The Role Of Melts/Fluids In Subduction Zones: Implications From Serpentinization And Fluxing/Metasomatic Processes

Subduction zone processes are of critical importance in deciphering global-scale material cycling,origin of arc magmas,the formation of continental crust and the mantle heterogeneity.Slab dehydration along the subduction channel produces various combinations of hydrous fluids,silicate melts and supercritical fluids through a complex continuum of metamorphic reactions,which carry incompatible and fluid mobile elements from slabs to the overlying mantle wedge.These enriched components are eventually transferred into arc magma.Therefore,the nature and behavior of these fluids/melts is helpful for understanding element transfer and material cycling in subduction zones.In this thesis,we studied two types of serpentinites from the Mianlue suture in the Qinling orogenic belt to explore fluids and fluid mobile element release in the shallow part of subduction channel.Cretaceous continental basalts in the North China Craton and adakitic rocks with high MgO content and lower crust affinity in island arc area are also investigated to explore melt metasomatism in subcontinental lithospheric mantle and melt-fluxed melting of the arc lower crust.Two types of serpentinites from the Mianlue suture zone in the Qinling orogenic belt,central China,are identified on the basis of detailed mineralogical and geochemical study.Serpentinites from the Jianchaling region?Group 1?are composed of lizardite/chrysotile+magnesite+magnetite.Most of these serpentinites?Group 1a?,consist of pseudomorphic orthopyroxene and olivine,and are characterized by low Al₂O₃/SiO₂,high MgO/SiO₂ and Ir-type PGEs to Pt ratios,suggesting a residual mantle origin.Meanwhile,the U-shape REE pattern and positive Eu,Sr and Ba anomalies of these serpentinites indicate that serpentinization fluids have interacted with gabbroic cumulates at moderately high temperatures.Considering the limited mobility of U in the hydrating fluids for the Group 1a serpentinites,hydrating fluids for these serpentinites are most likely derived from the dehydrated slab and have been in equilibrium with subducting sediments.There are also some serpentinites with low-grade metamorphic recrystallization from the Jianchaling region?Group 1b?,represented by recrystallized serpentine minerals?antigorite?.The Group 1b serpentinites display less discernable positive Eu anomaly and enriched U relative to Th and alkali elements,suggesting the hydrating fluids are relatively oxidized.Serpentinites from the Liangyazi region?Group 2?are composed of antigorite+dolomite+spinel+magnetite.The high Cr number?0.65–0.80?,moderate Al₂O₃ and low Ti concentrations of spinels in Group 2 serpentinites indicate a refractory mantle wedge origin.Fertile major element compositions?e.g.,high Al₂O₃ content and Al₂O₃/SiO₂?and conjoint enrichment in light rare earth elements and high field strength elements,however,suggest melt-rock interactions before serpentinization.Therefore,we conclude that the Jianchaling and Liangyazi serpentinites represent residual forearc peridotites that are derived from different parts of mantle wedge.These peridotites experienced different melt/fluid-rock interaction processes,resulting in distinctive geochemical compositions for the Jianchaling and Liangyazi serpentinites.In situ studies of chemical compositions of serpentine minerals indicate that a great deal of F,Cl and S are released into fluids during the lizardite to antigorite transition at³00–400°C.Meanwhile,a small amount of Pb,Ba and Sr may also be removed from serpentine,whereas B is released from the subducted slab and then incorportated into recrystallized serpentine minerals?antigorite?.As and Sb are transferred from slabs to fluids and then incorporated into serpentinites below 300°C,while the mobility of Li,Ba,U and La are greatly enhanced at the lower part of subduction channel.We apply our knowledge on trace element behaviors in fluids/melts in subduction zones to decipher the relationship between continental basalts in the North China Craton?NCC?and the westward subduction of the Paleo-Pacific plate.The results provided new constraints on the destruction of the NCC.There are voluminous Cretaceous continental basalts in the NCC,which are mainly composed of alkali basalts with minor sub-alkali basalts.Abrupt changes in chemical and isotopic compositions of these basalts were often ascribed to subcontinental lithosphere thinning of the NCC.However,processes responsible for such changes and its implications for subcontinental lithosphere evolution remain obscure.Here we report major geochemical changes at¹08 Ma in the north part of the NCC.The>108 Ma alkali basalts are characterized by negative?_Nd?t?and declining“island arc-like”geochemical characteristics from the east to the west,implying decreasing slab-derived components from westward subducting slabs in their metasomatized lithospheric mantle sources.In contrast,the<108 Ma Cretaceous alkali basalts have depleted Sr-Nd isotopic compositions and“OIB-like”geochemical features.These observations suggest that westward subduction of the Paleo-Pacific plate was responsible for the Cretaceous basaltic activities in the NCC.Combined with plate reconstructions and geophysical observations,we propose that flat subduction of the“extinct”ridge between the Izanagi and the Pacific plates controlled this major transition as well as the destruction of the NCC.The transition from a supra-subduction zone environment to a within-plate extensional environment around 108 Ma is probably due to the eastward slab rollback and the northward shift of the spreading ridge.Aqueous melts in subduction zones may also react with the lower arc crust.Adakitic rocks with high MgO content and lower crust affinity at convergent margins provide important constraints on arc crust evolution and regional tectonics.An integrated study on the adakitic pluton with high MgO content and its mafic microgranular enclaves?MMEs?in the Early Paleozoic North Qinling terrane was conducted in this study.Zircon dating of host rocks yielded identical weighted mean ages of 428 Ma.The MMEs contain two types of zircons:type 1 zircons share similar morphology,chemical composition and indistinguishable age with those from the host rock;type 2 zircons yielded an older age of 450 Ma.The similarity in zircon Hf isotopic composition for these zircons indicates a cognate origin of the MMEs.However,the lower Ce⁴⁺/Ce³⁺,higher crystallization temperature and the LREE overabundance of type 2 zircons indicate that mafic intrusions in the lower arc crust were subjected to an post-magmatic event at⁴50 Ma.Samples from the Tangzang pluton are characterized by typical adakitic geochemical features,and high MgO,Cr,Ni and Ba contents.Considering their similar isotopic compositions with the Neoproterozoic metabasites in the North Qinling terrane,we argue that the Tangzang pluton was generated by crustal anatexis of two sources,with contributions from the metasomatized mantle wedge.The mantle connection was achieved by aqueous melt-fluxed melting of a“hot zone”in the lower arc crust.This implies that andesitic continental crust may be also associated with aqueous melt-fluxed melting of the lower arc crust.