| The migmatites,granulites and granites in the collisional orogenic belts record the differentiation of continental crust after partial melting.In order to study the origin of crustal anatexis and related minerals in the continental subduction zone,migmatites experienced multiple stages of partial melting from the North Dabie orogen at Hubei province,and granites and pegmatites from the Gangdese orogen at Tibet are selected for this dissertation.By means of in-situ microanalysis,different types of minerals in the migmatite-granite-pegmatite have been analyzed by multiple methods of mineralogy and geochemistry.The results are used to distinguish the different origins of minerals,to ascertain the P-T-t conditions of multiple stages of partial melting,to reveal the migration and variation rule of elements in minerals during melt migration and evolution,and to play constraints on the genesis of rocks and tectonic evolution in the continental subduction zone.1.Genesis of different minerals in migmatites from the North Dabie high temperature/ultrahigh pressure metamorphic zoneAccording to the CL images,mineral inclusions,U-Pb ages and trace element characteristics of zircons,three groups of metamorphic zircons in migmatites at Luotian from the North Dabie zone into three groups.Zir-? zircon have the U-Pb ages of 272-1015 Ma,which is the inheritance and recrystallization of the protolith magmatic zircon.Zir-? zircon record two stages of zircon growth at Triassic.Subgroup Zir-?a was formed during prograde metamorphism till peak conditions,with the U-Pb ages ranging from 240 to 220 Ma.Subgroup Zir-?b was formed by peritectic reaction at granulite-facies during the early stage of continental exhumation,with the U-Pb ages of 220-200 Ma.Zir-? zircon records the migmatization at amphibolite facies during the continuous uplift of crust,with the U-Pb ages of 140-120 Ma.Garnet in migmatite only appears as residual skeletals in the quartz-rich leucosomes.They show two stages of discontinuous growth in the major and trace element profiles.The inner domains(Grt-?)have higher grossular contents,have positive Eu anomalies.or without Eu anomaly,suggesting that the Grt-? domains was formed at eclogite-facies conditions with plagioclase is not stable.The outer domain Grt-? have lower grossular contents,but higher almandine and spessartine,high contents of Sc,Y,MREE and HREE,and obvious negative Eu anomalies.These imply that Grt-II domains was formed at granulite-facies with plagioclase was stable during the initial exhumation.At this regard,the peritectic garnet Grt-II was overgrown in the rims by the dissolution of precursor Grt-I domains.Amphibole only occurs as porphyblasts in melanosome,and shows two distinct types of REE contents and distributions.Type ? amphibole in the rim has low REE contents and unexpected positive Eu anomalies,which can be attributed to the peritectic reaction of plagioclase and biotite.Thus Type ? amphibole is classed as peritectic origin.Type ? amphibole in the center has high REE contents and negative Eu anomalies,which is attributed to the crystallization from the anatectic melt with the synchronous crystallization of plagioclase.Thus Type ? amphibole is classed as magmatic origin.Two types of amphiboles has the similar Al contents,thus they shared the same Al-in-amphibole pressures of 3.8 to 7.1 kbar and hornblende-plagioclase temperatures of 720-780?.This conditions are consistent with that of hydration melting of biotite in the middle-upper crust.2.Genesis of garnet in granite and pegmatite in the Gangdese orogenTwo generations of garnet are recognized in a granite and a pegmatite from southeastern Gandese orogen on the basis of a combined study of petrography,major and trace element profiles,and garnet O isotopes.The first generation of garnet(Grt-?)occurs as residues in the centers of garnet grains.It is dark in BSE images,rich in spessartine,and poor in almandine and grossular,and its chondrite-normalized REE patterns show obvious negative Eu anomalies and depletions of HREEs relative to MREEs.Thus it represents an early stage of nucleation related to magmatic-hydrothermal fluids.The second generation of pegmatite garnet(Grt-?)occurs as the rims of euhedral garnets or as patches in the Grt-I domains of the pegmatites,It is bright in BSE images,poor in spessartine,and rich in almandine and grossular contents,and its chondrite-normalized REE patterns exhibit obvious negative Eu anomalies but enrichments in HREEs relative to MREEs.Compared with the Grt-I domains,the Grt-? domains have elevated grossular and HREE contents.On the other time,the garnets in the granite,from cores to rims,display homogenous profiles in their spessartine,almandine,and pyrope contents but increasing grossular and decreasing REE contents.These are typical of magmatic garnets that crystallized from a granitic magma.The Grt-? domains in the pegmatite have the same major and trace element compositions as the garnets in the granite,which suggests that the pegmatite Grt-II domains crystallized from the same granitic magma.Thus the patchy Grt-? domains crystallized after the dissolution of preexisting pegmatite garnet in the presence of granitic magma,which is classified as magmtic origin.It seems therefore,that the pegmatite crystallized firstly from early magmatic-hydrothermal fluids,producing small amounts of hydrothermal Grt-I,and that the fluids were then mixed with the surrounding granitic magma.U-Pb dating and Hf-O isotope analyses of zircons from the granite and pegmatite yielded almost the same U-Pb ages of 77-79 Ma,positive ?Hf(t)values of 5.6 to 11.9,and ?18O values of 5.19‰ to 7.05‰,and the Hf model ages of?350 Ma.These data indicate that the granite and pegmatite were both derived from the reworking of juvenile crust in a newly accreted continental margin.Although the pegmatite formed before the granite,both types of rock were sourced from the same juvenile crustal material during the Late Cretaceous.3.The significance of mineral genesis to the melt evolution during crustal anatexisDuring the process of crustal anatexis,rocks may be partial melted by adding water or dehydration.In generak,there are not only metamorphic minerals participating as reactants,but also peritectic minerals formed by peritectic reaction,as well as magmatic minerals crystallized from partial melts.By studying the genesis of different minerals in crustal anatectic rocks and the related veins,we have obtained the changing rules of minerals in different stages of crustal anatexis.Based on the mineral assemblage and trace elements of amphiboles in the melanosome,as well as the calculation of amphibolite-plagioclase geothermometer and Al-in-amphibolite geobarometer,it is indicated that the migmatites at Luotian in the North Dabie orogen experienced the hydration melting of biotite in the Cretaceous.The calculated temperature and pressure conditions of the peritectic amphibole constrains the migmatization in the early Cretaceous at the middle-upper crust,which was caused by the addition of external fluids.Based on the major and trace element zonings of garnets in the leucosome,metamorphic garnet(Grt-I)and peritectic garnet(Grt-II)are clearly identified.The distinguishing of peritectic garnet confirms that the North Dabie orogeny was overlapped by granulite-facies in the early stage of exhumation and experienced partial melting at high pressures.In addition,granites and pegmatites crystallize from the evolved melts after the crustal anatexis.It is generally accepted that pegmatites are formed in the late stage of crystallization differentiation of fluid-saturated granitic magma.However,pegmatites and granites in this study have similar magma crystallization pressure and temperature,whereas the differences in the major and trace elements between pegmatites and granite garnet indicate that pegmatites crystallize earlier and then mix with the surrounding intrusive granitic magma.The early generation of garnet in pegmatites crystalized from the magma-hydrothermal fluid,rather from the later stage of the adjacent granite.Nevertheless,the zircon Hf-O isotopes suggest that granite and pegmatites have the same source of magmatic source.Therefore,the refractory garnet and zircon may be important clues to the genesis of granites and pegmatites. |
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