Characteristics Of Migmatite In The Dabie Orogenic Belt And Its Petrogenesis

The crustal anatexis is fundamental during the process of orogenic belt formed by continental subduction and collision,and has significant impact on the rheology and thermal of the orogenic crust,which may eventually result in orogenic collapse.The migmatites in the orogenic belt are mainly formed by anatexis.Therefore,studies on migmatites are important to understand partial melting,metamorphic evolution and orogenic process.Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane?NDT?of the Dabie orogen.In the past,the nomenclature and classification of the North Dabie migmatites are rather chaotic,and there is no systematic observation and classification for them.Systematical works of field observations,petrographic analysis,mineral chemistry and zircon LA-ICP-MS U-Pb dating are undertook on the migmatites from Luotian and Yuexi dome.The North Dabie migmatites can be divided into two main types,namely metatexite migmatites with stromatic structure and dilation structure,and diatexite migmatites with schollen structure and schlieren structure.The Fe₂O₃,MgO,MnO,TiO₂,and CaO of the stromatic leucosomes,leucocratic veins and granites were significantly depleted,and the diatexite migmatites were slightly enriched in these elements.The whole rock major element composition of the stromatic leucosomes were tonalitic and granitic,and leucocratic veins were granitic.The whole rock major element composition of the diatexite leucosomes are similar to those of gneiss,were granodioritic.The leucosomes and granites showed enrichment of LREE and Ba,Th and U,Nb and Ti significant depletion.The stromatic leucosomes have a strong positive Eu anomaly,the granitic leucocratic veins and granites are negative Eu anomalies,and there is no obvious Eu anomaly in the diatexite leucosomes.All this indicates diatexite leucosomes,stromatic leucosomes and granitic leucocratic veins are genetic associated each other,which respectively crystallized from different evolution degrees of melt.The results of zircon U-Pb isotopic dating indicate that the protolith of North Dabie migmatites are mainly Neoproterozoic rocks and a little Paleoproterozoic crust.The migmatization from 148 Ma to 122 Ma and can be roughly divided into two phases,the first phase is 148-137 Ma,second 135-122 Ma.Based on the plagioclase-hornblende thermobarometers,the forming temperature and pressure of migmatites are estimated to be 723-768 °C and 3.7-5.2 Kilobar,which corresponds to an environment of middle and upper crustal.Petrogenesis of most migmatites is water-fluxed melting under a condition of the felsic gneiss being water-saturated,and the equation is Biotite + Plagioclase + Quartz + Fluid = Granitic melt+ Amphibole + Residual plagioclase.Besides,there are still a few migmatites which were formed by dehydration partial melting of hornblende,and the equation is Amphibole + Plagioclase + Quartz = Clinopyroxene + H₂O-bearing melt.