Origins Of Serpentinites In Hong’an Orogen, Central China: Records Of Multi-Stage Tectonic Evolution And Melt/Fluid Metasomatism In Subduction Zone

The serpentinites in orogenic belt originate from peridotites hydrated under hydrothermal process and eventually emplace within orogenic environment through complicated dynamic evolutions.These ultramafic components inherited the properties of pristine peridotites and recorded abundant information for crust-mantle interaction,providing critical clues to reveal the structure of the lithosphere,tectonic evolution,melt-fluid activities and the recycling of materials within the subduction-collision-orogenic system.The Hong’an orogenic belt is the integral section of the Qinling-Dabie-Sulu orogenic belt,which evolved as long-lived composite orogen well preserving both Paleozoic accretionary orogenic system and Early Mesozoic collisional orogenic system.A cluster of serpentinite bodies have been recognized tectonically emplaced within Hong’an terrene.These serpentinites documented the fluid activities during ocean-continent transition,which can be excellent objects to constrain the lithospheric evolution and crust-mantle interaction of subduction/collisional orogenic belt.This contribution was designed through detailed field work,petrology,petrography,mineralogy and geochronological discussion on selected outcrops including Yinshanzhai complex,Ximanshan and Wangchunwan serpentinites,aiming to:（1）discriminate their origin and geotectonic genesis,（2）revel the complicated fluid/melts activities in the multi-stage dynamic events,（3）discuss the possibility to discover a new ophiolitic setting in Hong’an orogenic belt.（1）Serpentinite of the Hong’an orogenic belt:multi-sourced mantle fragmentsSerpentinites in Hong’an orogenic belt are generally subdivided into two categories.The first group represented by the Yinshanzhai complex is lithological-heterogeneous with lenticular serpentinite“intruding”massive serpentinite matrix.Pseudomorphic textures are largely preserved.Both rock types are chemically depleted in Al₂O₃ and CaO,with high MgO/SiO₂ and low Al₂O₃/SiO₂ ratios,and low abundance of trace elements.The protolith is highly refractory forearc mantle peridotite,with CIPW normalized modes of dunite and harzburgite.Zoned chromite with magnetite overgrowth is identified as accessory phase in Yinshanzhai serpentinite.Three types can be characterized by compositional and morphological diversities:high Cr#（0.78-0.96）and intermediate Cr#（0.59-0.7）chromite coexist within the serpentinite matrix.Porous modified chromite（Ferritchromite）was recognized in serpentinite lenses.The former two types of chromite with high Cr#,Mg#and low Ti characteristics indicate a forearc mantle origin,Coexistence of compositionally-heterogeneous chromite propose that the protoliths experienced extensive melting.The second group is antigorite-serpentinite（>98%antigorite）represented by the Ximaoshan and Wangchunwan block with the protolith dominated by harzburgite.The whole-rock composition was elevated with the enrichment of HFSE and LREE indicating melt-rock interaction.They are geochemically similar with the refertilized abyssal peridotites or mantle wedge peridotites.（2）Serpentinization fluids and tectonic environmentSerpentinization fluids and tectonic environments of the two types of serpentinites are distinctive.Yinshanzhai mantle wedge serpentinites display U-shaped rare earth element distribution patterns with positive Eu anomalies indicating a forearc mantle wedge environment.Subjected to dehydration fluids from subduction slab,a reduced condition was proposed for the fluids and serpentinization environment（FMQ-1-FMQ-2）.The significant enrichment of U relative to alkalis,discernable negative Ce anomalies and weak negative Eu anomalies all point to more oxidized conditions（FMQ-FMQ-1）.Geochemistry indicates evolvement of seawater or seafloor hydrothermal fluids.The antigorite-serpentinites may originate:1)from abyssal peridotites,formed by seawater percolation and entrapped into the orogenic belt during subduction of ocean basin;2)from mantle wedge peridotites in the shallow forearc environment and serpentinized by seawater.（3）Metamorphism and deformation history recorded in the Yinshanzhai serpentinite:Two-stage subduction and roll-backThe olivine relicts in the Yinshanzhai serpentinite are extremely Mg-rich（Fo=96-97）,with lower NiO（0.23-0.38 wt%）and higher MnO（0.13-0.19 wt%）content compared to the mantle olivine,is consistent with the secondary olivine characteristics through serpentinite-dehydration.Serpentine in the rocks inherit the chemical features of the high-Fo olivine,which indicate that they are the products of secondary hydration after emplacement.The lizardite/antigorite transition with pervasive metamorphic deformation indicate widespread prograde metamorphism inferring a second-stage of subduction.Differential enrichment of Zn and Mn in chromite core and rim represents magmatic and metamorphic features respectively.The porous chromite in the serpentinite lenses,is recognized filled by chlorite in the pores.This modified core is enriched in Fe³⁺and Cr which corresponds to typical Ferritchromite（or Ferrian chromite）,indicating amphibolite-facies metamorphism.All the metamorphic fingerprints demonstrate that the Yinshanzhai serpentinites underwent two-stages of subduction and roll back.（4）Serpentinites in Hong’an orogenic belt indicate the receding of the paleo-oceanic basin and possible retention of paleo-oceanic debris.The formation of subduction-zone serpentinites in the Hong’an orogenic belt are closely related to the oceanic subduction events.Geochronological constrain the oceanic-type eclogites in Huwan and Gaoqiao county document the evolution of diverse ocean basin in Paleozoic and Proterozoic times,respectively.The present study suggests that the Ximaoshan and Wangchunwan serpentinites may represent the fragments of the ancient oceanic lithosphere,providing new constraints for the determination of the tectonic evolution of the Hong’an Complex orogenic belt.