite show that migmatie have different δ18O values and protolith ages. On one hand, Zircon with Neoproterozoic ages have low δ18O values suggest that part of the protolith experienced high-T meteoric-hydrothermal alteration, on the other hand, Zircon with Late Archean-Early Paleoproterozoic ages have high δ18O values suggest that ancient crust was evolved during the migmatization. The Neoproterozoic ages and low δ18O values are characteristic of South China, which indicates the protolith of North Dabie migmatites are originated from South China. The fluid which cause partial melting of the gneiss are came from dehydration of rocks similar to the protoliths in the deep crust, so the δ18O are inherited from source heterogeneity. The similarity of Sr-Nd isotope between migmatites and basement suggest that the formation mechanism of migmatite is in-situ partial melting. The ages of migmatization is early Cretaceous the transition from compression to extension in post-collisional orogenic belt causes an increase in heat flow, which leads to partial melting of orogenic crust. Zircon U-Pb ages of the North Dabie migmaties suggest that the transition of tectonic regime occurred pre-150Ma. The in-situ U-Pb dating and trace elements analysis of polyphase zircon in the North Dabie migmatie distinguish four groups of zircon. The protolith zircon exhibt high Th/U ratios of>0.1, steep HREE partterns with marked positive Ce and negative Eu anomalies, consistent with their magmatic origin. Their U-Pb dating yields Neoproterozoic ages, which indicate the protolith of migmatite are Neoproterozoic magmatic rocks. The metamorphic zircon show low Th/U ratios<0.1, flat HREE patterns (Yb/Gd= 1.7～7.2) with slightly negative Eu anomalies to positively Eu anomalies (0.85-1.44), low contents of HREE, Na+Ta, Y, Th, U. These features suggest their growth during eclogite-facies metamorphism. They U-Pb dating yield U-Pb ages of 211～230Ma, which indicates the migmatite suffered Triassic ulthrhigh-pressure metamorphism. The anatectic zircon with ages 188-206Ma exhibt high contents of HREE, Na+Ta, Y, U, low Th/U ratios0.1, steeper HREE patterns (Yb/GdN=79-738) with marked negative Eu anomalies, which are consistent with their anatectic origin. The Ti-in-zircon temperatures of this group zircon are 698～868℃, concordant with temperatures of phengite dehydration melting, suggest that the breakdown of phengite in the hot exhumational stage. The second anatexis is recorded in the fourth group zircon(116～141Ma). They characterized by middle contents of U, variable Th/U ratio(0.03～1.04), steep REE patterns with Yb/Gd ratios of 14-80 and negative Eu anomalies. The Ti-in-zircon temperatures of this group zircon are 699～821℃, which across the wet solidus of granite and reach the dehydration temperatures of biotite.These results indicate partial melting under both water-fluxed and fluid-absent conditions. The early migmatization is associated with hot exhumation stage, while the late migmatization is caused by regional extension of orogenic lithosphere.