Full-Text Search:
Home|About CNKI|User Service|中文
Add to Favorite Get Latest Update

Geochemical Characteristics and their Geological Implications of Meso-Cenozoic Basalts in the Northern Ordos Block,North China

ZOU Heping,ZHANG Ke,LIU Yuliang and LI Gang(Department of Earth Sciences,Sun Yat-sen University,Guangzhou 510275,Guangdong,China)  
Based on the geochemical and isotopic compositions of the Meso-Cenozoic basalts from the northern Ordos Block in North China,the implication of the possible mantle processes of the North China craton breakage beginning from the Mesozoic is discussed.The early Cretaceous basalts with Ar-Ar age of 126.2 Ma are distributed at Hangjin Banner in the northern Ordos and the late Miocene basalts with Ar-Ar age of 6.4 Ma are located at Helinger County in the northeastern Ordos.The early Cretaceous basalts from Hangjin Banner belong to alkalic potassic trachybasalts with lower SiO2 (46.93%~47.93%),higher alkali contents ( K2O+Na2O=5.75%~6.21% and K2O (Na2O-2)),and the serial index (σ)of 6.7~8.0.Their Mg indices are from 32.4 to 32.8 and contents of Ni-Cr are lower than 20μg/g,indicating that there had occurred crystallization differentiation of minerals such as olivine and monoclinic pyroxene in basaltic magmas.The early Cretaceous basalts show Nb-Ta depletions relative to La,and their initial 87Sr/86Sr ratios are 0.7062 and εNd(t)-12.25,indicating that the magma was mainly derived from the enriched EMI-type mantle.On the other hand,the late Miocene basalts from Helinger belong to the olivine tholeiite associations with Mg indices being 46.7~52.4 and the serial index (σ) 2.1~2.8.Their contents of Ni and Cr are 111.1~125.8μg/g and 123~178μg/g,respectively.Variation of the Mg indices and contents of Ni-Cr show a higher degree of original magma with the passage of time.The late Miocene basalts also show Nb-Ta enrichment that is similar to OIB in incompatible trace elements and relatively enriched characteristics of Sr-Nd isotope compositions with the initial 87Sr/86Sr ratios ranging from 0.7051 to 0.7053 and εNd(t)from-1.91 to-0.74,implying that the late Miocene magmas may have been the products as interaction of the asthenosphere and the EMI-type continental lithospheric mantle.Furthermore,both the basalts from Hangjin Banner and Helinger County are of low 207Pb/204Pb ratios with 207/204 being 6~6.5 and 208/20460,showing characteristics of the EMI-type Dupal isotope anomaly.Therefore,it is further indicated by the geochemical and Sr-Nd-Pb isotopic data of the basalts that a small-scale extension and thinning of the lithosphere beneath the Ordos block had taken place since the early Cretaceous.The extent of thinning and transformation of the lithosphere beneath the western North China,however,was far weaker than that of the eastern North China.
Download(CAJ format) Download(PDF format)
CAJViewer7.0 supports all the CNKI file formats; AdobeReader only supports the PDF format.
©CNKI All Rights Reserved