Multiple crust-mantle interactions during tectonic development from oceanic subduction to continental collision: Constraints from mantle wedge serpentinites in a continental subduction zone

• Published in: Lithos Vol. 482-483; p. 107713
• Main Authors: Yin, Zhuang-Zhuang, Chen, Ren-Xu, Gong, Bing, Zha, Xiang-Ping
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-10-2024
• Subjects: Crust-mantle interaction; Deserpentinization; Melt/fluid metasomatism; Serpentinite; Subduction zone; Crust-mantle interaction; Subduction zone; Serpentinite; Melt/fluid metasomatism; Deserpentinization
• ISSN: 0024-4937
• DOI: 10.1016/j.lithos.2024.107713

Detailed petrological and geochemical studies were carried out on serpentinites in the Dabie orogen of east-central China to provide insights into crust-mantle interactions at convergent plate margins. The serpentinites can be divided into two groups according to mineral assemblage: Ol-bearing serpentinite consisting of olivine + antigorite + lizardite + magnetite, and Ol-free serpentinite consisting of antigorite + magnetite + spinel + apatite. Petrographic observations indicate that the protoliths of these serpentinites are spinel dunite or harzburgite. They have high MgO/SiO2 ratios (0.92–1.08), low Al2O3/SiO2 ratios (0.01–0.03), and low TiO2 contents (<0.02 wt%), and their spinel exhibits high Mg# and Cr#. These features indicate that the protoliths of the serpentinites are residual mantle peridotites originating from the oceanic lithospheric mantle or mantle wedge. Based on the significantly enriched Nd isotope compositions of the serpentinites and field observations showing their association with metamorphic continental rocks rather than rodingite or oceanic eclogite, the protoliths of these serpentinites are suggested to be mantle wedge peridotites, although the possibility of oceanic lithospheric mantle cannot be completely ruled out. The enrichment of light rare earth elements (LREE) accompanied by an increase in high field strength elements (HFSE) indicates that they have experienced melt-rock interactions. Subsequently, the metasomatized peridotites were partially off-scraped into the oceanic subduction channel and transported upward to forearc depths, where they were hydrated by subducting oceanic crust-derived fluids to form antigorite serpentinite. These antigorite serpentinites were further dragged down to different depths during subsequent continental subduction and underwent dehydration. The dehydration of antigorite gave rise to metamorphic olivine with extremely high Fo values of 96.8–97.1, high MnO, and low NiO contents. Apatite and chlorite-rich veins in Ol-free serpentinites exhibit enriched SrNd 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 characterized by high SiO2, Al2O3, TiO2, CaO, P2O5, Th, U, REE and HFSE contents. These metasomatism results in the elevation of these elements in Ol-free serpentinites. The replacement of olivine by lizardite in Ol-bearing serpentinites indicates retrograde serpentinization at the shallow crustal level. These observations and interpretations lead to a model for multistage fluid metasomatism at convergent plate margins during the tectonic transition from oceanic subduction to continental collision.
•The Dongjiashan serpentinites are formed by serpentinization of residual mantle peridotites.•The serpentinites were dragged down in the subduction channel to deliver fluid to the mantle wedge.•The serpentinites were metasomatized by melt/fluid from both subducted oceanic and continental crust.