A cluster of serpentinite bodies has been recognized tectonically emplaced within the greenschist-amphibolite-facies metamorphic terrane in Hong’an, western Dabie orogen, central China. Two types of serpentinites are identified on the basis of integrated petrographic, mineralogical and geochemical study. The first type, represented by Yinshanzhai serpentinite complex (Group 1) comprises heterogeneous lithology as a massive serpentinite matrix “intruded” by antigorite-enriched serpentinite lenses. They are both pseudomorphic textured with different mineral assemblages indicating an increasing temperature-pressure condition. Serpentinite matrix (Group 1a) is chemically characterized by high MgO/SiO2 and low Al2O3/SiO2, Ti and Ca contents, suggesting a depleted mantle wedge origin. The coexistence of compositionally-heterogeneous chromite with high-Cr# (0.78–0.96) and intermediate-Cr# (0.59–0.70) pristine cores indicates extensive mantle melting. Meanwhile, extremely high Fo olivine relicts (96–97) with considerably higher MnO and lower NiO contents than mantle olivine indicate that they are metamorphic products from serpentine decomposition. Accordingly, we propose that Yinshanzhai serpentinite complex experienced two distinct episodes of hydration. The serpentinite lenses (Group 1b) show higher SiO2 and lower MgO concentrations. Nevertheless, the trace elements of groups 1a and 1b are consistent: U-shaped REE patterns, positive Eu anomalies and enrichment of LILE (i.e., Cs, U) are all identified as forearc properties. They are affected by reducing slab-derived fluids in forearc mantle, with fO2 of 1 to 2 logarithmic units below fO2 of fayalite-magnetite-quartz buffer (FMQ-2∼FMQ-1). The second type is antigorite-serpentinites (Group 2) represented by Ximaoshan and Wangchunwan blocks. They are non-pseudomorphic, with no primary framework silicate surviving. Fertile compositions (i.e., higher Al2O3 and Al2O3/SiO2, nearly flat REE patterns) and conjoint enrichment of LILE with HFSE suggest melt/rock interaction. Negative Ce, Eu anomalies, and enriched U relative to alkaline elements demonstrate interactions with more oxidized seawater or seafloor fluids (FMQ∼FMQ-1). Remarkable Sr negative anomalies may be attributed to Sr release during lizardite/antigorite transition in subduction zone, indicating interaction with low-Sr slab fluids. We propose that Group 2 serpentinites originate either from mantle wedge or abyssal peridotites, refertilized by mantle melts, then hydrated in seafloor or shallow forearc and entrapped into subduction channel. Combined with geochronology and tectonic constraints of associated eclogites in Hong’an terrane, the two types of serpentinites may correlate with subduction of different oceanic basins during the late Paleozoic and/or Proterozoic eras. It raises possibility of defining an ophiolitic setting in Hong’an Orogen for which further evidence is required.

Origins of Two Types of Serpentinites from Hong’an, Western Dabie Orogen, Central China and Associated Fluid/Melt-Rock Interactions: Implications from Geochemistry and Mineralogy

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2025年6月17日

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Yuchen Liu,
Changqian Ma

摘要

A cluster of serpentinite bodies has been recognized tectonically emplaced within the greenschist-amphibolite-facies metamorphic terrane in Hong’an, western Dabie orogen, central China. Two types of serpentinites are identified on the basis of integrated petrographic, mineralogical and geochemical study. The first type, represented by Yinshanzhai serpentinite complex (Group 1) comprises heterogeneous lithology as a massive serpentinite matrix “intruded” by antigorite-enriched serpentinite lenses. They are both pseudomorphic textured with different mineral assemblages indicating an increasing temperature-pressure condition. Serpentinite matrix (Group 1a) is chemically characterized by high MgO/SiO₂ and low Al₂O₃/SiO₂, Ti and Ca contents, suggesting a depleted mantle wedge origin. The coexistence of compositionally-heterogeneous chromite with high-Cr# (0.78–0.96) and intermediate-Cr# (0.59–0.70) pristine cores indicates extensive mantle melting. Meanwhile, extremely high Fo olivine relicts (96–97) with considerably higher MnO and lower NiO contents than mantle olivine indicate that they are metamorphic products from serpentine decomposition. Accordingly, we propose that Yinshanzhai serpentinite complex experienced two distinct episodes of hydration. The serpentinite lenses (Group 1b) show higher SiO₂ and lower MgO concentrations. Nevertheless, the trace elements of groups 1a and 1b are consistent: U-shaped REE patterns, positive Eu anomalies and enrichment of LILE (i.e., Cs, U) are all identified as forearc properties. They are affected by reducing slab-derived fluids in forearc mantle, with f_O2 of 1 to 2 logarithmic units below f_O2 of fayalite-magnetite-quartz buffer (FMQ-2∼FMQ-1). The second type is antigorite-serpentinites (Group 2) represented by Ximaoshan and Wangchunwan blocks. They are non-pseudomorphic, with no primary framework silicate surviving. Fertile compositions (i.e., higher Al₂O₃ and Al₂O₃/SiO₂, nearly flat REE patterns) and conjoint enrichment of LILE with HFSE suggest melt/rock interaction. Negative Ce, Eu anomalies, and enriched U relative to alkaline elements demonstrate interactions with more oxidized seawater or seafloor fluids (FMQ∼FMQ-1). Remarkable Sr negative anomalies may be attributed to Sr release during lizardite/antigorite transition in subduction zone, indicating interaction with low-Sr slab fluids. We propose that Group 2 serpentinites originate either from mantle wedge or abyssal peridotites, refertilized by mantle melts, then hydrated in seafloor or shallow forearc and entrapped into subduction channel. Combined with geochronology and tectonic constraints of associated eclogites in Hong’an terrane, the two types of serpentinites may correlate with subduction of different oceanic basins during the late Paleozoic and/or Proterozoic eras. It raises possibility of defining an ophiolitic setting in Hong’an Orogen for which further evidence is required.