A Petrological And Geochemical Study Of High-pressure Granulites From The Bohemian Massif

The genesis and evolution of high-pressure granulite and its related partial melting in collisional orogens are important to understanding of the geodynamics and crust-mantle interaction of continental collision.In this PhD thesis,an integrated study of petrology,geochronology and geochemistry was carried out for felsic granulites from the Moldanubian zone in the Bohemian Massif,the Variscan orogen of Europe.The results not only place constraints on the metamorphic evolution and protolith nature of these granulites,but also provide insights into the details of anatectic processes,anatectic conditions and antectic mechanisms as well as mineral reactions and mineral behaviors during the high-grade metamorphism.These have bearing on the geodynamic mechanism for superimposition of ultrahigh-pressure(UHP)eclogite facies metamorphic rocks by ultrahigh-temperature(UHT)granulite facies metamorphism in collisional orogens.The occurrence of coesite inclusions in garnet was determined in felsic granulites from the Gfohl unit in the Moldanubian,indicating that these granulites record the tectonic ransition from UHP eclogite facies to UHT granulite facies.Metamorphic mineral assemblages were categorized into four generations of with different P-T conditions for their formation.The first generation(M-I)is represented by chemical zoning in garnet cores and the inclusions of muscovite and rutile,recording an early prograde evolution from high-pressure(HP)amphibolite facies to UHP eclogite facies with low degrees of crustal melting.The second generation is represented by the peak UHP eclogite facies(M-?)and recorded by coesite-bearing garnet domains,kyanite and muscovite,with P-T conditions of 3.4-5.0 GPa and ca.950?.The third generation is represented by the garnet porphyroblast mantle or atoll-like/coronate garnet around kyanite or perthite(M-?a)during a transition from UHP eclogite facies to UHT granulite facies at 2.7 to 2.2 GPa at 900-1000?,with a pronounced growth of peritectic garnet at the expense of muscovite and omphacite.The formation of ternary feldspars(M-?b)reaveals considerable decompression at UHT/HP granulite facies under P-T conditions of 2.4-1.3 GPa and 880-1000? on the basis of reintegrated antiperthite/perthite,plagioclase,and compositional zoning in garnet.While some granulites were directly decompressed in the kyanite stability field under conditions of 1.0-1.2 GPa and 780-810?(M-?c),the other granulites were decompressed into the sillimanite stability field.The fourth generation is indicated by the occurrence of sillimanite and rutile(M-IVa)under conditions of 1.0-1.3 GPa and 880-970?,and retrograde minerals(ilmenite,spinel and plagioclase)and the crystallization of biotite,feldspar and quartz from later residual melts(M-?b)at medium-pressure(MP)granulite facies under conditions of 0.5-0.8 GPa and 760-810?.The different metamorphic trajectories of these felsic granulites may be caused by their different positions in the continental subduction zone in response to the change of thermal regime.The results indicate that these granulites have reached the very high temperatures during the peak UHP eclogite facies metamorphism,and subsequently reached the UHT conditions at the almost same time.Therefore,the UHT granulites in the collisional orogen were produced by prompt reworking of the UHP rocks subsequent to their formation by subduction of the continental crust to subarc depths.A combined study of geochronology,petrology and geochemistry for the felsic granulites from the Gfohl unit reveals anatectic processes,anatectic conditions and antectic mechanisms as well as mineral reactions and mineral behavior during the continental collision.During the prograde evolution from HP amphibolite-facies(M-I)to UHP eclogite-facies(M-?)with low degrees of partial melting,biotite and plagioclase were consumed to form garnet+muscovite+omphacite as well as zircon and monazite lack of Eu anomalies.The obtained U-Pb age of 340 ± 6 Ma is interpreted as the timing of peak UHP eclogite facies metamorphism(M-?).During the metamorphic transition from UHP eclogite facies to UHT granulite facies,muscovite underwent dehydration melting to form garnet porphyroblast mantle or atoll-like/coronate garnet around kyanite or perthite(M-?a)as well as zircon and monazite.Garnet formed in this stage has high HREE+Y content,zircon has high U content,and monazite have high HREE+Y,Th,U,Ba,Sr and Ca contents.With the gradual disappearance of muscovite,more omphacite was invoved in the peritectic reaction.Zircon formed in this episode has low U contents,and monazite has low contents of HREE+Y,Th,U,Ba,Sr and Ca.Garnet,zircon and monazite formed during the metamorphic transition exhibit relatively lack of negative Eu anomalies.Zircon and monazite formed during this stage yield U-Pb ages of 331±7 to 341 ±4 Ma.During the UHT granulite facies metamorphism,garnet was consumed and plagioclase was formed.Zircon formed in this stage exhibits a significantly negative correlation between HREE contents and Eu negative anomalies and yields a U-Pb age of ca.336±4 Ma.As the degree of partial melting increases,the previous zircon and monazite were dissolved into the anatectic melt.Finally,In the stage of decompressional cooling,biotite,feldspar and quartz as well as zircon and monazite were crystallized from the evolved anatectic melt.Zircon and monazite crystallized from the evolved melt have low HREE+Y contents and significant Eu negative anomalies,and yield U-Pb ages of 332±4 to 340±5 Ma.Garnet,zircon and monazite formed at these different stages have different trace element compositions,which are mainly controlled by metamorphic and peritectic reactions as well as paragenetic mineral assemblages.Zircon in the metapelite can grow at the different stages during the collisional orogeny,and its dissolution and growth mainly depend on anatectic conditions,anatectic extent,the property of peritectic reactions,and the stability of Ti-rich minerals.The anatectic melts produced during the metamorphic transition from the UHP eclogite facies to the UHT granulite facies underwent different degrees of fractional crystallization,resulting in the differences in the composition of residual,metamorphic,peritectic and magmatic minerals.Zircons grown in the different stages give similar U-Pb ages,but the zircon recording both UHP eclogite-facies metamorphism and UHT granulite-facies metamorphism in sample D72 gives U-Pb ages of 340±6 Ma for the UHP metamorphism and 337±3 Ma for the UHT metamorphism.There are only about 3-5 Myr intervals in age between the two extreme metamorphic events,indicating that the transition from the peak UHP metamorphism to the UHT metamorphism is a rapid process.Therefore,the UHT granulites in the collisional orogen were produced by prompt reworking of the UHP rocks subsequent to their formation by continental subduction to the subarc depths.The plate was still at the subarc depths when being heated,and it was heated in a short time to achieve the UHT condition.This lends support to the tectonic model in which contemporaneous UHP and UHT conditions would only be achieved by breakoff of the subducting slab at the lithosphere-asthenosphere boundary depth,where the asthenospheric upwelling has the capacity to provide the high heat flow for the superimposition of UHT metamorphism on the UHP metamorphic rocks.Such composite tectonic processes would have taken place in the Moldanubian zone during the Variscan orogeny,and the slab breakoff may have served as a common mechanism for contemporaneous UHP and UHT metamorphisms in collisional orogens.