Mesozoic mafic magmatism in the eastern South China Block and its implication
The eastern South China Block(SCB)was mainly influenced by Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic,accompanied by intensive magmatism and mineralization.However,the tectonic transition and evolution of these two dynamic domains remain debated.The tectonic transition has significant effects on the tectonic,sedimentary and magmatism activities in the eastern SCB.The mafic igneous rocks are excellent indicators for exploring the interaction among asthenosphere,lithosphere mantle and subducted slab.The spatio-temporal framework of Mesozoic mafic rocks is critical for unravelling the tectonic transition and evolution.Thus,this paper complied the geochronology and geochemistry data of Mesozoic mafic igneous rocks in the eastern SCB,aiming to constrain the tectonic transition and the influence of Paleo-Pacific Plate subduction.The formation ages of the Mesozoic mafic igneous rocks(228-69 Ma)mainly fall into five age-clusters at ca.228-223 Ma,ca.197-169 Ma,ca.168-146 Ma,ca.143-115 Ma,and ca.113-69 Ma.The Triassic mafic igneous rocks in the eastern SCB are enriched in large-ion lithophile elements(LILE)and high-field strength elements(HFSE)with significant Nb-Ta positive anomalies.They have high(87Sr/86Sr)i ratios and low εNd(t)values,deriving from partial melting of an enriched lithosphere mantle and forming in an post-collision setting associated with the Paleo-Tethyan dynamic domain.The Jurassic and Cretaceous mafic igneous rock can be geochemically classified into OIB-,OIB-like-and"arc"/arc-types,originating from different mantle sources.The OIB and OIB-like mafic rocks were derived from asthenosphere and asthenosphere-lithosphere interaction,respectively.The Cretaceous arc mafic rocks were derived from a mantle wedge which was modified by Paleo-Pacific subducted slab-derived melts/fluids.The Cretaceous mafic rocks show geochemical variations from the interior(OIB and OIB-like-types)to coastal provinces(arc-type).From the interior to coastal provinces,the Cretaceous mafic igneous rocks exhibit a decreasing trend of Nb/Yb,Nb/La,Ce/Pb ratios and εNd(t)values across the Ganjiang Fault,suggesting the increasing contribution of Paleo-Pacific subducted slab-derived components.Such spatio-temporal framework indicates that the slab metasomatism associated with Paleo-Pacific Plate subduction during the Cretaceous was probably limited to the east of the Ganjiang Fault.The Jurassic"arc"-type mafic igneous rocks are marked by enrichment in LILE and depletion in HFSE with significant Nb-Ta positive anomalies.However,they have Nb/La,whole-rock εNd(t)and(206Pb/204Pb)i,and zircon εHf(t)values higher than the Cretaceous arc mafic rocks.Furthermore,the Jurassic"arc"-type mafic rocks mainly occurred at the west of the Ganjiang Fault,distinct from the Cretaceous arc mafic rocks.Such spatial-temporal and geochemical signatures suggest that the Jurassic"arc"-type mafic rocks were not genuine arc rocks which were associated with Paleo-Pacific Plate subduction.Instead,their"arc"signatures appear to have been inherited from their mantle source.The Early-Middle Jurassic mafic rocks occurred in the EW-trending Nanling Range,and the contemporaneous sedimentary rocks share similar detrital provenance to the Late Triassic sedimentary rocks.The eastern SCB was not influenced by Paleo-Pacific Plate subduction during the Early-Middle Jurassic,and inherited the tectonic regime from Paleo-Tethyan dynamic domain.The Late Jurassic mafic rocks occurred as a NE-trending,and the basin type,detrital provenance and tectonic regime changed,suggesting a significant effect from Paleo-Pacific Plate subduction on the eastern SCB.Thus,the tectonic transition from Paleo-Tethyan to Paleo-Pacific dynamic domains likely occurred at ca.200 Ma initially,and completed at ca.170 Ma finally.The Paleo-Pacific dynamic domain has significant influences on the eastern SCB since Late Jurassic.
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