Origin Of The Late Mesozoic Alkaline Basalts And The Nature Of The Lithospheric Mantle In The Liaoyuan Area, Jilin Province

This thesis takes the Late Mesozoic alkaline basalts, as well as their deep-seatedxenoliths and xenocrysts in the Liaoyuan area of Jilin province as its research object.The formation times of these basalts were determined using K-Ar dating methods.The basaltic magama source and the tectonic setting of its formation are investigatedbased on the petrography, major-and trace-elements data. By researching the mineralchemical data from the deep-seated xenoliths and xenocrysts, the nature and deepprocess of Late Mesozoic Lithospheric Mantle are analyzed.1Formation times of the basaltsThe formation times of these basalts are determined using K-Ar dating methods.The whole-rock K-Ar dating results indicate that Liaoyuan basalts were formed in thelate Cretaceous (91.7±1.7Ma,82.5±1.9Ma,99.3±1.6Ma,82.2±2.4Ma), insteadof the previously concluded Neogene.2Basaltic magmatic source and tectonic setting of late Cretaceous basaltsLiaoyuan late Cretaceous basalts have Mg#[100Mg/(Mg+Fe2+)]=60-63, SiO2=45.1-45.9%and Na2O+K2O=4.50-6.72%, suggesting that they belong to thealkaline series. According to the standards of CIPW, they belong to basanite (Ne>5%)and alkaline olivine basalt (5%>Ne>0). Liaoyuan basalts are enriched in light rareearth elements (LREEs), and depleted in heavy rare earth elements (HREEs)[(La/Yb)N=12.0-14.0], and have no Eu anomalies. Their REE patterns are similar tothat of OIB, and their trace element geochemistry displays some characteristics ofcontinental intraplate volcanic rocks. Liaoyuan late Cretaceous basalts’ magmaticsource is asthenospheric mantle, which is formed during stretch mechanism of plate,related to the subduction of the Paleo-Pacific plate.3Nature and deep process of Late Cretaceous Lithospheric MantleThe entrained peridotite xenoliths consist mainly of spinel lherzolites and aminor amount of dunite xenoliths, in which olivines have relatively low Mg#(87.1-91.2). Combined with the major-and trace-elements geochemical characters ofthe mineral, Late Cretaceous Lithospheric Mantle of Liaoyuan is mainly composed ofthe new accreted lithospheric mantle, and may has residual older lithospheric mantle.The incompatible elements of clinopyroxenes from the peridotite xenoliths vary fromthe enrichment to the depletion: enriched in light rare earth elements (LREEs), whole types are flat, and depleted in light rare earth elements (LREEs), suggesting that thelithosphere mantle has experienced partial melting and fluid/melt metasomatism, andthe metasomatizing agents are mainly silicate melt and minor carbonate melt.