Petrogenesis Of Wehrlite And Pyroxenite Xenoliths In Early Cretaceous Igneous Rocks From Western Shandong, China

The thesis reports the petrographic, mineralogical and whole rock geochemical data forthe wehrlite and pyroxenite xenoliths from the Early Cretaceous Tietonggou intrusion,Jinling intrusion, Feixian and Fangcheng basalts in western Shandong Province, eastern partof the North China Craton (NCC), with the aim of determining their petrogenesis. In addition,based on the petrography, mineralogy and isotopic geochemistry for the selected wehrlite andpyroxenite xenoliths with the suggested genesis of melt-peridotite reaction, the nature andorigin of the melts modifying the lithospheric mantle are discussed. Based on the SIMS U-Pbdating and the oxygen isotopic data of zircons from the pyroxenite xenoliths in Fangchengbasalt, the destruction timing of the NCC has been determined. Taken together, this thesisfinally elucidates the deep processes and mechanism of the destruction of the eastern NCCduring the Mesozoic. Main achievements are as follows:1. The origin of wehrlite xenolithsThe petrographic and mineral geochemical data of the wehrlite xenoliths from theTiegonggou high-Mg diorites, Feixian and Fangcheng basalts indicate that the wehrlitexenoliths in our study area could be formed by melt-peridotite reaction. Evidences are asfollows:The olivines in the wehrlite xenoliths generally occur as an isolated residue withinclinopyroxene, and orthopyroxene was modified by clinopyroxene. The forsterite (Fo) contentand nickel concentration of the olivines from the wehrlite xenoliths mainly range from88.9to90.8and from2200to3285ppm, respectively, implying their mantle genesis. However, someof these values are lower than those of olivines derived from the lithospheric mantle.Combined with their oxygen isotopic compositions (δ18O=5.41‰-7.3‰) and the highCaO concentrations, it is suggested that the olivines experienced the modification of melts.The clinopyroxenes in the wehrlite xenoliths have Al2O3=0.7wt%-1.19wt%, TiO2=0.04 wt%-0.12wt%, Na2O=0.26wt%-0.41wt%, and Mg#=91.2-94.1(Mg#=100Mg/(Mg+Fetotal)), and are characterized by enrichment in the light rare earth elements (LREEs),depletion in the heavy rare earth elements (HREEs) and high field strength elements (HFSEs),and no Eu anomalies. The87Sr/86Sr,143Nd/144Nd, and187Os/188Os ratios of the wehrlitexenoliths range from0.70737to0.71403, from0.51218to0.51241, and from0.12661to0.57650, respectively. These lines of evidence suggest that the wehrlite xenoliths in our studyarea could be formed by the interaction between the recycled continental crust-derived meltsand the mantle peridotite.2. The origin of pyroxenite xenolithsThe pyroxenite xenoliths are found in Tietonggou and Jinling high-Mg intrusions,Fiexian and Fangcheng basalts. According to their petrography and mineral chemistry, thepyroxenite xenoliths can be subdivided into three types of origins:Cumulate pyroxenite:This type of pyroxenite xenoliths is found in Tietonggou and Jinling diorites, and arecharacterized by cumulus texture. Clinopyroxenes in the pyroxenite xenoliths have Mg#=81.7-90.1, Al2O3=1.01wt%-4.58wt%, Na2O=0.30wt%-0.92wt%, and TiO2=0.05wt%-0.39wt%, and are characterized by enrichment in LREEs and depletion in HREEs,similar to the clinopyroxene phenocrysts from Cenozoic basalts in eastern China.Pyroxenite xenoliths from deep continental crust:These pyroxenite xenoliths are found in Tietonggou and Jinling intrusions, and displayslaty structure and granoblastic texture. The clinopyroxenes from this type of pyroxeniteshave Al2O3=0.64wt%-2.92wt%, Na2O=0.48wt%-1.23wt%, TiO2=0.02wt%-0.26wt%, and Mg#=72.0-80.6, and have exhibit relatively flat rare earth element (REE) patterns,as well as negative Eu and Sr anomalies. They are also enriched in LILEs, and depleted inHFSEs (such as Nb, Ta, Zr, Hf), similar to the clinopyroxenes from the Archean granuliteterranes, suggesting that they could be originated from deep crust.Pyroxenites originated from melt-peridotite reaction:Abundant melt-peridotite pyroxenite xenoliths are hosted in Tietonggou and Jinlinghigh-Mg diorites, Feixian and Fangcheng basalts. The petrographic observations indicate thatolivine is replaced by orthopyroxene, and the orthopyroxene is subsequently replaced byclinopyroxene. In some xenoliths, metasomatic amphibole, mica and carbonate veins can bealso observed under microscope. Compared with the mantle-derived olivines, the olivines from this type of pyroxenitexenoliths have relatively lower Fo (79.1-86.4) and nickel contents (1167ppm-2468ppm),and distinctively high δ18O values (6.58‰-8.40‰), indicating that the mantle-derivedolivines had been modified by silica-rich melt. Similarly, the orthopyroxenes andclinopyroxenes from the pyroxenite xenolith have much higher nickel concentrations thanthose of orthopyroxenes and clinopyroxene from mantle-derived peridotite xenoliths andphenocrysts within Cenozoic basalts, the nickel contents of these orthopyroxenes andclinopyroxenes range from1097to1491ppm and from581to809ppm, respectively. Themuch high Ni contents in orthopyroxenes and clinopyroxenes from the pyroxenite xenolithsindicate that they inherited the composition of previous olivines, revealing the existence of thereaction between melts and the mantle-derived olivines.3. The modification timing of modifying lithosperic mantleZircons from the pyroxenite xenoliths via melt-peridotite reaction in Fangcheng basaltsare mainly euhedral-subheduel in shape and display fine-scale oscillatory growth zoning,implying their magmatic origin. SIMS U-Pb dating indicates that the youngest population of206Pb/238U ages rangs from153to157Ma. In addition, the metamorphic zircon with the ageof215Ma, and a large amount of Paleozoic (508Ma-512Ma) and Paleoproterozoic as wellas Neoarchean magmatic zircons are found in these xenoliths. Combined with the SHIMPU-Pb dating results of zircons from dunite xenoliths in the Tietonggou high-Mg diorites (theyoungest age populations are143and160Ma), it is suggested that the most intensivemodification of the lithospheric mantle beneath the eastern NCC took place during LateJurrassic to early Early Cretaceous.4. The nature and origin of the melts modifying the lithospheric mantleThe trace element compositions of cilnopyroxenes suggest that the wehrlite andpyroxenite xenoliths formed by melt-peridotite reaction in the Tietonggou intrusion were theproducts of Archen lithospheric mantle modified by silica-rich melts, whereas the pyroxenitexenoliths from Feixian and Fangcheng basalts represent the lithospheric mantle modified bycarbonate melt.The ISrand εNd(t) ratios of wehrlite xenoliths from the Tietonggou intrusion range from0.70596to0.70737and from0.512181to0.512416, respectively. Their initial187Os/188Os(125Ma) ratios range from0.12661to0.57650. The average δ18O values of olivines is6.5‰±0.4. These features imply that the metasomatic melts could be derived from the partial melting of recycled crust. The δ18O values of olivines in pyroxenite xenoliths from Feixianand Fangcheng basalts range from6.58‰to8.40‰. The87Sr/86Sr ratios of clinopyroxenesin the websterite xenoliths vary from0.70862to0.70979. Combined with the δ18O values ofzircons in pyroxenite xenoliths, it is suggested that the some of metasomatic melts could be atleast derived from the partial melting of the deep-subducted Yangtze slab.5. The process and mechanism of the modification of the lithospheric mantle in theeastern NCCThe petrography of wehrlite and pyroxenite xenoliths from western Shangdong Provincein the eastern NCC provides insights into the processes resulting in the destruction of theNCC during the Mesozoic. The following reactions can represent the deep processesmodifying the lithospheric mantle during the destruction of the NCC:Mg2SiO4(olivine)+SiO2—2MgSiO3(orthopyroxene)Mg2Si2O6(orthopyroxene)+2CaCO3+2SiO2—2CaMgSi2O6(clinopyroxene)+2CO2The above modification processes, together with in situ trace element and isotopecompositions of minerals from pyroxenite and wehrlite xenoliths, indicate that the mechanismmodifying the lithospheric mantle in the eastern NCC is a reaction between the melts derivedfrom the recycled continental crust and the mantle peridotite.