Geochemical Evidence For Recycled Crustal Materials In Mesozoic Intraplate Basalts,South China

Basalts are partial melting products of deep mantle,and their geochemical characteristics are a comprehensive reflection of various factors such as source composition,melting temperature and pressure,and magmatic evolution.Compared to mid-oceanic ridge basalts?MORBs?that are derived from the convective asthenosphere,the oceanic island basalts?OIBs?are usually formed in association with hotspot or mantle plume.These basalts have complex and diverse geochemical compositions and have been widely considered to reflect the recycling of crustal materials.However,small-scale OIB-type basalts are also widely found in continents,and their petrogenesis and deep dynamic backgrounds remain highly controversial,with the most intensive debate about the identification and derivation of those recycled crustal components in the source.The interior of South China is an ideal site to identify the recycled crustal components and discuss the possible origin of the recycled materials,since it experienced multi-stage small-scale OIB-type basaltic magmatism during Mesozoic time.In this thesis,I perform a comprehensive geochemical study on Jurassic Antang and Ningyuan basalts that have similar geochemical compositions to high-?(HIMU,?=²³⁸U/²⁰⁴Pb)oceanic island basalts,and Cretaceous Ji'an basalts with typical geochemical features akin to OIB.These analyses includes:?1?whole-rock major,trace element and Sr-Nd-Pb-Hf-Os-Ca isotopes;?2?chemical and oxygen isotopic compositions of olivine phenocrysts;?3?water content and chemical and hydrogen isotopic compositions of olivine-hosted melt inclusions;and?4?numerical modelling of thermal state of subducted slab and thinned lithosphere.Through the comprehensive study from mineralogy,petrology,geochemistry and numerical modelling of these two stages of intraplate basalts,the characteristics and main lithology of the mantle source can be constrained,and three types of recycled crustal components have been identified.The results provide important constraints for understanding the deep geodynamics of Mesozoic tectono-magmatic evolution in South China.The major conclusions are summarized below:?1?The primary magma for the Antang basalts was hydrous with the estimated water content more than 2.65 wt%and was probably derived from the mantle transitional zone?MTZ?,which comprised mainly old?>1.0 Ga?recycled oceanic crust.I identify the extremely high-water content?up to 5.0 wt%H₂O?in the unhomogenized melt inclusion.However,the relative depletion of fluid-mobile elements?K,Rb and Ba?and positive Nb-Ta anomalies in bulk rocks and melt inclusions,and the distance far away from the contemporaneous paleo-Pacific subduction zone,preclude an origin from a slab fluid-metasomatized mantle.The strong depletion of CaO in both olivine phenocrysts and whole-rock compositions indicates that the basaltic magmas originated from an eclogitic or pyroxenitic mantle source.Elemental and isotopic modelling further suggests that these basalts were probably derived from a hydrous source?0.16-0.21 wt%H₂O?,which was composed mainly of ancient recycled oceanic crust that had stagnated in the mantle transition zone?MTZ?for more than one billion years.Our results imply that the hydrous MTZ can preserve recycled crustal components for long periods of time and play an important role in the origin of H₂O-rich intraplate basalts.?2?The Ningyuan basalts that are characterized by low?^44/40Ca value were probably derived from an enriched mantle metasomatized by the old recycled carbonate.Similar to the Antang basalts,the Ningyuan basalts also show HIMU-like geochemical features,but with obviously higher CaO and CaO/Al₂O₃ and ²⁰⁶Pb/²⁰⁴Pb.Also,these lavas show positive Sr and weakly negative Hf-Ti anomalies in primitive mantle-normalized spidergrams.Additionally,the negative correlations of Sr/Pr and Sr/Nb with?^44/40Ca indicate an ancient carbonated mantle source.The?^44/40Ca shows a negative correlation with ²⁰⁶Pb/²⁰⁴Pb and a positive correlation with ²⁰⁷Pb/²⁰⁶Pb.The combined geochemical features suggest that the mantle source of Ningyuan basalts was possibly enriched by old recycled carbonate.?3?The recycled paleo-Pacific slab has been identified in the source of late Cretaceous Ji'an basalts??90 Ma?.The Ji'an basalts show geochemical characteristics akin to OIB and have depleted asthenospheric mantle-like Sr-Nd-Pb-Hf isotopic compositions with moderately radiogenic Os.Their olivine-hosted melt inclusions have low H₂O and highly negative?D values and olivine phenocrysts are mainly characterized by depletion of ¹⁸O with?¹⁸O values lowering to 3.9‰.These features are consistent with positive Sr and Eu anomalies in some whole-rock samples.The combined geochemical data suggest that the primary magmas were derived from an asthenospheric mantle enriched by melts from an altered gabbroic oceanic crust,which had experienced intensive dehydration.The low?¹⁸O preserved in olivine and the estimated slab age?<300 Ma?from the radiogenic whole-rock Os and Pb compositions also require the involvement of a recently recycled slab,probably represented by the subducted paleo-Pacific Oceanic plate.Further numerical modelling shows that melting of the dehydrated oceanic crust can occur along the torn flank of the subducting lithosphere,in the case that the slab is strongly thinned and fractured.Rollback of the subducting paleo-Pacific slab might create a slab window,in which melt from the torn/fractured slab reacted with the upwelling asthenosphere to form an enriched mantle source for the Ji'an basalts.Asthenosphere-slab interaction may be an important dynamic mechanism in the genesis of intraplate OIB-type basalts.?4?Considering the Mesozoic tectonic evolution in South China,there is a genetic relationship between the Jurassic-Cretaceous intraplate basaltic magmatism and the subduction of the Paleo-Pacific Ocean.During the early-middle Jurassic,the paleo-Pacific plate subducted to the East Asian continental margin,forming a strong mantle corner flow in South China interior,which triggered passive upwelling and melting of the MTZ material to produce the water-rich intraplate basalts.In the late Cretaceous,the paleo-Pacific plate rollbacked and retreated,leading to tearing or even breakoff of the subducted slab.Interaction between the melt of the tearing slab and asthenosphere formed the water-poor OIB-type mantle source.The results suggest that the subduction stage and mode of an oceanic plate exert different effects on the formation of small-scale OIB-type intraplate basalts.