<p>The convergent plate margins of Paleo-Tethys Ocean and Neo-Tethys Ocean are the two orogenic belts in the eastern Tethyan realm with the best exposures of igneous rocks. Previous studies have revealed that both belts have developed not only island arc basalts (IAB)-like mafic magmatism but also ocean island basalts (OIB) ± mid-ocean ridge basalts (MORB)-like mafic magmatism in various stages of their formation and evolution. These different types of mafic igneous rocks show both similarity and difference that were acquired in the various stages from oceanic subduction through continental collision to intracontinental reworking. Comprehensive examination and comparison of these igneous rocks can provide insights into the tectonic evolution of the eastern Tethyan realm. This study integrates the spatiotemporal distribution and geochemical compositions of Mesozoic and Cenozoic primitive mafic igneous rocks in the eastern Tethyan realm within China. The results show: (1) in addition to the stage of oceanic subduction, the stages of continental collision and intracontinental reworking are the primary periods of mafic magmatic activity at convergent plate margins, consistent across the Paleo-Tethyan and Neo-Tethyan domains. (2) OIB (±MORB)-like mafic igneous rocks are present not only in the stage of continental collision in both domains but also in the stage of intracontinental reworking in the Paleo-Tethyan domain. (3) In the stages from oceanic subduction to continental collision, the compositions of IAB-like mafic igneous rocks in both domains exhibit continuous variations, with remarkable similarities in lithology, elemental and isotopic characteristics, indicating their formation through similar deep geodynamic processes—partial melting of the mantle wedge metasomatized by subducting slab-derived fluids in the forms of aqueous solution and/or hydrous melts. As a consequence, the mafic igneous rocks display more compositional inheritance than difference. In the stage of continental collision, the mantle sources of mafic igneous rocks did not acquire geochemical signatures from the subducting continental crust. (4) during the continental collision, some IAB-like and most OIB (±MORB)-like mafic igneous rocks show more depleted radiogenic isotope compositions than their counterparts generated during the oceanic subduction, suggesting an increased contribution from asthenospheric mantle components and a relatively diminished influence from subducting crustal components. (5) extensive mafic magmatism in the compressional regime of continental collision occurs in the overriding plate above continental subduction zones, indicating the local extension of the overlying lithosphere due to upwelling of the asthenospheric mantle in response to rollback or breakoff of the subducting oceanic slab. This upwelling induces simultaneous melting of various mantle sources, leading to high compositional diversity of the mafic igneous rocks during this period. (6) during the intracontinental reworking in both domains, the geochemical characteristics of IAB-like mafic igneous rocks differ significantly from those formed in the stages of continental collision and oceanic subduction, likely due to contributions from subducting continental crust components to their mantle sources. Delamination and thinning of the thickened lithosphere are the most plausible mechanisms for triggering of the mafic magmatism in this stage. The geochemical and petrogenetic similarities between the Mesozoic and Cenozoic mafic igneous rocks from the Paleo-Tethyan and Neo-Tethyan domains are evident in these periods from oceanic subduction through continental collision to intracontinental reworking, suggesting the common features of tectonic evolution across the convergent plate margins of different eras. These features are primarily reflected in the processes of crust-mantle interaction, particularly via the formation of mafic magma sources in the mantle wedge due to metasomatism by fluids derived from subducting oceanic or continental slabs. Moreover, after the formation of the mantle source regions, the key geodynamic processes involved in their partial melting are similar, namely, the rollback of subducting oceanic slabs in the stage of oceanic subduction, the rollback or breakoff of subducting oceanic slabs during the continental collision, and the delamination and thinning of the lithospheric mantle during the intracontinental reworking. The heating caused by upwelling of the asthenospheric mantle is always the most effective mechanism for partial melting of the metasomatic domains in the mantle wedge. Therefore, the occurrence of mafic magmatism always involves two stages in the tectonic evolution of convergent plate margins. The first is the chemical metasomatism of the mantle wedge during the subduction of oceanic or continental slabs, generating the mantle source regions. The second is the partial melting of the metasomatic mantle domains, giving rise to the mafic magmas. The differences between the different stages and types of mafic magmatism lie in the composition of metasomatic agents and the time interval between these two stages.</p>

Geochemical evolution of Mesozoic and Cenozoic mafic igneous rocks in the eastern Tethyan realm: Implications for the tectonic evolution of convergent plate margins

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发表日期：

2025年3月12日

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Long Chen,
YongFei Zheng

摘要

The convergent plate margins of Paleo-Tethys Ocean and Neo-Tethys Ocean are the two orogenic belts in the eastern Tethyan realm with the best exposures of igneous rocks. Previous studies have revealed that both belts have developed not only island arc basalts (IAB)-like mafic magmatism but also ocean island basalts (OIB) ± mid-ocean ridge basalts (MORB)-like mafic magmatism in various stages of their formation and evolution. These different types of mafic igneous rocks show both similarity and difference that were acquired in the various stages from oceanic subduction through continental collision to intracontinental reworking. Comprehensive examination and comparison of these igneous rocks can provide insights into the tectonic evolution of the eastern Tethyan realm. This study integrates the spatiotemporal distribution and geochemical compositions of Mesozoic and Cenozoic primitive mafic igneous rocks in the eastern Tethyan realm within China. The results show: (1) in addition to the stage of oceanic subduction, the stages of continental collision and intracontinental reworking are the primary periods of mafic magmatic activity at convergent plate margins, consistent across the Paleo-Tethyan and Neo-Tethyan domains. (2) OIB (±MORB)-like mafic igneous rocks are present not only in the stage of continental collision in both domains but also in the stage of intracontinental reworking in the Paleo-Tethyan domain. (3) In the stages from oceanic subduction to continental collision, the compositions of IAB-like mafic igneous rocks in both domains exhibit continuous variations, with remarkable similarities in lithology, elemental and isotopic characteristics, indicating their formation through similar deep geodynamic processes—partial melting of the mantle wedge metasomatized by subducting slab-derived fluids in the forms of aqueous solution and/or hydrous melts. As a consequence, the mafic igneous rocks display more compositional inheritance than difference. In the stage of continental collision, the mantle sources of mafic igneous rocks did not acquire geochemical signatures from the subducting continental crust. (4) during the continental collision, some IAB-like and most OIB (±MORB)-like mafic igneous rocks show more depleted radiogenic isotope compositions than their counterparts generated during the oceanic subduction, suggesting an increased contribution from asthenospheric mantle components and a relatively diminished influence from subducting crustal components. (5) extensive mafic magmatism in the compressional regime of continental collision occurs in the overriding plate above continental subduction zones, indicating the local extension of the overlying lithosphere due to upwelling of the asthenospheric mantle in response to rollback or breakoff of the subducting oceanic slab. This upwelling induces simultaneous melting of various mantle sources, leading to high compositional diversity of the mafic igneous rocks during this period. (6) during the intracontinental reworking in both domains, the geochemical characteristics of IAB-like mafic igneous rocks differ significantly from those formed in the stages of continental collision and oceanic subduction, likely due to contributions from subducting continental crust components to their mantle sources. Delamination and thinning of the thickened lithosphere are the most plausible mechanisms for triggering of the mafic magmatism in this stage. The geochemical and petrogenetic similarities between the Mesozoic and Cenozoic mafic igneous rocks from the Paleo-Tethyan and Neo-Tethyan domains are evident in these periods from oceanic subduction through continental collision to intracontinental reworking, suggesting the common features of tectonic evolution across the convergent plate margins of different eras. These features are primarily reflected in the processes of crust-mantle interaction, particularly via the formation of mafic magma sources in the mantle wedge due to metasomatism by fluids derived from subducting oceanic or continental slabs. Moreover, after the formation of the mantle source regions, the key geodynamic processes involved in their partial melting are similar, namely, the rollback of subducting oceanic slabs in the stage of oceanic subduction, the rollback or breakoff of subducting oceanic slabs during the continental collision, and the delamination and thinning of the lithospheric mantle during the intracontinental reworking. The heating caused by upwelling of the asthenospheric mantle is always the most effective mechanism for partial melting of the metasomatic domains in the mantle wedge. Therefore, the occurrence of mafic magmatism always involves two stages in the tectonic evolution of convergent plate margins. The first is the chemical metasomatism of the mantle wedge during the subduction of oceanic or continental slabs, generating the mantle source regions. The second is the partial melting of the metasomatic mantle domains, giving rise to the mafic magmas. The differences between the different stages and types of mafic magmatism lie in the composition of metasomatic agents and the time interval between these two stages.