A Petrological And Geochemical Study Of Metabasites In North Dabie

The genesis and evolution of ultrahigh-pressure(UHP)metamorphic rocks and their concomitant partial melting in collisional orogens are crucial to understanding of the mechanism for exhumation of deeply subducted continental crust and post-collisional metamorphic superposition.In this PhD thesis,an integrated study of petrology,geochemistry and zircon U-Pb ages was carried out for metabasites in the North Dabie of the Dabie orogen.The results not only provide insights into the metamorphic evolution and the genetic mechanism for granulite-facies metamorphic superposition,but also place constraints on the the timing,P-T conditions,mechanisms of crustal anatexis,as well as mineral reactions during the high-grade metamorphism.The results provide new constraints on the mechanism for the post-collisional tectonic evolution of UHP metamorphic slices.Metabasites in North Dabie mainly contain garnet pyroxenite and garnet amphibolite,without fresh eclogite.Five generations of metamorphic mineral assemblages were distinguished for constraining the P-T evolution using pseudosection modeling calculated with Theriak-Domino and other geothermobarometry.LA-ICP-MS zircon U-Pb dating for metabasites further constrains the metamorphic ages.The first generation of assemblages(MO)is indicated by garnet and the crystal inclusions of muscovite,allanite,diopside and amphibole recording an early prograde evolution to the peak UHP eclogite facies stage,with P-T conditions of 2.0-2.2 GPa and 680-730?.The second generation of assemblages(M1)is characterized by omphacite-bearing garnet domains or chemical zoning in clinopyroxene porphyroblast cores(omphacite),recording the peak UHP condtions at>4 GPa at 886-915?in ages of 232 to 227 Ma.The third generation of assemblages(M2)is remarked by chemical zoning in clinopyroxene rims(diopside)and the crystal inclusions of garnet and rutile or diopside+plagioclase intergrowths,recording a HP granulite facies at 1.7-2.2 GPa at 800-930? in ages of 219 to 208 Ma.The first three generations of metamorphic mineral assemblages were produced during the continental collision in the Triassic.The fourth generation of assemblages(M3)is represented by inner corona around garnet consists of fine-grained vermicular orthopyroxene+clinopyroxene+plagioclase+magnetite,orthopyroxene+clinopyroxene+plagioclase around quartz,orthopyroxene rods in clinopyroxene rims or matrix amphibole closed to felsic veins.The P-T conditions of this stage are 1.0-1.2 GPa and>800? in ages of about 130 Ma,recording a high temperature(HT)/middle-pressure(MP)metamorphic superposition.In this stage,there is a simultaneous event of granulite-facies and amphibole-facies metamorphism.The last generation of mineral assemblages(M4)is marked by outer corona around garnet consists of coarse-grained amphibole+plagioclase+magmatite or matrix amphibole away from felsic veins,recording an amphibolite-facies metamorphism under the P-T condtions of 0.52-0.71 GPa and 697-791? in ages of about 130 Ma.The latter two generations of metamorphic mineral assemblages were produced in the post-collisional stage during the early Cretaceous.Four episodes of metamorphic/anetectic reactions during the exhumation and postcollisional stage have been recognized in metabasites.The first episode of metamorphic/anetectic reaction is represented by the appearance of diopside in the core of clinopyroxene during a transition from UHP eclogite-facies to HP granulite facies stage in ages of 227 to 219 Ma.This stage was accompanied by a pronounced growth of peritectic garnet at the expense of muscovite,omphacite and allanite.Most of omphacite was consumed into diopside by releasing Na,Si and Al compositions with little omphacite being luckily captured by new grown garnet to keep its compostion.This may be the basic reason for that eclogite cannot be found in North Dabie.Meanwhile,most of allanite transformed into epidote by releasing Th,U,LREE and Sr.This stage also resulted in depletion of Si,Na,Al,LREE,Th,U and Sr in the metabasites.The second episode of metamorphic/anetectic reaction is represented by the newly grown diopside rims around formed clinopyroxene and peritectic zircon during the HP granulite facies stage in ages of 219 to 208 Ma.The lasting 11 Myr HP granulite-facies metamorphism would have protected the omphacite included in garnet by newly grown diopside around formed omphacite from futher consumption in the later tectonic thermal event.The latter two episodes of metamorphic/anetectic reaction happened in HT/MP metamorphic superposition,corresponding to an episode of MP granulite and an episode of amphibolite faices metamorphism,respectively.The third episode of metamorphic/anetectic reaction is recorded by two types of zircon domains with indistinguishable U-Pb ages of ca.130 Ma,which are in good agreement with two types of titanite domains with consistent U-Pb ages of ca.130 Ma.Peritectic zircon contains multiphase crystal inclusions of Qz+Kfs,whereas anatectic zircon is free of crystal inclusions.Chondrite-normalized REE patterns for the peritectic zircon are similar in shape with steep HREE and negative Eu anomalies to those for the anatectic zircon,but higher in Nb,Ta,Th,U,P,REE and Y contents.Likewise,the metamorphic and peritectic titanites are clearly distinguished from each other by the differences in petrographic and geochemical compositions.The metamorphic titanite occurs in the cores and contains Qz+Amp+Rt+Ilm inclusions,whereas peritectic titanite occurs in the rims and is free of inclusions.The metamorphic titanite shows convex REE distribution patterns,low REE,Y,Th,U,Nb,Ta and Zr contents but high Sr contents.The peritectic titanite shows similar REE patterns,but high REE,Y,Th,U,Nb,Ta and Zr contents and low Sr contents.The last episode of metamorphic/anetectic reaction is represented by felsic veins which contain Opx-bearing mineral assemblages,green amphibole and amphibole closed to felsic veins during the amphibole facies stage in ages of about 130 Ma.The garnet amphibolite contains omphacite inclusions in garnet and the omphacite has high jadeite contents and lots of quartz porphyroblast significantly different from the garnet pyroxenite.This may be the result of either the differential exhumation between the two types of rock or the post-collisional superimposition at different levels.On all accounts,the garnet pyroxenite experienced higher temperature metamorphism and higher degree of anatexis.Most garnet pyroxenite exhibits low SiO2(<45 wt.%),Al2O3,Na2O,K2O,LREE,Th and U contents,but high CaO,MgO and FeO contents with unreasonable ?Nd(780 Ma)values.These features likely resulted from Triassic felsic melt extraction.As the whole-rock LREE contents increase,LREE patterns of garnet pyroxenite gradually appear slightly upward from Nd to La elements.Meanwhile,the LREE patterns of clinopyroxene and amphibole vary from depletion to enrichment.These results provide evidence that another fluid has injected into garnet pyroxenite after melt extraction and the injected fluid gradually varies from aqueous solutions to hydrous silicate melt.Most garnet amphibolite shows high SiO2(>45 wt.%),Al2O3,Na2O,K2O,LREE,Th and U contents,but low CaO,MgO and FeO contents with unreasonable ?Nd(130 Ma)values.Especially the garnet amphibolite with LREE enrichment shows Sm negative anomalies,which indicates that another felsic melt has injected into garnet amphibolite after melt extraction.The significant difference between garnet pyroxenite and gamet amphibolite suggests that the former is the residue of melt extraction and the latter is the product of melt injection.Cretaceous peritectic and anatectic zircon and metamorphic and peritectic titanite in metabasites further indicates that an episode of intense fluid/melt activity during HT/MP metamorphic superposition.Amphibole Al geobarmeter calculation results show that the pressure of amphibolite-facies metamorphism for the garnet pyroxenite is higher than that for the garnet amphibolite.The pressure for garnet pyroxenite is 0.70-0.71 GPa and that for the garnet amphibolite is 0.52-0.67 GPa.This pressure difference suggests that the garnet amphibolite was located above the garnet pyroxenite,which allows for that hydrous melts extracted from the garnet pyroxenite ascent into the garnet amphibolite.The aboved-mentioned evidence indicates that the garnet amphibolite and garnet pyroxenite are different results from the same period of HT/MP granulite-facies metamorphism in response to rifting orogeny.During the early Cretaceous,continental rifting in response to asthenospheric upwelling subsequent to lithospheric thinning of the collision-thickened orogen provides the high heat flow for partial melting of the garnet pyroxenite.The hydrous melts extracted from the garnet pyroxenite would migrate upwards into the garnet amphibolite,resulting in the high contents of Si,Al,Na,K and LREE with significant Sr-Nd isotope fractionation.