Element And Isotope Behaviors During Partial Melting Of Subducted Crustal Rocks

Partial melting of crustal rocks can not only affect the rheological behavior of the orogenic belt,but also be an important way of material migration and evolution in the crust.It is of great significance to the formation of various magmatic deposits and the transition of crustal composition from mafic to andesitic.As the products of partial melting,granulite-migmatite-granite associations provide a natural laboratory for studying the change of melt composition in the processes of partial melting,the origin of granite and the factors of influencing melting reactions.Recent studies indicate that the behavior of minerals in the melting processes is a direct factor affecting the chemical composition of melt and residue.The stability differences and composition changes of minerals under different conditions can also be used to retrieve the conditions of melting reactions.Post-collisional magmatism and migmatization are widely developed in the high temperature and ultrahigh pressure metamorphic belt of the North Dabie zone.Yuexi and Luotian migmatic domes provide a natural laboratory for the study of mineral behaviors during the crustal anatexis and the subsequent evolution of melt components This thesis presents a combined study of petrology and geochemistry for migmatites from Luoerling and Xinpugou as well as granites from Wenquan in North Dabie.The results are used to reveal the formation mechanism of several felsic bodies,different genetic types of the minerals in the melt and their influences on the elemental and isotopic compositions of anatectic melts.Thin leucosomes interbedded with melanosomes and coarse veins cross cutting stromatic migmatites were identified in the Luoerling outcrop of migmatites.Under microscope,the former has a granitic structure,which gradually contacts with the melanosome,and contains melting evidence such as low dihedral feldspar and estuarine-like quartz grain boundary,indicating that it is the in-situ leucosome formed during anatexis.The latter is in sudden contact with the adjacent migmatites,and has typical idiomorphic structure and large potassium feldspar particles under microscope,indicating that it represents the leucocratic vein of melt with short distance migration and evolution.Zircon U-Pb dating records not only the decompressional exhumation of the deep subducted protolith at the Late Triassic,but also the post-collisional anatexis in Cretaceous.The temperature and pressure conditions at 5.6 kbar and 688? were calculated by the Amp-Pl thermometer and Al-in-Amp barometer through identified peritectic minerals.The occurrence of peritectic amphibole and the inclusion of biotite in amphibole indicate that the melting mechanism for the formation of the migmatites is the fluid-fluxed melting of biotite.Amphibole in the migmatite and granies can be divided into two types:metamorphic amphibole with Eu/Eu*=0.48?0.57 and REE=817?875 ppm;and peritectic amphibole with Eu/Eu*=0.67?2.75 and REE=9?122 ppm,containing biotite,plagioclase and quartz inclusion.Metamorphic amphibole was only found in some melanosomes,while peritectic amphibole was only found in leucosomes.Thus,we can conclude that it is mainly peritectic amphibole that caused the change of melt composition.The addition of amphibole causes the increase of HREE in melt.However,the absence of amphibole in the leucocratic veins shows that amphibole has been completely dissolved in the process of melt migration.For the plagioclase in the migmatites and granites,three types of metamorphic,peritectic and magmatic crystallization are identified:peritectic plagioclase has the highest An value(An31-34),followed by metamorphic plagioclase(An28-31)and magmatic crystalline plagioclase(An8-27).Peritectic plagioclase and metamorphic plagioclase appear in the melanosome,while magmatic crystalline plagioclase appear in the leucocratic vein.The plagioclase in melanosome contain palaeofabric-aligned biotite inclusions and fine-grained quartz inclusions.On the contrary,the plagioclase in leucosome have no inclusions and are adjacent to melt pseudomorphs like myrmekite and low-dihedral quartz.Previous studies on incongruent melting reactions involving plagioclase in both reactant and product have found that the product has higher An values,which is consistent with our results.According to the Bowen reaction sequence,the An value of the peritectic plagioclase is higher than the magmatic crystalline one because of the early crystallization of the peritectic plagioclase.The very low An value of plagioclase in the leucocratic vein may reflect that the melt has experienced a certain degree of fractional crystallization of plagiocalse.The genetic types of monazite and titanite are metamorphic,peritectic and magmatic.Peritectic and crystallized monazite appeared in the in-situ leucosome.Metamorphic and peritectic titanite are found in the in-situ leucosome,while peritectic titanite is found in the melanosome and crystallized and metamorphic titanite are found in the granite.Their U-Pb dating results are consistent with the ages of Cretaceous zircons.Their Sm-Nd isotopic composition varies in different degrees:Monazite shows a smaller variation in ?Nd(t)from-15.5 to-21.3),whereas titanite exhibits a larger variation in ?Nd(t)from-12.4 to-22.3.The ?Nd(t)of titanite and monazite in the leucosome are in equilibrium with that of the whole rock,while the ?Nd(t)of titanite in melanosome are much lower than that of the whole rock.The Sm-Nd dating of titanite and monazite from the coarser veins gives a Cretaceous age of 114 ± 38 Ma,indicating that the reequilibrium of Sm-Nd system was realized during high degree melting.The monazite from the smaller veins gives a partial reset Sm-Nd isochron age of 493± 69 Ma,indicating that only partial reequilibrium of the Sm-Nd system occurs at low degree melting.Experimental petrology shows that zircon is more fusible than monazite at the same particle size and water content in melt.All the zircons in the fine in-situ leucosome are inherited grains,indicating that the monazite may not have been completely dissolved and record the information of protolith.The DPb<DNd in monazite also supports the U-Pb isotopic system records of the Cretaceous ages and the Sm-Nd isotopic system records of the protolith information.In view of the controlling effect of the water content in melt on the dissolution rate of monazite and titanite,we conclude that the low water content in the low-degree molten melts leads to the decoupling of the related isotopic systems.