Serpentinization And Deserpentinization Of Mantle Wedge Peridotites And Their Geochemical Effect

Serpentinite is an important composition of the subduction zone,and plays an important role in many processes such as fluid activity,crust-mantle interaction and the formation of arc magma in subduction zones.Studying the behavior of fluid mobile elements during serpentinization and metamorphic dehydration of serpentinite is necessary to understanding the geochemical cycle in subduction zones.This dissertation presents a combined study of petrology and geochemistry on orogenic serpentinites from the Dabie and Hong’an orogen in east-central China.The results are used to decipher the origin and evolution of the orogenic serpentinites,to determine the P-T conditions,time and mechanism of crustal metasomatism,to constrain the composition,nature and origin of metasomatic agents,to understand the Mg isotope fractionation during serpentinization and dehydration of serpentinite,and further to provide insights into the fluid activity and crust-mantle interaction in the convergent plate margin during tectonic development from oceanic subduction to continental collision.Serpentinites of the mantle wedge were identified from a composite oceanic-continental subduction zone that occurs as the Hong’an orogen in east-central China.They were comprehensively investigated by an integrated approach of petrological and geochemical studies and phase equilibrium modelling as well as zircon U-Pb ages,trace elements,and HfO isotopes.The results not only reveal the protolith and metamorphic evolution of serpentinite in the subduction zone,but also provide insights into multiple crust-mantle interaction during the process from ocean subduction to continental collision.These serpentinites show high Mg#and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents,resembling those from the mantle wedge rather than the subducting oceanic slab.Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge.Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic.The enrichments of Th,U,LREE and HFSE in the whole-rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids.The ca.3 1 0 Ma zircon domains exhibit positiveεHf(t)values of 4.5 to 19.1,variable δ18O values and high Th/U ratios similar to those of oceanictype eclogites in the Hong’an orogen but different trace element compositions,indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust.The ca.430 Ma zircons exhibit U-Pb ages,trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites.The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high to ultrahighpressure metamorphic rocks in the Hong’an orogen.Thus,these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust.The ca.220 Ma zircon domains show negative εHf(t)values of-16.0 to-2.7,indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during Triassic continental collision.In this context,the mantle wedge peridotites were firstly metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids.Afterwards,part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel,where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths.The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there.They were metasomatized by fluids derived from the subducted continental crust.Finally,the serpentinites occurred retrograde processes of serpentinization and carbonation on the subsurface.Therefore,the present observations and interpretations of orogenic serpentinites provide insights into the multistage of fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision.Detailed petrological and geochemical studies were carried out on serpentinites of the mantle wedge from the Dabie orogen in east-central China.Petrographic observations indicate that the protoliths of these serpentinites are spinel dunite or harzburgite.They have high MgO/SiO2 and low Al2O3/SiO2 ratios,low TiO2 contents,and high Zr/Nb and Hf/Nb ratios.Their spinel exhibits high Cr#but low TiO2 contents.These observations indicate that the protoliths of these serpentinites are mantle wedge origin.The enrichment of LREE accompanied with the increase of HFSE indicate that they have experienced melt-rock interaction.Afterwards,part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel,where they were hydrated by further subducting oceanic crustderived fluids to form antigorite serpentinite at forearc depths.The antigorite serpentinites were further dragged down to different depths during subsequent continental subduction and underwent dehydration.This gave rise to metamorphic olivine with extremely high Fo values of 96.8 to 97.1,high MnO but low NiO contents.Apatite and chlorite-rich vein in the Ol-free serpentinites exhibits enriched Sr-Nd isotopes and low δ18O values,indicating that the serpentinites were further metasomatized by subducting continental crust-derived fluids in the continental subduction channel.These fluids are halogen-rich and characteristic of high SiO2,Al2O3,TiO2,CaO,P2O5,Th,U,REE and HFSE.Their metasomatism results in the elevation of these elements in the Ol-free serpentinites.The replacement of olivine by lizardite in the Olbearing serpentinites indicates their retrograde processes of serpentinization on the subsurface.These observations and interpretations lead to a model for multistage of fluid metasomatism at different depths during tectonic transition from oceanic subduction to continental collision in the continental subduction zone.A combined study of petrography,whole-rock major and trace elements and Mg-C-O isotopes as well as mineral Mg isotopes was carried out on serpentinites and their associated talc-magnesite schists from Yinshanzhai in the Hong’an orogen.Petrography and field occurrence indicate that the Yinshanzhai serpentinite and talc-magnesite schist are genetically related.The talc-magnesite schist is formed through the infiltration of CO2 fluid from external sources to promote the decomposition of antigorite,and then precipitate to form magnesite and talc.Talc-magnesite generally has higher δ18O and δ13C values than normal mantle,indicating that the carbonate fluid may be derived from carbonate-bearing sedimentary rocks.Serpentinites have lower δ26Mg values of-0.14 to-0.30‰,and talc-magnesite schists have higher δ26Mg values of-0.26 to-0.04‰.δ26Mg values of the serpentinites exhibit a negatively correlation with MgO and total carbon(TC)contents,and the residues of serpentinite after HCl leaching have significantly higher δ26Mg values.These observations indicate that serpentinites may have experienced metasomatism by carbonate fluid which resulting in their lighter Mg isotope compositions.The δ26Mg values of the residues after HCl leaching are higher than normal mantle,and antigorite also has high δ26Mg values,implying that antigorite serpentinization resulted in heavier Mg isotope compositions of the serpentinites.Antigorite in serpentinite exhibits significantly higher δ26Mg values than olivine(δ26Mg is-0.39 to-0.06‰and-0.55 to-0.29‰,respectively),indicating that the metamorphic dehydration of subducted serpentinites led to large Mg isotope fractionation and released fluids with heavy Mg isotope compositions.These fluids would not only metasomatize the subducted crustal rock but also migrate into the overlying mantle to modify Mg isotope composition of the mantle source of arc magma.Such a mechanism provides a possible explanation for the observed heavy Mg isotope compositions of arc lavas and some subducted crustal rocks.