Water Contents,Nature And Evolution Of The Lithospheric Mantle Beneath The South China Block

The continental lithospheric mantle is the crust-mantle transition layer.It plays an important role in the interaction and evolution of the convection mantle-lithosphere mantle-crust system.The study of the composition,structure,formation and evolution of lithospheric mantle has become one of the frontier hotspots of solid earth science.Peridotite xenoliths entrained in alkali basalts and kimberlites are most direct samples to explore the composition,nature and evolution of the lithosphere mantle.The main minerals of peridotite xenoliths(e.g.,olivine,clinopyroxene,orthopyroxene and garnet)are called “nominally anhydrous minerals”(NAMs)whose ideal chemical formula does not contain hydrogen.Recent researches show that these NAMs can contain a certain quantity of hydrogen that mainly occurs as hydroxyl in crystal defects(known as “structural water”)and this structural water constitutes the most important water reservoir in the deep lithosphere.Although the absolute value of water contents is not high,they could still change the physical and chemical properties of minerals and/or rocks(e.g.,reduce the wave velocity,change rheological characteristics,enhance the electrical conductivity,lower thermal conductivity,change the optical properties,lower melting temperature,speed up the ion diffusion)and could be used to trace the operation of various geological processes.Therefore,this thesis investigated the peridotite xenoliths in the South China Block to use integrated studies of the occurrence mode,content and distribution of water to investigate the composition,nature,and structure of the lithospheric mantle and its geodynamic evolution.The peridotite xenoliths beneath the studied area(Daoxian,Xinchang,Xilong,Anyuan and Pingnan)are mainly(spinel)lherzolithes with minor(spinel)harzburgites.The major elements compositions of the minerals and whole-rocks show the same characteristics.The xenoliths beneath the Daoxian area are more fertile,while other areas are mainly fertile with minor moderately refractory peridotites.Clinopyroxenes in peridotites beneath the Daoxian and Pingnan areas show LREE-depleted pattern,and commonly exhibit negative anomalies in Nb and Ti,and clinopyroxenes in peridotites beneath the Xinchang,Xilong and Anyuan areas exhibit variable REE patterns from LREE depleted to LREE-enriched patterns and negative Ti anomalies.The fertile peridotite xenoliths could have been produced by low degree of partial melting,and fertile-moderately refractory xenoliths mostly experienced low to moderate degree of partial melting while only a few samples experienced high partial melting.Multiple melt-peridotite interactions were also recorded during the evolution process,followed by silicate metasomatism,and the metasomatic agent may originate from the asthenosphere.All of the clinopyroxene and orthopyroxene analyzed here exhibit several absorption bands in the typical OH-stretching vibration region.The analyzed olivine grains from Anyuan area display pronounced OH bands,unlike olivine in other areas,most of which have no detectable OH.According to water contents from calculated mineral and mineral modes,the whole-rock water contents of Anyuan peridotite xenoliths range from 63 to 120 ppm.The water contents of olivine from other areas are below the detection limit(< 2 ppm wt.H2O)but can be roughly estimated from the water contents of pyroxene and the previously reported partition coefficients between pyroxene and olivine.We use the Dcpx/ol = 10 to calculate water contents of olivine,which probably represent maximum estimates.The whole-rock water contents were calculated based on the water contents of olivine,orthopyroxene and clinopyroxene and their modal abundance.According to the calculated results,the whole-rock water contents of Pingnan peridotite xenoliths from Daoxian,Xinchang,Xilong and Pingnan areas are 35-75 ppm,12-21 ppm,15-28 ppm and 14-85 ppm,respectively.The water contents in Xinchang and Xilong is relatively low,and most of the samples in other areas are similar to the lower limit of the water content in the MORB(50-200ppm)source.The whole-rock water content in this study area has wide range of variation.The heterogeneity of the water content from Daoxian lithosphere mantle may be integrated controlled by the partial melting,metasomatism and redox state,and the other four areas are mainly affected by partial melting process rather than mantle metasomatism or redox state.The equilibrium temperature of the lithospheric mantle beneath the studied area are 877-1063?.Mg# in olivine does not obviously vary with equilibration temperatures,implying that the lithospheric mantle is compositionally heterogeneous,and the fertile and moderately refractory peridotite xenoliths coexist within the same depth ranges,similar to the South China Block.we suggest that the SCLM beneath the studied area,and even beneath the whole interior of the South China,could be dominated by young fertile lithospheric mantle which still preserves remnants of the ancient refractory mantle.The mantle extension and replacement of the lithosphere in South China Block have occurred as early as the Triassic.Combined with the study of water content showed no significant difference between the Cretaceous and Cenozoic beneath the South China Block,indicating that the lithosphere thinning and replacement in the Block occurred earlier than that in cretaceous.The South China Block has been affected by the Pacific subduction on the east side and accompanied by large-scale magmatic activity,which leads to the South China Block under the background of lithosphere extension and asthenosphere upwelling.It makes partial melting of ancient enrichment lithosphere mantle.During the early Cenozoic,the lithosphere in the South China Block was further extended.After the early enrichment of the lithospheric components,the mantle was transformed into the asthenosphere mantle.The fluid/melt derived from the asthenosphere mantle leads to lithosphere mantle hydration,resulting in relatively highwater content beneath the South China Block.Therefore,the coexistence of fertile mantle and refractory-moderate mantle beneath the South China Block might be ascribed to irregular erosion or peridotite-melt interaction by the upwelling asthenosphere mantle,and the replacement of juvenile and fertile lithospheric mantle during cooling of the upwelled asthenosphere.