The Mantle Evolution Of South China From Mesozoic To Cenozoic:A Causal Link With The Subduction Of The Pacific Plate

The South China Block had experienced multistage collisions with other blocks nearby during the Early Mesozoic,and during the subsequent Late Mesozoic,the tectonic regime of South China had changed from the Tethys tectonic system to the Paleo-Pacific tectonic system.The extensive arc-type magmatism occurring in the southeastern coast of China after 150 Ma implies that the Pacific slab had subducted into the mantle beneath South China intensively during that period,and the magmatic activity migrating oceanward to the southeast indicates that the slab dip angle of the Pacific plate was increasing.However,when and which style the Pacific plate subducted into the mantle beneath South China are still controversial,partly due to the lack of a complete understanding of geochemical evolution of the Mesozoic mantle.In this thesis,I tried to constrain these issues with the perspective of basalts,using the temporal geochemical and petrologic evolution of the mantle source to constrain the initial time of the subduction of the Pacific plate.The main work are as follow:1)Water has been expected to play an important role in the mantle evolution,because it can not only promote melting of the mantle but also serves as a special element to trace the source feature of basalts.The water content of basaltic magma can be evaluated by several methods,among which retrieving the water contents of magmas through phenocrysts has been developed in recent years.This method is based on the water contents of phenocrysts and the partition coefficients of H2O between phenocrysts and basaltic melts.The water contents of phenocrysts are usually analyzed by FTIR,based on the polarized spectra analyzing on the oriented section.However,the preparation of oriented samples is time consuming and not always possible in many petrologically interesting areas,so simplified methods are adopted in some situations.In this thesis,I reviewed the commonly-used methods and evaluated their uncertainties comprehensively to show that in which situations they could be applied.2)The Jitai Basin is the only known area in South China that contains multi-periods of Mesozoic basalts with negligible crustal contamination,making it a fantastic place to study the Mesozoic mantle evolution.In this thesis,I studied three periods(168 Ma,143-139 Ma,90 Ma)of basalts in the Jitai Basin through major and trace element and Sr-Nd isotope compositions of bulk rocks,major and minor element compositions of olivine phenocrysts and water contents of clinopyroxene phenocrysts.The Fe/Mn ratios of olivine phenocrysts increase from 168 Ma to 90 Ma,implying that more pyroxenites were involved in the mantle source of the late-stage basalts.On the trace element spider diagrams of bulk rocks,all the 168 Ma basalts have negative anomalies of K,which are absent for all the 143-139 Ma basalts but appear again for part of the 90 Ma basalts.Moreover,the negative anomalies of K always accompany with low degrees of partial melting.This evolution tendency implies that the recycled dehydrated sediments had decomposed and been involved in the mantle source from 168 Ma to 143 Ma,and then partly consumed in the subsequent period.Combining with the previous studies of the granites and tectonism,a flat-slab subduction process starting at?265 Ma is more likely to induce the temporal geochemical evolution of the Mesozoic mantle beneath South China.3)The seismic data have revealed the stagnant Pacific slab in the mantle transition zone beneath Zhejiang and north Fujian,but this slab is absent in central and south Fujian.The previous data of the Cenozoic basalts show that the negative anomalies of K on the trace element spider diagrams of bulk rocks always accompany with the absent of the stagnant Pacific slab,as well as the relatively low degrees of partial melting,and those bulk rocks with the presence of the slab in the mantle transition zone usually show the opposite features.Our data of the Cenozoic olivine phenocrysts in this thesis also show that the Fe/Mn ratios of olivines are higher with the presence of the stagnant slab in the mantle transition zone.Therefore,the geochemical and petrologic evolution tendency of the Cenozoic basalts in South China is similar with the Mesozoic ones,both of which suggest the influence of the subducted stagnant Pacific slab on the source characteristics of the basalts.